In vitro evaluation of early embryo viability and development in summer heat-stressed, superovulated dairy cows

Our study evaluated the viability and the development in vitro of embryos flushed from superovulated heat-stressed (hot season) and unstressed (cool season) Holstein cows 6 to 8 d after artificial insemination (AI). Plasma progesterone (P(4)) levels were measured in all cows. The incidence of unfertilized ova was significantly increased in hot-season cows (P < 0.05). There was no seasonal effect on the highly variable P(4) levels 6 to 8 d after AI. Morulae and blastocysts flushed from hot-season cows were significantly less viable in culture than morulae or blastocysts flushed from cool-season cows (P < 0.001 and P < 0.0025, respectively). Furthermore, there was a significant seasonal effect both on blastulation (P < 0.0025) and hatching of blastocysts developed from morulae in culture (P < 0.005) and on expansion of blastocysts placed in culture at the blastocyst stage (P < 0.05). These data lead us to suggest that the reduced fertility of summer heat-stressed dairy cows may result from decreased viability and developmental capacity of Day-6 to Day-8 embryos. In addition, the results indicate that the efficiency of embryo transfer procedures may be significantly lowered by the reduced embryo viability associated with hot weather and that reduced embryo viability may be the cause of the well-documented seasonal reduction in the efficiency of AI.