Adoption of adaptation protocols and feed additives to improve performance of feedlot cattle

ABSTRACT The evolution of ruminants was largely influenced by the symbiotic relationship between ruminal microbiota and the host. Within the rumen, these microorganisms degrade feedstuffs to produce organic acids and synthesize microbial protein, as energy and protein source for ruminants, respectively. This process is accelerated when these animals are transferred to feedlot diets. This review brings the most current studies that reported the most used nutritional managements in this transition process, in order to avoid metabolic disturbances, in which rumen development is a fundamental point. Also, this review brings the main biomolecules used as feed additive, which can in the help control of fermentation processes in order to minimize energetically inefficient processes within the rumen, as well as losses by excretion to the environment, which are widely questioned by be environmental pollutants. Furthermore, the main results of research on animal performance in response to these additives are reported, supplemented alone or associated with different feed additives when used on diets with high grain contents, as well as evaluating the potential of additives as antibiotics alternatives, a fact that has been discussed and is worrying since the use of ionophores or non-ionophores antibiotics represent practically the totality of feedlot diets.


Introduction
Ruminant animals evolved by ingesting fibrous food within their habitat.Due to the composition of grasses, whether from temperate or tropical climates that emerged in the Miocene, these animals developed a fermentation chamber for cellulolytic fermentation to extract energy from this type of food.This fact led these animals to develop a symbiotic process with their pre-gastric microbiota, which made them extremely efficient animals in extracting energy from roughages containing high content of structural carbohydrates (Van Soest 1994).According to the feeding habits of ruminant animals, Hofmann (Hofmann 1988) classified them as concentrate selectors, grass-eaters and intermediates types.Within this classification, cattle are grouped with grass eaters, where the rumen is a fraction of the gastrointestinal tract of great importance for the storage of large amounts of roughage, and consequent fermentation of this fibrous material to extract its energy.
Currently, intensive cattle management systems encourage the inclusion of high amounts of cereal grains in the diet in order to increase energy input to improve animal performance.Moreover, these ingredients may vary in their inclusion and/or extent of processing (Owens et al. 1997), being corn the most used cereal grain in feedlots (Samuelson et al. 2016;Silvestre and Millen 2021).Nevertheless, since ruminants did not evolve by ingesting large amounts of non-structural carbohydrates, a series of metabolic disturbance may become a concern, and negatively impact rumen and animal health (Nagaraja and Titgemeyer 2007).Furthermore, the increase in energy content of cattle diets, which may lead to digestive disorders without a proper adaption (Estevam et al. 2020), is attributed not only to higher inclusions of non-fibrous carbohydrates (NFC) (Galyean and Gleghorn 2002) but also to the extent of grain processing (Owens et al. 1997)and the reduction of fiber inclusion (Llonch et al. 2020), since there will be an increase on starch availability, resulting in a high rate of short-chain fatty acid (SCFA) release that, if they exceed the absorption capacity of the rumen papillae, promote accumulation and decrease of ruminal pH (Penner et al. 2009).Digestive disorders, such as ruminal acidosis (Owens et al. 1998;McCartney 2002) negatively impact the performance of cattle consuming high-grain diets (Krause and Oetzel 2006), and it has been reported as the second most common health problem affecting feedlot cattle (Millen et al. 2009;Oliveira and Millen 2014;Pinto et al. 2019;Silvestre and Millen 2021).
One of the management practices adopted by nutritionists worldwide is the use of nutritional protocols to adapt cattle to high-energy diets.This period of adaptation gives cattle time to adjust both the ruminal microbiota and epithelium.The gradual transition period from high-forage to high-concentrate diets should not be shorter than 14 days, regardless of the energy content of the finishing, breed, or grain processing (Brown et al. 2006;Barducci et al. 2019).It takes time, or at least 14 days for the ruminal papillae to grow and adapt to absorb the increasing amount of SCFA produced in the rumen.Therefore, adaptation protocols properly designed and conducted minimize rumen acidification and enlarge the absorptive surface area of the ruminal epithelium (Brown et al. 2006).
Among alternatives to control rumen acidification and manipulate rumen fermentation processes, the use of feed additives is popular among cattle nutritionists.These feed additives are classified as ionophores, non-ionophore antibiotics, probiotics and plant extracts with antimicrobial effects (MAPA 2004;Zeoula et al. 2008).Therefore, the improved animal health and performance through the dietary use of these feed additives consist of improving the ruminal ecosystem efficiency, reducing energy losses, lactate and methane production (Berchielli et al. 2006).
Thus, the objective of this review is to provide an overview of the adoption of adaptation protocols and feed additives to prevent ruminal acidosis in feedlot cattle, and their impacts on performance and metabolic disorders.

