Influence of protein percentage and degradability on performance of lactating cows during moderate temperature.

Sixty high producing Holstein cows (15/treatment) averaging 157 d postpartum were offered the following diets: high protein (18.5%), high rumen degradability (60%); high protein (18.0%), medium degradability (41%); medium protein (15.4%), high degradability (61%); medium protein (15.0%), medium degradability (46%). All diets contained 32% corn silage, 15% alfalfa haylage, and 10% whole cottonseed. Degradabilities were determined by the ficin method and were lowered by replacing soybean meal with meat and bone meal and brewers dried grains. Cows were milked three times daily and the study was conducted from May 11 to June 23 in Provo, UT during ascending day temperatures but cool nights. Milk yields were not different for the respective treatments (36.9, 35.4, 34.8, and 36.7 kg/d), but milk fat was higher at high protein degradability: 3.11, 2.89, 3.04, 2.78%, respectively, resulting in a tendency toward higher FCM. Lower acetate and propionate and acetate:propionate ratios were observed for medium degradability diets and were consistent with lower milk fat. Rumen NH3, butyrate, valerate, isovalerate, 2-methyl butyrate, and blood urea N were higher for cows on high protein diets but were not affected by degradability. Serum glucose and cortisol were not significantly affected by protein treatment, but serum triiodothyronine was lower on high protein; the interaction effect for thyroxine was significant.