DETERMINATION OF ANIMAL LOAD AND GRAZING SYSTEM IN THE WEIGHT GAIN OF BOVINES WITH SECONDARY VEGETATION FEEDING

The present study was to evaluate the weight gain of bovinesin three animal loads with two grazing systems and three levels of grazing, furthermore to evaluate the botanic composition of secondary vegetation areas. The study was carried out during two years, in the first year, the animal loads used were 0.25, 0.50 and 0.75 animal unit per hectare (AU/ha).The vegetation which animals were evaluated was secondary vegetation of 8 years old. Botanic composition was determined by transects at the beginning of experiment. The two systems used were rotational and alternate grazing with three animals per each load and system.The initial average weights were 175 kg and evaluation were carried out each 56 days. The experimental design used was random totally with factorial arrange of 3X2, the first factor was the animal load and the second was the grazing system. Results indicated changes in botanic composition caused

The present study was to evaluate the weight gain of bovinesin three animal loads with two grazing systems and three levels of grazing, furthermore to evaluate the botanic composition of secondary vegetation areas. The study was carried out during two years, in the first year, the animal loads used were 0.25, 0.50 and 0.75 animal unit per hectare (AU/ha).The vegetation which animals were evaluated was secondary vegetation of 8 years old. Botanic composition was determined by transects at the beginning of experiment. The two systems used were rotational and alternate grazing with three animals per each load and system.The initial average weights were 175 kg and evaluation were carried out each 56 days. The experimental design used was random totally with factorial arrange of 3X2, the first factor was the animal load and the second was the grazing system. Results indicated changes in botanic composition caused by grazing effect (P≤0.05), furthermore, grazing system and animal loads showed significative statistical differences between treatments and interaction. The best weight gain was 520 g of weight gain average per day detected in 0.50AU/ha with rotational grazing system (P≤0.05).

…………………………………………………………………………………………………….... Introduction:-
Animal production in extensive management is affected by climate, soil, plant, animal and human factors (Peclet al., 2017;Cavicchioliet al., 2019). In Mexican tropic the low production and productivity oflivestock is influence by land management (Manceraet al., 2018), moreover floristic composition in land management is usually not considered (Mejía, 2002) which can be adisadvantage for communities because theyproduce low technology application, generate health problems in cattle and obtain low economic benefits(Améndolaet al., 2016). In Quintana Roo state, the grazing livestock is usually basedon some introduced grass (Ramírez-Aviléset al., 2019). Accord sosaet al. (2000), it exists a surface of 1.5 million of hectares of low forest and secondary vegetation unused.Those hectares constitute an important forage sourcewhich are not used for animal feeding in dry season. Furthermore, many trees, herbaceous and shrubscontain important nutritive characteristics for animals (Sosa et al., 2004). Studies carried out in Quintana Roo have showed that shrubs can be used by cattlefeeding in dry season (Ziblimet al., 2019; Avornyoet al., 2018), additionally, it represents 60% of diet when grass are not available (Sosa et al., 2004;Ortega et al., 1999). Mohammed et al. (2015) mentioned that animals which graze in native pastures are superior than those which graze in forage species monocrops. Studies carried out in Yucatán peninsula by Ortega et al. (1999), showed that quality diet for cattle in deciduous tropical forest was heterogenous through the year.Vegetation growing in 442 raining season produced that animals were capable to select crude protein up to 16%, moreover, cattle that grazed in soil level, selected litter, seeds and fruits in dry season. Those results are the background to study anintegral system with nutritional value species of the regionfor improving the animal diet and determining the adequateload to avoid food resource scarcity (Mijares-Lópezet al., 2012). It is well known that forage productivity is not stable per year, but with a grazing system can be controlled (Nicarettaet al., 2020). Grazing system has as principal objective for improving the grassland and animal condition (Williams et al., 2019), like so their distribution and composition (Feria et al., 2002), therefore, animal load influencestheproductionand persistence of the species (Marchiet al., 2019). A range of animal load permits to estimate the potential in secondary vegetation areas, furthermore it helps to evaluate the reaction of the vegetation to the overgrazing and undergrazing.Accord to above mentioned the objective of this study was to determine the adequate animal load and the bestgrazing system in a secondary vegetation area.

Study localization, climate and soil conditions
The present study was carried out in el Consuelo ranch belongings toInstituto Nacional de InvestigacionesForestales, Agrícolas y Pecuarias (INIFAP), campo experimental Chetumal, located in Othon P. Blanco, Quintana Roo at 3.5 km of Xul-Ha community with 21°30' N and 89°29' W coordinates. Climate conditions are 27.6° C and 62.3% of relative humidity on average, annual medium precipitation is 1300 mm and the period with the most precipitation was from June to November. Soils of this area are chromic luvisols which are characterized to contain high organic matter.

