Selection of breeds, strains and individual pigs for prolificacy.

Prolificacy, defined as litter size at birth, is currently considered to be the most important component of sow productivity. However, in spite of a spectacular increase in productivity due to management advances, litter size at birth has remained constant for the past 20 years. This situation seems to question the long-term efficiency of the classical methods of genetic improvement such as within-herd selection and crossbreeding between European or American breeds. Some recent developments and research results suggest that one can be optimistic about the possibilities of increasing litter size in the near future. A survey of available breeds world-wide illustrates the important differences in average litter size (5-15 piglets), embryo mortality (15-40%) and heterosis (ranging from 5 to over 30%) on litter size. In particular the high prolificacy of some Chinese breeds can be used to speed up gentic progress in improving litter size either through systematic 3-way (3-4 additional piglets per litter in the F1 compared with European breeds) or 4-way crosses with Western breeds, or by developing composite lines selected for heritable traits such as growth rate and backfat thickness. The efficiency of this system might be improved by combining Chinese breeds with 'hyperprolific' western strains. When using Chinese breeds, special attention should be paid to the choice of the terminal boar, which should be as lean as possible, in order to produce acceptable carcasses for sale. Another potential solution would be to use modern computerized recording systems to detect extreme individuals and then to apply a strong selection intensity. Using this approach, it is then possible to develop a gene pool for prolificacy. Results obtained in France, Great Britain and Australia are encouraging. The expected progress is about 0.5 piglets per litter when strain selection is limited to one sex and about 1 piglet when it includes both sexes. Moreover, using crossbreeding, the heterosis effect seems to be cumulated with the genetic changes mentioned above. The computer can also be an aid in eliminating chromosomal translocations responsible for a reduction in prolificacy ranging from 5 to 50%.