Effect of season and exposure to heat stress on oocyte competence in Holstein cows.

Two experiments were conducted to evaluate seasonal variation in oocyte competence in Holstein cows and to test whether oocyte quality in summer is affected by the magnitude of heat stress. In the first experiment, ovaries of Holstein cows were collected from a slaughterhouse and used to harvest oocytes over 1 yr (n = 18 replicates). After in vitro maturation, fertilization, and culture, proportions of oocytes and cleaved embryos that developed to blastocysts by d 8 were lower in the warm season compared with the cool season. In the second experiment, nonlactating Holstein cows were housed in one of the following three environments for 42 d before slaughter: heat stressed (housed with shade cloth in summer; n = 14); cooled (housed in a free-stall barn with foggers and fans in summer; n = 14); and winter (housed similar to the heat-stressed group; n = 12). Cows were slaughtered at d 18 to 19 of the estrous cycle. Oocytes from the two largest follicles per cow were aspirated and cultured individually. Ovaries were then dissected to collect additional oocytes that were processed in a group for in vitro maturation, fertilization, and culture. Cleavage rates were similar among treatments, but none of the individually cultured oocytes developed to blastocysts. For other oocytes cultured in groups, proportions of oocytes and cleaved embryos that developed to blastocysts by d 8 were lower in summer than winter with no difference between the heat-stressed and the cooled treatment groups. Summer depression in oocyte quality in Holstein cows was evident, but cooling cows for 42 d did not alleviate that seasonal effect.