UREA VERSUS TRUE PROTEIN AS SUPPLEMENT FOR LACTATING DAIRY-COWS FED GRAIN PLUS MIXTURES OF ALFALFA AND CORN SILAGES

In trial 1, 12 cows averaging 36 kg/d of milk were fed 15.4 to 15.7% CP diets containing 30% com silage, 26% alfalfa silage with 60% DM, and 32 to 40% com grain. Four CP supplements were fed in 4 x 4 Latin squares: 1) 1.5% urea, 2) 9.3% soybean meal, 3) 8.2% meat and bone meal, or 4) 4.7% soybean meal and 4.1% meat and bone meal. Except for greater BW gain and lower milk protein on diet 3, production traits did not differ. True protein in the diet lowered plasma urea but did not alter essential AA in plasma. In trial 2, 16 cows averaging 38 kg/d of milk were fed 16.2 to 16.4% CP diets containing 27% com silage, 27% alfalfa silage with either 39 or 59% DM, and 35 to 43% com grain. Diets fed in 4 x 4 Latin squares were the following: 1) 1.8% urea, or 2) 5.5% soybean meal plus 5. 1 % meat and bone meal, fed with 39% DM alfalfa silage; or 3) 1.8% urea, or 4) 5.4% soybean meal plus 5.0% meat and bone meal, fed with 59% DM alfalfa silage. The DMI was greater on diets 2, 3, and 4; BW gain was greater with diets 2 and 4. Yields of milk and milk components were greatest on diet 2 and greater on diet 2 than on diet 1. Yields of milk and protein were not different between diets 3 and 4, but yields of fat and FCM were greater on diet 4 than on diet 3. Lower ruminal ammonia and urea in milk and blood were consistent with lower degradability of the protein meals. Results indicated that dietary true protein increased milk yield when low, but not high, DM alfalfa silage was fed with com silage as half of the forage.