Effect of heat stress on embryo production in superovulated cows and on the pregnancy rate in recipient cows

Objectives were determine the interaction effect of heat stress from embryo production season and embryo transfer season on pregnancy rates of recipient lactating cows, and evaluate effects of heat stress on embryo production and quality front superovulated cows. Lactating Holstein cows were superovulated during temperate (n=20) and warm (n=22) seasons. Embryos were collected, frozen and transferred into recipient lactating Holstein cows during temperate (n=54) and warm (n=53) seasons. Superovulatory response (85.1 %) and fertilization rates were similar (76.2 %) for both seasons (P > 0.05). The number of oocyles and embryos (10.6), and transferable embryos (7.4) collected per cow during the temperate season were superior to the warm season (6.1 and 4.4 respectively, P < 0.05). Pregnancy rates of recipient cows were higher during temperate season when they received an embryo produced during temperate season (45.0 %), than when they received an embryo produced during warm season (21.5 %, P < 0.05). Regardless of embryo collection season, pregnancy rates of recipient cows during the warm season were lower (13.9 %) than ones observed in recipient cows during the temperate season (33.2 %, P < 0.05). Heat stress compromised embryo yield and embryo competence for establishment of pregnancy, and reduced the ability of recipient cows to become pregnant after transfer.