Counteracting heat stress in poultry production



Birds are homeothermic. Their thermoneutral zone lies between 18 and 36°C, but the upper critical temperature is strongly dependent on the relative air humidity (RH%), which is lower at higher RH%, on breed and on production performance. Also the age of the parent stock and brooding conditions affect the heat tolerance of the offspring. As long as the ambient temperature is lower than the birds’ body temperature, heat loss from the core to the skin can be increased by radiation, depending on peripheral blood flow. Poultry responds to high environmental temperatures by behavioral changes, which allow them to re-establish heat balance with their surroundings. During periods of heat stress, broilers rest more, stand more quietly or simply sit near walls or waterers. Usually, they lift their wings in order to promote cooling by reducing body insulation. Hyperventilation or "panting" increases during periods of high environmental temperature, leading to increased CO2 loss.

Phytogenics can mitigate the consequences of heat stress in poultry

© Delacon: Phytogenics can mitigate the consequences of heat stress in poultry production


Consequences of heat stress



Reduction in feed intake is one of the first recognizable effects of heat stress in broilers. This reduction in feed intake during heat stress accounts for up to 30% of the reduced weight gain during heat stress. The major reduction is related to oxidative stress. During chronic heat stress plasma cortisol is increased and thyroid hormone levels are reduced (e.g. Sohail et al., 2010). These elevated plasma cortisol levels stimulate muscle catabolism and lipid peroxidation in muscle tissues, which was concluded from increased malondialdehyde (MDA) contents in breast muscle of broilers (Zhang et al. 2011). Azad et al. (2009) showed that lipid peroxidation in pectoralis muscle of broilers increased with the severity of heat stress during the last two weeks pre-slaughter.


Moreover, they showed that rectal temperature of heat stressed broilers was increased by approx. 2°C comparing broilers housed at thermoneutral temperature and broilers housed at constant 34°C. Niu et al. (2009) and Song et al. (2014) showed that heat stress additionally impairs immune response and intestinal integrity. The latter effect was related to lipid peroxidation in the enterocytes. Gu et al. (2012) indicated that heat shock proteins (HSP70, a group of highly conserved protective proteins, involved in cell protection and cell repair) play an essential role alleviating heat stress response, as they stimulate antioxidant enzyme activities, relieving oxidative damage in intestinal mucosal cells during heat stress. Adverse effects of heat stress on intestinal integrity may account for reported higher translocation of Salmonella enteritidis, resulting in intestinal inflammation and increased Salmonella counts in tissues after heat stress (Quanteiro-Filho et al. 2012). Additionally, Bonnett et al. (1997) showed that nutrient digestibilities were reduced during heat stress, which supports the need of using feed ingredients with a higher digestibility (and therefore dietary nutrient concentration will require the use of high quality feedstuffs) or feed additives that support nutrient digestion.

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Nutritional strategies to reduce heat stress in poultry production



Although effects of nutrient concentration on heat load of broilers are limited, dietary concentration reduces energy expenditure for nutrient intake and its positive effects are therefore similar to feeding good quality pellets. Although it is clear that limiting excess protein and optimizing amino acid profile minimize metabolic energy costs to excrete surplus nitrogen, the effect of heat stress on the optimum amino acid profile is not yet known. Gous (2010) indicated that although a higher fat content at the expense of carbohydrates will reduce metabolic heat production, effects are limited when relying on normal feed ingredients. It is well-accepted that management factors like feed withdrawal 4 to 6 h prior to the hottest period of the day limit heat increment of feeding. However, broilers will only benefit from temporary feed withdrawal if the ambient temperature during night-time is substantially lower than during the day (cyclic heat stress) to enable compensatory nutrient intake during the cooler periods of the day. Heat-stressed birds dissipate up to 80% of their heat production through evaporative cooling by panting (Van Kampen, as cited by Gous, 2010). As panting increases CO2 losses, heat-stressed birds will benefit from a higher cation: anion balance. Apart from optimizing feed composition and structure, several (classes of) feed additives have been mentioned in scientific literature to alleviate (the consequences of) heat stress. Papers indicate that the efficacy of such additives is focused on their antioxidant effects. Heat-stress induces oxidative processes in the enterocytes as discussed in ‘consequences of heat stress’. Therefore, increased levels of dietary antioxidants, like a combination of vitamins A and E, reduce lipid peroxidation during heat stress (Sahin et al., 2002).


Moreover, adding vitamin E improves the immune response of heat stressed broilers (Niu et al., 2009). Glutamine is considered to be a conditionally essential amino acid and has been shown to improve heat-stress resilience of broilers. Dietary glutamine improved growth performance and meat quality of broilers subjected to heat stress in a dose dependent manner (Dai et al., 2009). In addition, Gu et al. (2012) showed that glutamine enhanced the expression of HSP70 in jejunal mucosa after acute heat stress, protecting it from heat stress injury via increased levels of antioxidant enzymes in the jejunal tissue. Finally, Yesilbag et al. (2011) showed that an increased antioxidant status in meat by feeding broilers diets supplemented with rosemary or its essential oils, improved meat quality and shelf life.


Phytogenic feed additives (PFAs)



PFAs represent an efficient tool to meet the current and upcoming challenges of livestock production. Many plants (e.g. thyme, oregano) demonstrably show antioxidant efficacies that improve nutrient supply of cells, strengthen the cellular defence against oxidative substances and minimize damages caused by bacteria and oxidative stress, respectively. Consequently, these mechanisms lead to an improved health status of animals, allowing them to fully max out their genetic potential.


Feed additives that improve resilience against heat stress, among which phytogenic feed additives, generally exert clear antioxidant effects. Therefore, antioxidant effects seem to be the most important effects to focus on, when developing feed additives to improve heat stress resilience. Many aromatic plants, especially those from the plant family Labiatae (e.g. rosemary, thyme, oregano and sage), have been extensively studied for their antioxidant activity (Brenes and Roura, 2010). This activity is not only related to the phenolic compounds as also non-phenolic compounds may show considerable antioxidant activity by stimulating the antioxidant enzyme production (Mueller et al., 2012). Placha et al. (2014) showed that thyme oil improved intestinal antioxidant status, reduced MDA content in the enterocytes and improved intestinal integrity. A phytogenic feed additive, containing essential oils, herbs, spices and saponins (Biostrong® 510) positively influenced gut morphology in broilers and significantly increased nutrient digestibility in a study of Amad et al. (2013). Moreover, it stimulates the production of antioxidant enzymes.


Due to their proven beneficial characteristics, especially with respect to enhancing digestibility and antioxidant properties, phytogenic feed additives have the potential to become a new generation of feed additives for innovative livestock nutrition and welfare. They are foreseen to be a crucial tool when it comes to counteracting heat stress in poultry and thus, being able to contribute to a profitable animal production.


This article was published in World Poultry Volume 31, No. 6, 2015

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