Genetic improvement of litter size in pigs

Abstract

ABSTRACTDespite the low heritability (0·1) for litter size in pigs, quite high rates of genetic improvement are predicted theoretically using conventional selection methods. The highest rates are predicted from schemes with rapid generation turnover (1 year) and with selection of both males and females at breeding age on a family selection index. This index would combine litter records (two on each relative) of the dam, her full sibs and half sibs, and of the sire's dam and his full sibs and half sibs. Annual rates of genetic change of up to half a pig per litter (a proportion 0-05 of the mean) are predicted. This rate is substantially greater than the proportional rates of genetic change possible for growth and carcass traits. These predictions are fairly insensitive to maternal and rearing environmental effects which may affect litter performance. High response rates are also predicted from two-stage selection of males, first on family index as before and then on progeny test of daughters in a large artificial insemination bred population. Intense screening in a large population for females with high litter records (hyperprolific females) can be an effective way to start an improvement programme for litter size. However, continuous screening of hyperprolific females is less effective because of the long time taken to generate enough descendants needed for the next round of intense selection. Despite the high rate of genetic change possible for litter size, omission of the trait from an index which includes growth and carcass traits would result in only small losses (proportionally less than 0·05) in economic improvement of general purpose stocks under United Kingdom market conditions. However, the losses would be higher (proportionally 0·10 to 0·18) in specialized dam stocks and inclusion of litter size in an index when selecting such stocks would be worthwhile.