Characteristics of study area
The study area was 18 hectares which was constituted with vegetation classified as secondary forest (acahual). The average age of forest was eight years and wasconstitutedby medium sub-evergreen forest. Those areas were dominated by diversity of grasses, herbaceous, shrubs, trees and is used by wildlife and domestic animals.

Evaluation of floristic composition
Previously to establishment of animal load treatments and grazing system, the floristic composition in experimental area was estimated. The methodology to evaluate the floristic composition was the Canfield lines or transects (Bonhan, 1989),furthermore, the methodology of America United State Forestal Services (USDA, 1974) was applied. In this case, three samples were collected by species. Each sample included steams, leaves and fruits, then samples were dried in a laboratory oven and posteriorly identified in Instituto Tecnológico de Chetumal and INIFAP herbarium. Other factors considered in the methodology were cover percentage, density, frequency and vigor of species. Each three months were collected and separatedthe species obtainedinside of 0.5 m 2 in the rectangular quadrant. These cuts were carried out to determine the effect of treatments influenced by change direction in vegetation tendency.Each species was dried in a laboratory oven at 60°C for 72 hours.The variables registered were total dry matter, new buds and floristic composition.

Creation of reference laminas to determinethe diet botanic composition
Botanic composition was determined by histological technique. Samples per season (spring, summer, fall and winter) werecollected in two years. Then, a sample of each part of plantsidentified were ground in a Willey mill with mesh of 1 mm. Samples were put on slides to prepare the histological reference tissues. These references consisted in drawings and microphotographs by observation in a contrast microscopy of phases with camera. The drawings were vegetal tissue characteristics as stomata, trichomes, crystals etc. per species.

Evaluation of animal diet
Sampleswere collected in three consecutive days per month. The process consisted in collect animal feces samples at 8:00 to 9:00 in the morning directly in the rectum of animals. Then, samples were dried in a laboratory oven at 55°C for 48 hours, posteriorly were ground in a Willey mill with mesh of 1 mm. Afterward, 0.1 g of sample was put on a slide.Per each sample was carried out four replicates. Then, samples were observed in 25 fields per slide obtaining 100 fields per sample. Species number that appears in diet were obtained from slides lecture. Data frequency was obtained per sampling range. Those data were transformed in density using Curtis and Macintosh formula (Watson and whiteman, 1981).

Evaluation of grazing system
The two grazing systems evaluated werealternated grazing system (AGS) and rotational grazing system (RGS). The AGS had rest periods with occupation of 21 days and the RGS had 21 days of rest with occupation of 7 days. The 443 animal loads used in both systems were 0.25, 0.50 and 0.75 AU/ha. Three animals of 200 kg per treatment were used in experiment.The surface was modified to obtain the adjustment of animal in respect with load. Animals had access to enough mineral salts and water. Furthermore,they were weighted each 56 days and replaced when they had 350 kg. Animalswere subjected to an adaptation period of two weeks with electric fence.

Experimental design and variables evaluated
A complete randomized experimental design was used with factorial arrange of 3x2. The first factor was the animal load (0.25,0.50 and 0.75AU/ha) andthe second factor was the grazing system(rotational and alternate). The experimental unity was one animal per treatment. The variance analysis, correlation, regression and Duncan test (P≤0.05) were carried out with Statistical Analysis System (SAS) Software. The variable evaluated were floristic composition of study area, botanic composition of animal diet and weight gain /animal.

Floristic composition
The total of species detected were 65 which were distributed in 55 generous and 30 families. Woody species were dominants with percentages from 45 to 47% of presence, followed by herbaceous and grass. Changes in floristic composition (P≤0.05) were observed during the four-sampling season (Figure 1).These results could be due that many herbaceous species as Ipomeabatatas,Passiflorasp.andSabal yapa were disappearing when grazing increased. Contrary, woody species as Cydistapotosina, Dalbergiaglabra, hameliapatens andBahuiniadivaricateshowed dominance. On the other hand,herbaceous grew againin raining season and this effect showed no significant differences statistical both grazing systems and animal load. Many studies carried out in shrubs areas, showed the importance in animal diet both wild and domestics (Murgueito 2003;Senraet al., 2005).
Herbaceous species showed great dominance at the beginning,however, these tend to disappear because of grazing.On the other hand, woody species hada slow growth but tendedconstantly to increase.These results are similar to the observed in other studies of secondary vegetation areas (Sosa et al., 2000) when floristic diversity showeddominance in herbaceous and woody species which were preferred by animals. 444 mentioned that attributes that determine the forage quality as well as capacity of dry matter production, are not important if the species have not the capacity of persist in grazing system.

