The effects of thiouracil and thyroxine on resistance to heat shock.

Adult White Plymouth Rock hens were submitted to a heat shock of 42.2 C. It was demonstrated that .1% thiouracil fed for 4 weeks prior to heat stress significantly increased survival time. Administration of DL-thyroxine administered at the rate of 4 micrograms/100 g body weight injected on day of testing did not alter survival time. However, survival under heat shock was significantly reduced if DL-thyroxine was administered daily for 3 days prior to testing. White Rocks exhibited significantly longer survival time under heat shock than Single Comb White Leghorns, and females of both breeds exhibited significantly longer survival time their male counterparts. Treatment with DL-thyroxine greatly reduced the survival time differences between breeds and sex. The implications of these findings for applied breeding programs are discussed.