Department of Animal Science, Hebrew University, Rehovot 76100, Israel.
J Dairy Sci. 2010 Jan;93(1):242-8. doi: 10.3168/jds.2009-2601.
Heat stress is commonly relieved by forced evaporation from body surfaces. The mode of heat stress relief by heat extraction from the periphery is not clear, although it reduces rectal temperature. Radiant surface temperature (Ts) of the right half of the body surface was examined by thermovision in 4 lactating Holstein cows (30 kg of milk/d) during 7 repeated cycles of forced evaporation created by 30s of wetting followed by 4.5 min of forced airflow. Wetting was performed by an array of sprinklers (0.76 m(3)/h), and forced airflow (>3m/s velocity) over the right side of the body surface was produced by fans mounted at a height of 3m above the ground. Sprinkling wetted the hind legs, rump, and chest, but not the lower abdomen side, front legs, or neck. The animals were maintained in shade at an air temperature of 28 degrees C and relative humidity of 47%. Coat thickness was 1 to 2mm, so Ts closely represented skin temperature. Mean Ts of 5 x 20cm areas on the upper and lower hind and front legs, rump, chest, abdomen side, and neck were obtained by converting to temperature their respective gray intensity in single frames obtained at 10-s intervals. Little change occurred in Ts during the first wetting (0.1+/-0.6 degrees C), but it decreased rapidly thereafter (1.6+/-0.6 degrees C in the fifth wetting). The Ts also decreased, to a smaller extent, in areas that remained dry (0.7+/-1.0 degrees C). In all body sites, a plateau in Ts was reached by 2 min after wetting. The difference between dry and wet areas in the first cooling cycle was approximately 1.2 degrees C. The Ts of different body areas decreased during consecutive cooling cycles and reached a plateau by 3 cooling cycles in dry sites (front leg, neck, abdomen side), by 5 cooling cycles in the hind leg, and 7 cooling cycles in the rump and chest. The reduction in mean Ts produced by 7 cycles was 4.0 to 6.0 degrees C in wetted areas and 1.6 to 3.7 degrees C in sites that were not wetted. Initial rectal temperature was 38.9+/-0.1 degrees C; it remained unchanged during first 5 cooling cycles, decreased by 0.1 degrees C after 7 cooling cycles, and decreased to 38.4+/-0.06 degrees C after 8 to 10 cooling cycles, with no additional subsequent decrease. The concomitant reduction in Ts in dry and wet areas suggests an immediate vasoconstrictor response associated with heat extraction and later development of a cooler body shell. The reduction in rectal temperature represents a response involving transfer of heat from the body core to the body shell. This response mode requires consideration in settings of heat stress relief.
热应激通常通过身体表面的强制蒸发来缓解。从外围提取热量以缓解热应激的模式尚不清楚，尽管它可以降低直肠温度。在 7 个重复的强制蒸发循环中，通过在右侧身体表面上进行 30 秒的润湿，然后进行 4.5 分钟的强制气流，使用热像仪检查了 4 头泌乳荷斯坦奶牛（每天 30 公斤牛奶）的右侧的辐射表面温度（Ts）。润湿是通过一系列喷头（0.76m3/h）进行的，而在距地面 3m 高的风扇上产生的强制气流（>3m/s 速度）覆盖了右侧身体表面。喷头弄湿了后腿、臀部和胸部，但没有弄湿下腹部、前腿或颈部。动物在空气温度为 28°C 和相对湿度为 47%的阴影下饲养。皮毛厚度为 1 至 2 毫米，因此 Ts 非常接近皮肤温度。通过将各自在 10 秒间隔获得的单个帧中的灰度强度转换为温度，获得了上、下后腿、前腿、臀部、胸部、腹部侧面和颈部的 5x20cm 区域的平均 Ts。在第一次润湿期间，Ts 几乎没有变化（0.1+/-0.6°C），但此后迅速下降（第五次润湿时为 1.6+/-0.6°C）。在仍然干燥的区域，Ts 也会以较小的程度下降（0.7+/-1.0°C）。在所有身体部位，在润湿后 2 分钟达到 Ts 平台。在第一次冷却循环中，干燥区和湿区之间的差异约为 1.2°C。连续冷却循环中，不同身体区域的 Ts 下降，在干燥部位（前腿、颈部、腹部侧面）达到第 3 个冷却循环，在后腿达到第 5 个冷却循环，在臀部和胸部达到第 7 个冷却循环。在湿区，7 个循环产生的平均 Ts 降低了 4.0 至 6.0°C，在未润湿的部位降低了 1.6 至 3.7°C。初始直肠温度为 38.9+/-0.1°C；在前 5 个冷却循环中保持不变，在第 7 个冷却循环后降低了 0.1°C，在第 8 至 10 个冷却循环后降低至 38.4+/-0.06°C，此后没有进一步降低。干燥区和湿区的 Ts 同时降低表明，热提取后立即出现血管收缩反应，随后发展出更冷的体壳。直肠温度的降低代表了一种涉及从身体核心向体壳传递热量的反应。这种响应模式需要在缓解热应激的环境中加以考虑。
