Milk production during the complete lactation of dairy cows fed diets containing different amounts of protein

Milk production response to four different amounts of protein supplementation was measured in a complete lactation study utilizing 58 multiparous Holstein cows treated with bovine somatotropin. The four treatments were as follows (the first number is the dietary crude protein content (% of dry matter) during the first 16 wk of lactation and the second number is the protein content for wk 17 to 44 of lactation): 15.4-16.0, 17.4-16.0, 17.4-17.9, and 19.3-17.9. Diets were formulated to maximize the supply of amino acids to the intestine. High moisture ear corn was finely ground to promote ruminal fermentation and microbial protein synthesis, and roasted soybeans and expeller processed soybean meal were used as the protein supplements to provide relatively high amounts of rumen-undegradable protein. Alfalfa silage and corn silage (3:2) provided the forage. Milk production for the 308-d lactation for each of the treatment groups was 10,056, 10,831, 11,095, and 11,132 kg. Cows of this production level fed diets similar to those used in this experiment benefit from dietary protein of approximately 17.5% during the first 30 wk of lactation. A reduction in dietary protein to 16% can be made around wk 30 of lactation. This amount of dietary protein should, with prevailing feed prices, be compatible with maximum profit and a moderate amount of nitrogen excretion to the environment.