奶牛卧床冷水管局部降温系统应用效果分析

为缓解奶牛热应激，该研究基于奶牛胸颈部热敏感区产热及散热理论，开发了一套卧床冷水管局部降温系统。该研究在自然通风牛舍，通过水管壁温与试验组卧床表面温度变化测试了系统性能，并通过试验组与对照组奶牛核心体温、呼吸频率、产奶量及躺卧行为的差异分析了系统的应用效果。试验期间，系统水管壁温的变化范围为18～22 ℃，进出水温差1.7 ℃；试验组卧床表面平均温度维持在21～23 ℃，比对照组显著降低2.1 ℃（P<0.01）。结果表明，卧床冷水管局部降温系统水管壁温与距进水口的管道长度呈线性正相关关系。当温湿指数（Temperature-Humidity Index, THI）达到72时，试验组奶牛核心体温比对照组降低0.3 ℃（P<0.01），呼吸频率降低9次/min（P<0.01），证明卧床冷水管局部降温系统可有效缓解奶牛热应激。该研究为牛舍环境局部降温及缓解奶牛热应激提供了方法支撑。

Local cooling performance and application effect of cow bed with cooling water pipe

High surface temperature can easily cause heat stress in dairy cows in most tropic areas of China in summer. Heat stress can negatively affect the production and reproductive performance, as well as the immune capacity of cows. Mild and moderate heat stress has also gradually become a key factor constraining the healthy production of dairy cows in recent years, with the improvement of large-scale breeding. However, the current cowshed cooling technology is limited by environmental conditions or high energy consumption. It is highly demanding for the local cooling system in the whole bed suitable for a low-temperature cooling source. In this study, a local cooling system of water pipes was designed to improve the bed environment and alleviate heat stress of dairy cows, according to the heat dissipation of thermal sensitive areas. The experiment was carried out in Datong Experimental Station of the National Dairy Industry Technology System. DN40 galvanized water pipes were laid in front of and behind the breastplate positions of two-bed rows in a naturally ventilated cowshed. The water pipes were covered with a cushion layer about 3 cm thick to meet the safety and comfort needs of cows. A floor heating radiation film of 0.6 m wide was laid below to reduce cooling loss of the system, particularly for heat preservation and insulation. Water pipes were arranged in a single serpentine coil suitable for the narrow rectangle bed, with 2 tubes at the height of mouth and nose of cow, while 4 tubes at the contact position of chest and abdomen of cow, where the space between tubes was 60 mm. The cooling water of the system was first from 200 m deep groundwater in the milking hall, and then flowed through the treated beds in the cowshed to cool down, finally back to the storage tank in the milking hall for washing. The flow rate was 0.3 m/s in pipes. Performance indexes were selected to evaluate the system operation, such as the temperature of the water pipe wall and bed surface. The results showed that the pipe wall temperature varied between 18oC and 22oC under the pipe flow of 0.38 kg/s and flow rate of 0.3 m/s, indicating a positive linear correlation between the pipe wall temperature and the distance of water inlet (P<0.01). The surface temperature of the no-load bed decreased by 2.1oC in the treated group, compared with the control. Besides, the application of the system was tested in different core body temperatures, respiration rates, milk yields, and lying behaviors of cows. Specifically, the cows were detected in the moderate heat stress for 50.7% of the day, while the mild heat stress for 17.4% during phase one. The average core body temperature in the treated group was 0.3oC lower than that in the control group, indicating an extremely significant difference (P<0.01). The respiratory rate in the treated group was at most 9times/min lower than that in the control group. There was no significant decrease in the milk yield of cows in the treated group. Correspondingly, a similar trend was achieved in the daily changes of core body temperature and respiratory rate of cows for the treated and control groups. Specifically, the cooling system performed better, as the temperature increased. Furthermore, there was no significant difference in the lying-down time between the two groups, even though the local bed environment in the treated group was significantly better than that of the control group. More importantly, a 3 cm thick pad was laid on water pipes to improve the bed comfort, which was installed after the chest plate position to ensure that the region of the udder was not squeezed when cows laid down. Nevertheless, there was an inevitable impact on the bedrest comfort, where the single lying time fluctuated greatly in the initial operation of the system, because the galvanized water pipe was made of hard material, while the cushion was often opened by hooves. The lying time in the treated group gradually approached that in the control group, as the experiment went on. Anyway, the water-cooled bed system can widely be expected to effectively improve the local bed environment in the cowshed, further alleviating the heat stress of cows.