Dietary replacement of whole soybean grain with micronized defatted soybean meal in dairy cow diets: effects on intake, digestibility, ruminal parameters, and milk yield and composition

This study aimed to evaluate effects of replacing whole soybean grain with micronized defatted soybean meal (MDSM) in diets of lactating dairy cows on performance, digestion, metabolism, and milk yield and composition. Ten primiparous Jersey cows with an initial body weight (BW) of 410 ± 9 kg and 105 ± 5 days in milk were used. Five replacement levels (0, 25, 50, 75, and 100% of dietary DM) of whole soybean grain with MDSM were evaluated. The experiment was conducted as two simultaneous 5×5 Latin squares, each composed of five animals, five treatments, and five experimental periods, totaling 80 days. Each period lasted 16 days, including 12 days for adaptation and 4 days for data collection. No differences were observed among experimental diets for dry matter intake (mean = 14.58 kg day^-1; p = 0.08) or organic matter intake (mean = 13.60 kg day^-1; p = 0.08). Increasing dietary inclusion of MDSM resulted in greater crude protein intake, with the highest intake observed at 14% MDSM inclusion. A linear increase (P < 0.05) was observed in digestibility of dry matter, organic matter, crude protein, ether extract, and starch as MDSM inclusion increased. The inclusion level that maximized short-chain fatty acid production was 9.22%. Milk yield corrected to 3.5% fat increased linearly with increasing MDSM inclusion. Inclusion of MDSM did not affect milk fat (p = 0.41), protein (p = 0.86), or lactose (p = 0.88) concentrations. Complete replacement of whole soybean grain with MDSM in diets of Jersey cows increased milk yield, improved feed efficiency, and did not alter milk composition. Additionally, it enhanced digestibility of dry matter, organic matter, crude protein, and starch.