Mating Systems To Generate Merit

With the relative lack of investment by males in the raising of their progeny, they can concentrate on competing to father many progeny. This has sometimes given rise to extremely high potential fecundity in males, a situation we can exploit through mating systems that allocate many females to the few best males (Fig. 1). Assortative mating systems that allocate the best selected females to the best selected males give some benefits beyond the progeny generation, because the distribution of genetic merit is widened in the progeny generation.

We can boost the effective fecundity of males with artificial insemination. In dairy cattle, a single bull can father over a million progeny. This makes such individual bulls of critical value, and we must make every opportunity to select the best bulls. This can be done by starting in the previous generation with a mating strategy designed to produce elite young bulls, achieved by mating the very best cows in the land to the best available bulls. The resulting young bulls are progeny-tested for their milk traits, and the very best become the new champions. This is then a two-tier mating system with four pathways for genes to flow through the elite tier with MM (males to breed males) and FM, and the commercial or base tier with MF and FF.

Mating systems that promote the use of the same males across different farms and counties give us power to identify the average genetic merit of animals born in these different farms and countries. This means that we can make extra genetic progress by more confidently exploiting the genetic inventory that we have.

We can also boost the fecundity of females using multiple ovulation and embryo transfer (MOET). There is more potential to make gains from boosting female fecundity. If we were to double fecundity in both sexes, we would half the size of the shaded areas in Fig. 1, and it can be seen that this would give more improvement in females than in males, despite the wider distribution of male estimated breeding values (EBVs).

However, when we use MOET, the ideal mating system may change. Females become more important than they are in the four-pathway mating system. We can breed fewer females to support the same herd size, so that selection pressure can be higher. But we can also ''turn them over'' more quickly normally a cow has to be kept for two or three mating years just to be able to replace herself in the herd, but under MOET she can be used just once, reducing generation interval and speeding up progress per year.

It turns out that with this higher fecundity in both sexes, the normal two-pathway mating system (just two classes of animal M and F) is predicted to be more effective, with opportunistic use of elite progeny-tested sires available from existing four-pathway schemes. Generation intervals can be decreased dramatically by use of gametes from sexually immature animals (Fig. 2). Such practices have animal welfare implications that cannot be addressed here.

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