Feeding to Minimize Heat Stress
Dr. S V Rama Rao, D Nagalakshmi and V R Reddy
Animals are subject to stress from a number of sources, among them management/husbandry practices, nutrition and environment. For poultry in tropical countries, environmental temperature is one of the major stressors because this can range from -5 up to 35-43°C for prolonged periods. The ideal temperature for broilers is 10-22°C for optimum body weight and 15-27°C for feed efficiency
Layers will produce eggs constantly in the temperature range of 10-30°C. Above 30°C, performance will be depressed in terms of growth, feed intake, egg production, egg size and eggshell quality.
Nutritional imbalances can also result from poor quality control and lack of regulations for feeds, feed ingredients and feed additives. Furthermore, mycotoxins develop very quickly in hot and humid conditions, leading to loss of production, immunosuppression and higher mortality. Routine management practices, e.g. medication, vaccination, beak trimming etc, also add to the stress.
On the positive side, both the nutritionist and the farm manager can make changes to the feed and feeding practices to help minimize the effects of heat stress in broilers and laying birds.
Energy intake is the most important nutrient limiting bird performance at high temperatures. The energy requirement for maintenance decreases by about 30kcal/day with increase in environmental temperature above 21 °C. Although the energy requirement for maintenance is lower at higher temperatures, most of the energy is wasted in heat dissipation so the absolute energy requirement is not affected by heat stress.
The feed energy concentration should be adjusted to allow for the reduction in feed intake at higher temperatures. Feed intake changes about 1.72% for every 1°C variation in ambient temperature between 18 and 32°C. However, the decline is much faster (5% for each 1°C) when the temperature rises to 32-38°C. Measures to increase feed intake include the inclusion of fat in the diet. Feed consumption increased up to 17% by 5% fat supplementation in heat stressed birds because fat improves palatability. In addition, fat offers an extra calorific value by decreasing the rate of passage of digest a, thereby increasing the utilization of nutrients.
Fats or oils with more saturated fatty acids are preferred in hot humid climates. The concentration of energy should be increased by 10% during heat stress, whilst the concentration of other nutrients should be increased by 25%.
The requirements for protein and amino acids are independent of environmental temperature so heat stress does not affect bird performance as long as the protein requirement is met. However, heat stress reduces feed intake and the levels of protein/amino acids need to be increased with the environmental temperature up to 30°C. At even higher temperatures, heat stress has a direct effect on production and there is no benefit in raising the protein level.
The correct amino acid balance in the diet minimizes fat deposition in the liver, thereby increasing the survival of birds under heat stress. So, a low protein diet with balanced critical amino acids (methionine and lysine) is more beneficial than a diet high in total protein during hot periods. The oxidation of excess protein or amino acids generates metabolic heat.
Calcium and phosphorus
Heat stress reduces calcium intake and the conversion of vitamin D3 to its metabolically active form, 1,25(OH)2D3, which is essential for the absorption and utilization of calcium. In effect, the calcium requirement of layers, particularly older birds, is increased at high environmental temperatures. To overcome this effect, extra calcium should be provided at the rate of Ig/bird in the summer months in the form of oyster shell grit or limestone. Supplementation should be made over the normal dietary calcium level (3.75g/bird/d) recommended for layers.
However, excessive levels of calcium reduce feed intake due to the physiological limit of calcium appetite and also reduced palatability Instead of increasing the diet specification, the calcium should be offered separately as a choice feed. Better results are obtained by offering the calcium source in the afternoon. The optimum particle size is the one that supplies the required calcium at me time of shell formation. The minimum size to improve gizzard retention is about 1mm.
The phosphorus level in diet must not be forgotten as excessive phosphorus inhibits the release of bone calcium and the formation of calcium carbonate in shell gland, thereby reducing the shell quality.
Supplementing the diet with 0.5% sodium bicarbonate or 0.3-1.0% ammonium chloride or sodium zeolites can alleviate the alkalosis caused by heat stress. Sodium bicarbonate stimulates feed and .water intake at high environmental temperature. The body weight gain can be increased up to 9% by addition of these chemicals in the feed of heat-stressed broilers.
