Heat stress effects in primiparous and multiparous lactating crossbred cows under a warm environment and their responses to a cooling treatment

Abstract

Context In temperate climates, multiparous cows are more susceptible to heat stress (HS) than are primiparous cows. However, these differences in susceptibility may vary in warm environments, where the overall production level is lower, cow size is smaller, and adaptation to HS increases. Aim The different effects of HS and its alleviation on primiparous and multiparous cows was tested in crossbred cows (3/4 Holstein × 1/4 Brahman) in a warm environment. Method Twelve cows during the rainy season (August–September 2014) and 12 cows during the dry season (March–April 2015; six primiparous, six multiparous) were monitored for rectal temperature and respiration rate, indicators of HS, and milk yield (MY) in a dairy farm in El Salvador. Ambient temperature and relative humidity were recorded hourly to estimate temperature–humidity index throughout the day. During the rainy season, all cows were housed in a pen without cooling treatment. During the dry season, fans and sprinklers were installed in the pen and provided cooling to the herd in two 15-day periods, alternated with two periods without cooling (Control). Key results Primiparous cows were more susceptible to HS. Cooling alleviated HS, resulting in an increased feed intake and MY, with a greater impact on primiparous cows. During the cooling treatment, HS parameters increased overnight after the last cooling cycle of the day. This indicated that, despite the cooling treatment, cows still suffered from HS outside the hottest hours of the day. Furthermore, despite a higher temperature–humidity index during the rainy season, rectal temperature and MY remained similar in both the dry and the rainy season for cows without cooling, indicating that animals effectively lower their body temperature by increasing their respiration rate. Conclusions The cooling only partially helped alleviate HS in both multiparous and primiparous cows, with the effects being greater on the latter. However, the increments in MY were proportional to the increases in feed intake, indicating that the efficiency of feed utilisation was likely to be not improved. Implications More attention should be given to the susceptibility of primiparous cows to HS, as well as to the adjustment of cooling protocols towards alleviating HS in early morning and late afternoon hours.