SOURCES OF VARIATION IN BEEF CATTLE WEANING WEIGHT

Abstract

Milk production, body condition, body weight and individual feed records for 196 2-yr-old heifers and their calves were used to evaluate sources of variation in weaning weight and its two major components, total energy consumption for the year and efficiency of energy use, defined as weaning weight divided by total energy for cow and calf. Four breed groups of cows including 29 Hereford, 73 Simmental × Hereford, 68 Angus × Hereford and 26 Tarentaise × Hereford were represented over the 6-yr period. Long-horn bulls sired the calves in all but the first 2 yr when Angus bulls were used. Sources of variation were evaluated by fixed model least squares and multiple regression analyses. The least squares analysis indicated the Hereford and Angus × Hereford differed only in level of condition both at calving and at weaning, with the Angus cross slightly fatter. The Simmental cross was highest in calving condition and equal to the Angus in weaning condition. The Simmental and Tarentaise crosses were similar and significantly different from the other breed groups in milk production and weaning weight of first calves. The Tarentaise cross was different from the other three groups in cow weight at weaning, cow efficiency and condition at weaning. The Tarentaise cross and Hereford group had lowest total cow and calf energy consumption, with Simmental cross highest and Angus cross intermediate. The Simmental cross achieved a high weaning weight through high energy consumption and slightly above average efficiency, while the Tarentaise group utilized high efficiency along with low energy consumption. The breed of dam by sex of calf interaction was significant for cow efficiency which has some implication for efficiency experiments that depend on group feeding. Standard partial regressions indicated that year followed by cow condition at weaning and milk production were leading sources of variation in weaning weight and efficiency. Year and cow weight at weaning were leading sources for total energy consumption. Cow weight at weaning was of least importance in determining weaning weight and efficiency. Key words: Weaning weight, cow efficiency, energy consumption, beef cattle