Pigs are much more sensitive to heat than other animals so during periods of hot weather it is important to look at ways to reduce heat stress.
猪相比于其他动物对热更为敏感，高温季节，想方设法降低猪热应激的发生至关重要。
Why are pigs so sensitive to heat stress?
为什么猪容易发生热应激?
Most animals can transfer internal heat to the outside of the body by sweating and panting – these are the two most important tools for the maintenance of body temperature and form their inbuilt evaporative cooling system.
多数动物可将体内热量通过汗液和呼吸排出体外，这是动物维持体温恒定和保持自身调节能力的两种重要方式。
However, pigs do not sweat and have relatively small lungs. Due to these physiological limitations and their relatively thick subcutaneous fat, pigs are prone to heat stress. The two obvious symptoms observed when pigs are exposed to heat stress are increased respiration rate and loss of appetite. The latter reduces internal heat production.
然而，猪不出汗，肺也相对较小。这些生理上的局限，再加上猪皮下脂肪较厚，使猪易于发生热应激。当猪出现呼吸速率加快和食欲不振这两个明显的临床症状时，往往提示猪发生热应激，随后其机体产热相应减少。
If heat stress continues, pigs start to drink excessive amounts of water (increasing loss of electrolyte) and accumulate acids produced within the body (causing a loss of acid/base balance). This may eventually result in diarrhoea or death in severe cases.
持续的热应激使猪开始过量饮水(引起电解质流失)，体内酸不断累积(破坏体内酸碱平衡)，从而导致猪发生腹泻，严重时甚至引发死亡。
What does current research say about heat stress?
目前的研究对热应激怎么看?
A recent publication by Pearce et al. examined what happened to the intestinal structure when pigs were exposed to heat stress. The research showed that exposure to 35°C for 24 hours significantly damaged the intestinal defence function and also increased plasma endotoxin levels. The authors explained that when pigs are exposed to heat stress (even for as little as two to six hours) their intestinal defence systems are significantly compromised and this provides opportunity for infection as pathogenic bacteria can invade the body more easily. Therefore, heat stress can create secondary infection if sanitary conditions are poor.
Pearce等人研究了猪发生热应激时其肠道结构的变化，结果表明，猪在35°C环境下持续24小时会明显损伤其肠道防御功能，还会引起血浆内毒素水平升高。该研究显示，当猪发生应激时(即使只有2小时～6小时)，其肠道防御系统会大大受损，病原菌很容易侵入体内使感染率大大提高。如果环境卫生状况不佳，热应激还会引起继发感染。
Consequences of heat stress on the performance of pigs
热应激对猪生产性能的影响
Bigger pigs are more prone to heat stress and the reduction in growth performance is greater than for smaller pigs.
Figure 1 shows the magnitude of performance loss in 25, 50 and 75kg pigs when ambient temperature was increased from 14°C to 35°C. Average daily gain (ADG) starts decreasing when 75kg pigs were exposed to temperatures above 23°C, while for 25-kg pigs, ADG starts to decrease when exposed to temperatures above 27°C.
Figure 2 shows critical temperatures at various body weights - this information can be used as an index for the temperature management of different sheds housing varying age groups of pigs.
大猪比小猪更容易发生热应激，且热应激时生长性能比小猪下降得更明显。
图1显示了当温度从14°C升高到35°C时，体重分别为25kg、50kg和75kg猪生产性能的下降程度。重75kg的猪在温度超过23°C时，日增重开始降低;而重25kg的猪在温度超过27°C时，日增重才开始降低。
图2为不同体重猪生长发育的临界温度，可为控制各饲养阶段猪的环境温度提供参考。
Figure 1. Effect of ambient temperature on the average daily gain of grower-finisher pigs
图1 温度对生长肥育猪日增重的影响
(图中英文翻译，参照翻译修改图片)ADG：平均日增重，g/天，Ambient temperature: 环境温度，°C 体重75kg，体重50kg，体重25kg
Figure 2 Pig bodyweight has a significant effect on the critical temperature for average daily feed intake (ADFI) and average daily gain (ADG)
图2 猪体重对猪日采食量和日增重的临界温度有显著影响
（ADG：日增重 ADFI：日采食量）
At what temperature and humidity do pigs get heat stressed?
猪在怎样的温湿度环境下会发生热应激?
pigs generally develop heat stress at much lower temperatures when the humidity is high. Iowa State University recently released a heat stress index chart (Figure 3) which can be used as a decision tool for implementation of management strategies to reduce heat stress.
温度和湿度对热应激的发生影响很大。通常，猪在高湿环境中即使温度较低也会发生热应激。美国爱荷华州立大学近期发布了湿热应激指数图(图3)，可为制定减轻热应激的措施提供参考依据。
The chart shows that an average humidity of 40 percent coupled with temperatures higher than 28°C will significantly affect intestinal health and performance of grower-finisher pigs. The temperature tolerance is lower for breeding herds.
图3显示，湿度40%、温度超过28°C时，生长肥育猪的生长性能和肠道健康会受到显著影响。种猪对温度的耐受度较低。
Figure 3 Heat Stress Index for grow-finish swine
图3生长育肥猪湿热应激指数图
Room temp:环境温度 Relative humidity 相对湿度 No heat stress 无热应激区 Heat stress alert警戒区 Heat stress danger危险区 Heat stress emergency紧急区
Recommended management tools to reduce heat stress
减轻热应激的管理方法
· Increase ventilation and airflow and regularly check cooling system is in good working order, e.g. spray cooling.
· Reduce stocking density if possible.
· Maintain drinking water temperature as low as possible - around 10°C is ideal but difficult to achieve.
· Avoid feeding between 10.00am and 4.00pm - the hottest period of the day.
· Supplement electrolytes and antioxidants through the water supply.
· Increase dietary energy density.
· Minimise excess non-essential amino acids and fibre (minimising intestinal fermentation and therefore heat production).
· Increase availability of antioxidants through the diet such as vitamin E and betaine.
· 加强通风和空气流动，定期检查降温系统是否正常运作，如喷雾冷却;
· 若条件允许，减少饲养密度;
· 尽量降低饮用水温度，理想温度约10°C，但很难达到;
· 避免在每日高温时间(上午10点～下午4点)喂食;
· 通过饮水补充电解质和抗氧化剂;
· 增加能量供给;
· 减少多余非必需氨基酸和纤维的摄入(减少肠道发酵从而减少机体产热);
· 通过添加VE和甜菜碱等营养素使抗氧化剂有效发挥作用。
