Effects of the energy balance of dairy cows on lactational responses to rumen-protected methionine.

This trial was designed to investigate the interactions between level of dietary energy and the response of cows after supplementation of rumen-protected Met. We examined this interaction for dairy cows fed a diet that was deficient in Met. Two percentages of energy (87 or 100% of requirements) were supplied with two concentrations of rumen-protected Met (0 or 21 g/d). Twenty-four Holstein cows (58 d in milk) were assigned to an experiment with a split-plot design including five periods of 3 wk each. The lower energy level was obtained by limiting the amount of feed offered (18.4 vs. 20.1 kg of dry matter intake). Diets characterized by low or normal amounts of energy were composed of corn silage (69.4% vs. 69.7%), energy concentrate (18.5% vs. 22.1%), oil meals treated with formaldehyde (11.5% vs. 7.4%), and urea (0.8% vs. 1.0%), respectively. The interaction between energy level and Met supplementation did not affect yield or composition of milk. An increase in the supply of both energy and Met increased the true protein content of milk by 0.11 percentage units. The effects of the level of dietary energy on the protein content of milk augmented the effects caused by Met supplementation. The main practical conclusion was that rumen-protected Met can be used with diets based on corn silage and soybean meal to increase the protein content of milk, even for dairy cows that are in a negative energy balance.