THE RELATIONSHIP OF THE MAINTENANCE ENERGY REQUIREMENT TO HEIFER PRODUCTION EFFICIENCY

Thirty-three Hereford x Angus first-calf heifers were used to determin e the relationship between production efficiency (PE = calf weaning we ight/[12-mo dam + calf ME intake]) and nonlactating dam maintenance ME requirement/BW.75 (ME(m)) and its components, the efficiency of ME us e for maintenance (k(m)), and fasting heat production/BW.75 (FHP). Eac h heifer was kept in drylot from 19 mo of age until weaning of its fir st calf, during which time individual feed intakes were measured. Afte r the PE phase, heifers were moved to the metabolism facility and indi rect respiration calorimetry was used to determine maintenance energy metabolism. Maintenance metabolism of the dam, determined in controlle d conditions, contributed little to explaining PE variation (r2 < .04) . This may have been due to the high plane of nutrition provided and(o r) to the physiological state of the heifers during metabolism measure ments. Selection for lower ME(m), as determined by the procedures used in this study, is unlikely to improve heifer PE if nutrition is not l imited relative to requirements. Additionally, ME(m) was closely relat ed to FHP (r2 = .73), suggesting that it could be used as an indicator of fed maintenance requirements when determined within defined condit ions.