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Hereford and Friesian cows aged 12-15 months in 1970, were inseminated with semen from Hereford, Friesian, Charolais and Brahman bulls each year from 1970 to 1973 to produce a total of 736 calves. All cows and calves were grazed together, and the total of 650 surviving calves weaned at c. 8 months of age. Cow age and year of calving were confounded. Hereford cows had longer gestations, smaller calves, more calving difficulty and greater calf mortality than Friesian cows. Hereford cows provided their calves with less than half as much milk as did Friesian cows; the weaning weight advantage of calves from Friesian cows varied from 48 % in 2-year-old cows to 22-24 % in 3-, 4- and 5-year-old cows. Relative to Hereford sires, the use of Charolais and Brahman sires on Hereford and Friesian cows resulted in increases in gestation length, birth weight, calf mortality and calving difficulty; the use of Friesian sires tended to increase calving difficulty, but did not increase gestation length, birth weight or calf mortality. In Hereford cows, the use of Charolais and Friesian sires, as compared with Hereford sires, increased weaning weight by 8 % on average over the 4 years (the increase was zero in 2- and 3-year-old cows and 11-18 % in 4- and 5-year-old cows); the use of Brahman sires did not increase weaning weight. In Friesian cows, the use of Charolais sires, as compared with Hereford sires, increased weaning weight by 5-10% in all years; the use of Friesian sires did not influence weaning weight whilst the use of Brahman sires reduced it by 5%. Heterosis from crossing the Hereford and Friesian breeds increased the weaning weight of the first-cross calves by 3.4%. |
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