Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/30/2001
Publication Date: 3/2/2002
Citation: Freetly, H.C., Nienaber, J.A., Brown Brandl, T.M. 2002. Relationships among heat production, body weight, and age in finnsheep and rambouillet ewes. Journal of Animal Science. 80:825-832. Interpretive Summary: Feed energy that an animal eats is used to either maintain its current size or for weight gain. Predicting how much weight an animal will gain requires a knowledge of the amount of energy that it uses to maintain itself. In sheep, the amount of energy required to maintain weight decreases as an animal ages, and the rate at which this decrease occurs differs with its genetic background. These genetic differences in energy usage have made it difficult to estimate the amount of energy that sheep require to maintain weight at a given age. This difficulty results in the need to develop unique equations for each genetic type. However, the findings from this study have demonstrated that genetic differences can be reconciled if age is expressed as a function of mature size rather than chronological age. After expressing age as a function of mature size, a single equation can be used to estimate the energy required to maintain body size.
Technical Abstract: It was the objective of this study to quantify heat production across ages of Rambouillet and Finnsheep ewes and to evaluate the previous hypothesis that breed differences can be accounted for by scaling for proportion of mature body weight. Seventy-two Finnsheep and 55 Rambouillet ewes were sampled. Heat production was estimated based on individual animal gaseous exchange, which was determined from 55 through 71 h of the feed removal. Heat production per unit body weight (BW) decreased as sheep aged, and the breed-specific functions fit the data better than the pooled functions. The rate of decrease in heat production was greater in Finnsheep ewes until 37 wk of age. The rate of growth of Rambouillet ewes was greater than Finnsheep ewes over the first 52 wk of age, and Rambouillet ewes reached 95% of their mature weight at an earlier age (71 wk) than did Finnsheep ewes (113 wk). At any given age, Rambouillet ewes had achieved a greater proportion of their mature BW and have a lower heat production per unit BW than Finnsheep ewes. Across Finnsheep, Rambouillet, Suffolk, and Texel ewes, daily heat production per unit BW (kcal/kg) was best described by the function: f(BW, MBW) = 59.5e**(-0.797(BW/MBW)); where BW = body weight and MBW = mature body weight. This study suggests that breed differences can be accounted for by scaling for proportion of mature body weight. Further more, this study demonstrates the necessity of accounting for both age and breed when estimating metabolic rate in sheep.