PARENT SESSION
Oocyte Development

EFFECT OF SEASON AND EXPOSURE TO HEAT STRESS ON OOCYTE QUALITY OF HOLSTEIN COWS.

Al-Katanani, Yaser¹, Paula-Lopes, Fabiola¹, Hansen, Peter¹, ¹

ABSTRACT- In cattle and sheep, heat stress before insemination is associated with decreased fertility. Objectives were to evaluate seasonal variation in oocyte competence in cattle and test whether oocyte quality in summer is affected by the magnitude of heat stress. Non-lactating Holstein cows (n=12-14/group) were killed at d 18-19 of the estrous cycle, oocytes harvested by slashing the ovaries, and oocytes from all but the largest and second largest follicle subjected to in vitro maturation, fertilization and culture. Estrous cycles were synchronized by GnRH (100 g, i.m.) and, 7 d later, PGF₂ (25 mg, i.m.). In summer, cows were housed in one of two environments for 42 d prior to expected slaughter. One group (Heat Stress; HS) was housed in a lot provided with shade cloth. The other (Cool; C) was housed in a free stall barn with foggers and fans. In winter (W), cows were housed in the same lot as used for the C group. Sperm used for fertilization were from the same bulls in summer and winter. Rectal temperature was higher (P<0.001) in summer (39.8±10.8°C and 39.2±10.8°C for the HS and C groups) than winter (38.7±11.0°C) and was lower for C than HS (P<0.001). There was no effect of treatment on cleavage rate (88.8±2.2%, 88.4±2.0% and 88.4±2.2% for HS, C and W, respectively). However, the proportion of cleaved embryos that developed to blastocyst at d 8 was lower (P<0.001) for summer than winter; there was no difference between HS and C. Least-squares means ± SEM were 12.0±4.1%, 11.9±4.1% and 34.3±4.2% for HS, C and W, respectively. The same differences were seen when development was expressed as oocytes that developed to blastocyst (10.9±3.6%, 10.8±3.4% and 29.3±3.7% for the HS, C and W; summer vs. winter, P<0.001). In conclusion, there was a summer depression in oocyte quality in non-lactating cows. However, cooling cows during the summer did not alleviate this seasonal effect. This latter result suggests that either 1) the degree of cooling was not sufficient to prevent heat stress effects, 2) heat stress acted to damage oocyte quality earlier in oocyte development than when cooling was initiated or 3) seasonal effects are due to factors other than heat stress.

KEY WORDS: Oocyte quality, Heat stress, IVF