Influence of dietary protein and starch on production and metabolic responses of dairy cows.

The effects of starch and CP degradability on ruminal digestion, milk yield, and milk composition were evaluated using a 4 x 4 Latin square design. Thirty-six Holstein cows (15 multiparous; 21 primiparous) and 4 ruminally cannulated cows were assigned randomly to four diets. The experiment was conducted during wk 4 through 16 postpartum. Diets contained 60% concentrate and 40% bromegrass hay. Treatments were barley plus canola meal and soybean meal; barley plus corn gluten meal, fish meal, and meat and bone meal; corn plus canola meal and soybean meal; and corn plus corn gluten meal, fish meal, and meat and bone meal. Intake of DM was not influenced by starch source, but CP intake was higher for diets based on corn than for those based on barley. Yields of milk, milk protein, and lactose were higher for diets based on fish meal than canola meal but were not altered by dietary starch source. Milk protein percentage and 4% FCM yield were not altered by dietary protein source. The 4% FCM and milk fat yields were higher for cows fed diets based on corn than for cows fed diets based on barley. Cows fed diets based on corn had significantly lower concentrations of most short- and medium-chain fatty acids and elevated concentrations of C18:0, C18:1, and C18:2 in milk. Protein source significantly influenced milk yield, and milk composition was altered by starch source. However, no interaction was observed between starch and protein sources.