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.
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.
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