Slow-release urea and highly fermentable sugars in diets fed to lactating dairy cows.

This experiment was designed to test the inclusion of highly fermentable sugars (FS) in dairy rations and their interactions with a slow-release urea (SU) product. The FS are a blend of liquid coproducts from the corn milling and cheese industries, and the SU is calcium chloride urea. Eight multiparous and 4 primiparous Brown Swiss cows (117 +/- 46 d in milk) were blocked by parity and utilized in a multiple Latin square design. Basal diets were formulated for 16.6% crude protein and 1.55 Mcal/kg of net energy for lactation and contained 35% of dietary dry matter as corn silage, 15% alfalfa hay, 34% of a concentrate mix containing varying proportions of ground shelled corn and soybean meal, and 16% of a constant concentrate premix. The premix consisted of equal proportions of corn distillers grains, soybean hulls, expeller soybean meal, vitamins, and minerals across all diets. Diets contained either no supplemental FS (NFS) or FS (8.64% RationMate) and either no SU (NSU) or SU (0.61% Ruma Pro) in a 2 x 2 factorial arrangement of treatments. Feeding FS tended to decrease milk production compared with feeding NFS. Milk fat percentage was increased for cows fed FS compared with NFS. Feeding SU decreased dry matter intake and increased feed efficiency compared with cows fed NSU. Dietary treatment had no effect on energy-corrected milk, milk fat yield, milk protein percentage, or milk urea N. Feeding FS increased the molar proportion of ruminal butyrate and decreased the molar proportion of propionate; however, no other effects were observed on ruminal fermentation. No interactions between FS and SU were observed. It was concluded that the replacement of corn and soybean meal with dietary FS increased milk fat percentage and that the replacement of soybean meal with SU significantly improved feed efficiency.