PROTEIN AND ENERGY SUPPLEMENTATION OF HIGH ALFALFA SILAGE DIETS DURING EARLY LACTATION

Protein versus energy as a first-limiting nutrient for milk yield and milk protein content was studied in cows fed high alfalfa silage diets. In Experiment 1, 12 cows during wk 3 to 12 of lactation were fed diets containing 48.2% alfalfa silage or 98.2% silage with or without infusion of 1 kg/d of casein into the abomasum. Infusion of casein resulted in a 22% increase in milk yield, 180-g/d increase in milk protein yield, and no change in DMI. In Experiment 2, 20 cows during wk 3 to 6 of lactation were fed 98.2% alfalfa silage diets and received no infusion, 1 kg/d of glucose infused into the abomasum, 1.2 kg/d of soy protein into the abomasum, or a mixture of the glucose and protein infusion into the abomasum. Infusion of glucose had no effect on milk yield but decreased DMI, milk fat, and milk protein. Infusion of protein increased daily milk yield by 17% and protein yield by 144 g/d. In Experiment 3, 20 cows during wk 9 to 12 of lactation were fed 98.2% alfalfa silage and received no infusion 75 kg/d of propionate infused into the rumen, 1.2 kg/d of soy protein infused into the abomasum, and the propionate and soy protein infusions combined. In Experiment 4, 20 cows were fed 78.2% alfalfa silage diets with or without 5% propylene glycol. Infusion of propionate or supplementation of propylene glycol in Experiments 3 and 4 reduced DMI and milk yield. Milk yield response to protein infused into the abomasum and lack of response to energy infused into the abomasum or included in the diet in a nonfermentable form suggest that protein, not energy, is the first-limiting nutrient for milk yield in cows fed high alfalfa silage diets.