Modern broiler breeds have a
continuously increasing growth rate and feed efficiency, which coincides with a reduced heat tolerance. Housing broilers at high ambient
temperatures adversely affects
performance, intestinal integrity, immune response, and meat quality. Feed additives that alleviate the
consequences of heat stress, among which phytogenic feed additives,
generally exert clear antioxidant effects.
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 behavioural 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.
Consequences of heat stress
Reduction in feed intake is one of the first recognisable 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 . 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. Research showed that lipid peroxidation in pectoralis muscle of broilers increased with the severity of heat stress during the last two weeks pre-slaughter.
Moreover, it showed that rectal temperature of heat stressed broilers was increased by approx. 2oC comparing broilers housed at thermoneutral temperature and broilers housed at constant 34oC. Further research showed that heat stress additionally impairs immune response and intestinal integrity. The latter effect was related to lipid peroxidation in the enterocytes. Also indicated was 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. Additionally, nutrient digestibility was reduced during heat stress, which supports the need to use 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.
Nutritional strategies to reduce heat stress
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. It is clear that by limiting excess protein and optimising the amino acid profile, metabolic energy costs will be minimised to excrete surplus nitrogen. The effect of heat stress on the optimum amino acid profile is not yet known.
A study by Gous 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. As panting increases CO2 losses, heat-stressed birds will benefit from a higher cation: anion balance.
Apart from optimising 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. Moreover, adding vitamin E improves the immune response of heat stressed broilers.
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. In addition, 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, 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) show antioxidant efficacies that improve nutrient supply of cells, strengthen the cellular defence against oxidative substances and minimise 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.
Effect of ascorbic acid in heat-stressed poultry
Two reviews are available in published literature relating to the effects of ascorbic acid on poultry performance. Both lack aspects of the condition. While this review takes into account past and present knowledge.
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. 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. 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. 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 and thus, being able to contribute to profitable animal production.
References available upon request.