Corn is the primary source of energy in dairy herds, particularly those that are intensively managed. Approximately 75% of the gross energy in corn comes from starch. However, the starch digestibility is lower than other cereals grains (Ferraretto et al., 2013). Therefore, increasing the starch availability in corn grain has the potential to improve lactation performance and feed efficiency and to reduce feed costs, especially during periods of high grain prices (Ferraretto & Shaver, 2012).
The effect of processing on starch digestibility varies among cereal grains and depends on the processing method used (Svihus et al., 2005). Grain processing methods reduce the interaction between the protein matrix and the starch granules and alter site of digestion, ruminal fermentation characteristics, passage rate and feed efficiency (Huntington & Givens, 1997; Ferraretto et al., 2013). Milling is one of the most utilized processing methods and granulometry of milled corn grain is determined by geometric mean particle size (GMPS) and the geometric standard deviation (GSD). Reducing corn grain mean particle size is known to increase starch digestibility by increasing the surface area for ruminal bacterial attachment and intestinal digestibility, especially in the small intestine (Owens et al., 1986; Huntington & Givens, 1997).
The aim of this study was to evaluate the productive performance of lactating cows fed diets with ground corn grain to give two particle sizes. Fifty Holstein lactating cows from a commercial farm in Carambeí, Paraná State, Southern Brazil, were blocked according to parity, DIM, and milk yield in the covariate period, and randomly allocated to two dietary treatments varying in corn grain particle size, in a complete randomized block-design trial. A 4-day covariate period (or 12 consecutive milkings) with cows fed the same coarsely ground corn (CGC) diet was followed by a 28-d treatment period with cows fed their assigned experimental diets. Mean BW, milk yield, and DIM (mean ± SD) were 658 ± 64 kg, 38.8 ± 7.3 kg d-1, and 155 ± 80, respectively, at the beginning of the covariate period. Ingredient and nutrient composition of experimental diets are provided in Table 1. Cows were fed diets containing either finely (FGC) or coarsely (CGC) ground corn grain, which were processed to differ in geometric mean particle size (660 μm and 915 μm, with standard deviations of 1.9 and 1.8, respectively) but not in nutrient composition.
To achieve the desired particle sizes, a hammermill (Van Aarsen® model HM GD HPS, Heelderweg, Panheel, Netherlands) with 240 hammers and sieves with mesh size of 3 mm was used to process the corn grain.
Cows were housed in individual tie-stalls in the same barn, with controlled temperature (18.5°C) and humidity (74%), and milked thrice daily (07h00, 14h00 and 23h00). Water was available ad libitum, forages (mixed in a mixer wagon) and concentrates were weighed separately for each cow and then mixed to be offered four times daily (09h00, 13h00, 18h00 and 00h00) at 110% of the expected intake based on the intake from the previous day. Tie-stalls were equipped with wooden side panels to prevent cows from stealing feed from their neighbors. Diets were adjusted for DM concentrations once weekly to maintain the desired composition.
Total feed refusals were removed twice daily (09h00 and 18h00) and weighed. Daily DMI for individual cows was calculated as feed delivered minus feed refused multiplied by the daily feed DM concentration. Milk yield was recorded by a milk meter (Ezi Test – True Test®), and milk samples were obtained from 15 consecutive milkings in the last 5 days of the treatment period (d 24, 25, 26, 27, and 28). Milk samples were analyzed for fat, total protein, lactose, casein, total solids, and milk urea nitrogen (MUN) with infrared spectroscopy (AOAC, 1990; method 972.160) by Associação Paranaense de Criadores de Bovinos da Raça Holandesa (APCBRH – Curitiba, PR, Brazil).
Samples of feed ingredients were obtained once weekly and during the last 5 days of the experimental period. Samples of the diet and orts were collected weekly to determine their nutrient profile. Samples of the total diet, forage, concentrate, and orts samples were analyzed for DM (AOAC International, 2000, method 930.15), CP (AOAC International, 2000, method 990.03), ADF (AOAC International, 2000, method 973.18), NDF (Van Soest et al., 1991), EE (AOAC International, 2000, method 2003.05), and ash (AOAC International, 2000, method 942.05).
