3rd International Poultry Meat Congress

Consequences of heat stress
Lara and Rostagno (2013) summarized the effects of chronic heat stress on broiler performance,
being:
- reduced feed intake and body weight gain;
- increased feed conversion ratio;
- decreased breast muscle and increased thigh muscle proportions;
- reduced protein;
- reduced meat quality.
Reduced feed intake of broilers during heat stress only accounts for 25% to 35% of the lower
body weight gain as was shown in studies with pair-fed broilers. Feed intake effects therefore
seem to be overestimated. The major reduction in growth performance is related to stress as
such (Bonnet et al., 1997; Daghir, 2009). 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
increases 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. 2C comparing
broilers housed at thermoneutral temperature and broilers housed at constant 34C. 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,
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.
Additionally, it is demonstrated in many papers that heat stress exerts direct adverse effects
on meat quality. For instance Dai et al. (2009) showed that housing broilers at 28C from 35 to
42 days of age reduced breast meat pH and water holding capacity and increased breast meat
discoloration, compared to broilers housed at 23C and Zhang et al. (2012) showed that both
constant (34C) and cyclic high temperatures (36C during 6h and 23C during rest of the day)
from 29 to 42 days adversely affected these meat quality parameters, whereas effects of constant
high temperature was much more detrimental than during cyclic high temperatures. Therefore,
heat stress has a negative impact on consumer perception of the final product.
Nutritional strategies to reduce heat stress
In 2010, Gous summarized main nutritional strategies to alleviate heat load of broilers:
1) the use of high density feeds;
2) increased protein to energy ratio;
3) improving amino acid profile;
4) changing the energy source from carbohydrates to fat;
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