Use of high-energy diets to cattle and its association with ruminal acidosis
The improved performance associated with the reduction of the slaughter age is often achieved due to the use of diets containing high levels of concentrate feedstuffs in order to meet energy demands and maximize production (Ye et al. 2016).According to Foreign Agricultural Service's report Livestock and Poultry: World Markets and Trade, Brazil is the world largest beef exporter (USDA, 2022), in response to an increase in international market demand for Brazilian beef.As a result, there was an increase in the number of cattle finished in feedlots, where Nellore cattle are the predominant beef breed type in Brazil.However, the Brazilian feedlot industry is relatively recent when compared to American industries, but still evolving (Silvestre and Millen 2021).Moreover, the range in roughage levels is much larger in Brazilian feedlots than in North America.The caloric density of finishing diets varies due to differences in the level of grains used.North American surveys show that most nutritionists adopt inclusions greater than 78.2% (Samuelson et al. 2016), which is different from what is adopted in Brazil, where most nutritionists (50%) still work with levels between 51 to 65% (Silvestre and Millen 2021).
Furthermore, the energy source of diets and also the degree of grain processing play an important role in impacting ruminal fermentation (Owens et al. 1997).Corn, sorghum, barley and wheat are among the most used grains in Brazilian feedlot diets (Silvestre and Millen 2021), which are usually processed to increase starch degradability in the rumen and starch digestion in the total gastrointestinal tract, resulting in an increased concentration of metabolizable energy (ME) in the diet (Owens et al. 1997).
The texture of the corn hybrid used in diets can also directly impact its digestibility in the gastrointestinal tract of cattle (Correa et al. 2002;Corona et al. 2006), so the processing extension may have a greater influence on the hard texture hybrids.McCann et al. (2016) reported that corn hybrids with floury endosperm (dent corn) were digested more efficiently than those with hard endosperm (flint corn) when dry rolled; however, when grains went through the high-moisture ensiling processes, this difference in digestion disappeared.Caetano et al. (2015) evaluated starch levels and processing methods (dry-rolled vs high-moisture flint corn silage) in diets of Nellore bulls, and reported that the use of high-moisture corn associated with dietary starch levels promoted a linear increase in net energy for maintenance (NEm) and net energy for gain (NEg), however, when corn was processed as dry rolled, dietary starch levels did not affect diet NEm and NEg.Likewise, the smaller the particle size of flint corn, the greater the digestion of starch in the rumen (Philippeau and Michalet-Doreau 1998;Correa et al. 2002)and the total gastrointestinal digestive tract (Corona et al. 2006), since the reduction in particle size reduces the particle colonization time by the rumen microbiota.
Forage sources are usually included in high-energy diets to promote rumen buffering and offset ruminal acidification; however, because of their low energy content and digestibility values (Allen 1997;Mertens 2002), diet inclusions are limited and associated with stimulation of rumination (Allen 1997).Forage inclusion levels recommended in finishing diets by feedlot cattle nutritionists are 9% (Samuelson et al. 2016) and16.8 (Silvestre andMillen 2021) in the United States and Brazil, respectively.The physical characteristics of this fiber have great relevance in carrying out its function.Beauchemin and Yang (Beauchemin and Yang 2005) evaluated the effects of three levels of physically effective neutral detergent fiber (peNDF) in dairy cow diets, 11.5, 10.3 and 8.9% of dry matter intake (DMI), and noted that chewing time (feed + rumination) increased linearly with the increase in peNDF; however, ruminal pH data did not differ between treatments.Likewise, Llonch et al. (2020) evaluated the levels of peNDF: 6.4, 10.4, 13.6 and 15.4% for beef cattle, and also found no difference between mean ruminal pH; however, the time under the limits of pH 5.8, 5.7, 5.6 and 5.5 linearly decreased with the increase of the peNDF, which made the authors recommend levels of 10.4% of peNDF, a value lower than the 15.3% found by Mertens (Mertens 2002) in its regression to maximize the average daily gain (ADG).Weiss et al. (2017) working with forage levels and particle size noted that the inclusion of 5% forage had a greater pH area below 5.6, greater production of total SCFA, as well as a shorter rumination time when compared to cattle receiving 10% of roughage treatments.Thus, when cattle are fed diets containing lower levels of forage and higher grain content, the capacity of the rumen epithelium to remove SCFA may be exceeded without the proper inclusion of peNDF (Penner et al. 2009) to avoid the accumulation of H + ions (Stone 2004), which may lead to ruminal acidosis.
The literature classifies ruminal acidosis as acute or subacute based on the pH within the compartment (Nagaraja and Titgemeyer 2007).A ruminal pH of 5.6 or below is generally considered the threshold for ruminal acidosis; a pH range between 5.0 and 5.6 is considered to be subacute; and rumen pH below 5.0 is considered acute acidosis (Britton and Stock 1989;Owens et al. 1998;Krause and Oetzel 2006).
Regarding subacute acidosis, ruminal pH tends to depress as a result of the increased fermentation rate due to the addition of quickly fermentable substrates, replacing dietary fiber (Therion et al. 1982; NASEM-National Academies of Sciences E, and Medicine 2016) causing overproduction of SCFA.As a consequence of the reduction in fiber levels, rumination, and therefore the entry of bicarbonate via saliva into the rumen is reduced (Maekawa et al. 2002).In this situation, the fermentation pathway may be shifted, and propionate production may occur using the acrylate pathway, which contains lactate as an intermediate product (Hino and Kuroda 1993).As the pH approaches 5.0 or below for a period of time, the growth of lactate-utilizing bacteria is inhibited (Therion et al. 1982), and therefore lactate begins to accumulate (Hino and Kuroda 1993).Therefore, subacute acidosis has the potential to become acute acidosis as lactate builds up.Lactic acid is about 10 times stronger than the most common SCFA (pKa 3.9 vs 4.9), hence, it is less protonated than others SCFA, accumulating and contributing to depression of rumen pH.
This change in ruminal SCFA production is due to a change in microbiota metabolism as a result of pH.Hungate et al. (1952) were the pioneers in reporting this effect on ruminal flora, showing the impact of grain addition, and consequent pH reduction, on the ruminal communities of microorganisms.In their findings, they noted an increase in gram-positive bacteria, in which Streptococcus bovis was the major cause of ruminal acidification.This strain of bacteria is considered the main etiological agent of acute acidosis (Nagaraja and Titgemeyer 2007), due to the enzymes, lactate dehydrogenase (LDH) and pyruvate formate lyase (PFL), which makes it a mixed fermenter, so the regulation of its fermentation pathway is due to ruminal pH and allosteric effect, in which LDH is regulated by fructose-1,6-diphosphate and PFL by dihydroxyacetone phosphate and glyceraldehyde-3-phosphate (Asanuma et al. 1999;Asanuma and Hino 2002).As the ruminal pH decreases to values below 5.6, with rapidly fermentable substrates in large quantities, LDH is activated, and this bacterium then starts to produce lactic acid (Russell and Hino 1985), which may result in a drop in ruminal pH.
As ruminal pH decreases and remains low, there is a negative effect on the rumen microbiota and the rumen epithelium (Penner GB 2010).In this context, even those microorganisms responsible for the fermentation of lactic acid, such as Megasphaera elsdenii and Selenomonas ruminantium ssp.Lactilytica (Huber TC et al. 1976), may be eliminated from the rumen.Among these lactic acid fermenters, Megasphaera elsdenii, a gram-negative coccus, is probably the most important ruminal organism specialized in lactic acid utilization.It is estimated that Megasphaera elsdenii ferments around 60-80% of the DL-lactate produced in the rumen (Counotte et al. 1981), and also it is more tolerant to low pH.
However, if lactate production and release rates exceed the capacity of lactate-utilizers bacteria, ruminal pH drops drastically, increasing the occurrence of lesions in the rumen epithelium due to acidification (Nagaraja and Titgemeyer 2007).This fact can be a gateway for epimural bacteria translocation, such as Fusobacterium necrophorum, which can cause systemic inflammation, or in the case of Fusobacterium necrophorum reach the liver, generating abscesses (Nagaraja and Titgemeyer 2007;Loor et al. 2016;Plaizier et al. 2017), reducing animal performance and carcass production.
Thus, among nutritional management tools to mitigate such risks of excessive acidification, adaptation protocols should be adopted for the gradual transition of animals to diets containing high content of grains (Bevans et al. 2005), in order to allow enough time for the microbiota, and also for the rumen epithelium, to adapt.So, a well designed adaptation protocol and properly conducted is a key factor to maximize the effects of feed additives to improve health and animal performance.

