Breeds can be considered as mildly inbred lines. Inbreeding and genetic uniformity (homozygosity of genes) have gradually and inevitably increased within pure breeds since their formation. Even in breeds with a large population size, it is not uncommon for inbreeding levels to increase about 0.5% per generation. Heterosis, the difference between the mean of reciprocal F1 crosses (A x B and B x A) and the mean of two parental breeds (breeds A and B), is the reverse of inbreeding depression. Diallel crossing experiments with Bos taurus (nonhumped cattle) breeds in temperate climates have demonstrated that weaning weight per cow exposed to breeding was increased by about 23%. This increase was due to beneficial effects of heterosis on survival and growth of crossbred calves and on reproduction rate and weaning weight of calves from crossbred cows. More than half of this advantage is due to the use of crossbred cows. Effects of heterosis are greatest for lifetime production of cows (30%), longevity (15%), and calf crop percentages weaned (5 to 7% for reproduction rate and 3 to 5% for calf survival). Effects of heterosis are important, but they are of more intermediate magnitude for growth rate (3 to 5%) and maternal performance of F1 dams. Effects of heterosis on carcass and meat traits have been relatively small (3% or less). Crossing of Bos indicus (thoracic-humped cattle) and Bos taurus breeds (e.g., Brahman x Hereford) yields even higher levels of heterosis, averaging about twice as high as those reported for corresponding traits in crosses of two Bos taurus breeds.
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