Inbreeding in Cattle

Genetic and Phenotypic Effects Inbreeding can have dramatic effects on a herd. These effects are the result of individuals receiving identical genes from each parent. If the parents are related, it is more likely that they have genes that are identical. An individual receiving identical genes from each parent is said to be homozygous for that pair of genes. This would be desirable if the gene the individual received from each parent leads to superior performance. However, most animals carry undesirable genes that usually remain hidden unless the animal is homozygous. An inbred individual is more likely to be homozygous for any gene, so the animal is more likely to express undesirable genes, and hence, undesirable traits. Inbreeding does not create undesirable recessive genes, but it does tend to bring to light these unfavorable genetics. This leads to a decline in average phenotypic performance called inbreeding depression. This phenomenon is well documented in all the major livestock species. Inbreeding depression has the greatest effect on reproductive traits, followed by growth traits, with little or no effect on carcass traits. This pattern is the reverse of the magnitude of heritability for these classes of traits. Inbreeding depression is essentially the opposite effect of heterosis (hybrid vigor), which is the advantage gained from crossing lines or breeds. Figure 1 illustrates the effect of inbreeding depression on percent open and percent dead at birth. An increase in the inbreeding coefficient results in a change in the performance for the two traits. This change in the percent of open cows as a result of inbreeding would be in addition to the usual percentage open in a non-inbred population. This would also be true for the percentage of calves dead at birth. Other traits in which inbreeding depression has been documented for beef cattle include percentage weaned, birth weight, preweaning gain, weaning weight, postweaning gain, and final weight. Inbreeding in Cattle