Net Energy Value for Lactation of a High and Low Concentrate Ration Containing Corn Silage

Rations containing corn silage and either 30 or 60% of the dry matter from concentrate were fed to Holstein cows in amounts calculated to minimize body tissue change. Complete energy balance measurements were made at 6-week intervals throughout the lactation cycle. Digestibility of energy in the 30 and 60% concentrate ration declined from 72 and and 75% at maintenance down to 63 and 64% at 4X maintenance. Smaller losses of energy as methane and urine partially compensated for greater fecal loss, but metabolizability of both rations decreased (P < .05) as the amount of ration consumed increased. The partial efficiency of milk synthesis from metabolizable energy was 61% for the 30g ration compared to only 54% for the 60% concentrate ration. The low efficiency was associated with the synthesis of milk of depressed fat concentration. When the NEmilk required for maintenance is assumed to be 73 kcal/wt -z~ per day, the average NEm~lk value of the 3070 concentrate ration is 1.66 .11 Mcal/kg dry matter compared to 1.70 ~ .17 Mcal/kg dry matter for the 60% concentrate ration. Substitution of concentrate for corn silage over the range studied did not increase net utilization of energy.