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Energy Utilization in Mature Ewes from Seven Breeds with Diverse Production Potentials¹

University of Nebraska, Lincoln 68583-0908 and U. S. Department of Agriculture, Clay Center 68933

Abstract

Effects of genetic changes in reproduction, growth, body composition or lactation on the efficiency of market lamb production depend partly on the associated changes in feed intake requirements for maintenance and for protein and fat deposition. To evaluate these relationships, feed intake and body weight changes were monitored for six pairs of open, dry, mature ewes from each of seven diverse breeds fed pelleted alfalfa (53% TDN) ad libitum (AL) or restricted (MN) to 64% of ad libitum levels, for an average of 41 d. After a 56-h fast, heat production (FHP) was measured calorimetrically for 16 h before slaughter and analysis of empty body composition. The estimated daily metabolizable energy intake/kg^.75 of body weight for no change in body energy (ME_m) was 167 kcal for the AL vs 147 kcal for MN ewes, and ranged from 139 to 169 among breeds (P < .05). Estimated above-maintenance ME requirements, kcal/g tissue deposited, were 30 to 50 for protein and 10 to 14 for fat deposition. Mean FHP/d, adjusted by regression to zero activity, was 72 kcal/kg^.75 wejght and was nonsignificantly higher (3.3) for the leaner MN than for AL ewes. Thus, the lower total ME_m for MN than for AL ewes was necessarily derived from reduced metabolic and physical activity and(or) higher digestibility. Genetic increases in lean vs fat deposition would reduce above-maintenance feed by one-third to one-fourth because of the high water content of lean, but more lean mass may increase maintenance costs.

¹ Published as Paper No. 8692, Journal Ser., Nebraska Agric. Exp. Sta., Lincoln. Partial publication of the Ph.D. dissertation of the senior author.

² Present address: Agric. Dept., Dordt College, Sioux Center, IA 51250.

³ Roman L. Hruska U.S. Meat Anim. Res. Center, ARS, USDA, A218 AnS, Univ. of Nebraska, Lincoln 68583-0908.

⁴ Roman L. Hruska U.S. Meat Anim. Res. Center, ARS, USDA, P. O. Box 166, Clay Center, NE 68933.