Heat stress is responsible for large economic losses in the world’s poultry industry each year. Broiler production in hot conditions is currently a major challenge in many regions and is forecasted to become more widespread in the future. The projected change in global average temperature is likely to be around 0.3 °C to 0.7 °C, for the period 2016–2035, relative to the reference period 1986–2005. Consequently, there will be more frequent hot extreme temperature episodes globally; even in places where normally temperatures are milder (FAO, 2015). Heat stress costs the industry billions of US dollars in terms of loss of productivity and implementation of control strategies – a situation that seems to be worsening.
Scientists have described heat stress as the result of “a negative balance between the net amount of energy flowing from the animal’s body to its surrounding environment and the amount of heat energy produced by the animal” (1).
Broilers have for decades been selected for high feed intakes and growth rates. Consequently they have high metabolic activity, leading to greater production of heat and moisture. Nevertheless, as homeothermic animals under thermo-neutral zone conditions (18-27°C), they are able to balance heat production and loss. They use mechanisms including: thyroid hormone regulation, respiratory evaporation, convection, radiation and conduction. In this way they efficiently maintain body temperature and can fully express their genetic growth potential. However, when the environmental temperature rises beyond the comfort threshold, these mechanisms lose their effectiveness and broilers experience heat stress.
During heat stress, several adaptive strategies are used to dissipate excess heat. Firstly, when high temperatures occur, broilers significantly increase water consumption and decrease feed intake (thus reducing nutrient uptake). Secondly, they dissipate energy by increasing heart rate, panting and other metabolic adaptations. All these events have a detrimental effect on performance in term of average daily gain, feed efficiency and liveability.
In terms of metabolic adaptions, several endocrine and immune reactions occur. In order to increase heart rate, blood pressure and panting, high quantities of adrenaline and noradrenaline are released by the sympathetic–adrenal medullar system. Increasing circulation of these hormones causes an over-stimulation of the immune system, upregulating circulating lymphocytes and the production of the pro-inflammatory cytokine IL-6, for example. When high levels of adrenaline continue to circulate, cortisol and corticosterone release pathways are activated. High levels of glucocorticoids and in particular, a high level of corticosterone, can decrease the levels of thyroid hormones: TSH (thyroid-stimulating hormone), T4 (thyroxine) and consequently T3 (triiodothyronine). These hormones are linked to body temperature regulation as well as growth. Several studies have shown that thyroid hormones are involved in the control of thermoregulation in birds and mammals: reporting that T3 concentrations consistently decrease in high temperatures (1, 2, 3).
At a cellular level, all these metabolic changes induce an over-production of free radicals. These include reactive oxygen species (ROS), which are well known to dramatically damage biological molecules such as lipids, protein and DNA – consequently impairing cell functions. To counteract these challenges, the cell needs to be protected by boosting antioxidant protection.
During heat stress conditions (cyclic or chronic) it has been shown that supplementing diets with antioxidants can help birds to prevent or cope with the negative effects. Selenium (Se), defined as the chief executive of the antioxidant system, is involved in several levels of antioxidant defence (4). As such, it is best placed to protect against heat stress. However, to achieve the best results, the form of selenium should be considered.
Glutathione peroxidase (GSH-Px) is a key antioxidant, protecting the body from free radical damage at all times. It is thought that GSH-Px activation is a particularly important adaptive response to the oxidative stress caused by heat stress. Increased levels of GSH-Px have been observed in heat stressed broilers, in the liver, serum and thigh muscle. However, if the stress is too severe, then the body cannot adapt and GSH-Px activity decreases. Other studies have shown that when broilers are fed organic selenium, GSH-Px activity in both blood and liver increases (4).
By replacing sodium selenite with organic selenium, researchers have demonstrated a significant increase in GSH-Px activity in the blood of broiler chicks (5). This difference in GSH-Px activity in blood was maintained after heat stress, and the response of heat shock protein (HSP70) to heat stress was different between birds fed different sources. In the selenite-fed group it increased, but in the organic-Se fed group it decreased, indicating that birds overcame heat stress more easily. Heat shock proteins are produced by cells in response to heat stress. In order to protect cells against oxidative damage, they act by refolding proteins that have been damaged by cell stress. Therefore, a lower level indicates less ROS-mediated damage and in turn reduced negative effects on the immune system.
Under heat stress conditions, a pure form of organic selenium (100% hydroxy-selenomethionine, OH-SeMet; Selisseo® – Adisseo France S.A.S.) would be a valuable dietary supplement, helping birds to create tissue selenium reserves that are available when required. The potential of OH-SeMet to counteract the negative effects of heat stress in broiler has been investigated.
