SUMMARY Three groups of fifteen Palouse gilts each were used to test the effects of altered thyroid activity on ovulation and fertilization rates and embryonic mortality in swine. Two groups received 0·15% thiouracil and 0·0123% thyroprotein, respectively, in a basal ration, and the third served as controls. All gilts were assigned to treatment sets at first oestrus and were bred to fertile boars at second oestrus. One-third of the gilts in each group were slaughtered 12–24 hr after the end of second oestrus, one-third slaughtered 25 days post coitum and the remainder allowed to farrow. An analysis of variance revealed no significant differences between groups for: ovulation rate, fertilization rate, embryonic mortality during the first 25 days of gestation, number of normal embryos at 25 days post coitum, or number of pigs born alive. A significant increase in the duration of gestation was found for thiouracil-treated females. Pigs born to thyroproteinsupplemented mothers were significantly heavier at birth than pigs born to thiouracil-treated mothers. Under the conditions of this experiment, X2 analysis indicated that thiouracil produced a highly significant embryonic mortality during gestation from the 25th day of pregnancy until farrowing. Thyroprotein treatment tended to lower embryonic mortality during the same period.

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Three lots of 10 Palouse gilts each were used to test the effect of altered thyroid activity on embryonic mortality in swine. Two lots received 0.15% thiouracil and 0.0123% thyroprotein, respectively, in a basal ration; the third lot served as the control. All gilts were assigned to treatment groups at first estrus, bred to fertile boars at third estrus and slaughtered 38 days post coitum. An analysis of variance revealed no statistically significant differences among lots for: ovulation rate, embryonic mortality during the first 38 days of gestation, or number of normal embryos at 38 days post coitum. Crown-rump lengths of normal embryos carried by thyroprotein-treated and control gilts were longer than those of normal embryos carried by thiouracil-treated gilts ( P < 0.01). There was a trend toward lowered ovulation rate and reduced embryonic mortality in the gilts receiving thyroprotein. The significance of these results is discussed in relation to BMR and pregnancy in women.