EFFECTS OF INITIAL AGE AND WEIGHT ON TEST DAILY GAINS OF STATION-TESTED BULLS

Abstract

Average daily gain (ADG) of 8620 bulls of the Angus, Charolais, Hereford, Limousin, Simmental and Shorthorn breeds, tested during 1969–1980 from 15 performance test stations across Canada was used to assess the effects of initial age and weight on test ADG and to estimate the environmental correlations between pretest and on-test ADG. From a linear model which included effects of station-years, breeds and the within-breed linear covariates of initial age and weight, the linear partial regressions for initial age were only significant for Angus (P < 0.05) and Limousin (P < 0.01), suggesting initial age has little effect on on-test ADG within the age range covered by these data. The linear partial regression values of on-test ADG on initial weight, ranging from 0.30 (for Charolais) to 0.86 (for Angus) g/kg were significant for all breeds, except for Simmental, indicating bulls with the highest initial weight for a given age would be expected to make the greatest gain on test. The phenotypic correlations between pretest and on-test ADG were low with values ranging from 0.02 to −0.15. Highly significant (P < 0.01) environmental correlations were found, ranging from −0.27 for Angus to −0.62 for Limousin when the same set of genetic parameters was assumed for all breeds, ranging from −0.22 for Shorthorn to −0.52 for Charolais when breed-specific genetic parameters were used. These environmental correlations suggest that differential degrees of compensatory growth likely occurred during the test period and bulls from a poor pretest environment tended to gain more than bulls from a better pretest environment. Results of this study support frequently expressed concern about the existence of pretest farms effects. Key words: Station test, gain on test, age, weight, environmental correlation