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 die ts containing 48.2% alfalfa silage or 98.2% silage with or without inf usion of 1 kg/d of casein into the abomasum. Infusion of casein result ed in a 22% increase in milk yield, 180-g/d increase in milk protein y ield, 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 infus ion, 1 kg/d of glucose infused into the abomasum, 1.2 kg/d of soy prot ein into the abomasum, or a mixture of the glucose and protein infusio n into the abomasum. Infusion of glucose had no effect on milk yield b ut decreased DMI, milk fat, and milk protein. Infusion of protein incr eased daily milk yield by 17% and protein yield by 144 g/d. In Experim ent 3, 20 cows during wk 9 to 12 of lactation were fed 98.2% alfalfa s ilage 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 prop ionate and soy protein infusions combined. In Experiment 4, 20 cows we re fed 78.2% alfalfa silage diets with or without 5% propylene glycol. Infusion of propionate or supplementation of propylene glycol in Expe riments 3 and 4 reduced DMI and milk yield. Milk yield response to pro tein infused into the abomasum and lack of response to energy infused into the abomasum or included in the diet in a nonfermentable form sug gest that protein, not energy, is the first-limiting nutrient for milk yield in cows fed high alfalfa silage diets.