INFLUENCE OF BIRTH DATE, SEX OF CALF, BREED GROUP AND AGE OF DAM ON PREWEANING PERFORMANCE OF RANGE BEEF CALVES

Abstract

Preweaning data on 2856 calves covering a 14-yr period in three breed groups of range beef cattle: Hereford (HE), Beef Synthetic (SY) and Beef Crossbred (XB) were analyzed for birth weight, weaning weight and preweaning average daily gain (ADG). Year of birth, age of dam, sex of calf and breed group were considered as main effects. Calf birth date was used as a covariate in the least-squares analyses. Year of birth was a significant source of variation. The correlation coefficients between the least-squares constants for years and year of birth indicated a steady improvement in the preweaning traits over the 14-yr period. Bull calves exceeded heifers (P < 0.05) in all traits except for ADG in HE. Preweaning performance peaked in calves from 5- to 6-yr-old dams. Though heavier at birth, XB calves were excelled by SY calves at weaning primarily because of superior ADG recorded in SY, while HE was the lightest of the breeding populations. Breed group by sex of calf interactions for weaning weight and ADG were significant. In addition, declining multiplicative sex adjustment factors from birth to weaning in the HE and XB populations suggested that in view of their greater potential for growth, male calves might have been marginally disadvantaged by the poor nutritional environment provided by the dams. In the HE and SY populations, calf birth weights significantly increased with the progression of the calving season, but the stage of the calving season did not affect calf birth weights in XB. The linear effects of age of calf were generally significant for weaning weight while quadratic effects were significant only in the XB population. Key words: Beef cattle, preweaning traits