Adaptation protocols to control rumen acidification
As previously mentioned, diets consumed by beef cattle before feedlot arrival are typically forage-based, and therefore, protocols for adapting ruminal microorganisms to effectively use readily fermentable carbohydrate is necessary since an abrupt and rapid transition of cattle from ad libitum access to a forage-based to a cereal-based diet may precipitate metabolic disorders, resulting in a negative impact on animal performance (Cheng et al. 1998;Owens et al. 1998).Usually, an adaptation protocol that consists of multiple step-up diets, in which the level of concentrate feedstuffs is increased gradually, is the most popular among nutritionists in North America and Brazil (56.3% and 61.1%,respectively),where the recommended adaptation length is 24 days and 19.2 days, respectively (Samuelson et al. 2016;Silvestre and Millen 2021).Hungate et al. (1952) reported that cattle ingesting an amount of grain capable of inducing acute indigestion in hay-fed animals showed no ill effects when properly and gradually adapted to the very same amount.Bartle and Preston (Bartle and Preston 1992) adapted steers to 90% concentrate diets based on steam-flaked sorghum by feeding adaptation diets containing approximately 65, 75 and 85% concentrate for 7 each, and reported that the transition between diets was less dramatic by providing the first day of each new diet the same amount (Mcal) of net energy (NE) offered on the last day of the previous one.Bierman and Pritchard (Bierman and Pritchard 1996) adapted cattle to a 92% concentrate diet by either allowing ad libitum access to 45, 65, 75 and 82% concentrate diets throughout 11 days or by restricting intake of the final diet initially (to 1.74% of initial BW) followed by gradual increases until ad libitum intake was achieved.These authors indicate that ADG did not differ among treatments during the first 29 days, but cattle fed restricted quantities of the final diet initially consumed 20% less dry matter and were 19% more efficient.Considering the entire 121-day feeding period, ADG was similar but cattle fed the final diet at restricted intakes initially were 11% more efficient (Bierman and Pritchard 1996).
This gradual transition of cattle to high-concentrate diets is important to provide time for the microbiota, and also for the rumen epithelium to adapt to an increased amount of quickly fermentable carbohydrates in the rumen.The adaptation of the rumen to high-energy diets involves the increase of ruminal bacteria populations, including those capable of utilizing lactic acid (Huber TC et al. 1976;Counotte et al. 1981), and also the increase of the ruminal absorptive surface area from 55 to 85% (Perdigao et al. 2018;Barducci et al. 2019;Parra et al. 2019;Estevam et al. 2020;Pereira et al. 2020;Rigueiro et al. 2020;Rigueiro et al. 2021).Therefore, both ruminal microbiota and epithelium have a central role in the prevention of ruminal lactic acid accumulation in grain-adapted animals.In support of the fact that the ruminal microbial population is responsible for tolerance to grain, Allison et al. (1964) working with lambs, reported that when these animals were inoculated intra-ruminally with ruminal contents from sheep that had been adapted to a wheat diet did not get as sick as control lambs following feeding of cracked wheat through the ruminal cannula.This shows that the interaction between the microbiota inside the rumen tends to balance in adverse conditions in the short term.
However, the long-term impact is related to the ruminal epithelium development based on its slower rate of adaptation when compared to ruminal bacterial communities (Nagaraja and Titgemeyer 2007).Moreover, a gradual adaptation to high-concentrate diets is important to assure sufficient time for the ruminal epithelium to adapt to an increasing level of SCFA production, since the more developed the ruminal epithelium is, the faster the SCFA clearance becomes, decreasing the risk of ruminal acidosis.Brown et al. (2006) reported that ruminal microbiota takes 2-3 days to adapt to a new diet, whereas the ruminal epithelium takes 5-7 days.
In a study conducted by Burrin et al. (1988) with individually fed Hereford-Angus animals previously fed with a corn silage diet, and subsequently fed a 75% concentrate diet for 6 days followed by a 95% concentrate diet based on high-moisture corn to cattle receiving graded concentrations of monensin, was reported that all steers exhibited reduced pH and HCO 3 and increased lactate in the blood samples after 4 days on a 75% concentrate diet when compared to day 0. In a second experiment, evaluating the increasing levels of concentrate in diet from 55 to 95% in either fast (55, 75 and 95% concentrate for 4, 2 and 15 days, respectively) or slow (55, 65, 75, 85 and 95% concentrate for 4, 2, 5, 2 and 8 days, respectively) stepup period fed during a 21-day grain adaptation, these authors reported that cattle that stepped-up slowly had higher gains (1.31 vs 1.18 ± 0.06 kg, P < 0.10) and improved feed efficiencies (0.166 vs 0.151 ± 0.004, P < 0.05) than those stepped-up fast.According to Brown et al. (2006) fast diet transitions in 14 days or less, while allowing ad libitum access to the diet, generally results in reduced performance during adaptation or over the entire feeding period.
This increase in the concentration of grains in cattle diets promotes changes in feeding behavior.According to Missio et al. (2010), the time spent eating and ruminating, the number of bouts per meal, and the eating rate of neutral detergent fiber decreased linearly as the concentrate levels in the diet increased; on the other hand, time spent resting increased linearly as diet concentrate's level increased from 22% to 79%.This change in feeding behavior of cattle by decreasing fiber content in the diet reduces the buffering capacity of the rumen, and therefore the absorptive surface area of rumen epithelium must be properly enlarged to remove more rapidly the SCFA produced to avoid problems related to ruminal acidification.
Thus, adaptation over periods of 24-27 days, depending on the protocol, has been recommended in the USA (Samuelson et al. 2016).However, cattle stay longer periods on feed at North-American feedlots until reach the desirable slaughter weight when compared to Brazilian feedlots (201 vs 106.8 days; (Samuelson et al. 2016;Silvestre and Millen 2021).All studies involving adaptation protocols to high-concentrate diets until 2010 used Bos taurus-based animals.
Based on the facts just described, a series of studies have also examined the impacts of shorter periods of adaptation on feedlot performance, carcass characteristics, and rumen morphometrics of Bos indicus cattle fed diets containing a moderate amount of energy when compared to North-American diets (Perdigao et al. 2018;Barducci et al. 2019;Parra et al. 2019;Estevam et al. 2020).
In order to study adaptation protocols for Nellore cattle, Parra et al. ( 2019) evaluated the effects of adapting Nellore bulls to high-concentrate diets in 14 or 21 days, using either a step-up or restriction protocol, on feedlot performance, and reported that cattle adapted for 14 days had smaller ruminal absorptive surface area than those adapted for 21 days at the end of adaptation period; however, this fact had no influence in rumenitis score.This lower development of the ruminal epithelium shown by cattle adapted for 14 days is because the rumen papillae take 5-7 days to adapt themselves to a new diet, whereas rumen microorganisms take 2-3 days (Brown et al. 2006).Moreover, Parra et al. (2019) reported that bulls adapted for 14 days that had 7 more days of ad libitum access to the finishing diet increased hot carcass weight (HCW) by 6.9 kg.Thus, the authors concluded that, despite the negative effects of a 14 day-adaptation on the absorptive surface area of the rumen at the end of adaptation period, these effects were not strong enough to negatively impact growth rate and carcass traits overall, showing that Nellore animals can be safely adapted in 14 days, regardless of the protocol adopted: restriction or step-up.
Based on facts just described, and considering that no negative impact from the shorter adaptation period was observed, Barducci et al. (2019) evaluated the effect of the decrease in the periods of adaptation, from 14 to 9 days, on performance and the rumen epithelium of Nellore cattle.As in the previous study, bulls from restriction protocol presented lower DMI, in kilograms and percentage of body weight (BW) from 0 to 28 days when compared to animals from step-up protocol (9.0 vs 9.6 ± 0.26 kg, and 2.35 vs 2.51 ± 0.05% of BW, respectively; P = 0.01).Moreover, bulls on restriction protocol had an improvement on Gain-to-Feed ratio (G:F; 0.165 vs 0.160 ± 0.005; P = 0.04).The ruminal development, regarding number of papillae (NOP) and absorptive surface area (% ASA), for animals adapted in 14 days were higher than those adapted in 9 days, with no differences during the adaptation and finishing periods.However, the ASA was lower for animals adapted at 9 days, both being lower than the values observed at the end, showing that 9 days can negatively impact rumen papillae development at the end of the adaptation.In addition, at the end of the finishing period, ASA from animals adapted for 14 days was similar to those adapted for 9 days, as well as animals adapted for 9 days had a higher cell proliferation index (77.28vs 68.40 ± 1.4%; P < 0.01), which means that rumen papillae were still adapting (Melo et al. 2013).Based on results of feedlot performance and carcass characteristics of this study, bulls should not be adapted to high-concentrate diets in less than 14 days, since no improvement in performance was observed, and there was strong evidence that the rumen epithelium is not fully adapted after only 9 days of adaptation.
The shortening of the adaptation period from 14 to 9 days, although it did not provide complete rumen development, it did not influence the performance of the animal.Perdigao et al. (2018) conducted a similar study, where Nellore cattle was adapted to high-concentrate diets for 6 or 9 days, and reported that cattle adapted by step-up protocol presented greater DMI (9.26 vs 8.72 ± 0.23 kg; P = 0.009) as previously reported (Barducci et al. 2019;Parra et al. 2019), which implied in greater ADG throughout the study when compared to those adapted by restriction protocol, regardless of adaptation length (1.43 vs 1.33 ± 0.04 kg; P = 0 04).Regarding rumen morphometrics, cattle adapted by a step-up protocol for 9 days had a lower mitotic index than those adapted for 6 days (13.36 vs 18.13 ± 0.37%; P < 0.01).Moreover, at the end of the finishing period, the mitotic index was lower for animals adapted by step-up protocol (7.77 vs 14.06% ± 0.37; P < 0.01).These results led the authors to conclude that the step-up protocol led to an improved feedlot performance, and the adaptation for 9 days promoted better rumen epithelium development without positively impacting cattle performance.
Evaluating the different days for adaptation previously tested (6, 9, 14 and 21) (Perdigao et al. 2018;Barducci et al. 2019;Parra et al. 2019), Estevam et al. (2020) assessed linear, quadratic and cubic relationships between days of adaptation and the dependent variables using a step-up protocol.The authors observed a quadratic increase in final BW (512.01,521.95, 527.62 and 514.84 ± 5.15 kg, respectively; P = 0.05), ADG (1.37, 1.48, 1.56 and 1.41 ± 0.07 kg respectively; P = 0.05), HCW (277.28,283.28,287.8 and 283.30 ± 2.29 kg respectively; P = 0.04) and G:F (0.142, 0.153, 0.153 and 0.146 ± 0.005 kg/kg respectively; P = 0.04) in response to increasing days of adaptation, in which bulls adapted by 14 days presented better results.Regarding rumen morphometrics, bulls adapted in 14 days presented larger ASA (37.72,32.53,40.94 and 39.56 ± 2.56 cm 2 , respectively; P = 0.03), and a larger papillae area, expressed as % of ASA, than cattle adapted by 9 days.Consequently, there was a smaller cell proliferation index, cell death index and keratinized layer thickness at the end of the adaptation for cattle adapted for 14 days, indicating better rumen development, which may contribute to faster SCFA clearance and reduce papillae keratinization.The shorter the adaptation length, the riskier the acidosis problem becomes; therefore, is not worth the risk when performance when no performance or economic benefits are promoted by shortening the adaptation period.Thus, based on the results, the authors recommended that Nellore cattle should be adapted for 14 days to high-concentrate diets.
As previously recommended by Parra et al. (2019), Barducci et al. (2019) and Estevam et al. (2020), the adaptation period required for Nellore cattle was 14 days.However, in all the studies just cited, cattle were fed at maintenance prior to starting on feed.Thus, Pereira et al. (2020) evaluated the impacts of three nutritional statuses prior to feedlot arrival and their impact on rumen epithelium and cattle performance: 1) Maintenance (forage-fed ad libitum); 2) Restriction (forage offered at 1.4% of BW plus supplement); and 3) Concentrate (forage ad libitum and 0.5% of BW of concentrate feedstuffs).The authors reported that the days for adaptation had no difference across treatments, and the general mean was 14.5 days.However, cattle submitted to restriction treatment had greater NOP (72.76 vs 62.99 ± 3.41; P = 0.08), % of ASA (26.89 vs 22.30 ± 1.36 cm 2 /cm 2 ; P = 0.08) and papillae height (3.38 vs 3.17 ± 0.06 mm; P < 0.01) compared with cattle from concentrate group.
Furthermore, as previously mentioned, Bos taurus feedlot cattle presented lower incidence of rumenitis than Bos indicus when fed with a high-concentrate diet (Millen et al. 2015), which is directly related to a lower susceptibility of Bos taurus cattle to ruminal acidosis.Watanabe et al. ( 2022) developed a study in order to shorten the period of adaptation from 14 to 9 days, using crossbreed Angus x Nellore bulls and Nellore in high-concentrate diets using the step-up protocol.Feedlot performance and rumen macroscopic variables evaluated were not affected by shortening de adaptation from 9 to 6 days regardless of the genotype, but Nellore cattle presented higher rumenitis scores when compared to Angus x Nellore cattle.However, the keratinized layer thickness was bigger for crossbreed Angus bulls than Nellore cattle (21.6 vs 18.38 ± 0.698 nm; P < 0.01) in response to increased DMI of a diet containing rapidly fermentable carbohydrates (12.50 vs 10.04 ± 0.260 kg; P < 0.01).In conclusion, the most appropriate adaptation period in this scenario for Bos indicus feedlot cattle is 14 days, regardless of the protocol adopted.The shortening of the adaptation period to less than 14 days does not improve animal performance characteristics.Therefore, the use of adaptation protocols during periods shorter than 14 days, despite the risk, should be based on economical evaluations and the capacity of the feedlot operation to provide proper feeding management.

