Thus, far fewer sows are required to maintain a population than cows, so that sowscan be selected more intensely than cows and contribute more to geneticimprovement. But much of the variation in rates of genetic improvement reflectsdifferent breeding goals, different incentives for genetic change, different industry
AGRI 21 Course syllabus - animal breeding portion8structures (or lack of them) and differing degrees of use of genetic and reproductivetechnologies.Systems of matingMating systems(or systems of mating) are alternative strategies for themating of animals selected to be parents. These includeinbreeding andoutbreedingas they influence gene combinations received by progeny and,consequently, the degree of homozygosity or heterozygosity of the population. Theprimary forms of outbreeding that have the greatest application to livestockproduction arecrossbreeding, outcrossing, linecrossing, grading-up, andspecies hybridization.InbreedingInbreeding is generally something to be avoided in animal breeding.Inbreedingrefers to mating of related animals. What this does is increase thechance or probability that more of the alleles at each gene in offspring or progenywill be identical. The level of inbreeding is measured by theinbreeding coefficientwhich is a measure of the probability that alleles carried by that individual will beidentical. Increased inbreeding causes reductions in fertility, litter size and generalviability (i.e.inbreeding depression) and leads to increased incidence of geneticabnormalities. It also leads to loss of genetic variation, and without variation nogenetic progress can be made. Overall, therefore, uncontrolled inbreeding should beavoided.However, one of the reasons that pure breeds of livestock are similar inappearance is due to a low level of inbreeding. During the history of creating eachbreed,line breedingis used (i.e. mating of animals in the same genetic line orselected groups which are not highly related). Line breeding, if used with care, canhelp in establishing particular characteristics.CrossbreedingCrossbreedingrefers to the crossing of two different breeds or lines ofanimals. It is used in livestock production to take advantage ofheterosis(sometimes called hybrid vigor) and complementarity.Hybrid vigor or heterosis occurs when the genetic merit (and henceperformance) of the cross is greater than the average of the parents. Crossbreedingis widely used in both plant and animal agriculture to take advantage of hybrid vigor.In pig production, for example, commercial sows are often crossbreds of LargeWhite (also known asYorkshire) and Landrace pigs because they exhibit heterosisfor all aspects of reproduction. The crossbred sows therefore produce more litterswith more piglets per litter than either of the parent breeds.
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