ARTICLE FEED & ADDITIVE MAGAZINE August 2022 67 Oxidative stress damages the intestinal mucosa, prevents the efficient digestion and absorption of nutrients and adversely influences average animal growth. High ambient temperature causes oxidative stress leading to reduced aerobic metabolism and enhanced glycolysis, resulting in poor meat quality characterized by low pH and high drip loss (Vandana et al., 2020). ROLE OF HEAT SHOCK PROTEINS DURING HEAT STRESS To maintain thermal homeostasis, the bird's body must enter a stage of oxidative stress. It starts producing and releasing heat shock proteins (HSP) to protect itself from the delirious cellular effects of ROS. Numerous studies in broiler chickens exposed to heat stress reported an upregulated gene expression of HSP70 and HSP90 has been observed in muscle, liver, heart, kidney, and blood vessels during acute heat stress (Surai et al., 2015). METABOLIC HEAT PRODUCTION IS REDUCING FEED INTAKE AND NUTRIENT DIGESTIBILITY As an adaptive mechanism to minimize metabolic heat production, reduced birds' feed intake is one of the biggest causes of economic losses. With every increase of 1°C above 30°C, feed intake is expected to decrease by 4-5%, causing a deficiency of nutrients needed for optimum production. A study from Awad et al. (2019) on broilers reported around 8% reduction in feed intake, 17% reduction in body weight gain, and around 10% increment in feed conversion ratio when the birds were exposed to 34°C for 6 hours per day from 22 to 35 days of age. Another study from de Souza et al. (2016) on broilers revealed that heat stress significantly compromises growth performance by reducing protein digestibility up to 9.7%. It also showed an increase in metabolizable energy intake (20.3%) and heat production (35.5%) and a decrease in energy retention (20.9%) and energy efficiency (32.4%). Poor nutrient availability disrupts normal lipid metabolism (lipolysis) and causes a sharp decline in muscle glycogen reserves, leading to dark, firm, and dry (DFD) meat. HEAT STRESS DETERIORATES MEAT QUALITY Heat stress reduces the deposition of protein and intramuscular fat in the breast muscle, significantly increasing the muscle's lactic acid production, resulting in pale, soft, and exudative meat (PSE), which has become a significant concern in the poultry industry (Vandana et al., 2020). Heat stress during transport has been associated with increased mortality rate, decreased meat quality, and reduced welfare status. Similarly, before slaughtering, heat stress may increase the metabolic rate and rigor mortis resulting in protein denaturation (Nawaz et al., 2021). This hinders the ability of proteins to bind water and culminates in a poor water-holding ability characterized by higher drip loss and cooking loss. HEAT STRESS NEGATIVELY AFFECTS THE SMALL INTESTINE AND GUT MICROFLORA Heat stress may result in impaired peripheral blood circulation, locally depriving the small in-
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