Feed additives for cattle health and performance
Ruminal fermentation is intrinsically inefficient, with greater than 10% of dietary carbon being converted to methane and heat, and up to 50% of dietary crude protein is degraded into ammonia and lost in the urine (Blaxter 1962).The addition of antimicrobials affects this energy metabolism, increasing its efficiency, and improving nitrogen metabolism and digestion; including also reductions in both bloat and lactic acidosis.Moreover, antimicrobials can affect the ratio of ruminal SCFA, increasing propionic acid production and reducing the molar percentages of butyric and acetic.Increased production of propionic acid from the rumen will increase the hepatic gluconeogenic flux, providing greater gains to animals (Janssen 2010).
Within the classification of antimicrobials, they can be divided into the following groups: ionophores and non-ionophore, depending on their mode of action in the ruminal bioenergetic process.In the group of ionophores, stand out monensin, lasalocid, narasin and salinomycin, being produced by bacteria Streptomyces cinnamonensis, Streptomyces lasaliensis, Streptomyces aureofaciens and Streptomyces albus, respectively.Among them, monensin is widely used in ruminant animal feeds, whether in commercial operations or research (Duffield et al. 2012), whether by feedlot nutritionists in the United States (77.3%; (Samuelson et al. 2016) or Brazil (86.1%; (Silvestre and Millen 2021).
Ionophores passively interact with cations (K + , Na + , Ca ++ , Mg ++ ), which act in the intra and extracellular chemical balance of bacteria across cell membranes.They bind to their respective higher affinity cations and transport them through the cell membrane into bacteria.The bacteria, in its turn, uses the ion pump mechanism in an attempt to maintain its osmolarity, using its energy excessively, thereby creating energy loss in bacterial cells, resulting in bacterial death.This molecule selectively inhibits gram-positive bacteria rather than gramnegative bacteria because of differences in bacterial cell-wall structure, since the Gram-negative bacteria contain an outer membrane formed by proteins, lipoproteins and liposaccharides with a hydrophobic characteristic.Since ionophores are extremely hydrophobic, it disorganizes the transport of cations through the membrane of Gram-positive bacteria, promoting greater energy expenditure in order to maintain the osmotic balance between inner and outer membrane cells.As these bacteria produce less ATP per mole of fermented glucose, they end up energetically depleted, resulting in a process called autolysis.The result of this shift in rumen bacterial populations has several impacts on ruminant metabolism.
A summary of studies using monensin and its effects on performance in growing and finishing cattle are presented in Table 1.On average, the use of different doses of monensin reduced the DMI and improved Feef-to-Gain ratio (F:G) by 2.10 and 3.32%, respectively, and increased the ADG by 2.05%.It is important to note that among the studies evaluated in this article, the main adaptation protocol used was the stepup protocol.However, few studies have evaluated the effect of different additives during the adaptation phase and the effects on performance and rumen microbiota (Rigueiro et al. 2020;Rigueiro et al. 2021).
In a meta-analysis developed by Duffield et al. (2012) with 64 papers published between 1972 and 2010, it was reported that the average concentration of monensin in feed across studies was 28.1 mg/kg feed, resulting in an improved F:G (0.530 kg/ kg of BW gain; P < 0.001) and increased ADG (0.029 kg; P < 0.001) by 6.4 and 2.5%, respectively, and decrease in DMI (0.270 kg; P < 0.001) by 3%.A 0.008 kg reduction in DMI per kilogram of BW gain was identified per 1 mg/kg increase in dose of monensin in the feed, where this translates into an improvements of feed efficiency (F:G) of -0.55, -0.64 and -0.73 kg DMI per kilogram of gain for doses of 22, 33 and 44 mg/kg, respectively (P = 0.048).
Similarly, in a study of meta-analysis developed from a database with 16 studies from published literature, Ellis et al. (2012) aimed to model the change in SCFA profile in response to monensin doses in high-grain-fed beef cattle.In summary, the authors reported that the change in total SCFA was not related to monensin dose, but the results indicated that the shift in SCFA profile may be dose-dependent, where the proportional change in acetate, propionate and butyrate was −0.0634 (± 0.0323; P = 0.068), 0.260 (± 0.0735; P = 0.003) and −0.335 (± 0.0916; P = 0.002) mol/100 mol of total SCFA, respectively, for each mg/kg DM of monensin.The profile of SCFA absorbed from the rumen has consequences on the host animal's post-absorptive metabolism and, consequently, animal productivity.In this context, increased propionate production can be important to animals with high requirements for glucose (Janssen 2010).Moreover, changing the metabolic pathway of fermentation for the production of propionate makes animals more efficient (Bannink et al. 2008), since propionate competes with methane as a hydrogen sink in rumen fermentation.In other words, inhibiting methanogenesis can shift fermentation toward propionate production (Habib et al. 2022), and propionate production implies a net incorporation of hydrogen, whereas acetate and butyrate are associated with the net production of hydrogen (Wolin 1960).
In another meta-analysis, Golder and Lean (Golder and Lean 2016) evaluated the effects of lasalocid on cattle performance (n = 31 studies), carcass traits (n = 14 studies) and rumen variables (n = 10 studies), and reported that lasalocid increased ADG by 0.040 kg/day (P = 0.05) and improved F:G by 0.410 kg/kg (P = 0.03).Values similar to the means found by the studies reviewed are shown in Table 2.However, the lasalocid, unlike monensin, did not affect DMI.In this context, the authors noticed larger heterogeneity in DMI influenced by the duration of lasalocid supplementation and the linear effect of initial BW, where cattle with initial BW at ≤275 kg fed lasalocid for more than 100 days had the lowest DMI.In the rumen measures, lasalocid increased total SCFA and ammonia concentrations by 6.46 and 1.44 mM, respectively, and increased propionate (P = 0.042) and decreased acetate (P = 0.005) and butyrate (P = 0.017) molar percentage by 4.62, 3.18 and 0.83%, respectively.
Methane, in turn, represents an energy loss to the animal, where observations can range from 2 to 12% of gross energy intake, and it is also a greenhouse gas (Johnson and Johnson 1995).Guan et al. (2006) evaluating the effects of feeding a single ionophore (monensin) or rotation of two ionophores (monensin and lasalocid) with high and low concentrate levels in the diet on enteric CH 4 emissions, reported that ionophores decreased enteric CH 4 production, expressed as liters per kilogram of DMI or as a percentage of gross energy intake, by 27% in the initial 2 weeks of ionophore supplementation for animals assigned to the high-concentrate diets, and by 30% in the initial 4 weeks of supplementation for the lowconcentrate diets.In addition, rotation of monensin and lasalocid does not increase the extent or duration of depression in enteric methane emissions.Gibb et al. (2001) evaluated the effect of supplementing monensin (26 mg/kg DM) or salinomycin (13 mg/kg DM) on cattle performance and reported that steers fed monensin exhibited lower DMI (8.2 vs 9.3 and 9.3 kg; P < 0.05) and rates of gain (1.21 vs 1.62 and 1.56 kg; P < 0.05) than those fed control or salinomycin.Cattle fed salinomycin required fewer days (93.3) to reach the targeted fat thickness (5 mm of backfat) than those fed control or monensin (105.8 days).Moreover, the average of the reviewed studies using salinomycin showed an increase in DMI and ADG by 0.30, and 7.89%, respectively, improving the F:G by 6.57% (Table 3).Regarding ionophores, these antimicrobial compounds become essential management tools in order to prevent or control ruminal acidosis and consequently improving feed efficiency in feedlot cattle.As mentioned previously, monensin is the most widely used ionophore in feedlot diets, and the main effects of ionophores are a decrease in DMI and an increased feed efficiency.Despite the beneficial effects of monensin shown above, several nutritional alternatives to monensin have been studied, since the EU Commission, on 25 May 2002, proposed to ban antibiotic growth promotants, including monensin (McCartney 2002;23rd CoS 2003;Castillo et al. 2004).In this context, among the most used non-ionophore antibiotics for growth promoters, can be highlight virginiamycin and flavomycin, which are produced by bacterial strains Streptomyces virginiae and Streptomyces spp., respectively.Regarding mode of action, the virginiamycin penetrates the bacterial cell wall, binding to the 50 S ribosomal subunit and blocking protein synthesis through the inhibition of peptide bond formation (Cocito 1979).The M and S factors act synergistically, where factor S potentiates the activity of factor M. The binding of both factor M and factor S to the ribosomal subunit causes an interruption of metabolic processes within the bacterial cell, resulting in a bactericidal effect, which includes the inhibition of growth of Gram-positive, particularly lactic acid-producing bacteria (Nagaraja and Taylor 1987).Regarding Flavomycin (bambermycin), it inhibits the synthesis of peptidoglycan on the bacterial cell wall (Volke et al. 1997) and may have indirect benefits on gut tissue protein turnover by suppressing Gram-negative pathogenic bacteria, specifically Fusobacterium spp.(Edwards et al. 2005).
