Effects of Cow size and Milk Yield on Beef Cow Production Efficiency

Abstract

Data on 5 groups of cow and Angus-sired first-calf pairs representing year and season of calving subclasses were analysed within groups for effects of cow size and milk yield on 180-day weaning weight and efficiency of production. There were 15, 20, 22, 32 and 38 covrcalf pairs in groups 1 to 5 respectively. There were 3 criteria for size: height at withers, post-calving weight and length of cow. The milk criteria were 180-day milk yield and 180-day milk fat yield. Efficiency was estimated as the ratio of calf weaning weight to total TDN consumption of cow and calf during lactation. Interactions between the effects of milk and size were examined and the data were analysed for relationships between the various criteria for cow size and milk yield. Milk yield was generally positively associated with the various measures of cow size. With regard to 180-day weaning weight, interactions between height at withers and total milk yield were significant in 2 of 4 groups. When efficiency was considered, interaction was significant in 1 of 4 groups. The effect of sex was generally non-significant. Regression coefficients of 180-day weaning v/eight on height at withers where cows were classified into low, medium and high classes within group ranged from -4.7 to 5.7 Kg/cm. The regression coefficients were generally positive and significant indicating an increase in weaning weight associated with increased height at withers. When efficiency estimates were regressed on height at withers, the regression coefficients were very low and generally negative. Regression of 180-day weaning weight and efficiency on total milk yield for the different height classes showed that both 180-day weaning weight and efficiency increased with the amount of milk produced by the dam. There was no interaction effect between total milk yield and post-calving weight of cow on 180-day weaning weight and efficiency in any of the 5 groups. Regressions of 180-day v/eaning weight on weight of cow were positive and significant in 4 of 5 groups indicating an increase in weaning weight associated vtith increased weight at calving. Regressions ranged from -0.1 to 0.4 Kg calf weight/Kg cow weight. None of the regressions of efficiency on post-calving weight was significant. Interaction between total milk yield and length of cow was significant in 1 of 5 groups. When fat yield was considered, there was no interaction effect between yield of fat and the 3 size criteria in any of the 5 groups, but fat yield had a significant effect on both 180-day weaning weight and efficiency in 3 of 5 groups.