Author
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Macneil, Michael |
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Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings Publication Acceptance Date: 2/20/2007 Publication Date: 6/15/2007 Citation: MacNeil, M.D. 2007. Retrospective analysis of selection applied to a ratio. Western Section of Animal Science Proceedings 57:85-88. Interpretive Summary: Use of ratios to adjust one correlated trait for another is fairly commonplace both in the scientific community and by livestock producers. However, there are statistical arguments that restrict the appropriate use of ratios to certain circumstances. Use of the ratio of calf weaning weight to cow weight as a selection criterion has theoretical defects and places inconsistent emphasis on the component traits resulting in variable responses to selection. Despite these issues the practice is pervasive. These problems are most severe with high selection intensity which occurs, for instance, in the choosing bulls for use in AI. Other commonly used ratios likely pose similar problems. Thus, it is suggested that ratios of two traits not be used in describing individual animals or as a criterion to choose among candidates for selection. Technical Abstract: Use of ratios to adjust one correlated trait for another is fairly commonplace. However, there are statistical arguments that restrict the appropriate use of ratios to certain circumstances. The ratio of a calf’s weaning weight to that of its dam has been used as an indicator of cow efficiency and an evaluation criterion. Objectives of this study were to retrospectively assess selection applied when bulls were selected based on the ratio of their weaning weight to the coincident weight of their dam and predict correlated responses in these weights to that selection. The variance of the ratio tended to increase (P = 0.11) as weaning weight increased, but was independent of cow weight. The observed selection differential for weaning weight was independent of the selection differential for the ratio (P > 0.10). However, the selection differential for cow weight was inversely related to the selection differential for the ratio (P = 0.01). The emphasis on cow weight relative to that given weaning weight ranged from 3% to 109% (SD = 31%) and the variance of the index in retrospect fluctuated more than 5-fold. It follows that the correlated responses of weaning weight and cow weight to selection for their ratio would be variable. These results contraindicate use of the ratio of calf weaning weight to cow weight either as an evaluation criterion for beef cows or as a selection criterion upon which to choose bulls or heifers. By inference, the use of ratios of traits to evaluate animals or as selection criteria is generally likely to be inappropriate. |