As an alternative to ionophores, virginiamycin is an antimicrobial non-ionophore that is also effective against the growth of gram-positive bacteria, including ruminal lactic acid-producing bacteria, reducing the possibility of lactic acidosis.A study with seven field experiments was conducted by Rogers et al. (1995), which used approximately 3100 feedlot steers and heifers to evaluate the best dose-response for virginiamycin.The modeling indicated that the effective dose range for virginiamycin in feedlot diets was 19.3-27.3mg/kg DM for increasing ADG by 4.6%, 13.2-19.3mg/kg for improving F:G by 3.6% and 16.5-19.3mg/kg for reducing liver abscess incidence by 38%.Values are higher than the averages among the studies reviewed here (Table 4), in which there was a reduction in the DMI by 0.53%, an increase in the ADG by 1.77%, and improved F:G by 1.86%.
In order to investigate the dose-response of virginiamycin in calf-fed Holstein steers, Salinas-Chavira et al. ( 2009) used doses of 0, 16, 22.5 mg/kg DM in two periods: 0-112 and 113-340 days.Across the 340-day feeding period, treatments did not affect ADG or DMI.However, virginiamycin improved F:G by 2.2 and 3.8% at 16 and 22.5 mg/kg DM, respectively.Moreover, the virginiamycin supplementation at 22.5 mg/kg increased the estimated dietary NEm and NEg by 3.8 ± 0.055 and 4.9 ± 0.048% (P = 0.04), respectively.Furthermore, Salinas-Chavira et al. ( 2016) evaluated the influence of balancing diet formulations to meet AA requirements during the initial 112 days on growth performance, the efficiency of energy utilization, and characteristics of digestion, and reported that virginiamycin (22.5 mg/kg DM) supplementation did not affect initial or overall (112-308 days; P = 0.62) DMI.During the initial 112day period, virginiamycin supplementation improved F:G and increased dietary NE by 5.3 and 4.3% (P < 0.01), respectively.However, there was no effect of virginiamycin supplementation on ADG, as previously reported by Salinas-Chavira et al. (2009).When measuring the digestibility of nutrients (Salinas-Chavira et al. 2016), virginiamycin supplementation slightly decreased non-ammonia N flow to the small intestine by 6.1%, as well as for diets balanced to meet 100% of metabolizable AA with virginiamycin supplementation decreased post ruminal N digestion by 5.4%.Furthermore, with the diets balanced to meet 100% of metabolizable AA, the feeding of virginiamycin did not affect post ruminal starch digestion.However, with urea as the only source of dietary N, virginiamycin supplementation decreased post ruminal starch digestion by 4.8%.
Virginiamycin usually improves energy efficiency associate of energy utilization, increasing the net energy of the diet for maintenance and gain as previously reported (Rogers et al. 1995;Salinas-Chavira et al. 2009;Salinas-Chavira et al. 2016).2017) evaluated the differential response to virginiamycin supplementation (28 mg/kg DM) on growth performance and characteristics of digestion when grain content of the diet was reduced by 25%, decreasing dietary NEm from 2.22 to 2.10 Mcal/kg.The authors reported that virginiamycin tended to reduce the DMI in animals with higher NEm (7.48 vs 7.85 ± 0.18 kg; P = 0.09) and increase it in those with lower NEm (8.38 vs 8.70 ± 0.18 kg; P = 0.09).In general, virginiamycin increased the estimated NE of high-energy diets by 5 and 7%, respectively, for NEm and NEg, without impacting ruminal or total tract digestion.However, according to the authors, virginiamycin-induced enhancement in dietary NE appears to be largely associated with increased efficiency of energy utilization, since supplementation did not influence characteristics of ruminal and total tract digestion.
At this point, the replacement or combination of ionophores with non-ionophores can be an alternative to improve the performance of animals.Although both ionophore and non-ionophore additives inhibit the growth of Gram-positive bacteria, they possess different modes of action.Therefore, the synergistic effect of the combination of additives may optimized the performance of the animals.In order to investigate this, Lemos et al. (2016) evaluated the effect of the combination of ionophore (monensin) with non-ionophore feed additives (virginiamycin and flavomycin) on shifts in rumen microbial populations, risks of digestive disorders, such as acidosis, and nutrients utilization in cattle fed no-roughage diets.The authors reported that supplementation with monensin (30 mg/kg DM), virginiamycin (25 mg/kg DM), monensin (20 mg/kg DM) plus virginiamycin (25 mg/kg DM), flavomycin (4.4 mg/kg DM) or monensin (20 mg/kg DM) plus flavomycin (2.2 mg/kg DM), did not affect cattle performance and carcass characteristics.Similarly, the rumen fermentation characteristics, such as pH, ammonia nitrogen, SCFA and protozoa, were not influenced by feed additives.Benatti et al. (2017) evaluated increasing doses of monensin (0, 10, 20 and 30 mg/kg DM) in diets containing virginiamycin (25 mg/kg DM) on cattle performance, and reported that in the adaptation period, the diet with virginiamycin increased NEm by 8% and NEg by 12% compared to the control diet (P = 0.01).Comparing the levels 0 and 30 mg/kg DM of monensin, there was a reduction of 30% in DMI (P = 0.04), 6 kg in BW (P < 0.01) and 0.200 kg in ADG (P = 0.01).Overall, the combination of additives led to a linear decrease in DMI of 0.700 kg, improved F:G by 0.02 kg/kg, and did not change ADG.
Similarly, Rigueiro et al. (2020) aimed to determine the effects of different combinations of monensin and virginiamycin during the adaptation and finishing periods on feedlot performance.The authors reported that, during the adaptation period, bulls fed only virginiamycin (25 mg/kg DM) had greater DMI than animals from other treatments, both in kg (6.68 vs 8.35 ± 0.38 kg; P < 0.01) and in % of BW (1.70 vs 2.05 ± 0.05%; P < 0.01), and consequently presented greater ADG (0.54 vs 1.12 ± 0.11 kg; P = 0.03) and improved G: F (11.90 vs 6.67 ± 0.06; P < 0.01) when compared with cattle fed only monensin (30 mg/kg DM).Furthermore, feeding monensin combined with virginiamycin during the adaptation period and only virginiamycin during the finishing period improved feedlot performance overall by increasing ADG, HCW and dressing percentage.Based on the results, the authors concluded that cattle should be fed high-concentrate diets containing monensin and virginiamycin during the adaptation period, and only virginiamycin during the finishing period to improve overall feedlot performance.
Due to the higher DMI of animals on diets with virginiamycin in the period of adaptation (Rigueiro et al. 2020), and considering the DMI as an important indicator to evaluate how well cattle are either accepting or adapted to the diets (Brown et al. 2006), Rigueiro et al. (2021) proposed another study to shorten the period of adaptation from 14 to 9 or 6 days by using virginiamycin (25 mg/kg DM) as the sole feed additive in the diet.Authors reported that during the first 28 days on feed, the DMI decreased linearly as the adaptation was shortened, and overall, the DMI was affected quadratically, where animals adapted for 9 days presented the greatest intakes.Furthermore, the shortening of the adaptation period decreased carcass fat deposition linearly, without negatively impacting HCW.Regarding feed efficiency, cattle fed either monensin (27 mg/kg DM) adapted by 14 days or monensin combined with virginiamycin adapted by 14 days improved F:G by 10.4 or 8.1%, respectively, when compared to bulls fed virginiamycin and adapted also for 14 days.Besides that, the shortening of the adaptation period for cattle fed only virginiamycin did not negatively impact rumenitis score.However, cattle receiving virginiamycin and adapted for 14 days presented a higher incidence of rumen lesions when compared to those fed monensin combined with virginiamycin (0.85 vs 0.38 ± 0.83; P < 0.01).Regarding rumen variables, it was reported that cattle fed virginiamycin and adapted for 9 days had lower rumen development in terms of the number of papillae, mean papillae area, ASA and papillae area expressed as % of ASA.Therefore, the authors recommended that feedlot cattle fed virginiamycin as the sole feed additive should not be adapted to high-concentrate diets in less than 14 days.

