Animal Breeding

Breeding is one of the most important aspects of animal husbandry, because it is through breeding that farmers raise both the yield of their animals and the quality of what those animals produce — more milk, better meat, finer wool, and so on.

What is a breed?

A group of animals that share a common ancestry and resemble one another closely in appearance, features, size and general configuration is called a breed. Breeding programmes work by choosing which animals within (or across) breeds are allowed to mate, and how closely related those animals are. There are two broad strategies: inbreeding, mating within the same breed, and out-breeding, mating between animals that are not closely related.

Inbreeding

Inbreeding means mating more closely related individuals of the same breed for 4–6 generations. Breeders identify superior males and superior females of a breed and mate them in pairs; the offspring are evaluated, and the best among them are chosen for the next round of mating.

• In cattle, a superior female is a cow or buffalo that yields more milk per lactation.
• A superior male (bull) is one whose progeny outperform those sired by other males.

This is essentially the same logic Mendel used to develop homozygous purelines in pea plants — inbreeding steadily increases homozygosity, which is exactly what is needed to fix a pureline. It also exposes harmful recessive genes so selection can eliminate them, and lets superior genes accumulate while undesirable ones are weeded out. Because selection happens at every step, this raises the overall productivity of the inbred population.

However, continued and especially close inbreeding can backfire — it tends to reduce both fertility and productivity, a problem called inbreeding depression. When this happens, breeders mate the affected animals with unrelated superior animals of the same breed, which usually restores fertility and yield.

Out-breeding

Out-breeding covers all forms of mating between animals that are not closely related, and takes three forms.

Out-crossing — mating individuals within the same breed that have no common ancestor for 4–6 generations. Their offspring are called an out-cross. This is the method of choice for animals whose productivity — milk yield, growth rate, and so on — is below the breed average; even a single out-cross can help overcome inbreeding depression.

Cross-breeding — mating superior males of one breed with superior females of a different breed, combining the desirable traits of both breeds in the offspring. The resulting hybrids may be used directly for commercial production, or may themselves be inbred and selected further to fix a new, stable breed that surpasses both parent breeds. Many of today's animal breeds were created this way — for example, Hisardale, a sheep breed developed in Punjab by crossing Bikaneri ewes with Marino rams.

Interspecific hybridisation — mating male and female animals from two different but related species. The offspring can sometimes combine desirable features of both parents and be of real economic value — the best-known example being the mule, produced by crossing a horse and a donkey. (Figure 9.2 pictures a mule, illustrating this cross between species.)

Controlled breeding technologies

Modern breeding relies heavily on artificial insemination: semen collected from a chosen male parent is injected directly into the reproductive tract of a selected female, either fresh or after being frozen and transported. This makes it possible to mate animals that could never physically meet, and removes many practical difficulties of natural mating. …