Estimation of net energy requirements (NEm and NE delta) of lactating beef cows.

Spring-calving Angus cows (n = 24) in moderate body condition were assigned to either a high (H), maintenance-high (MH) to support superior milk, maintenance-low (ML) to support average milk, or low (L) energy diet at 12 d (SD = 4) postpartum. Energy balance for individual cows was determined by body condition change, weight change, and weigh-suckle-weigh milk production estimates. High energy intake increased (P < .05) BW, body condition score (BCS), and megacalories of body energy (BE) by 94 d postpartum. Neither dietary nor BCS accounted for significant (P > .30) variation in days to first ovulation or conception. Fasting heat production was estimated to be 72.5 kcal/BW.75 from the regression of log daily heat production/BW.75 on daily ME intake/BW.75. Rate of daily BW change did not affect concentration of energy in the weight change. Body condition score change was highly correlated (r = .98) to BW change, with each unit of BCS (1 to 5 scale) change associated with 68 kg of BW change. Two methods were used to determine NE for weight change (NE delta) based on empty body weight (EBW) change. Method 1 used the equation: BCS change = -.404 + .0147 (BW change) and Method 2 used only the regression coefficient of this equation to predict daily BCS change. Methods 1 and 2 resulted in similar regression equations: NE delta (Mcal/kg EBW change) = 1.590 + 1.251 (BCS) and NE delta (Mcal/kg EBW change) = 1.317 + 1.251 (BCS). Ranges of estimated protein and lipid in the EBW change, respectively, were 10.0 to 13.7% and 17.1 to 77.2%.