Farm Technology Group, Wageningen University and Research, 6700 AH Wageningen, the Netherlands.
Wageningen Livestock Research, Wageningen University and Research, 6708 WD Wageningen, the Netherlands.
J Dairy Sci. 2022 Aug;105(8):7061-7078. doi: 10.3168/jds.2021-21683. Epub 2022 Jun 7.
The focus of this study was to identify the effects of increasing ambient temperature (T) at different relative humidity (RH) and air velocity (AV) levels on heat loss from the skin surface and through respiration of dairy cows. Twenty Holstein dairy cows with an average parity of 2.0 ± 0.7 and body weight of 687 ± 46 kg participated in the study. Two climate-controlled respiration chambers were used. The experimental indoor climate was programmed to follow a diurnal pattern with ambient T at night being 9°C lower than during the day. Night ambient T was gradually increased from 7 to 21°C and day ambient T was increased from 16 to 30°C within an 8-d period, both with an incremental change of 2°C per day. A diurnal pattern for RH was created as well, with low values during the day and high values during the night (low: RH_l = 30-50%; medium: RH_m = 45-70%; and high: RH_h = 60-90%). The effects of AV were studied during daytime at 3 levels (no fan: AV_l = 0.1 m/s; fan at medium speed: AV_m = 1.0 m/s; and fan at high speed: AV_h = 1.5 m/s). The AV_m and AV_h were combined only with RH_m. In total, there were 5 treatments with 4 replicates (cows) for each. Effects of short and long exposure time to warm condition were evaluated by collecting data 2 times a day, in the morning (short: 1-h exposure time) and afternoon (long: 8-h exposure time). The cows were allowed to adapt to the experimental conditions during 3 d before the main 8-d experimental period. The cows had free access to feed and water. Sensible heat loss (SHL) and latent heat loss (LHL) from the skin surface were measured using a ventilated skin box placed on the belly of the cow. These heat losses from respiration were measured with a face mask covering the cow's nose and mouth. The results showed that skin SHL decreased with increasing ambient T and the decreasing rate was not affected by RH or AV. The average skin SHL, however, was higher under medium and high AV levels, whereas it was similar under different RH levels. The skin LHL increased with increasing ambient T. There was no effect of RH on the increasing rate of LHL with ambient T. A larger increasing rate of skin LHL with ambient T was observed at high AV level compared with the other levels. Both RH and AV had no significant effects on respiration SHL or LHL. The cows lost more skin sensible heat and total respiration heat under long exposure than short exposure. When ambient T was below 20°C the total LHL (skin + respiration) represented approx. 50% of total heat loss, whereas above 28°C the LHL accounted for more than 70% of the total heat loss. Respiration heat loss increased by 34 and 24% under short and long exposures when ambient T rose from 16 to 32°C.
本研究的重点是确定在不同相对湿度（RH）和空气速度（AV）水平下环境温度（T）升高对奶牛皮肤表面和呼吸散热的影响。20 头荷斯坦奶牛参与了这项研究，平均胎次为 2.0±0.7，体重为 687±46 公斤。使用了两个气候控制呼吸室。实验室内气候按照昼夜模式编程，夜间环境 T 比白天低 9°C。在 8 天的时间内，夜间环境 T 逐渐从 7°C升高到 21°C，白天环境 T 从 16°C升高到 30°C，每天递增 2°C。还创建了昼夜 RH 模式，白天 RH 值较低，夜间 RH 值较高（低：RH_l=30-50%；中：RH_m=45-70%；高：RH_h=60-90%）。在白天，还研究了 AV 的影响，分为 3 个水平（无风扇：AV_l=0.1m/s；风扇中速：AV_m=1.0m/s；风扇高速：AV_h=1.5m/s）。只有在 RH_m 时，AV_m 和 AV_h 才会结合。总共，每个有 5 个处理，每个处理有 4 个重复（奶牛）。通过每天两次（上午：1 小时暴露时间；下午：8 小时暴露时间）收集数据来评估短期和长期暴露于温暖条件的影响。在主实验的 8 天前，让奶牛适应实验条件 3 天。奶牛可以自由进食和饮水。通过放置在奶牛腹部的通风皮肤盒测量皮肤表面的显热损失（SHL）和潜热损失（LHL）。通过覆盖奶牛口鼻的面罩测量呼吸产生的这些热量损失。结果表明，随着环境 T 的升高，皮肤 SHL 降低，RH 或 AV 对降低速率没有影响。然而，在中高 AV 水平下，平均皮肤 SHL 较高，而在不同 RH 水平下，皮肤 SHL 相似。皮肤 LHL 随环境 T 的升高而升高。RH 对 LHL 随环境 T 的升高速率没有影响。与其他水平相比，在高 AV 水平下，皮肤 LHL 随环境 T 的升高速率更大。RH 和 AV 对呼吸 SHL 或 LHL 均无显著影响。与短期暴露相比，奶牛在长期暴露下损失更多的皮肤显热和总呼吸热。当环境 T 低于 20°C 时，总 LHL（皮肤+呼吸）约占总热量损失的 50%，而当环境 T 高于 28°C 时，LHL 占总热量损失的 70%以上。当环境 T 从 16°C 升高到 32°C 时，短期和长期暴露下呼吸热损失分别增加了 34%和 24%。
