Relationships among heat production, body weight, and age in Suffolk and Texel ewes.

The objective of this study was to test the null hypothesis that heat production (HP) was equivalent between Suffolk and Texel ewes at common fasted BW and at common ages. Open-circuit respiration calorimetry was used to estimate HP in 48 Suffolk and 48 Texel ewes at eight ages (10, 15, 20, and 28 wk of age and 1.5, 2.5, 3.5, and 4.5 yr of age). Sheep were individually penned in an environmentally controlled building 6 to 10 d before sample collection. Feed was removed 55 h before sample collection. Heat production measurements were made from 55 and 71 h of the feed restriction. Suffolk ewes had greater mature weights (P < .01) and reached maturity more rapidly than Texel ewes (P < .05). At a common weight, Texel ewes had a lower HP than Suffolk ewes. The ratio of HP:BW declined as ewes aged, as indicated by the fact that the slope of the log-transformed data was negative and less than 1 (P < .01). The models predict that Suffolk ewes had a lower HP:BW than Texel ewes at a common age, but when ewes were compared at common proportions of mature weight HP:BW did not differ between breeds. Breed differences in HP seem to be the result of differences between breeds in maturing rate.