These losses include non-productive days for sows and economic losses in growing-finishing pigs. Even in mild climate conditions such as The Netherlands pigs have problems in the summer with performance losses due heat stress (Figure 1).
Figure 1. Seasonality and the impacts of heat stress on swine in mild climates.
Source: Agrovision 2013
Pigs more heat stressed
Pigs are much more sensitive to hot weather than other livestock animals—largely due to the fact that pigs hardly sweat and their lungs are relatively small compared to body size.
Physiological responses to heat stress
Exposure to heat stress causes several physiological responses, including:
- Respiration rate increases
- Pulse rate falls
- Pigs start heavily panting
- Animals stop eating, because this contributes to further heat production.
Size, temperature and performance
The fact that heavier pigs are more sensitive to heat stress can be clearly seen in growth performance parameters. Investigation of different weight classes (75, 80 and 28 kg body weight) showed a direct negative correlation on average daily gain (ADG) with increasing room temperature.
While 75 kg pigs start to decrease their ADG at around 23°C, pigs with 25 kg can compensate up to 27°C (Langridge, Western Australia, 2014).
A commonly accepted temperature range for sows in the farrowing house typically spans between 21°C and 25°C —though this range is too high. Nursery sows begin to show signs of heat stress starting at 22°C (Table 1). The feed intake drops almost 0.5 kg/day as temperature increases to 25°C.
Table 1. Effect of the level protein of ration in the behavior of lactating sows in atmospheres of thermal comfort and caloric stress (Noblet et al, 2000).
This lactating trial under heat stress showed that sows lose less weight with a feed with lower crude protein. One of the explanations for this fact is that during digestion, proteins generate more metabolic heat than fats (26% against 9%), due to the complex reactions for the metabolism of the amino acids that compose them (Church and Pond, 1982).
Negative effects on gut and immune systems
Effects of heat stress can be explained by changes in the intestinal barrier. If the barrier is impaired through heat stress, this impairment can lead to an increased permeability for endotoxins, which in turn will result in local or systemic damage or inflammatory reactions (Lambert, 2009) (Figure 2).
Figure 2. Summary of pigs’ reactions to increasing temperatures.
A significant increase in serum endotoxin concentrations was shown by Pearce et al (2013) when pigs were assigned to acute heat stress (35°C, 24-43% humidity) for 24 hours. Especially effects on milk production can be brought in context with endotoxin circulation as it is described that endotoxin decreases prolactin concentrations in postpartum plasma (Smith and Wagner, 1984), which in turn has a negative impact on piglet development.
Capital intensive solutions
Technical solutions to reduce heat stress are often time consuming and capital intensive investments, e.g. building cooled stables.
Addressing heat stress through nutrition
A nutritional approach to addressing heat stress in swine can prove more adaptable and quicker to implement.