Maia A S C, daSilva R G, Battiston Loureiro C M
Laboratory of Bioclimatology, Department of Animal Science, College of Agricultural and Veterinary Sciences, São Paulo State University, Jaboticabal Campus, Via Paulo Donato Castellane, km. 5, 14884-900 Jaboticabal, São Paulo, Brazil.
Int J Biometeorol. 2005 Sep;50(1):17-22. doi: 10.1007/s00484-005-0267-1. Epub 2005 Apr 29.
The general principles of the mechanisms of heat transfer are well known, but knowledge of the transition between evaporative and non-evaporative heat loss by Holstein cows in field conditions must be improved, especially for low-latitude environments. With this aim 15 Holstein cows managed in open pasture were observed in a tropical region. The latent heat loss from the body surface of the animals was measured by means of a ventilated capsule, while convective heat transfer was estimated by the theory of convection from a horizontal cylinder and by the long-wave radiation exchange based on the Stefan-Boltzmann law. When the air temperature was between 10 and 36 degrees C the sensible heat transfer varied from 160 to -30 W m(-2), while the latent heat loss by cutaneous evaporation increased from 30 to 350 W m(-2). Heat loss by cutaneous evaporation accounted for 20-30% of the total heat loss when air temperatures ranged from 10 to 20 degrees C. At air temperatures >30 degrees C cutaneous evaporation becomes the main avenue of heat loss, accounting for approximately 85% of the total heat loss, while the rest is lost by respiratory evaporation.
热传递机制的一般原理广为人知，但对于荷斯坦奶牛在田间条件下蒸发散热和非蒸发散热之间的转变，尤其是在低纬度环境下，仍需深入了解。为此，在热带地区对15头在开阔牧场饲养的荷斯坦奶牛进行了观察。利用通风胶囊测量动物体表的潜热损失，同时根据水平圆柱体对流理论和基于斯特藩 - 玻尔兹曼定律的长波辐射交换估算对流热传递。当气温在10至36摄氏度之间时，显热传递从160瓦每平方米变化至 -30瓦每平方米，而皮肤蒸发导致的潜热损失从30瓦每平方米增加至350瓦每平方米。当气温在10至20摄氏度之间时，皮肤蒸发散热占总散热的20% - 30%。当气温高于30摄氏度时，皮肤蒸发成为主要散热途径，约占总散热的85%，其余部分通过呼吸蒸发散失。
