UTERINE CAPACITY AND PRENATAL SURVIVAL IN MEISHAN AND LARGE WHITE-PIGS

Uterine capacity was estimated in Chinese Meishan (MS), Large White (L W) and crossbred (LW X MSI sows, using unilateral hysterectomy-ovariec tomy (UHO) methods. In the first two parities, 20 sows of each genotyp e, whose left uterine horn and ovary had been removed in the post-pube rtal period, were mated, their ovulation rate recorded by laparoscopy and allowed to farrow normally. In the third parity the mated sows wer e slaughtered at around 30 days of pregnancy and ovarian, uterine and embryo traits were recorded. The ovulation rate in UHO females teas si milar to that observed previously in intact females of the same popula tion, MS sows producing 4.2 (s.e. 1.0) more ova at parities 1 and 2 an d 7.0 (s.e. 1.9) more ova at parity 3 than LW sows. At parity 3, MS so ws had 5.9 (s.e. 2.0) more viable embryos at 30 days of pregnancy than LW sows. Crossbred sows were intermediate for both ovulation rare and the number of viable embryos at 30 days. At the first two parities, l itter size was higher in the MS by 1.0 (s.e. 0.8) piglets per litter t han in the LW, while the crossbred sows exceeded the mid-breed mean by 1.7 (s.e. 0.7) piglets. Uterine capacity, estimated as twice the numb er of piglets carried to term in a single horn, was 12 foetuses in the LW, 14 in the MS and 16 in the crossbred sow. Piglets born to MS sows were 0.4 (s.e. 0.1) kg lighter than those born to LW sows, with a sig nificantly lower (MS: LW = 0.68, P < 0.01) within-litter standard devi ation, which was found to be independent of the smaller mean weight. A t parity 3, MS sows were found to have uteri of similar length with bo th lighter (-0.36 (s.e. 0.14) g) and shorter (-1.54 (s.e. 0.76) mml em bryos with smaller spaces (-7.8 (s.e. 2.7) cm) between embryo sites th an LW sows. Within-litter standard deviations for embryo size traits w ere lower in MS than in LW sows, especially for the distance between e mbryo sites (MS : LW = 0.45, P < 0.02). The reduced variability of emb ryo traits in litters in MS sates was independent of their smaller lit ter means. Third parity litters from crossbred sows were similar to th e mid-breed mean for means and variability of embryo traits but crossb red sows had significantly longer uteri (48.5 (s.e. 18.5) cm) than the mid-breed mean, being longer than either pure breed. It was concluded that differences between the pure breeds in uterine capacity may have arisen through better within-uterus organization, whereas the observe d heterosis effects may result in part from physically longer uteri in crossbred sows and that uterine capacity contributes to the genotype differences in prenatal survival and prolificacy, especially in crossb red sows.