Primiparous sows were fed to appetite during lactations that occurred during winter or summer, and 11.4 ± 0.4 pigs per litter were weaned at 23.5 ± 0.1 days of age. Sows were slaughtered at 0 or 72 h after weaning or blood samples were collected until 24 h after onset of oestrus. Sows that lactated during summer consumed less food and lost more (P < 0.05) weight, heartgirth and backfat than those that lactated during winter. Weaning-to-oestrus interval was greater (P < 0.05) in summer (224 ± 25 h) than in winter (93 ± 13 h). Content of GnRH in the hypothalamus and concentrations of LH in the anterior pituitary and serum were lower (P < 0.05)after weaning in summer than winter. The numbers of visible ovarian follicles < 5 mm in diameter at weaning were lower (P < 0.05) in summer than in winter. In contrast to LH, FSH concentration in serum was higher (P < 0.10) in summer than winter, but FSH values in the anterior pituitary were lower (P < 0.05) in summer than in winter. Post-weaning patterns of secretion of oestradiol and follicular development differed between winter and summer. For example, in some sows weaned during the summer, transient surges of oestradiol occurred repeatedly during 0 to 280 h after weaning without provoking surges of LH. These results indicate that the period of post-weaning anoestrus in summer is prolonged because of altered activity of the hypothalamic-pituitary axis, possibly because of changes in sensitivity to the feedback of oestradiol. Lower feed intake during lactations that occur during summer may predispose the endocrine system to the aberrations.



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