Feed additives as alternatives to antibiotics
As previously mentioned, antibiotics are commonly offered in feedlot diets to prevent diseases and metabolic disorders, as well as to improve feed efficiency and performance of the animals.However, their inclusion has been questioned, since some European nations have banned the use of ionophores because they are being classified as antibiotics (23rd CoS 2003), and this fact brings concern about the risk of these products in increasing bacterial resistance to antibiotics and, consequently, possible risks to human health (Rivaroli et al. 2016).This has instigated a search for natural alternatives to ionophores, such as tannins, essential oils (EO) and functional oils (FO), polyclonal antibodies preparations (PAP) and enzymes.
An alternative to antibiotics is passive immunization with PAP against specific groups of ruminal bacteria (Ikemori et al. 1997;DiLorenzo et al. 2008;Blanch et al. 2009), such as lactate-producing ruminal bacteria (Streptococcus bovis) and bacteria related to liver abscesses (Fusobacterium necrophorum).Avian antibodies supplementation is a viable alternative since the main source of immunoglobulins (immunoglobulin Y) is resistant to heat, acid digestion and proteolysis (Shimizu et al. 1988).Vaccination and oral doses of antibodies against lactate-producing ruminal bacteria have led to a reduction in the concentration of rumen lactate (DiLorenzo et al. 2006), in response to a reduced population of Streptococcus bovis, which is the main agent responsible for lactic acidosis.Consequently, this increased the ruminal pH of steers fed high-grain diets (Shu et al. 1999), improving the DMI (Shu et al. 1999;Gill et al. 2000).It is important to note that the microbial resistance in animals fed PAP does not typically occur because the antibodies do not directly modify the RNA or DNA of the target organism, or if it does occur, it is possible to create new antibodies from the resistant microorganism (Kelley and Lewin 1986;Cassiano et al. 2021).The results of the use of PAP are presented in Table 5, in which when compared to the non-use of feed additives, the PAP reduced the DMI by 0.41% and improved the F:G by 3.17%; but when compared to monensin, increased the DMI by 3.90% and improved F:G by 1.16%, increasing ADG by 2.71%.
In this context, DiLorenzo et al. ( 2006) evaluated the effect of PAP supplementation against S. bovis (PAP-Sb) and F. necrophorum (PAP-Fn), in steers fed high-grain diets, on reducing ruminal counts of target bacteria in beef steers supplemented or not with antibiotics (300 mg of monensin/day and 90 mg of tylosin/day).The authors reported that the PAP-Sb reduced ruminal counts of S. bovis in doses of 2.5 ml or 7.5 ml, as well as ruminal pH, was increased by feeding PAP-Sb, antibiotics, and PAP-Sb plus antibiotics.In the same way, ruminal F. necrophorum counts were reduced by feeding PAP-Fn and antibiotics, although the reduction in ruminal F. necrophorum counts was greater when feeding antibiotics alone than when feeding PAP-Fn and antibiotics together.In conclusion, the authors reported that PAP of avian origin and against S. bovis or F. necrophorum were effective in reducing target ruminal bacterial populations.These PAP could be effective in preventing the deleterious effects associated with these bacteria, and possibly in enhancing animal performance, since these bacteria play a key role in the development of ruminal acidosis and abscessed livers, respectively.
Similarly, DiLorenzo et al. (2008) evaluated the effects of feeding PAP-Sb and PAP-Fn on the performance, and carcass characteristics of steers in high-grain diets.The authors reported that PAP-Sb when used alone improved F:G compared to control, as well as steers receiving PAP had a decreased severity of liver abscess, but, no differences were observed in any other carcass characteristics compared to control.In a companion metabolism trial, it was observed that decreased ruminal counts of S. bovis (P = 0.02) when compared with steers from control, as well as the supplementation with PAP-Fn decreased the ruminal counts of F. necrophorum by 98% after 19 days (P = 0.01), as already reported by DiLorenzo et al (DiLorenzo et al. 2006).Therefore, supplementation with PAP was efficient in controlling the S. bovis population in the rumen and can represent an alternative new technology with the potential to enhance feed efficiency and decrease liver abscesses.
In order to evaluate the effect of including PAP in cattle crossbred, Pacheco et al. (2012) compared the effect of supplementation with monensin (300 mg/day) or PAP (10 ml/ day) in high-grain diets, and the authors reported that no feed additive main effects were observed for any of the feedlot performance variables and carcass characteristics, except dressing percentage, in which supplementation with PAP resulted in a decreased dressing when compared with cattle receiving monensin (53.4 vs 54.5 ± 1.12, P = 0.04, respectively).However, for rumenitis scores, cattle receiving PAP had lower rumenitis scores than those receiving monensin (2.08 vs 3.17).Furthermore, Millen et al. (2015) evaluated the inclusion of spray-dried PAP (3 g/day) in replacing of monensin (300 mg/day) on feedlot performance and carcass characteristics and reported an increase only for DMI in kg or % of BW for cattle receiving PAP (7.24 vs 7.60 ± 0.14 kg, P = 0.02; 2.03 vs 2.11 ± 0.021% of BW, P < 0.01, respectively), as well as final LM area (61.8 vs 64.5 ± 1.08 cm 2 , P = 0.02), and no effects were observed for ADG, F:G, HCW, or dressing percentage.Considering both studies, it can be concluded that feeding PAP provides an alternative technology with the potential to be utilized in an association with ionophores;  however, further investigation would be needed to determine the mechanisms by which beneficial effects of PAP are achieved.
Besides PAP, another alternative that has been studied to replace antibiotics in diets is plant strata (Table 6).EO are plant-derived compounds, such as thymol, with known antimicrobial, anti-inflammatory, antioxidative and coccidiostatic properties.The use of plant extracts, such FO and EO, have compounds that have been shown to modulate ruminal fermentation to improve nutrient utilization in ruminants (Hristov et al. 1999).Antimicrobial activities of EO have been attributed to several terpenoids and phenolic compounds (Helander et al. 1998;Chao et al. 2011), as well as the chemical constituents and functional groups, contained in the EO.The proportions in which they are present and also the interactions with processes associated with the bacterial cell membrane include electron transport, ion gradients, protein translocation, phosphorylation and other enzyme-dependent reactions (Dorman and Deans 2000).Furthermore, they were effective against a wide variety of microorganisms, including Gram-positive and Gram-negative bacteria.This fact can lead to changes in the production and proportion of SCFA produced in the rumen (Calsamiglia et al. 2007).In addition, there is evidence that some EO reduces the rate of deamination of amino acids, the rate of ammonia production and the number of ammonia-hyperproducing bacteria (Wallace et al. 2002;McIntosh et al. 2003).
In this context, Meyer et al. (2009) evaluated the effect of replacing monensin by comparing it with two EO blends in feedlot cattle diets, using 90 mg/kg DM of EO blend and 26,4 mg/kg DM of monensin.A blend contained thymol, eugenol, vanillin, guaiacol, limonene, and another blend contained guaiacol, linalool, and α-pinene.Steers fed monensin had improved F:G (6.62 vs 6.41; P = 0.05); however, ADG was not different among treatments (P = 0.59), and monensin reduced the DMI (12.0 vs 11.4 ± 0.11 kg; P < 0.01), as well as the prevalence of total liver abscesses, was reduced for steers fed monensin.
Similarly, Silva et al. (2019) investigated the effects of the inclusion of monensin at 30 or 40 mg/kg DM, monensin (30 mg/kg DM) plus virginiamycin (25 mg/kg DM) and EO (400 mg/kg DM; castor oil and cashew nut shell liquid), in cattle with an abrupt diet transition from 0 to 92% concentrate in the total mixture.The authors reported that the animals showed no difference in performance and rumen parameters.However, as previously mentioned (Navarrete et al. 2017), the animals supplemented with virginiamycin presented higher NEm and NEg.So, that led the authors to conclude that the abrupt transition to high concentrate diets did not affect performance, and the EO did not cause negative effects on performance compared to conventional additives for finishing Nellore cattle in a feedlot.Likewise, Zotti et al. (2017) working with the same treatments in a metabolism experiment, reported that during the first 60 h after transition there was no change in acetate molar proportion and time that the rumen pH remained below 5.6.So, the authors concluded that the use of the EO and monensin (40 mg/kg DM) did not change most of the rumen fermentation variables, especially in the first week after the abrupt transition.
Similar results were found by Melo et al. (2020), by including monensin (27 mg/kg of DM) or EO (500 mgmg/kgf DM; blend of castor oil acid and cashew nut shell liquid) and its effects on the performance of feedlot steers.The authors reported that when monensin was added to the diet, cattle had lower DMI overall (8.9 vs 10.4 ± 0.19 kg; P < 0.01) and improved F:G (8.33 vs 9.09 ± 0.005; P = 0.038), as already demonstrated by Duffield et al. (2012).The addition of functional oil reduced the DMI variation in the first 28 days on feed (12.5 vs 9.9 ± 0.36%; P = 0.034).The authors reported that both feed additives, when added to the finishing diets caused positive impacts on cattle, but in different magnitudes.
