Influence of heat stress on pregnancy rate of beef cattle heifers and cows using ovsynch plus CIDR followed by timed artificial insemination in Thailand

This study aimed to evaluate the pregnancy rate in beef cattle heifers and cows in Thailand under different heat stress conditions using the Ovsynch plus controlled internal drug release (CIDR) followed by timed artificial insemination (TAI) protocol. Eighty-one beef heifers and ninety-five postpartum cows were tested. These cattle were divided into three groups. Group 1 (29 heifers and 33 cows) was examined in winter, while Group 2 (25 heifers and 30 cows) and Group 3 (27 heifers and 32 cows) were examined in summer and rainy season, respectively. All cattle were treated with Ovsynch plus CIDR for 7 days followed by TAI protocol. The TAI was performed on Day 10. Pregnancy rate was determined circa 60 days after TAI. Temperature and relative humidity for each season were classified according to the temperature and relative humidity index (THI). Winter was classified as a mild stress condition, while summer and rainy season were categorized as moderate stress conditions. The pregnancy rate of the heifers and cows in the mild stress condition (67.74%) was higher than of those in the moderate stress condition (49.12%) (P<0.05). The pregnancy rates of heifers compared to cows, however, were not significantly different in all the seasons year-round. In conclusion, this study suggests that heat stress affects the pregnancy rate of beef cattle in Thailand, namely the pregnancy rate of heifers and cows in the mild stress condition was better than in the moderate stress condition (using Ovsynch plus CIDR followed by TAI protocol).