The geometric mean particle size (GMPS) of the ground corn grain was measured using a dry sieving technique and automatic sieve shaker with sieve diameters of 4, 2, 1.2, 0.6, 0.3, 0.15mm and a bottom pan. Approximately 100 g of each sample was placed on the top sieve and the stack was shaken for 10 minutes until the distribution of the materials did not change (Zanotto & Bellaver, 1996). The GMPS and geometric standard deviation were calculated according to the equations given by the Granulac® software by Embrapa (2013). The distribution of corn grain particle size during the particle size analysis is shown in Table 2.
Blood samples were collected once on d 26, by venipuncture of the tail vein into a vacuum tube containing sodium fluoride as an antiglycolytic and potassium EDTA as an anticoagulant for plasma (Vacuette do Brasil, Campinas, SP, Brazil). Blood was centrifuged and plasma separated and stored at -20°C until analysis. The acidosis indicator (D-lactate) was determined using a D-Lactate colorimetric assay (commercial kit from Sigma-Aldrich®) at Enzymology Laboratory of Federal University of Lavras.
For fecal starch (FS) determination, rectal grab samples from individual cows were collected once from each cow on day 26. Starch concentrations were analyzed at Fundação ABC Laboratory by enzymatic methodology (Demiate et al., 2001). Fecal starch values were used to estimate total-tract starch digestibility (TTSD) with the following equation (Fredin, Ferraretto, Akins, Hoffman & Shaver, 2014):
% TTSD = (100% - (1.25 x % FS))
Four percent fat-corrected milk (FCM; kg/d) was estimated by the following equation (NRC, 2001):
FCM = 0.4 × Milk Production + 15 × (Milk Fat / 100) × Milk Production
Milk energy output (MilkE; Mcal/d) was estimated by the equation (NRC, 2001):
MilkE = 9.29 × fat (kg) + 5.63 × true protein (kg) + 3.95 × lactose (kg)
Thereby, energy corrected milk (ECM; kg/d) was estimated by the equation (NRC, 2001):
ECM = (MilkE × Milk Production)/0.70
All data were tested for normal distribution by the Shapiro-Wilk test. For the variables without repeated measures over time (fecal starch, D-lactate, and BW) statistical analyzes were performed using the GLM procedure of SAS (v. 9.4). The fixed effects were block and treatment. For all other variables analyzed as repeated measures over time, statistical analyzes were performed using the MIXED procedure of SAS (v. 9.4). Block, treatment, time (day), and the interaction between treatment and time were analyzed as fixed effects. The same variables collected during the covariate period were included as covariates. The model also included cows nested within treatment as the random effect. The covariance structures evaluated were first-order autoregressive (AR-1), unstructured (UN), compound-symmetry (CS) and toeplitz (TOEP) and were defined according to the lowest value obtained for "Akaike's Information Criterion Corrected" (AICC). For results interpretation and discussion, effects were declared significant when p ≤ 0.05. Tendencies were declared when 0.05 < p ≤ 0.10. Table 3 shows performance least square means when cows were fed FGC and CGC diets.
Processing and the subsequent particle size of grain directly affect ruminal and total tract starch digestibility (Firkins et al., 2001; Ferraretto et al., 2013). Reducing corn grain mean particle size increases starch digestibility by increasing the surface area for bacterial attachment in the rumen and enzymatic digestibility in the intestines. Besides, large particles are more resistant to water uptake which is essential for microbial and enzymatic nutrient digestibility (Huntington & Givens, 1997).
