Inbreeding and family index selection for prolificacy in pigs

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Abstract
The use of family selection indices has been proposed as a promising selection method for increasing prolificacy in pigs. Responses of up to 0-50 pigs per litter per generation are predicted from selection programmes using information from the dam and from relatives (full and half-sibs) of the dam and sire. In order to test this method in populations of finite size, the rates of selection response and the accumulation of inbreeding have been studied by simulation along 10 generations in a selected herd of 10 sires and 100 dams using five different family selection indices (Id, Ifd, Ihd, Ifs, Ihs). Two undesirable features were evident: (a) the standard deviations of indices were lower than those expected; and (b) the increase of inbreeding was up to three times that expected without selection. Both effects increased with the complexity of the family index reducing the rates of selection responses per generation, when a genetic model with dominance was assumed, to values close to 0·20 piglets, similar to those obtained with the basic index (Id). Some results of inbreeding effects on reproductive traits in three strains of an old closed herd of Iberian pigs are also presented. Multiple regression analysis of data from 4657 litters indicated a decrease in the number of live born ranging from 0·14 to 0·35 piglets per 10% increase in dam or litter inbreeding coefficient.