PROTEIN AS THE 1ST-LIMITING NUTRIENT FOR LACTATING DAIRY-COWS FED HIGH PROPORTIONS OF GOOD QUALITY ALFALFA SILAGE

Milk yield and composition response to dietary supplementation of protein or energy was determined in cows fed high proportions of good quality alfalfa silage. In Experiment 1, 40 multiparous Holstein cows were assigned to one of five groups at parturition. Cows were fed experimental diets during wk 4 to 15 of lactation. The control diet contained 75% alfalfa silage and 23.2% high moisture ear com (DM basis). The following supplements partially replaced corn in the treatment groups: protein, 6% fish meal plus 2.1% blood meal; fat, 5% prilled animal fat; protein plus fat, the protein and fat treatment combined; and protein plus glucose, the protein treatment plus 1.6 kg/d of glucose infused into the abomasum. Supplementation of protein increased milk yield by 18%, and supplementation of fat alone only marginally increased milk yield. Infusion of glucose decreased milk fat content and thereby reduced FCM yield. Cows fed fat or infused with glucose had slightly lower milk protein content. In Experiment 2, 27 primiparous Holstein cows were fed 98.2% alfalfa silage, 91.2% alfalfa silage plus 7% fish meal, and 61.2% alfalfa silage plus 7% fish meal plus 30% dry shelled corn in treatments 1 to 3, respectively. Daily milk yields for treatments 1 to 3 were 18.8, 22.2, and 25.8 kg/d. The milk yield response to protein supplements in both experiments and relative lack of response to energy supplements alone suggest that protein, and not energy, was first-limiting for milk yield with diets containing a large proportion of alfalfa silage.