Periods of high heat, particularly in the summer, affect the comfort of farm animals, which are thus subjected to heat stress. This stress affects the physiology of livestock differently depending on the species.
Complex mechanisms of response to heat stress
Heat stress affects the whole metabolism of animals. Farmers can easily spot severe heat stress with signs such as hyperventilation, panting or a decrease in feed intake and vitality (less movement and activity).
But these observations are only the tip of the iceberg. Outside the thermal neutrality zone, the organism adopts a global strategy to reduce the internal temperature with physiological, behavioural and also metabolic modifications. These changes can have a number of repercussions:
- Respiratory alkalosis and metabolic acidosis,
- Impaired intestinal integrity due to altered blood flow to peripheral organs,
- High levels of oxidative stress and inflammation.
Variable sensitivity to stress
All species are affected by heat stress with varying degrees of sensitivity mainly due to
- The species: thermoregulatory mechanisms vary according to the species.
- The size of the animal: the larger the animal, the greater the impact.
- The age of the animal: the younger the animal, the less sensitive it is to heat stress.
- Metabolic stress: the more intense the metabolism, the greater the sensitivity to heat stress
- Genetics: breeding for productivity may have a higher sensitivity to heat stress.
The cost of heat stress
These thermoregulatory mechanisms also have a cost: nutrients are diverted to maintain the temperature of the animals. This leads to a decrease in growth performance or reproductive parameters; it can even lead to an increase in mortality during intense heat waves.
In poultry farming, the negative impact of heat stress on poultry production has been demonstrated in numerous studies in real or simulated situations. The zootechnical observations vary according to the level of heat stress and the production concerned, but overall it is observed that:
- An increase in mortality
- A decrease in ADG
- A deterioration of the FCR
In pig farming, high heat leads to a decrease in animal consumption, a deterioration of the index, a drop in the ADG and an increase in mortality.
This results in lower carcass weights in the summer period, when the price of pork is at its highest.
Furthermore, animals recover slowly. For long-cycle animals, performance may decline several months after the heat stress period. For breeding stock, heat stress leads to increased sensitivity of the offspring: birth weight, immunity.
An American study (St-Pierre et al., 2003) estimated the total annual economic losses for the livestock sectors. These ranged from 1.69 to 2.36 billion dollars in the USA. Of these losses:
- $897 million to $1.5 billion is for the dairy industry,
- $370 million in the cattle sector,
- $299 to $316 million the pig sector,
- $128 million to $165 million for the poultry sector.
In France, the ITAVI estimated the cost of the 2003 heatwave at 44.5 million euros for the poultry industry.
Heat stress, a major issue in livestock breeding
To anticipate the future economic impact of heat stress, recent models have been built to estimate annual losses due to heat stress. In the United States, these estimates amount to $1.5 billion for the dairy industry and nearly $1 billion for the hog industry (Pollmann, 2010; Key and Sneeringer, 2014).
These estimates probably underestimate the losses:
- They do not take into account the climate change expected in the coming years.
- They are based on temperate environments. Actual heat stress losses due to prolonged exposure to heat in tropical climates are proportionately greater.
- They mainly take into account the quantitative impact on production. The qualitative impacts on products and veterinary costs in particular are not taken into account.
In conclusion, as temperatures rise and production performance increases, economic losses will be more pronounced in the future, requiring the implementation of appropriate preventive measures to reduce them.
For more information on heat stress in livestock breeding, see: Article – Strategies for managing heat stress