In collaboration with the INRA (URA, France), the effect of pure organic selenium (OH-SeMet) supplementation was evaluated on growth performance and meat quality of broilers exposed to heat stress. 960 day-old male Cobb 500 chicks were randomly assigned to one of four treatments in a 2×2 factorial design:
The animals were fed the experimental diets for five weeks. Half of the birds were kept in a thermo-neutral environment for the whole period. The other half were kept in a thermo-neutral environment for the first two weeks and then at 32°C for the last three weeks of the trial.
Heat stress induced a drop in growth performance for both selenium treatments. For broilers fed OH-SeMet, this decrease was reduced – they performed numerically better than SS fed broilers under heat stress. OH-SeMet fed broilers were 52g heavier at 35 days (Figure 1), and the feed conversion ratio was one point better (Figure 2).
Figure 1: Body weight at day 35 (g)
Figure 2: Feed conversion ratios between days 21 and 35
The diet also impacted the chemical composition of the meat. The groups fed the OH-SeMet diet showed significantly higher breast protein content in both thermal environments (Figure 3). Whilst under heat stress, protein content decreased for the group fed the SS diet, it did not with the OH-SeMet diet. Additionally, the difference between the SS and the OH-SeMet results after heat stress was 2.36 times greater than the difference without heat stress.
Figure 3: Protein content (%) of breast muscle at 36 days
This study suggests that during heat stress, pure organic selenium is better able than mineral selenium to support performance and meat quality of broilers.
Maintaining the antioxidant balance, by ensuring an adequate production of selenoproteins, is a key factor in allowing full expression of a broiler’s genetic potential. During heat stress, increased selenoproteins expression requires additional selenium. However, due to a reduction in feed consumption, dietary uptake is less. Selenium reserves in the body (as SeMet) are essential to maintain an effective antioxidant defence by enabling selenoproteins production.
Mineral selenium sources, such as sodium selenite, cannot be stored in tissues. Therefore, it can only guarantee selenoproteins expression and production, when feed intake is as expected. In contrast, feeding animals a highly bioavailable source of selenium, such as OH-SeMet, allows the creation of selenium tissue reserves. This selenium insurance policy enables animals to produce sufficient amounts of antioxidant enzymes during heat stress – when feed intake is reduced. Dietary supplementation with OH-SeMet can help broiler producers fight the negative effects of heat stress on performance and health.
A trial at the University of Ghent used a cyclic heat stress model to study the effects of OH-SeMet on the FCR of broilers; along with some heat stress-related biomarkers. In the finisher phase of the breeding cycle (day 25 to day 39) the temperature in the experimental poultry house was increased from 22°C to 34°C for six hours per day, with a relative humidity between 50 and 60%. Broilers were reared for 39 days and fed a diet with either 0.3 ppm Se kg/feed from sodium selenite (SS) or 0.3 ppm Se kg/feed from OH-SeMet.
When the cyclic heat stress was applied, a significant improvement in feed efficiency was observed for the OH-SeMet compared to the SS group (1.56 vs. 1.62, respectively; Figure 4).
Figure 4: Effect of selenium sources on FCR 25-39 days during chronic cyclic heat stress
Mortality in the finisher period, which increased substantially with cyclic heat stress, was numerically lower (-4.2%pts) for the OH-SeMet-fed birds compared to the SS group (Figure 5).
Figure 5: Effect of selenium sources on mortality rate during cyclic heat stress and for the overall trial period.
Biomarker results demonstrated that Se levels in plasma were significantly higher in the OH-SeMet group than the SS group – confirming the higher bioavailability of the organic form. On day 39, broilers fed OH-SeMet maintained higher serum T3 level significantly more than in those fed SS (Figure 6). Moreover, broilers fed OH-SeMet showed numerically lower (not significant) plasma levels of heat shock protein 70 (HSP70) than those fed SS – at both day 26 and day 39 (Figure 7).
Figure 6-7: Effect of selenium sources on serum triiodothyronine (T3; day 39) and on plasma heat shock protein 70 (HSP70; day 26 and day 39) concentration in broiler.
These results indicate a better adaptation to heat stress of broilers fed OH-SeMet.
In poultry production, stress conditions are a common factor in decreasing bird performance. Heat stress is one of such conditions that can occur in a variety of situations and environments. There are oxidative stress-related changes in broilers subjected to heat stress conditions. The impact of feeding antioxidants may therefore be greater when exposed to stressors, including heat. Research has shown that under heat stress conditions, supplementation with OH-SeMet can increase antioxidant protection. It improves broilers’ Se status and positively modulates the endogenous redox system to a greater extent than sodium selenite, demonstrating the importance of feeding growing broilers with an effective selenium source in order to improve adaptation to heat stress.
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