EFFECT OF EXOGENOUS THYROXINE ON THE INTERVAL FROM WEANING TO ESTRUS OF THE PRIMIPAROUS SOW 1'2

A total of 159 primiparous sows were fed 0 (C) or 675 mg/d of thyroprotein (TP) from d -2 until d 14 postweaning. Sows received 8 (LE) or 14 (HE) Meal of ME/d during a 28-d 3,5,3 -trnodothyronme and 3,5,3 ,5 -tetraldothyronm e lactation period. Plasma levels of ' '" " (T3) ' ' " " (T4) were determined for d -4, -2, 0, 1, 2, 3, 5 and 7 postweaning. Sows consuming LE lost more weight and backfat during lactation (P < .01) than HE sows. Litters of sows consuming LE gained less weight than litters of sows consuming HE (P < .01). Thyroprotein increased plasma levels of both T3 and T4 (P < .01), whereas energy intake had no effect (P > .10) on either of the two hormones. The response to thyroid hormone was not consistent across days postweaning (TP x day, P < .01). Feeding TP increased T4 (P < .01) on all days, and T3 was increased (P < .05) on d 0, 2 and 3 postweaning. Thyroid hormones rose marked- ly following weaning regardless of TP level. Percentage of sows in estrus by d 7 postweaning for LE-C, LE-TP, HE-C and HE-TP were 80, 78, 92 and 90, respectively. Energy restriction during lactation resulted in fewer sows returning to estrus by d 7 (P < .03) than those fed HE. However, feeding TP postweaning had no effect on return to estrus, suggesting that the effect of dietary energy restriction during lactation on the interval from weaning to first estrus is not mediated through postweaning thyroid hormone concentrations. (Key Words: Pigs, Sow Feeding, Energy Intake, Estrus, Thyroid Hormones.) J. Anim. Sci. 1989. 67:157-167