Introduction:

Heat stress is one of the most important stressors affecting poultry, especially in the hot regions of the world, and is a major contributor to growth restriction. It has been suggested that modern broiler breeds be more susceptible to heat stress than earlier genotypes. In addition, heat-induced suppression of growth is now being seen at lower ambient temperatures in commercial strains. Feed efficiency, growth rate, mortality, and other important traits governing the prosperity of the poultry industry are adversely affected by severe heat stress, e.g. broilers at the age of six weeks exposed to 32oC showed a 24% decrease in feed intake (Geraert et al. 1996). It has been established that high environmental temperatures affect the development of a specific immune response in chickens (Thaxton et al. 1968, Subba Rao 1970; Thaxton and Siegel 1972). When chickens were exposed to temperatures ranging from 32.2–43 oC for short intermittent period of constant high temperatures or cycling high temperature conditions, the resulting antibody response to sheep red blood cells (SRBC) was reduced significantly.

Several methods are available to alleviate the effect of high environmental temperature on the performance of poultry. Because it is expensive to cool animal buildings, such methods focus mostly on dietary manipulations. The concentration of nutrients required to maintain health and productivity in poultry is challenged due to the reduction in feed intake under heat stress conditions. Studies have shown a redirection of nutrient flow to meet the metabolic requirements of an immune or inflammatory response (Bauman and Currie 1980).

Heat Stress induces oxidative stress in animals:

When animals suffer from heat stress, they adopt several strategies to dissipate excess heat. They increase their water consumption significantly and decrease their feed intake (thus reducing their nutrient and micronutrient uptake). Moreover they dissipate energy by increasing their heart rate, panting and by means of other metabolic adaptations. All these events have a detrimental effect on their performance, in terms of average daily gain, feed efficiency, liveability. These metabolic changes induce an over-production of free radicals at the cellular level. These radicals include reactive oxygen species (ROS) and reactive nitrogen species (RNS), which are known to seriously damage such biological molecules as lipids, protein and DNA. To counteract these challenges, the cell needs to be protected by means of the boosting of antioxidant activities.

Role of Selenium in Heat Stress:

Selenium (Se) is an essential trace element for human and animal health. Dietary Se supplementation has been shown to positively affect the growth performance, antioxidation and immune functions of birds, particularly those reared under heat stress condition.

Selenium could be supplemented in diets as inorganic mineral salts, typically as sodium selenite (SS) or in organic forms such as Se-enriched yeast (SY), or pure selenomethionine forms. Usually, the organic forms of Se have higher bioavailability and antioxidant properties than inorganic forms (Mahan et al., 1999; Mahmoud and Edens, 2003).

The antioxidant systems in the body contain numerous antioxidant enzymes, such as superoxide dismutase (SOD), thioredoxin peroxidase (TPx) and glutathione peroxidase (GSH-Px), while numerous non-enzymatic substances are also employed to protect the body from oxidative stress (Flohé, 2010). Drip loss and meat color are important indexes for evaluation of meat quality.

Selenium is an indispensable component of the antioxidant system, and it involves all the levels of antioxidant defence. For instance, Se can be used to synthesize GSH-Px, a key antioxidant that protects the body from free radical damage.

Increased levels of GSH-Px have in fact been observed in heat stressed animals. Selenium has been demonstrated to significantly increase serum GSH-Px activity. This difference in GSH-Px activity in blood was observed to be maintained after heat stress, and the response of the heat shock protein (HSP70) to heat stress was different for animals fed different sources of selenium: it increased in a selenite-fed group, but it is maintained lower in an organic-Se fed group, thus indicating that the animals overcame heat stress more easily.

Efficacy of various sources of Selenium in broiler diet under heat stress conditions:

In various heat stress experimental models, different forms of selenium sources were studied. In collaboration with the INRA (URA, France), the effect of pure organic selenium, OH-SeMet (SO 0.2 ppm Se, Selisseo®) or sodium selenite (SS 0.2 ppm Se) supplementation has been evaluated on the growth performance of COBB 500 broilers exposed to heat stress.

The animals were fed experimental diets for five weeks. Half of the birds were kept in a thermo-neutral (TN) environment for the whole period, while the other half was kept in a thermo-neutral environment for the first two weeks and then at 32°C (Heat stress, HS) for the last three weeks of the trial.

Heat stress induced a drop in growth performance for both Se treatments. However, the OH-SeMet (SO) fed broilers were 52g heavier at d35 (Figure 1), and the feed conversion ratio was nine point better than the SS-fed broilers (Figure 2). This study suggests that OH-SeMet can better support and ensure the performance of broilers than mineral selenium during heat stress.

The diet impacted the chemical composition of the meat. The tissues were more enriched in Se when OH-SeMet was used, compared with selenite (Table 1).

Another experiment was designed, in collaboration with the University of Ghent (Belgium), in which a cyclic heat stress model was used to study the effects of dietary supplementation of OH-SeMet on ROSS 308 broilers. The broilers were reared for 39 days and fed a diet with either a 0.3 ppm Se feed from SS or a 0.3 ppm Se feed from OH-SeMet (Selisseo). In the finisher phase of the breeding cycle (day 25 to day 39), the temperature was increased from 22°C to 34°C (50-60% humidity) for 6 hours per day.

When the birds were challenged with high temperatures, Selisseo resulted in ADG of 105.1 g/d versus 104.4 for SS. FCR was significantly improved for Selisseo® to 1.56, 6 point improved compared with 1.62 for SS (Figure 3). It's worthy to note that mortality in Selisseo group also dramatically increased, a sign that the challenge was well coped. As shown in Figure 4, Selisseo significantly reduced mortality rate by 4.2 points as compared to SS, respectively.

The selenium insurance policy

Maintaining the antioxidant balance, by ensuring an adequate production of selenoproteins, is the key to ensuring the performance of animals under heat stress. Feeding animals with a pure form of organic Se, such as OH-SeMet, enhances selenium deposition in the tissues, thus creating Se tissue reserves that act as a 'selenium insurance policy'. This selenium insurance enables animals to produce sufficient amounts of antioxidant enzymes under heat stress, when feed intake is reduced, and results to be an effective way of ensuring growth and productive performance of livestock and reducing the mortality rate in broilers under this challenging condition.

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Article made possible through the contribution of Dr. Sumit Saxena, Dr. Michele De Marco and Adisseo

Hydroxy Selenomethionine: an answer to oxidative stress in broilers