The excretion of potassium through urine is significantly higher at 35°C than at 24°C. The potassium requirement increases from 0.4-0.6% with a rise in temperature from 25 to 38°C. A daily potassium intake of 1.8-2.3g potassium is needed by each bird for maximum weight gain under hot conditions.
To compensate for the reduced feed intake under heat stress, dietary allowances for electrolytes (sodium, potassium and chloride) may be increased by 1.5% for each 1°C rise in temperature above 20°C. Electrolytes are also present in the drinking water and these levels need to be taken into consideration. Excess intake of electrolytes can lead to wet droppings, Potassium chloride can be added to the drinking water (to give 0.24-0.30% K) but care must be taken to avoid imbalances. Excess chloride is known to decrease the blood bicarbonate concentration.
During heat stress, the bird tries to maintain its body temperature by increased respiration, i.e. evaporation of metabolic water, which may considerably increase the water requirement. The addition of electrolytes (and/or vitamin C) to cold water helps to increasing feed intake by heat stressed birds.
Additional allowances of ascorbic acid (vitamin C), vitamins A, E, and D3 and thiamine can improve bird performance at higher temperatures. However, the loss of vitamin activity either in premix or in feed during storage particularly at elevated environmental temperature is a prime concern and probably explains the conflicting results on the effects of vitamin supplementation during heat stress. High temperature, moisture, rancid fats, trace minerals and choline speed up the denaturation of vitamins. Vitamin activity in feeds can be maintained by using feed antioxidants, gelatin encapsulated vitamins, appropriate storing conditions and adding choline and trace minerals separately from other vitamins.
Ascorbic acid synthesis is decreased at elevated environmental temperature, making it an essential dietary supplement during the summer. The vitamin helps to control the increase in body temperature and plasma corticosterone concentration. It also improves eggshell quality via its role in the formation of the shell's organic matrix. Furthermore, it protects the immune system and reduces mortality in growing birds infected with IBD in a hot environment by protecting the lymphoid organs and thyroid activity. Supplementation of ascorbic acid (200-600mg/kg diet) improves growth, egg production, number of hatching eggs, feed efficiency, egg weight, shell quality and livability during heat stress.
The absorption of vitamin A declines at high temperatures. In broiler breeders, a three fold increase in supplementation has been found to be beneficial.
Vitamin E protects the cell membrane and boosts the immune system so additional dietary supplementation may be advantageous during hot weather. Mortality due to E. coli infection reduced significantly by supplementation of vitamin E in diet.
Heat stress is known to interfere with the conversion of vitamin D3 to its metabolically active form, i.e. 1,25(OH)2D3, so higher dietary levels maybe justified during periods of high temperature. The active form of vitamin D3 is involved in the synthesis of calcium binding protein, essential for calcium and phosphorus homeostasis.
A number of compounds are effective in reducing the ill effects associated with hyperthermia although their cost may be prohibitive. Antipyretic compounds, e.g. salicylic acid and aspirin, minimize the levels of catecholamine in the blood during heat stress. The performance of heat stressed birds can be increased with magnesium aspartate, zinc Sulphate, diazepam, metyrapone or clonidine in the feed. Aureomycin has been found lo alleviate the stress (growth depression) caused by injection of foreign protein or salmonella end toxin but it has not always been found to be beneficial. Acetylsalicylic acid (3% of the diet) increased me weight gain and shell quality in some reports but the effects are inconsistent. Resin pine, an alkaloid from the Rawolfia plant is known to prevent the loss of carbon dioxide from birds subjected to high environmental temperature, thus stabilizing the blood acid base balance. Flunixin, an anti inflammatory analgesic drug at 0.28-2.2mg/kg bodyweight per day increased water consumption by 150-300ml/bird/day. The anticoccidial compound, nicarbazine (at the standard dose of 125mg/kg), has increased the mortality of broilers to up to 90% during heat stress. Adding potassium chloride in drinking water can ameliorate the toxic effects.
Change in feeding practices
Under hot and humid conditions, feed should not be stored for longer than a week.
The bird's body temperature increases after feed ingestion due to the thermogenic processes of digestion and metabolism. With morning feeding, the thermogenic effect coincides with the rising environmental temperature, aggravating heat stress, The thermogenic effect lasts for 8-10 hours at 35°C, compared to just 2 hours at 20°C. