Inter-animal radiation as potential heat stressor in lying animals

Abstract

A model for predicting inter-animal radiant heat exchange in shaded animals is presented, with emphasis on mature cattle. When a cow's surface temperature is 35 °C, as is common in warmer climates, it loses ∼510 Watt m(-2) as radiant heat. Net radiant heat balance depends on radiation coming from bodies in the vicinity. In the 30 °C radiant temperature shaded environment typical of warm climates, net radiant loss from a lactating cow is ∼60 Watt m(-2), i.e., 30 % of its ∼173 Watt m(-2) heat production. Cows rest for 8-14 h day(-1). The heat exchange of a lying cow differs from that of a standing one: the body center is low and 20-30 % of its surface contacts a surface of relatively low heat conductance. Lying reduces the impact of the surrounding shaded area on heat exchange but increases that of heat radiating from neighboring cows. When a cow rests adjacent to other cows, with 1.25 m between body centers when in stalls, it occupies about 140° of the horizontal plane of view. Heat emitted from the animal's surface reduces the net radiant heat loss of a resting cow by ∼30 Watt m(-2). In contrast, the presence of cows at 5 and 10 m distance, e.g., cows resting on straw in loose yard housing, reduces the net radiant heat loss of the resting cow by 9 and 5 Watt m(-2), respectively. Radiant heat input increases with animal density, which is beneficial in cooler climates, but acts as a stressor in warm climates.

A review of thermal stress in cattle

“…Another study observed a maximal rate of water loss due to evaporative cooling in 600 kg lactating dairy cows of 1.5 kg h −1 6 . Using an estimate of surface area for such a cow of 6.54 m 2 , that value of 1.5 kg h −1 implies maximal sweating at only 230 ml m −2 h −1 , which is quite a bit lower than that reported by Berman 39 . When Bos indicus and Bos taurus heifers of similar body mass (335 kg) were placed into a climate chamber and exposed to the same, gradually rising, ambient temperature and humidity, the core temperature of the Bos indicus did not increase significantly until the wet‐bulb temperature exceeded 30°C, while the core temperature of the Bos taurus increased 4 days earlier, when the wet‐bulb temperature was 26°C 30 .…”

“…A lactating dairy cow with a skin temperature of 36 C will radiate around 500 W m À2 , equating to 3.27 kW for a 600 kg cow, which is about one-third of their metabolic heat production. 39 Because radiant exchange occurs between any two surfaces that "see" each other, heat is transferred between animals by radiation. Thus, stock density impacts the distance between animals and subsequently the rate of between-animal radiant heat transfer.…”

“…6 Using an estimate of surface area for such a cow of 6.54 m 2 , that value of 1.5 kg h À1 implies maximal sweating at only 230 ml m À2 h À1 , which is quite a bit lower than that reported by Berman. 39 When Bos indicus and Bos taurus heifers of similar body mass (335 kg) were placed into a climate chamber and exposed to the same, gradually rising, ambient temperature and humidity, the core temperature of the Bos indicus did not increase significantly until the wet-bulb temperature i exceeded 30 C, while the core temperature of the Bos taurus increased 4 days earlier, when the wet-bulb temperature was 26 C. 30 It is not known whether the better capacity to achieve heat balance in the Bos indicus was because of a higher capacity for evaporative heat loss, or because of a lower metabolic heat load, or a combination of the two. In those same experiments, the Bos taurus heifers increased their respiratory frequency when the wet-bulb temperature was 26 C, while Bos indicus increased only when the wet-bulb reached 28 C. When the respiratory frequency did increase, the respiratory frequency of the Bos indicus remained lower than that of the Bos taurus until the wet-bulb temperature has been at 32 C for 5 days.…”