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.
Why are pigs so sensitive to heat stress?
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.
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.
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.
What does current research say about heat stress?
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.
Consequences of heat stress on the performance of pigs
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.
Figure 1. Effect of ambient temperature on the average daily gain of grower-finisher pigs
(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)
At what temperature and humidity do pigs get heat stressed?
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.
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.
Figure 3 Heat Stress Index for grow-finish swine
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
· 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.