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Effects of Litter Size and Light Regimen Imposed during Lactation on Ovarian Follicular Population at Weaning in Third-Parity Sows^1,2,

Université Laval, Québec, Québec, Canada G1K 7P4 and Université de Sherbrooke, Sherbrooke, Québec, Canada, J1K-2R1

Abstract

Sixteen third-parity sows maintained under rwo 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 < .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 < .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 < .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 < .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 < .05). In Class 1, both L 16 h and S 4 h sows had more atretic follicles (litter size x photoperiod interaction, P < .05). Weaning-to-estrus intervals and ovulation rates were not different between groups, but the L 16 h group took 2.1 d more to reach estrus. These results suggested that the number of antral follicles increases with litter size, but that a 16 h light:8 h dark light regimen applied during lactation increased the rate of atresia of large follicles.

¹ Supported by the Conseil des Recherches et Services Agricoles du Québec.

² The authors wish to thank Ferme Olympique Enr., St-Pierre de Broughton, Québec, who provided us with buildings, animals and the assistance of their qualified staff.

³ Dept. de Zootechnie, Faculté des sciences de l'Agriculture et de l'Alimentation, Université Laval, Ste-Foy, Québec, Canada G1K 7P4.

⁴ Dept. de Biologie, Faculté des Sciences, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1.

⁵ Dept. de Pathologie, Faculté de Médecine, Université Laval, Ste-Foy, Québec, Canada G1K 7P4.

⁶ Reprint requests to J. J. Dufour.