Responses of bovine oocytes and preimplantation embryos to elevated temperature: Possible causes of embryonic loss and strategies to improve fertility under heat stress conditions

Pregnancy rates in lactating dairy cows per insemination are decreased during the summer in hot climates. The goals of this dissertation were (1) to determine effects of heat stress on oocyte competence, (2) to develop strategies to protect the early embryo from the adverse effects of elevated temperatures and (3) to develop strategies to improve fertility in lactating dairy cows under heat stress conditions.;An initial study was done to evaluate the magnitude of heat stress associated infertility using Dairy Herd Improvement Association records on first service from 8124 Holstein cows from three locations. The decline in 90-d non return rate to first service was of lower magnitude and shorter duration in South Georgia than in North or South Florida.;To determine effect of season and magnitude of heat stress on oocyte competence in Holstein cows, two experiments were carried out where oocytes were collected and subjected to in vitro maturation, fertilization and culture. In the first experiment, there was a summer depression in the proportion of oocytes and cleaved embryos that developed to the blastocyst stage. In the second experiment, non-lactating Holstein cows were housed in one of three environments for 42 d before slaughter; two groups in summer [heat stress (HS) or cool (C)] and one group in the winter (W). There was a summer depression in oocyte quality in Holstein cows, and cooling cows for 42 d during the summer did not alleviate this seasonal effect.;A field trial was conducted to confirm beneficial effects of timed embryo transfer on d 7 after anticipated ovulation as compared to timed artificial insemination (TAI) and to determine the efficacy of vitrification as a method for embryo cryopreservation. Non-pregnant, lactating Holstein cows in the summer were subjected to the Ovulation Synchronization (OvSynch) protocol and were inseminated either on the day of anticipated ovulation (TAI) or received a fresh or vitrified embryo on day 7 after anticipated ovulation. Pregnancy rate following embryo transfer of fresh embryos was higher than that after TAI and pregnancy rate following transfer of vitrified embryos was no better than that following TAI. (Abstract shortened by UMI.).