ABSTRACT
Eight early lactation Holstein cows, used in a replicated 4 x 4 Latin square design, were fed the following diets: control; control plus ruminally protected amino acids (15 g methionine and 20 g lysine); control plus added fat (.32 kg 60:40 animal and vegetable blend and .36 kg of Ca salts of fatty acids); control plus ruminally protected amino acids plus added fat. The objective was to examine the effect of ruminally protected forms of lysine and methionine and dietary fat on milk yield and composition. Cows were fed for ad libitum consumption of total mixed diets consisting of 50% forage and 50% concentrate on a DM basis. Added fat increased milk, fat, and 4% FCM yield but decreased milk protein percentage. Ruminally protected amino acids increased milk protein percentage. The combined effect of fat and ruminally protected acids increased milk fat percentage and yield more than the sole addition of either supplement. Added fat increased the percentage and yield of long-chain fatty acids in milk. Plasma free fatty acids were also increased by fat addition. Adding ruminally protected amino acids to fat-supplemented diets may help alleviate the milk protein depression found with added fat.
