The relationship of the maintenance energy requirement to heifer production efficiency.

Thirty-three Hereford x Angus first-calf heifers were used to determine the relationship between production efficiency (PE = calf weaning weight/[12-mo dam+calf ME intake]) and nonlactating dam maintenance ME requirement/BW.75 (MEm) and its components, the efficiency of ME use for maintenance (km), and fasting heat production/BW.75 (FHP). Each heifer was kept in drylot from 19 mo of age until weaning of its first calf, during which time individual feed intakes were measured. After the PE phase, heifers were moved to the metabolism facility and indirect respiration calorimetry was used to determine maintenance energy metabolism. Maintenance metabolism of the dam, determined in controlled conditions, contributed little to explaining PE variation (r2 < or = .04). This may have been due to the high plane of nutrition provided and (or) to the physiological state of the heifers during metabolism measurements. Selection for lower MEm, as determined by the procedures used in this study, is unlikely to improve heifer PE if nutrition is not limited relative to requirements. Additionally, MEm was closely related to FHP (r2 = .73), suggesting that it could be used as an indicator of fed maintenance requirements when determined within defined conditions.