Effects of protein level and forage source on milk production and composition in early lactation dairy cows.

Twenty multiparous and 12 primiparous Holstein cows were assigned at calving to one of three grass hay-based diets containing either 14, 18, or 22% CP or an alfalfa hay-based diet containing 22% CP to examine the effect of protein level and forage source on milk yield and composition. The diets contained 23% ADF during wk 1 to 4 postpartum, which was lowered to 11% for wk 5 to 12 postpartum. Cows fed the 18 and 22% CP grass-based diets produced higher yields of milk, 4% FCM, fat, protein, and SNF than those fed the 14% CP diet during the high fiber period. In addition, cows fed the 22% CP grass-based diet had higher milk fat tests than those fed the 14% CP diet during the high fiber period, due primarily to an increase in short-chain fatty acid synthesis. Milk fat depression was more severe when cows were changed to low fiber diets while fed the 22% CP alfalfa-based diet than when fed the 22% CP grass-based diet. Depression in milk fat content was 15.0, 17.0, 15.6, and 27.0% for 14, 18, and 22% CP grass-based and 22% CP alfalfa-based diets, respectively. Cows receiving the 18 and 22% CP grass-based diets exhibited higher blood NEFA during the high fiber feeding period than those fed the 14% CP diet. After fiber was lowered, changes in rumen acetate:propionate ratios were unaffected by treatment. Lowering fiber level resulted in an increased milk CP percentage regardless of treatment. Grass hay appeared to be more effective than alfalfa hay in preventing depression in milk fat test upon the change to a low fiber diet.