Sows’ responses to increased heat load – A review

There is a comprehensive body of literature on how increased air temperature affects the physiology, production and behaviour of sows, while very few studies consider the thermal effects of air humidity and air velocity. This review summarises studies that have investigated effects of air temperature by reviewing published literature in which sows were exposed to at least two different levels of air temperature ranging from 15 °C to 39 °C. Increased rectal temperature was investigated in the majority of the studies (26) and on average, the rectal temperature increased by 0.099 °C per °C increased air temperature above 25 °C. The increase was smaller at lower air temperatures, and it was suggested that rectal temperature is practically unaffected by air temperatures in the range of 15 °C–21 °C. This review elucidates how air temperature also affects performance indicators such as respiration rate, vaginal temperature, skin temperature, feed intake, milk yield, body weight loss during lactation, mortality, litter daily weight gain during lactation and sow behaviour. One study reported how respiration rate, rectal temperature, vaginal temperature and skin temperature were affected by both air temperature and air humidity, and the results suggest that the relative significance of air temperature and humidity may be similar for sows and finishing pigs (e.g. an increase of 40% relative humidity at an air temperature of 30 °C has a similar effect as a 1.9 °C increase in temperature). Studies on mitigation methods against the effects of high temperature and humidity such as snout cooling, drip cooling and floor cooling were reviewed to extract knowledge related to the effects of air velocity, temperatures of surrounding surfaces and the opportunity for sows to moisten their skin. •Review of the comprehensive literature related to heat stress among sows.•Many studies of air temperature - very few studies of velocity and humidity.•Respiration rate is a very useful and fast-reacting indicator of heat stress.•Floor cooling may be an effective method to reduce heat stress in lactating sows.