Effects of litter size and light regimen imposed during lactation on ovarian follicular population at weaning in third-parity sows.

Sixteen third-parity sows maintained under two photoperiod treatments (16 h light: 8 h dark [16 h] or 4 h light: 20 h dark [4 h]) were assigned to be unilaterally ovariectomized (left ovary) at weaning on d 28 after farrowing a small litter (12 or fewer piglets; S) or a large litter (more than 12 piglets; L). Antral follicles were classified as atretic or nonatretic and divided into six size classes. The right ovary was obtained at slaughter, 16 d after estrus was detected. Number of antral follicles in the left ovary was greater (P less than .05) for L sows (540) than for S sows (427). Sows exposed to 16 h of light with large litters had a higher number of atretic follicles than all other groups (litter size x photoperiod interaction, P less than .05). Number of nonatretic follicles were similar between groups. However, when nonatretic follicles were classed by size (1 = smallest; 6 = largest), their distribution differed. In Class 1, L sows had more follicles than S sows (P less than .05). For Classes 3 and 4, photoperiod interacted with litter size to decrease the number of follicles in L 16 h sows (litter size x photoperiod interaction, P less than .05). These lower numbers of nonatretic follicles were related to a higher rate of follicular atresia in L 16 h sows. In Classes 2, 3 and 4, L 16 h sows had more atretic follicles than any other group (litter size x photoperiod interaction, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)