In this context, there have been some studies conducted to evaluate the effectiveness of essential oils fed to beef cattle on animal performance, as well as on meat quality (Meschiatti et al. 2019;Silva et al. 2019;Melo et al. 2020).However, generally cattle performance was similar among animals receiving no additives, monensin or essential oil.In addition, the composition of EO are dependent on plant type, as well as the method of extraction and the harvest conditions.Consequently, this can impact the functionality of these oils (Meschiatti et al. 2019).Based on results, the EO has been shown to be an additive with the potential to be utilized in an association with monensin or other additives.To assess the effect of replacing monensin with the use of essential oil, as well as associating this with the use of exogenous enzymes, Meschiatti et al. (2019) developed two experiments (performance and metabolism).Their treatments consisted of the use of monensin (26 mg/kg DM), essential oil (90 mg/kg DM; contained thymol, eugenol, limonene and vanillin on an organic carrier), an association of monensin and essential oil, and association of essential oil with exogenous amylase (560 mg/kg DM) or exogenous amylase plus protease (840 mg/kg DM).The combination of EO with exogenous amylase resulted in an increase in DMI, ADG, BW and hot carcass weight by 810 g (± 0.235; P < 0.01), 190 g (± 0.054; P < 0.01), 18 kg (± 12.7; P < 0.01) and 12 kg (± 8.0; P < 0.01), respectively, when compared to monensin.In metabolism results, feeding EO increased the total tract digestibility of crude protein compared to monensin, but no differences in total tract nutrient digestibility between these two treatments.Thus, it was reported by the authors that diets containing the EO enhanced DMI compared with a basal diet containing monensin without impairing feed efficiency.In addition, a synergism between EO and exogenous amylase was detected, further increasing cattle performance and carcass production compared to monensin.
The use of exogenous enzymes has been adopted in ruminant diets in order to improve the digestibility of food com-Along with the maintenance of the ruminal environment through the use of alternative compounds to antibiotics, the use of exogenous enzymes has been studied since the 1990s in order to improve feed efficiency.Their effects are related to improvement in the digestibility of forage cell walls.Usually, cell wall digestibility in the total digestive tract is less than 65% (Van Soest 1994).Furthermore, when conditions in the rumen are suboptimal for fiber digestion, which is the case for cattle fed high grain diets, cell wall digestion in the total tract is about or less than 50%, with only 35% of ruminal digestion (Beauchemin et al. 2001), considering that this can vary according to the rumen pH and temperature (Adesogan et al. 2014).This improvement in digestibility of either the fibrous fraction or the soluble carbohydrates can increase animal performance.The addition of fibrolytic enzyme may increase fibrolytic activity by stimulating the growth of ruminal microorganisms through the release of cell wall compounds, increasing the hydrolytic capacity of the rumen mainly due to increased bacterial attachment (Beauchemin et al. 2004).However, it is important to note that the beneficial impact of the exogenous fibrolytic enzymes on fiber digestibility depends on several factors, such as the basal diet composition, dose-response effect, as well as the types of enzymes (Mendoza et al. 2014;Elghandour et al. 2016;Souza et al. 2021), and ruminal retention time and pH, which affects the ruminal activity and enzymatic stability (Meale et al. 2014).Romero et al. (2015) evaluated the effects of 12 fibrolytic enzymes on ruminal in vitro neutral detergent fiber digestibility and pre-ingestive hydrolysis of bermudagrass haylage.The authors found several promising exogenous fibrolytic enzyme candidates that increased the digestibility of bermudagrass haylage.Increases of up to 4.8 percentage units (13.5%) in NDF digestibility due to exogenous fibrolytic enzyme treatment were detected, this fact also increased SCFA concentrations and decreased acetate:propionate ratio of ruminal fluid in some cases.The improved digestibility was partly explained by hydrolysis of fiber fractions leading to the release of water-soluble carbohydrates and phenolic compounds from the cell wall, The performance data of animals fed with enzymes are shown in Table 7, where their use increased DMI, ADG and F:G by 1.26, 3.23 and 1.57%, respectively.These results show that certain exogenous fibrolytic enzymes or amylase improved the nutritive value of the diets of bermudagrass haylage (Romero et al. 2015) and suggested that they may be effective at increasing the digestibility of other bermudagrasses and tropical or subtropical forages, as well as the better use of starches in diets (Meschiatti et al. 2019).However, some studies showed no effect of exogenous enzyme supplementation on animal performance (DiLorenzo and Galyean 2010; He et al. 2014;He et al. 2015), and this inconsistency of results can be attribute, in part, to the characteristics of the enzymes, as mentioned above, as well as the chemical and physical properties of the substrates.Regarding tannins, this additive is a complex group of polyphenolic compounds found in a wide range of plant species commonly consumed by ruminants, and tannins are distinguished from other polyphenols by their ability to bind minerals and reduce the bioavailability of minerals (Naumann et al. 2017) and to form complexes and precipitate proteins (Hagerman 2012).They are classified into hydrolyzable (HT) and condensed tannins (CT) whose structures are distinctly different (Besharati et al. 2022), where condensed tannins have lower toxic effects than hydrolysed tannins (Fraga-Corral et al. 2020), because more protein is bound to hydrolysed tannins than condensed tannins, with can leads to a reduced rumen digestibility and protein solubility (Jayanegara et al. 2015).Typically, high concentrations of tannins may be toxic, reducing voluntary feed intake and nutrient digestibility.However, at low to moderate concentrations, tannin supplementation may shift the site of protein degradation, increasing metabolizable amino acid flow to the small intestine (Min et al. 2003;Ávila et al. 2015;Orlandi et al. 2015) and reducing methane emissions (Carulla et al. 2005;Grainger et al. 2009) in ruminant production.Therefore, the effect of feeding the tannin extract is the manipulation of ruminal protein metabolism, reducing ruminal protein degradation and urinary N excretion.This is due to the formation of hydrogen bridges with plant proteins, forming insoluble and stable complexes, making the use of these complexes in the rumen unavailable, so that in the abomasum at pH < 3.5 this complex is dissociated, releasing proteins for digestion.
Besides that, tannins are used in diets to reduce the availability of protein within the rumen of animals, in order to reduce deamination and increase the flow of metabolizable protein to the intestines (Min et al. 2003), which can improve animal performance (Table 8).To determine the effects of feeding the condensed tannins (CT, 2.5% from Acacia mearnsii) extract in high protein diets, Koenig and Beauchemin (Koenig and Beauchemin 2018) worked with dried distillers and soluble grains replacing barley grains, with crude protein concentrations of the diets increasing from 12.9, 16.8, 20.4 to 20.5%.The authors reported that the microbial N flow reduced with the inclusion of CT, as well as a shift in the route of N excretion resulting in an increase in fecal N-output and a reduction in apparent total tract N digestibility.The excretion of total urinary N and N-urea in urine decreased by 17 and 21%, respectively, in heifers fed CT.The reduction of N digestibility reflected the protein binding effects of CT within the gastrointestinal tract and the shift in excess N excretion from labile urea N in urine to bound neutral detergent insoluble N and acid detergent insoluble N in feces in heifers fed condensed tannin.In a similar experiment, with the same treatments, Koenig et al. (2018) observed that there was no effect of the inclusion of CT on final body weight, DMI, ADG, F:G and carcass traits.However, cattle fed CT tended to have lower NH3-N emissions compared with cattle that were not supplemented (95.1 vs 72.7 g N/(steer•days)), which contributes to mitigating the emission of this greenhouse gas.
These performance results differ from those found by Rivera-Méndez et al. ( 2016), which worked with supplemental CT (Quebracho) on feedlot growth performance during the finishing phase, and reported an increase of ADG by 6.5% (P = 0.05), improved F:G by 5.2% (P = 0.04), and tended to increase dietary NE by 3.2% (P = 0.06), values that are different from the means found in the reviewed studies, in which there was a 6.24% reduction in the DMI, an increase of 2.14% in the ADG and an improvement of 1.56% in the F:G.Likewise, in a Trial 2, these authors evaluated the effect of tannin sources, condensed (Quebracho) and hydrolyzable (Chestnut), in supplementation of level 0,6% DM.Consistently, tannin supplementation tended to increase ADG by 6.8% (P = 0.08), and DMI by 4% (P = 0.06), without the effect of the source of supplementation, which led the authors to conclude that tannin supplementation promoted greater DMI and ADG of steers during the finishing feedlot phase and that the effect on feed intake is not certain, but is apparently independent of potential tannin effects on metabolizable protein supply since during the finishing phase performance is not limited by metabolizable protein supply (Zinn et al. 2000;Carrasco et al. 2013).

