Effect of dietary starch concentration and direct-fed microbial supplementation on lactation performance, total-tract nutrient digestibility, and enteric methane emissions by dairy cows.

The objective of this experiment was to evaluate the effects of dietary starch concentration by replacing high-moisture corn with soy hulls and a direct-fed microbial (DFM) containing Bacillus subtilis 810 and Bacillus licheniformis 809 on the lactation performance, feeding behavior, total-tract nutrient digestibility, and enteric CH₄ emissions of dairy cows. Sixty-four multiparous Holstein cows (67 ± 24 DIM and 729 ± 60 kg of BW at the beginning of the covariate period) were randomly assigned to 1 of 4 treatments with a 2 × 2 factorial arrangement for a 12-wk treatment period, after a 1-wk acclimation period and 2-wk covariate period. Treatments consisted of diets with 21% starch and no DFM (RS-CON), 21% starch and DFM (RS-DFM), 27% starch and no DFM (NS-CON), and 27% starch and DFM (NS-DFM). Data were analyzed as a completely randomized design using a mixed model including a covariate and dietary starch, DFM supplementation, week, and their 2- and 3-way interactions as fixed effects. Cow within treatment was a random effect. Diets with 21% starch increased DMI and milk fat concentration but decreased feed efficiency. Cows fed DFM tended to have greater lactose concentration than those not fed DFM (CON). Milk yield and milk protein concentration did not differ. The eating rate was lower and meal length was greater for DFM than CON. Cows fed 27% starch had a lower eating time, meal length, intervals between meals, and smaller meal sizes than 21% starch diets. Meal frequency increased with greater starch. Diets with 27% starch reduced CH₄ emissions and intensity and tended to reduce CH₄ yield. Digestibility of DM, OM, CP, ether extract, and NDF was greater for DFM than CON diets. The replacement of high-moisture corn with soy hulls increased milk fat concentration but not milk yield. Diets with 27% starch decreased CH₄ production and improved feed efficiency compared with 21% starch diets. Additionally, feeding cows DFM improved total-tract nutrient digestibility compared with CON.