FACTORS INFLUENCING WEANING WEIGHTS OF RANGE BEEF AND DAIRY-BEEF CALVES

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Abstract
June and September measurements of lactation were made using 206 (1976) and 242 (1977) cows ranging from 2 to 10 yr of age from the University of Alberta beef herd representing four beef and dairy-beef breeds and lines. Influences on calf weaning weight investigated were age and breed of dam, sex of calf, and the continuous variables of calf age, calf birth weight, cow winter weight loss, cow post-calving average daily gain, milk yields and constituent yields of butterfat, protein and lactose. Milk and constituent yields had similar relationships to calf weaning weight (r = approximately 0.6). Stepwise regressions involving age and breed of dam accounted for 48 and 45% of the variance in weaning weight. When age and breed of dam were ignored, milk or constituent yields accounted for approximately 40% of weaning weight variance. After removing main effects, milk variables explained from 6.2 to 10.4% of the total variance. A unit increase of 1 kg in average daily milk yield was associated with 7.7 kg increase in weaning weight. Birth weight of calf and cow weight change during lactation had minor effects on calf weaning weight. No significant source of variation in weaning weight was explained by cow winter weight loss. As a significant proportion of the variation in weaning weight is accounted for by milk and constituent yields, selection for increased lactation performance and the introduction of dairy breeds into the breeding program of a beef herd can effect meaningful increases in calf weaning weights.