DESARROLLO EMBRIONARIO EN OVEJAS PELIBUEY Y SUFFOLK EN CONDICIONES DE ESTRÉS CALÓRICO EMBRYONIC DEVELOPMENT IN PELIBUEY AND SUFFOLK EWES UNDER HEAT STRESS CONDITIONS

Heat stress before and after ovulation affects embryonic development in sheep, and genetic differences in heat tolerance have been observed. In the present study, it has been assessed whether heat stress during the postovulatory period may reduce the proportion of transferable embryos and blastocysts in ewes, and if the effect on sheep adapted to warm climates (Pelibuey) is lower than that on unadapted ones (Suffolk). Thirty-two ewes were superovulated. On day 2 of the cycle (estrus=day 0), the ewes were randomly assigned to two treatments: 1) heat stress (n=16, 8 Pelibuey and 8 Suffolk), from day 2 to 6 they stayed in a climatic chamber 6 h a day, at 372.5 °C and 25 % relative humidity (HR); 2) control (n=16; 8 Pelibuey and 8 Suffolk sheep), kept at environmental temperature (18 3 °C and 50 % HR). Progesterone concentrations were determined daily from day 0 to 7. The embryos were collected on day 7. There was no difference (p>0.05) between the group exposed to heat stress and the control with respect to the proportion of transferable embryos (55 vs 69 %), nor to the blastocyst proportion (34 vs 42 %); between breeds there were no differences either (p>0.05). A larger proportion (p 0.05) of ewes having been under thermal stress (26.7 %) had premature luteal regression, but none of the control group. It is concluded that Pelibuey and Suffolk ewes, exposed to heat stress for 6 h daily from day 2 to 6 after estrus, are not affected in their percentage of transferable embryos, nor of embryos having reached the blastocyst stage, and no difference between breeds was observed. Heat stress increased the proportion of ewes with premature luteal regression.