Heat stress affects the pig industry in tropical climates and temperate regions as well. Economic losses due to heat stress in the US swine industry alone are estimated at US$316 million annually.
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. When pigs are exposed to heat stress, their respiration rate increases, pulse rate falls, they start heavily panting and they stop eating because this contributes to further heat production. 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).
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. Source: BIOMIN
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.
Addressing heat stress through nutrition
Technical solutions to reduce heat stress are often time consuming and capital intensive investments, e.g. building cooled stables. A nutritional approach can prove more adaptable and quicker to implement. Based on current knowledge there are some aspects which can improve swine productivity during periods of heat stress.
Smaller, more frequent meals per day and/or night feeding.
A sufficient supply of fresh, clean water. Eliminate bacterial growth in the water by adding acids, such as Biotronic® SE forte liquid or Biotronic® Top liquid, to avoid infections through the system.
Physical rations adjustment
Wet the feed with water.
Use pelleted feed instead of mash.
Lower crude protein
Noblet showed in a lactating trial under heat stress that sows lose less weight with a feed with lower crude protein (see Table 1: Noblet et al., 2000). 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).
Replace starch with fat as an energy source
Fats are excellent sources of energy for swine to compensate for lower feed intake. Fat is also a more digestible ingredient that generates less metabolic heat during digestion compared to starch.
The higher the tenor of fiber of an ingredient, the worse the digestibility. Undigested fiber goes to the large intestine where it stimulates the growth of microorganisms that will generate heat in fermentation processes.
Maintain the right electrolytic balance
As temperatures increase, an animal’s breathing tension increases. Faster respiration takes more carbon dioxide out of the blood stream which is then exhaled. This changes the pH levels in the blood, leading to metabolic acidosis and lower feed intake. Blood ‘buffers’ such as sodium bicarbonate or potassium can restore the electrolytic balance and support feed intake.
Use Digestarom® to aid protein digestion
Certain plant-based compounds can promote better enzyme secretion and reduce protein loss by lowering pigs’ inflammatory response. A heat stress trial for lactating sows in Thailand showed an almost 10% increase in feed intake using a phytogenic Digestarom® P.E.P. which resulted in a 20% reduction in weight loss in the different parities (Figure 3).
Figure 3. Digestarom® improves feed intake by sows. Source: Khon Kaen University in Thailand, 2008
Combat toxins that negatively impact animal health
Hot and humid weather increases the chance of mycotoxin contamination on the field and under storage conditions. In heat stress conditions the liver is often under stress.
This often appears as poor nutrient utilization and/or chronic inflammation of the liver. It is important to keep the liver as healthy as possible and avoid additional stress from toxins, e.g. mycotoxins. Many trials have shown the negative effect of heat stress on development of endotoxins in the gut.
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. Innovative feed additives such as Mycofix® can actively combat major mycotoxins and decrease the production of pro-inflammatory cytokines.
Heat stress affects swine performance much of the time in tropical climates and seasonally in more moderate ones.
While there are many ways to improve management and feed formulations to address heat stress, the techniques listed above provide a good start based on the most current scientific knowledge. These reflect a greater focus on the reduction of toxins and gastrointestinal inflammation under these stress conditions. The best strategies combine different modes of action or techniques that reduce animal stress, improve animal performance and boost the financial result for the farmers.