Reproductive Activity of Socorro Island Merino Ewes and Their Crosses with Pelibuey under Heat Stress Conditions.

An experiment was carried out to evaluate the effect of spring and autumn seasons on the reproductive activity of Merino Socorro Island ewes and their crosses with Pelibuey under heat stress ( HS ) conditions in the tropics. All ewes ( n = 80) were randomly assigned to one of two breeds during the first and second periods, respectively: (1) Twenty Socorro Island Merino ewes ( SIM ) and (2) 20 Pelibuey Crossbred ewes ( PBC ). Animals were fed the same diet and given water ad libitum. All statistical analyses were performed using SAS statistical software 9.12 procedures. In both seasons, a mean of more than 80 U of maximum THI was obtained, while in spring and autumn, the minimum THI exceeded 30 and 40 U, respectively. All animals were in oestrus and ovulated in both seasons. The frequency of animals in spring during the first 48 h of oestrus expression was greater ( p < 0.05) than 48-55 h but similar ( p > 0.05) than 55-65 h; in autumn during the first 48 h and 48-55 h were similar ( p > 0.05), but different ( p < 0.05) than 55-65 h. The duration of oestrus expression was longer in the spring than in the autumn ( p < 0.05). The frequency of animals was higher ( p < 0.05) in SIM than in PBC ewes during the first oestrus cycle (1-17 d) and was also higher ( p < 0.05) in PBC than in SIM ewes during the second oestrus cycle (18-35 d). The SIM ewes produced more ( p < 0.05) progesterone ( P 4 ) than the PBC ewes. During the sampling days of the oestrus cycle, more P 4 was created in autumn than in spring ( p < 0.05). Both breeds showed severe HS. In the future, ewes treated under assisted reproductive programs in the tropics may improve reproductive efficiency.