Inter-animal radiation as potential heat stressor in lying animals.

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.