ORIGINAL PAPER Impact of heat stress on the reproductive performance and physiology of ewes: a systematic review and meta-analyses Cinthya B. Romo-Barron 1 & Daniel Diaz 1,2 & Jesus J. Portillo-Loera 1 & Javier A. Romo-Rubio 1 & Francisco Jimenez-Trejo 3 & Arnulfo Montero-Pardo 1 Received: 4 August 2018 /Revised: 25 February 2019 /Accepted: 4 March 2019 # ISB 2019 Abstract 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 7.09 h (95% CI, - 10.75, - 3.43) the duration of estrus in cycling ewes but increased 0.57 days (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 183.62 g (95%CI, - 252.25, - 114.99) and 1665.18 g (95% CI, - 2136.82, - 1193.55). Heat stress increased ewes’ body temperature 1.13 °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. Keywords Heat stress . Reproductive performance . Reproductive physiology . Systematical review . Meta-analysis Introduction Several factors, including management procedures (Smith and Dobson 2002), nutrition (Dobson et al. 2012), and environ- mental conditions (Collier et al. 2006), affect livestock because they cause stress, which is defined as the inability of an animal to cope with its environment (Dobson and Smith 2000). Ambient conditions (temperature, relative humidity, and solar radiation) are among the main abiotic factors that negatively affect livestock because they induce heat stress under certain circumstances (Sejian 2013), i.e., when the am- bient temperature exceeds body temperature, causing a failure of heat loss due to evaporation that leads to a deficient ther- moregulatory response of the animals (Collier et al. 2006; Renaudeau et al. 2012). Therefore, physiological and behav- ioral responses rapidly confront the deleterious effects of heat stress, which may be defined as a physiological condition in which the core body temperature of an animal exceeds its range specified for normal activity (Bernabucci et al. 2010). In heat-stressed animals, thermoregulation becomes a pri- ority; therefore, the dynamic process to reach an equilibrated body temperature disrupts several other physiological func- tions (Hansen 2009). In the particular case of sheep, heat stress affects their reproductive performance and cause economic losses, primarily due to impaired reproduction (Dobson et al. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00484-019-01707-z) contains supplementary material, which is available to authorized users. * Arnulfo Montero-Pardo arnulfo.montero@uas.edu.mx 1 Laboratorio de Reproducción Animal, Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Sinaloa, Boulevard San Ángel 3886, Predio Las Coloradas, 80246 Culiacán Rosales, Sinaloa, Mexico 2 Centro de Ciencias de la Complejidad (C3), Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Mexico City, Mexico 3 Instituto Nacional de Pediatría, 04530 Mexico City, Mexico International Journal of Biometeorology https://doi.org/10.1007/s00484-019-01707-z