Supplemental dietary protein for grazing dairy cows: Effect on pasture intake and lactation performance

One hundred twenty-four cows (92 multiparous and 32 primiparous) were used to evaluate the effect of grain supplements containing high crude protein [ (22.8% CP, 5.3% rumen undegradable protein (RUP), dry matter basis], modera te CP (16.6% CP, 6.1% RUP), and moderate CP with supplemental RUP (16.2% CP , 10.8% RUP) on lactation performance of Holstein cows rotationally grazing annual ryegrass-oat pastures. Supplemental protein was provided by solvent extracted soybean meal in the high CP and moderate CP supplements and as a corn gluten meal-blood meal mixture (2.8:1) in the moderate CP, high RUP s upplement. Cows were blocked according to previous mature milk equivalent p roduction and calving date (partum group; 0 d in milk or postpartum group; 21 to 65 d in milk) and randomly assigned to dietary treatments. Grain was individually fed, at approximately a 1:3 grain to milk ratio, before a.m. a nd p.m milkings. The study was replicated during two grazing seasons that a veraged 199 d. Cows had ad libitum access to bermudagrass hay while on past ure (dry matter intake = 1.3 kg/d). Protein supplementation had no effect o n study long pasture dry matter (12.7 +/- 1.0 kg/d) or total dry matter (23 .9 +/- 1.2 kg/d) consumption. Protein concentration did not affect actual m ilk yield of either calving group (high CP vs. moderate CP); however, postp artum group cows receiving high CP grain supplements maintained greater mil k fat concentrations (3.34 vs. 3.11%), which led to higher fat-corrected mi lk (FCM) yields than control cows receiving moderate CP grain diets (30.3 v s. 28.9 kg/d). Crude protein concentration in milk of high CP-supplemented, postpartum group cows was also higher than moderate CP cows (3.42 vs. 3.27 %). Additional RUP did not increase FCM yield above that generated by moder ate CP grain diets for partum (34.3 vs. 32.9 kg/d) or postpartum-group cows (28.9 vs. 28.2 kg/d). Increasing CP concentration of grain supplement did not affect milk yield of Holstein cows grazing immature winter annual pastu res. Supplementing additional RUP was without benefit, indicating that in t his study energy deprivation may have been the major nutritional constraint for high-producing dairy cows grazing lush pastures.