Nutritional adjustments under heat stress in poultry production

High ambient temperature is one of the most common stressors in commercial poultry production, resulting in reduced feed intake and body weight gain, and increased mortality. Because of their physiological state and greater metabolic activity, broilers are more susceptible to temperature-associated environmental challenges.

Reza Vakili
Solutions Deployment Team, Makian Phosphate, Iran
& Associate Professor, Department of Animal Science, Islamic Azad University, Iran

STRESSORS IN MODERN POULTRY PRODUCTION
Heat stress is of great concern in all types of poultry production. Poultry production is associated with a range of stressors including environmental, nutritional, and biological, and such stress factors inevitably influence the animal’s physiology and performance with varying degrees.

HEAT STRESS
High ambient temperature is one of the most common stressors in commercial poultry production, resulting in reduced feed intake and body weight gain, and increased mortality. Because of their physiological state and greater metabolic activity, broilers are more susceptible to temperature-associated environmental challenges.

It also causes alterations in intestinal nutrient transporters, gut permeability and function, immune response and the endocrine system including cortisol and thyroid hormones. Moreover, elevated ambient temperatures have been reported to cause undesirable changes in carcass characteristics, bone mineralization, and meat quality.

Exposure to high ambient temperature elevates the level of reactive oxygen species (ROS) and causes biological and physiological disturbances in cellular functions. An uncontrolled increase in ROS level leads to free radical mediated chain reactions, which further causes lipid peroxidation and oxidative damage to proteins, DNA, and RNA. Birds respond to such conditions by reprograming several defense mechanisms including antioxidant enzymes, heat-shock proteins, and cytokines to alleviate or reduce the negative effects of Heat Stress.

WHEN IS CHICKEN “HEAT STRESS” A GOOD THING?
Environmental and nutritional strategies are available to improve the efficiency of these defense mechanisms. Among the strategies for coping with the impact of HS in animals, dietary interventions through supplementation of several feed additives including vitamins (e.g. A, E and C) and minerals (e.g. zinc, selenium) can be utilized to improve the host’s antioxidant defense mechanism.

Vitamin E is a biological antioxidant and a free radical scavenger that protects the cells and lipid rich-membranes from oxidative damage and is considered as the core of the antioxidant system. Among the four tocopherols and four tocotrienols (designated as α-, β-, γ-, and δ-), only the α-tocopherol form has the biological activity to meet the animal’s Vit E requirements. Dietary supplementation of various levels of Vit E can improve broiler performance and reduce the oxidative stress experienced under high environmental temperatures.

Several studies have shown a positive effect of n-3 on bone mineral density (BMD) and bone mineral content (BMC) in animals.

Zinc is used in poultry diets because of its antistress effects. Moreover, its requirement increases and its retention decreases during stress. Zinc deficiency also causes shortness and thickness in long bones of legs and wings. There is also some evidence suggesting supplemental Zinc can alleviate negative effects of heat stress in broiler chickens.

Bone breaking strength is measured by evaluating the reaction of the bone to stress and force. An increase in bone mineralization is accompanied by an increase in bone stress and bending moment values. Bone mechanical strength is affected by nutrition, genes, age, sex, environment and etc., for the expression of collagen and proteins, quantity and quality of the organic and inorganic material, content and size of the mineral material, and design and structure of the bone (Boskey et al., 1999). Velleman (2000) explained the structural complexity and composition of bone associated with strength, which varied according to the age and nutritional status of the bird: femur compression strength = 11.9 + 0.0141 (bird BW).

However, contradictory data regarding the effect of vitamin E on collagen synthesis exists, and few studies have reported its influence on cartilage development. In fact, increasing in thickness of the growth plate was proportional to DL-α-tocopheryl acetate dose. The effect of vitamin E on chondrocytes was observed by Xu et al. (1995) in chicks fed by two levels of DL-α-tocopheryl acetate and two dietary lipids. The thickness of the entire growth plate cartilage and of the lower hypertrophic chondrocyte (mineralized) zone was significantly wider in animals fed the greater level of vitamin E. α-tocopherol protects PUFA from lipid oxidation. However, the effect of α-tocopherol and zinc supplementation on bone strengthen in poultry has rarely been studied and is rather controversial. The relation between dietary fat, zinc and vitamin E with heat stress on femur breaking strength has been studied separately.

EFFECTS OF DIETARY FAT, VITAMIN E, AND ZINC SUPPLEMENTATION ON TIBIA BREAKING STRENGTH
Performance
High environmental temperature reduced the feed intake, body weight and increased feed to gain ratio in broiler. The diets contain saturated fat have been a significant effect on feed intake and body weight although the same result on feed to gain was not convenient. Addition of zinc and vitamin E to the unsaturated oil ration have been increased in body weight of broilers though it was less than body weight of tallow ration.

Bone Strength
High temperature causes excretion of some minerals for example Ca, Fe, Zn, and leads to decrease bone strengthen. In chickens, bone mineralization as determined through dual-energy x-ray absorptiometry (DEXA) is positively correlated with bone breaking force (r = 0.58 to 0.68; P < 0.001) and bone ash weight (r = 0.73 to 0.99; P < 0.001). Higher increases in stress occur with smaller increases in ash content. Decrease in bone mineralization is accompanied by a decrease in bone breaking force and bone ash weight. Oil supplementation can affect the TBS and weight of dry and wet bone. It is unknown whether the potential effects of n-3 fatty acids on bone are due to specific longer chain derivatives, such as DHA and EPA or not. Thus, the differences in specific n-3 sources on bone have to be clarified. The highest dry bone weight, width and length of bone, maximum force for breaking bone and ash value obtained in the group fed by diet supplemented with zinc and vitamin E. In fact, the increase in thickness of the growth plate was proportional to DL-α-tocopheryl acetate dose.

The thickness of the entire growth plate cartilage and of the lower hypertrophic chondrocyte (mineralized) zone was significantly wider in animals fed the greater level of vitamin E. The authors maintained that the increased thickness of the mineralized zone may be due to decreased cartilage resorption and phagocytic activity on the metaphyseal side. The lowest bone strengthen was observed in the group fed by diet supplemented with tallow. It seems that, high level of arachidonic acid in supplemented dietary with tallow, which is the precursor of PGE2 and may lead to decrease in bone formation, consequently decrease in bone strengthen (Watkins et al. ,1997). Our results are compatible with the results of Liu et al (2003). The highest bones strengthen, dry matter and ash content were observed in broilers fed by supplemented diet with zinc and vitamin E in normal temperature. Afterward the highest dry bone weight was observed in broiler under heat stress, but fed by diet supplemented by tallow. Higher growing rate in this group could be the reason of this fact. Feeding diet supplemented with zinc and vitamin E and diet supplemented with canola and fish oil in normal temperature resulted to maximum bone strength and ash content in broiler, respectively. It seems this high bone strength is because of high level of EPA, DHA, omega-3 and Zn in diet, which might increase Ca precipitation and collage formation. The lowest ash value and force for breaking bone were observed in broiler under heat stress, which fed by tallow. That would probably be due to high environmental temperatures and high omega-6 content in the diet. It might increase excretion some minerals and PGE formation, but decrease in collagen formation.

CONSIDER TO DIETARY FAT, VITAMIN E, AND ZINC SUPPLEMENTATION
In conclusion, the higher environmental temperature and feeding chickens with tallow lead to decrease in bone strengthen and in contrary supplementing diet with fish oil, zinc and vitamin E lead to increase in bone strength in broiler.

References available upon request.