Botanical composition of animal diet
The most important species in diet were shrubs with 68% of average consumption (Table 1). In the first samplingperiod, the shrubs contain detected in diet was 72.5% without significative statistical differences (P≥0.05), but in second and third sampling-period, the shrubs contain were 66 and 65.5% respectively. In both periods with R x 0.50 AU/ha were the only different (P≤0.05). The grazing system A x 0.25AU/ha showed the highest consumption of average shrubs for three periods with 68.27%, followed by A x 0.25 AU/hawith 66.3% and R x 0.50AU/hawith 68% of average consumption.The lowest consumptions were detected in A x 0.75AU/ha with 67.9%. The second species detected with 15.5% of consumption was the herbaceous ( Table 1)

Weight gain inalternateate grazing system
In the first evaluation of dry season was observed an increasing in weightgain forthe first two months, however from third month, significative statistical differences were detected (P≤0.05).The load of 0.75 AU/ha showed gains of 310 g/month while0.50 AU/ha was440 g/month and 0.25 AU/ha was 451 g/month. For animal load of0.50 AU/ha, disminution of weightgain was not abrupt compared with 0.75 AU/ha. Similar gains was observed in 0.25 AU/ha.This result could be because animal maintain their weight through dry season.
In the first period of evaluation of rain season, weight gainsfor the firtst month were 425,475 and 480 g/month for0.75, 0.50 and 0.25 AU/harespectively. However, infrom the second month,animal showed weightloss for 0.75AU/ha, butfor 0.50 and 0.25AU/ha the gains were mantained until the third month and then showed weight loss.

445
In second evaluation of dry season, it was observed that 0.75AU/haand 0.50 AU/ha loadsare not efective to mantain weight gains in animals. This result was observed when animals were remove from grazing area at90 days. Furthemore at 60 days of evaluation , 0.75 and 0.50 AU/ha loadsshowed weight loss (P≤0.05)of 300 and 390 g/month respectively, contrarly in 0.25 AU/ha which showedweight gains of 445 g/ month.

Interaction of load animal with grazing system
The best treatment observed was RGS with 0.5 AU/ha, which showedweight gainsof 520g/month.The interaction with animal load and grazing system obtained in this study are similar to the reported by Brown (1977), who mentioned that weight gain per animal is better when the animal load is low (Souza et al., 2020), furthermore, the gain is mantained when the animal load increasses gradually.Other studies Reyes, 2003) indicated that reduction of animal load, increases the weight gain per animal, moreover this weight gain is related with consumption of forage maturity and animal metabolism capacity (Hackney et al., 2021). Weight gains obtained in this study were low, however in dry season of tropic conditions , many animals die because of forage sorthage.
In dry season, many studies have indicated the stational grassand legumes consumption of animalds(Craine, 2021; Jean et al., 2020; Watter et al., 2020). They usualy have the oportunity to select their diet, which impact in weight gain (Pfister and Malecheck, 1986). Other reason of low weight gains, could be due to animals maintain low selection of grasses which have growing excessively and loss their quality. Moreover, in raining season, died grasses increase more than 50% which represent negative weight gains . These results are accord with Cowlishaw (1969) and Wheeler et al. (1973), who mentioned that weight gains in RGS was similar that AGS evaluated. Moreover , RGS neither improved the grassland nor increased the weight gains, however, they are important systems because of their botanic composition and conditions of natural lands(Peyraudet al., 2019;Hennessy, 2019; Senra, 2005).
Utilization of 0.75 AU/ha in RGS has resulted better than AGS, nevertheless AGS has showed better weight gains thatRGS.This result could be because of animal seleccion and consumption of species. Studies reported by Baumont et al. (2000) and Baumont et al.(2004) showed that independently of system used, the animal load permited better weight gains in dry season. However it is important considered that those studies have been carried out in monocrops lands orinassociation with two species, differently that our studies which contained more than 120 species.

Conclusions:-
Native vegetation was modified by animals grazing, this modification induced changes in botanic composition. The first species to decrease their population were herbaceous because ofanimal consumption preference, however,shrubes and forest species are not considered less important in the system because of high nutritional value and availability during the year.

446
Animal load is the most important factor to determine the persistence and prevalence of secondary vegetation species, furthermore, the adecuategrazing system it is neccesary to improve the efficiency in the use of forage and animal production.
The AGSwith 0.75 AU/ha showed negative effect in weight gain, however, in 0.5 and 0.25 AU/ha the weight gains were constant. The best weight gain was obtained in RGS with 0.25 and 0.50 UA/ha.