A COMPARISON OF ALTERNATIVE GENETIC MODELS FOR LITTER SIZE IN PIGS

Predictions of two models were compared. The models relate ovulation r ate (OX) and prenatal survival (PS) to litter size (LS): the uterine c apacity model (UCM), where maximum LS is limited by uterine capacity ( UC), and the threshold model (TM) whereby PS is modelled as a binary t hreshold trait. Records were simulated according to both models using statistics from French Large White gilts. Both models were able to rep roduce closely the observed curvilinear relationship between OR and LS , with LS reaching a plateau at high OX. Several genetic correlations (Pg) fulfilling the conditions h(OR)(2) = 0.34 and h(LS)(2) = 0.12 (th e residual maximum likelihood estimates in the population) were studie d by means of stochastic computer simulation. The generic correlation between OX and LS was very sensitive to changes in h(UC)(2), whereas r ho(gLs,PS) was always positive, and rho(gOR,PS) was always negative. T he correlation between PS and UC was larger than 0.90, except for very small h(UC)(2). This suggests that genes affecting PS have a strong i nfluence on LTC and that PS can be a good indirect criterion to select for UC. Both models predicted that the advantage of an index combinin g OX and LS with respect to direct selection on LS diminishes in succe ssive generations of selection and that the size of the experiment nee ded to detect significant differences is very large. Records were also simulated by halving the mean and variance of LTC, so as to mimic uni lateral hysterectomy-ovariectomy (UHO). If the UHO treatment results i n halving UC, LS of UHO females should behave very much as half the UC of intact females.