Heat stress: Poultry health, performance and potential mitigation strategies

Heat stress represents a major challenge for maintaining health and performance levels within the poultry industry.

In response to such demands, there have been great improvements in chicken genetics during the past decade. Broiler chickens, which weighed around 900 g in 56 days in the 1950s, were around 4202 g in 2005. Similarly, laying hens in the early 1900s used to lay 150 eggs per year while current commercial laying hens lay around 300 eggs annually . Such genetical improvements in broilers and laying hens, have also brought upon higher metabolic rates and production performances in these animals. The higher metabolic rates of these animals generate larger amounts of body heat, making them much more susceptible to heat stress. The risk of heat stress in poultry is made even worse due to current rearing conditions with high stocking density of birds, along with high environmental temperatures.

Heat stress is one of the major environmental stressors in the poultry industry resulting in substantial economic loss. It causes several physiological changes, such as oxidative stress, acid-base imbalance, and suppressed immunocompetence. Leading to increased mortality and reduced feed efficiency, body weight, feed intake, and egg production. While also affecting meat and egg quality.

As of 2003, the poultry industry suffered annual economic losses of $128 to $165 million, due to the negative impacts of heat stress. When subjected to this extreme condition, birds are unable to maintain a physiological balance between body heat production and heat loss.  This condition involves the interaction of several factors such as:

•  high environmental temperature
• humidity
• radiant heat
• and airspeed

The normal body temperature of chickens ranges around 41–42 °C. Meanwhile, the thermoneutral temperature to maximize growth ranges between 18–21 °C . Studies have shown that any environmental temperature higher than 25 °C can cause heat stress in poultry. There is extensive scientific literature covering the negative impacts of heat stress on poultry health and performance. There is also a significant amount of studies carried out in search of potential mitigation strategies.

This article aims to review some of the most relevant published information regarding the physiological and behavioral changes seen in poultry when subjected to heat stress, as well as mentioning (2) potential mitigation strategies for broilers and laying hens.

 _{Biological changes in poultry associated with heat stress}

Source:(Wasti, S.; Sah, N.; Mishra, B. 2020)

Oxidative Stress

Suppressed Immune function

Heat stress is known to suppress birds’ immune function,  which in turn results in a higher prevalence of different infectious diseases, such as: Newcastle disease and Gumboro disease.

It has also been observed that birds subjected to heat stress suffered a reduction in the size of immune-related organs, such as:

• spleen,
• thymus,
• and lymphoid organs

Antibody titers have also been shown to be lower in heat-stressed birds as well as total white blood cell counts (WBC). Meanwhile,  the heterophils to lymphocytes (H/L) ratio has proven to be higher for birds under heat stress conditions.

_{Potential nutritional strategies for mitigating heat stress in poultry}

The following are just some examples of nutritional strategies, from the many existing ones that have been developed and tested to counter heat stress in poultry production.

 

 Feed Restriction

Restricting the feed during the hotter period of the day has been a common practice in poultry production. In this practice, feed intake is reduced by withdrawing feed for a certain period  to reduce the metabolic rate of birds. Feed restriction has been shown to reduce rectal temperature, minimize mortality  and reduce abdominal fat in heat-stressed broilers.

Dual feeding regime

Practical observations have shown that feed restriction results in overcrowding and rush at a re-feeding time resulting in some additional mortality. Thus, the dual feeding regime has been devised to ensure birds have access to feed throughout the day. The thermic effects of proteins are higher than carbohydrates and produce higher metabolic heat. Taking this into account, the protein-rich diet is provided during cooler times and the energy-rich diet during the warmer period of the day.

Wet feeding

Birds lose a significant amount of water through the respiratory tract when subjected to heat stress. Therefore, they increase their water intake under this conditions, in order to restore their thermoregulatory balance. Adding water in the feed helps increase water intake and reduces viscosity within the gut resulting in a faster feed passage. Wet feeding stimulates pre-digestion, improves nutrient absorption in the gut, and accelerates the action of digestive enzymes on the feed.

Conclusions 

•  Heat stress has become a major challenge affecting the development of the poultry industry during the last decade. This scenario has become even more complicated due to rising global temperatures.
•  Heat stress in poultry results from the interplay of several factors, such as: high environmental temperatures, humidity, radiant heat, and airspeed. Causing significant physiological, neuroendocrine, and behavioral changes in the animals.
• Several strategies have been set forth and tested to mitigate the deleterious effects of heat stress in poultry. However, only a few of these are widely implemented within the poultry industry.
• Due to its multifactorial nature, single strategy approaches are not effective at counteracting the negative impacts of heat-stress on poultry. Therefore, a holistic approach towards heat stress mitigation in poultry is required.
• The use of nutritional strategies, modifications in housing and management should all prove to be beneficial in mitigating heat stress. However, greater research evaluating the combination different approaches to counteract heat-stress in poultry is still needed. In order to evaluate the efficiency of such combinations and assess their cost to benefit relation.