Implications
The adoption of adaptation protocols for periods shorter than 14 days does not result in any metabolic or economic benefit, either for Bos taurus or Bos indicus cattle, regardless of the energy content of the finishing diet.Furthermore, 14 days seems to be the limit associated with the response to increased  5.62, 5.67, 5.77 1.43, 1.46, 1.47 3.93, 3.88, 3. rumen fermentation and consequent development of the rumen epithelium to properly support SCFA absorption.Thus, regardless of either adaptation length or feed additive, feedlot cattle need at least 14 days to adapt to finishing diets.
Regarding the use of feed additives in high-concentrate diets, ionophores have shown consistent results, and alternative compounds to antibiotics have shown potential in replacing them, in order to maintain or even increase the performance, but the search for alternatives has revealed that further studies are needed to develop a feed additive of similar efficiency in terms of production and economics.Moreover, the use of associations of feed additives for long periods may either negatively affect performance or result in no economic benefit.Finally, studies associating feed additives and level of either energy or starch may be desirable to adjust doses and phases of supplementation.

Disclosure statement
No potential conflict of interest was reported by the author(s).

e
Number of animals.fAdaptation protocol, RP = restriction protocol, SP = step-up protocol, n.s.= data not shown.g F:G: Feed efficiency, values are feed-to-gain ratio.h Quadratic effect to F:G (P < 0.05).

a
Mean result for control and for each doses of aditive.Means without a common letter in their superscripts differ (P < 0.05).protocol, RP = restriction protocol, SP = step-up protocol, n.s.= data not shown.g F:G: Feed efficiency, values are feed-to-gain ratio.

93 a
Mean result for control and for each doses of aditive.Means without a common letter in their superscripts differ (P < 0.05).protocol, RP = restriction protocol, SP = step-up protocol, n.s.= data not shown.g F:G: Feed efficiency, values are feed-to-gain ratio.

Table 1 .
Summary of studies using monensin and its effects on the performance of growing and finishing cattle.
a Mean result for control and for each doses of aditive.Means without a common letter in their superscripts differ (P < 0.05).b References in chronological order.c Treatment period.d Concentrate level.e Number of animals.f Adaptation protocol, RP = restriction protocol, SP = step-up protocol, n.s.= data not shown.g F:G: Feed efficiency, values are feed-to-gain ratio.h Review, weighted mean difference is estimate of actual effect of treatment in units measures, significantly different P < 0.01.i Effect values obtained from monensin and virginiamycin treatments.

Table 2 .
Summary of studies using lasalocid and its effects on the performance of growing and finishing cattle.Mean result for control and for each doses of aditive.Means without a common letter in their superscripts differ (P < 0.05).Adaptation protocol, RP = restriction protocol, SP = step-up protocol, n.s.= data not shown.Feed efficiency, values are feed-to-gain ratio.h Review, weighted mean difference is estimate of actual effect of treatment in units measures, significantly different P < 0.05 except to DMI (P = 0.09).
a b References in chronological order.c Treatment period.d Concentrate level.e Number of animals.f g F:G:

Table 3 .
Summary of studies using salinomycin and its effects on the performance of growing and finishing cattle.
a Mean result for control and for each doses of aditive.Means without a common letter in their superscripts differ (P < 0.05).b References in chronological order.c Treatment period.d Concentrate level.

Table 4 .
Summary of studies using virginiamycin and its effects on the performance of growing and finishing cattle.
a Mean result for control and for each doses of aditive.Means without a common letter in their superscripts differ (P < 0.05).b References in chronological order.c Treatment period.d Concentrate level.e Number of animals.f Adaptation protocol, RP = restriction protocol, SP = step-up protocol, n.s.= data not shown.g F:G: Feed efficiency, values are feed-to-gain ratio.h Effect values obtained from monensin and virginiamycin treatments.

Table 5 .
Summary of studies using polyclonal antibodies preparations (PAP) and its effects on the performance of growing and finishing cattle.

Table 6 .
Summary of studies using essential and functional oils and its effects on the performance of growing and finishing cattle.Adaptation protocol, RP = restriction protocol, SP = step-up protocol, n.s.= data not shown.Feed efficiency, values are feed-to-gain ratio.* h Abrupt diet transition from 0 to 92% concentrate in the total mixture.
b References in chronological order.c Treatment period.d Concentrate level.e Number of animals.f g F:G:

Table 7 .
Summary of studies using exogenous enzyme and its effects on the performance of growing and finishing cattle.
a Mean result for control and for each doses of aditive.Means without a common letter in their superscripts differ (P < 0.05).b References in chronological order.c Treatment period.d Concentrate level.e Number of animals.f Adaptation protocol, RP = restriction protocol, SP = step-up protocol, n.s.= data not shown.g F:G: Feed efficiency, values are feed-to-gain ratio.

Table 8 .
Summary of studies using tannins and its effects on the performance of growing and finishing cattle.