Effects of feeding rumen-degradable valine on milk production in late-lactating dairy cows.

The study objective was to determine if feeding the rumen-degradable AA Val can increase milk production comparable to recombinant bovine somatotropin (bST). Eight multiparous late-lactating (255±26.4 d in milk) Holstein dairy cows were blocked by milk yield (34.1±8.25 kg/d) and randomly assigned to 1 of 4 treatments in a replicated 4×4 Latin square design with 21-d periods (7 d for dietary adaptation and 14 d for data collection). Treatments were control (CON), a single injection of recombinant bST (rbST), and Val fed at 40 (V40) and 80 g/d (V80). Cows were fed a total mixed ration with a distillers dried grains carrier at 113.4 g/d containing none or added AA. Dry matter intake (21.3, 22.0, 22.8, and 21.5 kg/d for CON, rbST, V40, and V80, respectively) was similar among treatments, except cows receiving V40 had greater dry matter intake than cows receiving V80. Milk yield (22.0, 26.1, 25.2, and 24.9 kg/d), 3.5% fat-corrected milk (22.1, 25.4, 24.4, and 24.3 kg/d), and energy-corrected milk (22.7, 26.1, 25.1, and 24.9 kg/d) were increased at similar amounts for cows receiving rbST, V40, and V80 compared with CON cows. Milk fat percentages (3.51, 3.36, 3.32, and 3.38%) were greatest for CON cows compared with cows receiving V40, whereas cows receiving other treatments were intermediate and similar. Milk protein percentages (3.20, 3.12, 3.15, and 3.13%) were greater for CON cows compared with cows receiving rbST and V40, whereas cows receiving V80 were intermediate and similar. Ruminal isobutyrate (1.19, 1.24, 1.44, and 1.74 mol/100 mol) concentrations were increased for cows receiving V40 and V80 compared with CON and rbST cows, with cows receiving V80 having greater concentrations than cows receiving V40. Plasma growth hormone concentrations (1.78, 1.99, 1.55, and 1.45 ng/mL) were greater for cows receiving rbST compared with cows receiving V40 and V80, whereas CON cows were intermediate and similar. Plasma insulin-like growth factor-1 concentrations (60.4, 106.1, 65.9, and 58.3 ng/mL) were greater for cows receiving rbST compared with cows receiving other treatments. This study suggests that feeding rumen degradable Val can increase milk yield comparable to recombinant bST.