IN VIVO ENVIRONMENTAL STRESS ON BOVINE FERTILITY AND IN VITRO KINETIC EFFECT OF HEAT SHOCK ON NUCLEAR OOCYTE'S MATURATION

As known, breakthrough of global warming, changes in nutrition and expansion of metabolic disorders affect human and animal reproduction. Environmental stress had reduced fertility in cattle and these cares mainly due to hyperthermia with a great impact on cow's conception, especially during the hot months of the year, with a dramatic reduction in fertility rate. The present study was performed to evaluate the effect of weather factors on bovine (Holstein) fertility, in Terceira Island (Azores) which is a dry summer sub-tropical climate. Successively, in vitro kinetic heat shock was performed on bovine oocytes during maturation, simulating cow exposed to heat. For such purpose the success of the first artificial insemination (AI) after calving of 630 cows were recorded for one year. Cows were considered pregnant if they were not observed in estrus at least 90 days after the first AI. In parallel, climatic data was obtained from CLIMAAT (Centre of Climate, Meteorology and Global Change) at different elevation points from 0 to1000m and grazing points of cows. THI was calculated using the formula purposed by Garcia-Ispierto: THI = (0.8 x Tmax + (RHmin (%) /1 00) × (Tmax – 14.4) + 46.4) For in vitro experiments, oocytes (n=598) were maturated in vitro (IVM) for 24 hours, divided in five groups: Control at 38.5 ºC for 0 - 24hr and heat shock groups in which oocytes were exposed at 39.5ºC in four different periods 0-6hr; 0-12hr; 0-18hr; 0-24hr during IVM. Oocytes from each group were used for meiotic assessment. For weather records, it was observed differences of THI according the altitude in the Island, being higher near the sea and minimum above 500 m high (Figure 1). THI of grazing points were intermediate between maximum and minimum altitude. Relating THI and fertilizing success, it was observed a negative correlation between cow's conception rate (CR) and THI (-91.3%; p<0.05) (Figure 1). Mean THI in summer was 72.3±1.5 with a CR of 36.8%, while in winter THI was 56.6±2.0 and the CR was 65%. Regarding in vitro maturation, results showed an effect on nuclear oocyte maturation rate (NMR) for every 6hr of heat stress at 39.5 ºC (Figure 3). As observed in vivo, a high negative correlation (-0.96%; p<0.05) was observed between the time which oocytes were submitted to stress and their ability to develop to the stage of metaphase II. For every 6hr of heat shock a significant decrease in nuclear maturation rate was observed. Further studies will be performed to evaluate the ability of heat shock matured oocytes to develop after in vitro fertilization to the stage of blastocyst. As THI values in hot months are lower in highest elevations, above 500m altitude, one could purpose, to reduce the impact of heat stress in cow conception rate, to locate the cows in high elevation point during the warmest seasons.