Impact of heat stress on the reproductive performance and physiology of ewes: a systematic review and meta-analyses

High values of ambient temperature and relative humidity negatively affect livestock because they induce heat stress. In heat-stressed ewes, the reproduction is affected, although there apparently is a different effect depending on their physiological status. Therefore, we conducted a systematic review and meta-analyses to estimate the effect of heat stress on the reproductive performance and physiology of cycling or pregnant ewes. We included studies in which pregnant or cycling ewes were exposed to heat stress and included a comparison group. We assessed the risk of bias and used meta-analyses that included the period of exposure to calculate pooled estimates. We included 36 studies comprising 20,626 ewes, and the meta-analyses demonstrated that heat stress decreased 7.09h (95% CI, -10.75, -3.43) the duration of estrus in cycling ewes but increased 0.57days (95% CI, 0.23, 0.92) the length of the cycle, although differentially affecting the expression of sexual behavior. Heat-stressed cycling ewes had greater odds of embryo mortality (OR=12.36; 95% CI, 7.10, 21.54) and a decreased chance of impregnation (OR=0.41; 95% CI, 0.23, 0.75). In heat-stressed pregnant ewes, placental and fetal weights were reduced, respectively in 183.62g (95%CI, -252.25, -114.99) and 1665.18g (95% CI, -2136.82, -1193.55). Heat stress increased ewes' body temperature 1.13 degrees C (95% CI, 0.85, 1.42) and respiratory rate 93.53 breaths/min (95% CI, 63.5, 123.5) without affecting heart rate. In a sub-group analyses, the period of exposure was shown to cause a significant effect. In conclusion, our results revealed a differential effect of heat stress that depends on both the physiological status of ewes and the period of exposure, and identify susceptible reproductive stages during which the severe effects of heat stress can be prevented/ameliorated.