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Primiparous sows that farrowed on a commercial farm during late summer in 1980 (n = 65) or late winter in 1981 (n = 62) and lactated 3 to 4 wk were used. Sows were assigned in a factorial experiment to one of two lactation diets (control or 10% fat-supplemented) and one of three periods (0, 2 or 5 d) of early weaning of the heaviest one-half of the litter. Days from weaning to estrus averaged 16.7 +/- 1.5 and 8.7 +/- 1.6 in summer and winter, respectively, and the season X diet interaction was significant for days from weaning to estrus and percentage of sows that exhibited estrus after weaning. In summer, supplementing diets with fat reduced the weaning-to-estrus interval from 20.9 +/- 2.1 to 12.6 +/- 2.1 d and increased percentage of sows in estrus by 10 d postweaning from 34 to 59. In winter, days from weaning to estrus tended to be greater and percentage of sows in estrus by 10 d postweaning tended to be less in sows fed fat-supplemented diets (10.3 +/- 2.5, 74%) than in sows fed control diets (7.1 +/- 2.9, 82%). Over both seasons, weaning one-half the litter 2 d early (2-d group) increased the percentage of sows in estrus by 10 d after weaning (77%) compared with 5-d (58%) or 0-d (51%) groups. Total litter weight at weaning was not affected by season, diet or days with one-half litter. However, piglets of lightest weight remaining on sows two or five extra days gained .09 +/- .03 kg/d more than lightest weight pigs in control (0-d) litters over the same interval. Early weaning of heaviest pigs may improve reproductive performance of primiparous sows in summer and winter, while supplementing lactation diets with fat may improve performance in summer, when weaning-to-estrus interval is longer than in winter.

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