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.