Impact of heat stress on follicular size, oestradiol concentration and oestrus expression in Nigerian Zebu cows

The study was conducted to evaluate the effect of heat stress on follicular size, oestradiol concentrations and oestrus expression in different seasons on Zebu cows. Twelve (n=12) matured, Zebu cows were utilized for over a year. The year was divided into 3 phases. Ambient temperature, relative humidity and rectal temperature were determined using. Cows were synchronized for oestrus. Ultrasonic follicular studies were carried out daily. Blood samples were collected after administration of a second dose of PFG2α to assess serum concentration of oestradiol. The enzyme-linked immunosorbent assay technique was used to determine the concentration of oestradiol. Oestrus activities were monitored by visual observation. Follicle diameters at 24 hr were lower (P< 0.05) in the cold, dry season (6.34± 0.68 mm) than hot dry (8.09±0.52 mm) and the rainy season (8.62±0.9 mm). At 72 hr, follicular diameters were significantly higher P<0.05 in the hot, dry season (17.01±1.41 mm) than cold dry (12.90±1.22 mm) and the rainy season (12.08±0.82 mm). Time to peak of oestradiol concentrations was shorter P<0.05 in the cold, dry season (31.50±5.41 hr), followed by a hot, dry season (35.42±4.25 hr) and then rainy season (52.25±7.08 hr). The amplitude of oestradiol was higher P<0.05 in the cold, dry season (39.13±5.27 pg/mL) than hot dry (19.50±2.52 pg/mL) and rainy seasons (17.63±1.89 pg/mL). Durations of proestrus oestradiol surges were significantly higher P<0.05 in the rainy season (40.88±7.10 hr) followed by a hot, dry season (35.50±7.71 hr) and then cold dry season (24.25±3.27 hr.). The total number of mounting activities for two hr. was higher P<0.05 in the cold, dry season (7.6±0.93/hr) followed by rainy season (6.2±1.28/hr) and then hot dry season (4.4±0.81/hr). In conclusion, the study showed that heat stress affected oestrus expression by lowering the number of mounts per hour, increasing follicular size and reducing oestradiol concentration of these cows.