Genetics of Reproduction in The Pig

Abstract

Reproductive performance depends both on the genotype of the pig and the environment which it encounters. Some aspects of this environment, for example the feeding regime, can be specified; other random factors affecting individual pigs cannot. Similarly, there are genetic differences among breeds and animals of the same breed such that there is both genetic and non-genetic variability between pigs of the same breed under the same management system. For traits of reproduction, such as litter size, and its components, such as ovulation rate and embryonic survival, genetic variability is due to segregation of many genes which have, presumably, different effects on each trait and also exist with different frequencies in the population. Whatever type of management system, feeding regime and disease control the pig producer adopts, he can still benefit from the best choice of breeds and crosses among them, and from genetic changes within them. Genetic and non-genetic improvement are not alternatives for they can, and should, proceed together. An awareness of genetic differences may also enable the research worker in other disciplines to use them to advantage in his experiments or, at least, not confound genetic and other effects. Firstly in this chapter some of the evidence on genetic variability in reproductive performance is reviewed and then consideration is given to how it is being used in genetic improvement programmes and how it might be used in the future. Most of the information refers to simple measures such as litter size because data on some aspects, e.g. reproductive performance, are scanty. It is necessary to distinguish between genetic differences in performance observed among and within identifiable populations. These populations can be different breeds, or lines of the same breeds in different herds, as long as they can be identified so that stock can be repeatably drawn from them. For example, breed differences in litter size reflect deviations in the frequency of genes affecting litter size among the populations, and can be utilized by selection between them. However, once this selection is made and populations substituted, the gains are maintained without further cost,