Nutritional strategies to target heat stress

Heat stress is currently considered to be a major environmental factor impairing the welfare and productivity of poultry. The damaging effects of heat stress on broilers and laying hens are reduced growth rates, decreased egg production and poor meat and egg quality. The burden exerted on the profitability of poultry farming will grow worldwide in the future as genetic selection for fast growth increases sensitivity to heat stress. In addition, poultry markets of warm regions are forecasted to grow in the following decades. Strategies to alleviate the detrimental effects of heat stress on the productivity of poultry are, therefore, sound and should be based on several complementary approaches. Such approaches include housing conditions, management practices and nutritional strategies. This review focuses on the latter.

What is heat stress?

Heat is produced in the body as a consequence of various chemical reactions associated with intermediary metabolism. Homeostatic mechanisms regulating body heat production and body heat loss allow keeping the body core temperature relatively constant. No additional energy is required to dissipate or create heat when birds are in the thermoneutral zone (Figure 1).

Figure 1 – Thermoregulation in poultry.

Birds have, however, to be active in dissipating body heat when ambient temperature exceeds the upper critical temperature threshold. Such physiological state is called heat stress. Upon heat stress, heat loss is increased through radiation, convection, conduction and evaporation. While it is generally accepted that poultry farming can be achieved between 10 and 27°C, maximum performance is achieved in more narrow range of temperatures: around 18-22°C for growing broilers and 19-22°C for layers. There are some discrepancies in the literature concerning the range of ambient temperature that allows optimum performance and health of poultry. This can be explained by the fact that many factors influence the sensitivity of poultry to heat stress, e.g. breed, relative humidity, air speed or stocking density.

Behavioural and physiological consequences

Under high ambient temperature and high stocking densities, it becomes increasingly difficult for birds to lose heat by conduction or convection. A lot of heat can, however, be dissipated by evaporation, i.e. the change of liquid water to vapour. Because poultry lack sweat glands, such reaction does not occur on the skin of birds but in the lungs. Hence, respiratory evaporation is a major heat loss mechanism for poultry in heat stress situations. In such situations, birds also limit activities that may generate additional heat: e.g. eating (diet-induced thermogenesis) or moving (muscle contraction thermogenesis). Water intake is concomitantly increased due to changes in endocrine regulation. In a nutshell, birds subjected to heat stress show a reduced activity and feed intake, while having an increased respiratory rate (panting) and higher water intake. At the physiological level, heat stress leads to several dysfunctions:

1. Respiratory alkalosis: increased respiratory rate induces an increase in blood pH.
2. Hormonal changes impairing innate immunity and growth: high ambient temperatures activate several neuroendocrine mechanisms. There is an increase in plasma corticosterone concentration. This hormone suppresses the innate immune system of poultry. Also, high ambient temperatures decrease the concentration of the thyroid hormone triiodothyronine. Such changes are detrimental for lean tissue accretion and, therefore, for growth and
3. Decreased gut wall integrity: Heat stress has been shown to increase intestinal permeability to various pathogens and, consequently, inflammation. This leaky gut syndrome is explained by osmotic stress – Heat stress induces body water losses and, therefore, deregulates osmotic pressures. Intestinal integrity is decreased as a consequence of such osmotic stress and oxidative stress – High temperatures are responsible for stimulating free radicals production, such as reactive oxygen species (ROS). High levels of ROS can damage biological molecules such as DNA, proteins, lipids or carbohydrates. The overall consequences are reduced performances such as feed efficiency, growth rate or egg production.

Nutritional strategies

Nutritional solutions can help poultry to cope with heat stress. Such strategies have a twofold objective. First, to reduce diet induced thermogenesis by selecting nutrients having a low heat increment. Secondly, to provide birds with specific bioactive nutrients that correct the physiological dysfunctions associated with heat stress (e.g. oxidative stress or leaky gut).

