Effect of source and amount of protein on milk production in dairy cows

Forty multiparous Alpine does (mean BW of 61.5 kg) were utilized in a 13-wk trial to investigate the effects of a TMR differing in CP amount (13 or 17%) and source (solvent-extracted soybean meal or heat-treated soybean meal with or without urea) on lactational performance. Protein supplements contributed 30% of the N in 13% CP diets and 50% of the N in 17% CP diets. All diets were isoenergetic (2.5 Mcal of metabolizable energy/kg of DM) and were fed for ad libitum intake for the entire trial. Mean DMI (2.88 kg/d), milk production (2.65 kg/d), milk fat (4.05%), milk protein (2.68%), milk lactose (4.54%), and milk SNF (7.81%) did not differ among dietary treatments. Plasma urea N was greater (23.2 vs. 10.9 mg/dl) in does receiving the 17% CP diets; however, blood hematocrit (27.4%), beta-hydroxybutyrate (843 microM), plasma glucose (68.8 mg/dl), NEFA (600 mu eq/dl), and plasma total protein (74.% g/L) were not significantly affected by treatment. The apparent absence of a dietary effect on lactational performance may be due to the high DMI of the does (4.7% when expressed as DMI per kilogram of BW) and high CP intake providing a surfeit of protein relative to requirements.