Effect of dietary protein concentration and degradability on response to rumen-protected methionine in lactating dairy cows

An incomplete 8 x 8 Latin square trial (4-wk periods; 12 wk total) using 32 multiparous and 16 primiparous Holstein cows was conducted to assess the production response to crude protein (CP), digestible rumen-undegraded protein (RUP), and rumen-protected Met (RPM; fed as Mepron; Degussa Corp., Kennesaw, GA). Diets contained [dry matter (DM) basis] 21% alfalfa silage, 34% corn silage, 22 to 26% high-moisture corn, 10 to 14% soybean meal, 4% soyhulls, 2% added fat, 1.3% minerals and vitamins, and 27 to 28% neutral detergent fiber. Treatments were a 2 x 2 x 2 factorial arrangement of the following main effects: 15.8 or 17.1% dietary CP, with or without supplemental rumen-undegraded protein (RUP) from expeller soybean meal, and 0 or 9 g of RPM/d. None of the 2- or 3-way interactions was significant. Higher dietary CP increased DM intake 1.1 kg/d and yield of milk 1.7 kg/d, 3.5% fat-corrected milk (FCM) 2.2 kg/d, fat 0.10 kg/d, and true protein 0.05 kg/d, and improved apparent N balance and DM and fiber digestibility. However, milk urea N and estimated urinary excretion of urea-N and total-N also increased, and apparent N efficiency (milk-N/N-intake) fell from 33 to 30% when cows consumed higher dietary CP. Positive effects of feeding more RUP were increased feed efficiency and milk fat content plus 1.8 kg/d greater FCM and 0.08 kg/d greater fat, but milk protein content was lower and milk urea N and urinary urea excretion were elevated. Supplementation with RPM increased DM intake 0.7 kg/d and FCM and fat yield by 1.4 and 0.06 kg/d, and tended to increase milk fat content and yield of milk and protein.