Energy sources

Maintenance energy requirements increase when ambient temperature rises above 28°C as more energy is required for panting. Feed intake is concomitantly reduced. Increasing dietary fat levels at the expense of carbohydrates may reduce diet induced thermogenesis, while increasing energy density. This may compensate for the lower feed intake.

Crude protein content

Crude protein has a high heat increment effect. Imbalanced amino acids profiles exacerbate the heat increment induced by protein consumption as due to the energy costs associated with poor nitrogen retention and nitrogen excretion. Hence, reducing crude protein content using feed grade amino acids may be a sound strategy to cope with heat stress. Such strategy has, however, inconsistent effects on the growth performance of poultry in the scientific literature.

Increase dietary electrolyte balance

Electrolytes are lost by hyperventilation and excretion in urine. The resulting imbalance in electrolytes negatively affects the metabolism of birds. It is possible to supply extra electrolytes, such as sodium and potassium, to restore the DEB of poultry.

Feeding specific bioactive nutrients

Betaine is a trimethyl derivative of glycine having zwitterion and methyl donor properties. It acts, therefore, as an osmoprotectant. Betaine helps cells to maintain their structural integrity and functions by regulating the movements of water through the membrane. Betaine accumulates in cells exposed to osmotic stress, such as gut cells. Biological antioxidants react with ROS to convert them into less potent molecules. This prevents or delays the adverse effects of ROS. The combined use of lipophilic and hydrophilic antioxidants maximises the efficacy of this strategy.

Trial with betaine and antioxidants

Betaine and antioxidants are, therefore, important nutrients in the context of heat stress. Such nutrients are available as a fat coated blend (product name beTaHit). The fat matrix is protecting the ingredients over time in order to ensure maximum efficiency. Such dietary strategy was tested. A trial was conducted at the MiXscience Research Center (MRC).

The study was designed as a randomised complete block design with 3 groups:

1. CTR (no dietary supplementation),
2. B1 (1000 g/T betaine HCl) and
3. B2 (750 g/T BeTaHit) in 16 blocks.

Ross 308 broilers (n=192) were kept under standard conditions from one1 till 20 days. High temperatures were applied every day from 21 till 35 d (28˚C for 3 hours; 30˚C for four4 hours; 28˚C for 3 hours; 24˚C for 12 hours). Birds fed B2 had significantly lower FCR and higher breast meat yields than their CTR counterparts (Figure 2).

Figure 2 – Effect of dietary supplements (CTR= no supplementation; B1: 1000 g/T betaine HCl; B2: 750 g/T beTaHit) on growth performance of broilers at 35 d of age.

In comparison to CTR, filet production costs were reduced by almost 2% when feeding B1 and by 6% when feeding B2. In this trial, osmotic stress sensitivity was evaluated using hemolysis scores. Red blood cells were put in hypotonic solutions, inducing inflow of water into cells. Bursting threshold is used as an indicator of the capacity of cells to withstand osmotic stress. The concentrations at which cells explode were reported in Figure 3. Birds fed beTaHit (B2) had a significantly lower hemolysis score than their control (CTR) fed counterparts, thereby indicating an improved osmotic stress resistance.

Figure 3 – Effect of dietary supplements (CTR= no supplementation; B1: 1000 g/T betaine HCl; B2: 750 g/T beTaHit) on hemolysis score at 35 d of age.

Conclusion

In conclusion, different nutritional strategies can be implemented in a comprehensive programme aiming at correcting the negative effects of heat stress on the growth performance of poultry. Such nutritional strategies should be paired with adequate housing and management practices. Changes in macronutrient composition and use of specific bioactive nutrients supplementation may alleviate partly the detrimental effects of heat stress on growth performance. Various environmental stress sources may activate identical conserved stress pathways in poultry. Hence, specific bioactive nutrients supplementation may be beneficial under broader stress circumstances.

References are available upon request. All information only for export outside Europe, USA and Canada.

Second author: Pierre Moquet, poultry specialist, MiXscience, France