Disruption of Ovarian Follicular Dynamics by Environmental Heat Stress in Zebu Cattle in Kano State

Thermal stress during the hot dry season in Kano State, North West Nigeria, significantly impairs reproductive performance in cattle herds. Reduced conception rates, delayed return to oestrus, and poor embryo survival are well-documented consequences of hyperthermia in dairy and beef cattle, yet the direct effects of heat stress on follicular dynamics in Nigerian Zebu cattle have not been systematically investigated. This study used transrectal ultrasonography to characterise ovarian follicular wave dynamics in Bunaji cows maintained under seasonal heat stress conditions and compared these with cows in thermoneutral conditions. Thirty cows were monitored during the hot dry season when ambient temperatures exceeded 38 degrees Celsius for more than six hours daily, and the same animals were monitored during the cool dry season as their own controls. Follicular dynamics were assessed by daily ultrasonographic scanning over two complete inter-oestrous cycles per period. Concurrent measurements included rectal temperature, respiration rate, and serum progesterone and oestradiol by ELISA. During heat stress, the dominant follicle showed significantly impaired growth rate and reduced maximum diameter, with a mean of 12.4 mm versus 15.3 mm in thermoneutral conditions. Ovulatory follicle size was also significantly smaller under heat stress. Oestradiol concentrations at ovulation were significantly lower during hot season. Cycle length was significantly shorter during heat stress. These findings provide mechanistic insight into heat-induced reproductive failure in Zebu cattle and support interventions including shade provision, cool water access, and cycle cooling strategies for cattle management in Kano and the North West during dry hot months. Keywords: follicular dynamics, heat stress, Zebu cattle, ultrasonography, Kano.