EFFECTS OF AMOUNT OF PROTEIN AND RUMINALLY PROTECTED AMINO-ACIDS IN THE DIET OF DAIRY-COWS FED SUPPLEMENTAL FAT

The objectives of this experiment were to investigate the effects of a mount of dietary CP and ruminally protected AA supplementation on prod uction of milk and milk components, ruminal fermentation, and nutrient digestibilities by cows fed diets containing high oil com and tallow. Holstein cows in midlactation producing 22 to 25 kg/d of milk were us ed in a 5 x 5 Latin square design. Treatments were 1) control (16.8% C P, no added fat); 2) 14.2% CP, no AA; 3) 14.2% CP, with AA; 4) 17.5% C P, no AA; and 5) 17.5% CP, with AA. Diets 2 to 5 contained supplementa l fat from high oil com and tallow. Diets consisted of 33% alfalfa hay lage, 17% com silage, and 50% concentrate DM. Intake of DM was not dif ferent among treatments. Dietary fat increased yields of milk, fat, SN F, and total solids and percentages of fat and total solids. Increasin g CP from 14.2 to 17.5% did not alter production or composition of mil k. Supplemental AA increased yields of 4% FCM, milk fat, milk CP, true protein, and casein protein and percentages of CP, true protein, and casein protein in milk when either 14.2 or 17.5% CP was in the diet. S upplemental fat chd not alter ruminal fermentation, but increases in d ietary CP increased total VFA concentration in the rumen without affec ting proportions of individual VFA. Apparent digestibilities of DM, OM , CP, starch, and energy in the total tract were greater for cows fed the 17.5% CP diets. Addition of AA to the 14.2% CP diet increased appa rent digestibilities of DM, OM, ADF, NDF, and energy in the total trac t but decreased digestibilities for cows fed the 17.5% CP diets. Feedi ng AA to midlactation cows in diets containing supplemental fat may al leviate the decrease in milk protein percentage associated , with fat supplementation; this response was similar for cows fed diets that con tained either 14.2 or 17.5% CP.