Selection within and across populations in livestock improvement.

Genetic evaluations within and across populations (countries, breeds, herds) allow ranking on estimated genetic merit and selecting breeding individuals across populations. Selection within and across populations (combined selection) should by definition always be as good as, or better than, within-population selection, the limiting case. The advantage depends on the sizes of the populations, the number of populations, the initial genetic means, and the correspondence of the breeding objectives in the different populations, as measured by the genetic correlation for economic merit. The advantages of combined selection are evaluated deterministically for a simple case of selecting the best males for use across populations by using a common truncation line over the distributions of EBV for the different populations. Combined selection increases overall response rates in the cooperating populations. Where the initial genetic means are the same, small populations (100 males tested) benefit greatly from combined selection. Large populations (500 to 1,000 males tested) also benefit, but less. The results depend on the increased selection response to scale, response being approximately linear with the logarithm of the number tested. When the initial means differ, the genetically poorer population can catch up in three to five generations and then contribute to the increased responses with combined selection. When breeding objectives differ, selection usually gradually pulls the populations apart and they make less and less contribution to each other and finally become separate. These results have implications for breeding strategies. Their application would affect structures of populations and rates of genetic change possible by selection.