Investigation of nitrogen balance in dairy cows and steers nourished by intragastric infusion. Effects of submaintenance energy input with or without protein.

Two dairy cows were maintained by intragastric infusion of volatile fatty acids and casein. Except when fasting, the casein-nitrogen was held constant, while total gross energy supply was varied from zero during fasting to 650 kJ/kg body-weight (W)0 . 75. One cow was estimated to attain zero N balance at an energy intake of 255 kJ/kg W0 . 75 and the other at 307 kJ/kg W0 . 75, which was calculated to be substantially below the estimated energy required for zero energy balance. When the cows were later given an N-free infusion for a period preceding the trial, N balance occurred at 98 kJ/kg W0 . 75 for one cow and 115 kJ/kg W0 . 75 for the other. Four steers were similarly nourished by intragastric infusion and the energy nutrient increased from 0 at fasting to 450 kJ/kg W0 . 75. The protein was held constant at 1 g N/kg W0 . 75 except at fasting. The energy level at which N balance occurred was 154 (SE 38) kJ/kg W0 . 75 or approximately equal to the energy content of the protein. The practical implications of these findings are discussed.