Impact of nonfiber carbohydrate on intake, digestion, and milk production by dairy cows

Four concentrations of dietary nonfiber carbohydrate (42, 36, 30, and 24%) were evaluated using eight multiparous Holstein cows (662 kg of BW; 63 DIM) in replicated 4 x 4 Latin squares with 28-d periods. Shelled corn and soybean meal were partially replaced with wheat middlings, dried brewers grains, and soy hulls to formulate 36, 30, and 24% nonfiber carbohydrate diets. Decreasing dietary nonfiber carbohydrate decreased DMI, did not affect actual or fat-corrected milk production, increased milk fat percentage, and decreased milk protein percentage and production. Apparent total tract DM and OM digestibilities were highest for 36% and lowest for 24% nonfiber carbohydrate diets. Digestibility of NDF was lowest for the 42% nonfiber carbohydrate diet. Ruminal pH and acetate:propionate increased, and total VFA concentrations decreased, as dietary nonfiber carbohydrate decreased. Ruminal degradation of alfalfa hay DM and NDF were higher for low nonfiber carbohydrate diets. Significant depression of DMI (.9 to 1.8 kg/d) coincided with dietary nonfiber carbohydrate concentration at or below 30%. Results indicate that diets for cows producing 40 kg/d should contain more than 30% nonfiber carbohydrate and suggest little benefit of 42 over 36% nonfiber carbohydrate diets.