INFLUENCE OF BODY SIZE ON THE BIOLOGICAL EFFICIENCY OF COWS: A REVIEW

Abstract

Relationships between body size and the biological efficiency of cows are reviewed in three parts: (1) size and biological efficiency of milk production; (2) size and biological efficiency of beef production; (3) a discussion of some important factors in experimental design, when cattle are of different sizes. Ranges of correlation estimates from different experiments were not always small. The overall relationship between milk yield and body size was 0.33 within breeds of dairy cattle; genetic correlations averaged 0.14. The average correlation of measures of body size with dairy efficiency (milk yield/feed intake) was −0.18, and genetic correlations averaged −0.37. Overall correlations between dairy efficiency and milk yield were large and positive (0.81), and genetic correlations averaged 0.89. In dairy cattle the relationship between milk production and body weight change during lactation was negative. Similarly, in beef cattle, the relationship between calf weaning weight and weight change of cow during lactation was negative. Weaning and yearling weights of calves generally increased with weights of their dams. Biological efficiencies of cow and calf to weaning or yearling weights were superior for small cows, where only combined cow and calf feed requirements and calf weights were considered. When feed requirements for replacements to the breeding herd were included (with extra salvage weight of saleable beef from the breeding herd as cow weight increased), biological efficiency was affected little by cow size unless reproductive performance also changed. Problem areas related to experimental design were the definition of size of cow, the definition of efficiency, and the selection of equitable diets for any given stage in the life cycle.