Effect of Varying Protein and Energy Densities in Complete Rations Fed to Cows in First Lactation

Abstract

Holstein cows (63) in 1st lactation were allotted randomly at 28 days postpartum to receive 1 of 9 rations in a 3 .times. 3 factorial design. Rations had crude protein 12, 15 or 18% in each of 3 energy densities from ratios of forage:concentrate of 75:25, 55:45 and 35:65. Corn silage and hay crop silage were combined in a ratio of 2:1 (dry basis) in each case, and diets were offered for ad lib intake as complete feeds for an 11-wk experimental period plus 2 wk for a digestion trial. In regression analysis as energy density increased, feed intake, milk yield, milk protein, and lactose yields increased linearly, urea N in blood plasma decreased linearly and milk fat percent and yield decreased curvilinearly. As dietary protein increased, feed intake, fat-corrected milk, milk fat yield and plasma urea N increased linearly, and yields of milk, solids-corrected milk, milk protein and lactose increased curvilinearly. Actual milk yield adjusted for dry matter intake increased with greater energy density but not with higher protein concentration. Increases of energy and protein each resulted in linear increases of digestibility of dry matter, crude protein and energy of diets. There were no interactions of energy .times. protein for these measurements. Cows in 1st lactation will increase milk production in response to increasing protein over a range of energy densities when feed intake is increased.