Cows fed with FGC diet had lower (p < 0.01) DMI than cows fed with CGC diet, but DMI as a percentage of BW and milk efficiency were not affected by treatment (p = 0.61 and p = 0.15, respectively). Animals from FGC group show lower (p < 0.01) fecal starch and higher (p < 0.01) TTSD. Starch digestibility is known to affect dry matter intake (Allen et al., 2009). The hepatic oxidation theory (HOT) may explain the lower DMI observed in cows consuming FGC (Allen et al., 2009). Each 1%-unit change in TTSD was related to a 3%-units change in ruminal digestibility in the meta-analysis review by Ferraretto et al. (2013). If considering TTSD values from the current study, FGC would have approximately 9%-units greater ruminal starch digestibility than CGC. Enhanced ruminal starch fermentation in cows fed FGC likely increased the production of volatile fatty acids (VFAs), mainly propionate. Propionate is metabolized extensively by the liver and increases the liver energetic status, sending a satiety signal to the brain. According to Allen (2000), stimulated distention receptors in the rumen epithelium and hepatic oxidation could explain the DMI decrease observed in cattle consuming FGC. It is unclear if greater starch digestibility was the driver of reduced intake by cows fed FGC as ruminal starch digestibility was not measured and magnitude of other variables were not of large magnitude, further research is warranted to elucidate this premise.
Reducing corn grain particle size directly affected FS concentrations, indicating greater starch digestibility. It is recommended that FS concentrations be lower than 5%. It is estimated that for each percentage-unit increase in FS above 5%, milk yield is reduced by 0.45 kg d-1 (Ferguson, 2003). Cows fed FGC had 2%-units lower FS concentrations than cows fed CGC in the present study, but milk yield was not affected by diet (p = 0.12), no effects on FCM and ECM were detected (p = 0.72 and p = 0.60, respectively) and MilkE was also not affected (p = 0.36) by diets. Total-tract starch digestibility (TTSD) was 2%-units greater for FGC compared to CGC (94 vs. 91% of starch, respectively). These results are similar to those reported by Maulfair et al. (2011) and Ferraretto et al. (2013).
In our study, milk fat, protein, casein, lactose, and total solids contents were not affected (p > 0.15) by diets, as well as yields of these solids. Lack of milk yield and milk components response to corn grain particle size contrasts with Firkins et al. (2001). In their meta-analysis review of literature, increasing starch digestibility increased milk and protein yields and reduced milk fat content. Possibly, the inclusion rate of corn grain in our experimental diets was too modest (3.1 kg d-1) to alter milk yield and efficiency. Future research evaluating these treatments with a greater inclusion of corn grain in the diet is warranted to elucidate this premise.
MUN was greater (p = 0.04) for FGC diets. Charbonneau et al. (2006) reported lower MUN concentrations in cows fed diets containing finely ground corn. According to Vagnoni & Broderick (1997), increased ruminal starch digestion should promote microbial protein synthesis and milk protein secretion leading to reduced concentrations of MUN. Fine starch particles could have greater ruminal escape rates and hence, partially shift digestion to the small intestine. Moreover, FGC diet would increase rumen and small intestinal starch digestion and, with that, decrease hindgut fermentation (Owens et al., 1986). Some studies show that with less amount of starch fermented in the hindgut, there is a higher concentration of urea nitrogen in the milk (Reynolds et al., 2001). Endosperm type also is well-known to affect TTSD; corn grain with a floury endosperm has a 6% greater TTSD compared to corn with a vitreous endosperm (Lopes et al., 2009). For floury corn grains, particle size is less critical since starch not digested ruminally can be thoroughly digested in the small intestine. However, ruminal starch escape of flint corn, as typically fed to cattle in Brazil, does not necessarily translate into greater post-ruminal digestibility (Rémond et al., 2004).
Cows fed FGC had increased concentrations of plasma D-lactate, suggesting a potentially higher degree of SARA (sub-acute ruminal acidosis) with FGC compared to CGC. Although D-lactate concentrations were higher in FGC cows, these levels are not critical in agreement with the lack of effect on milk fat concentration.