An increase in selection intensity, results in an increase in genetic gain. Fast genetic gain can thus be achieved by selecting only the very few, the very best animals for breeding. Apart from the fact that the reproductive capacity will determine the minimum number of animals that need to be selected in order to maintain population size, there is another important issue: inbreeding. The rate of inbreeding in a population can be predicted by 1/8Nm + 1/8Nf and that a smaller number of parents thus results in a higher rate of inbreeding. This is especially the case with unbalanced numbers of males and females. If we use the recommendation of the FAO not exceed the rate of inbreeding of 0.5 to 1% for the population to remain viable, this may have consequences for the selection strategy.
In a large population of 20,000 animals (half male, half female) a selected proportion of 1% would result in 100 selected animals. Equal selected proportions in males and females would result in a rate of inbreeding of 0.25%. In a small population of 2000 animals a selected proportion of 1% in males and females would result in a rate of inbreeding of 2.5%, which is too large. Often the selected proportion of males is much smaller than in females. If we take the population of 20,000 animals again, a selected proportion of 0.1% in males (select the best 10 males) and use all 10,000 females for breeding, results in a rate of inbreeding of 1.25%, which is still too high.
Breeding companies are competing companies who want to provide the same market of genetic material. Therefore, they try to make as much genetic progress as possible to keep (or increase) market share, but restrict the rate of inbreeding to 1%.
Thus: Decisions on the intensity of selection depend on the consideration of genetic gain versus rate of inbreeding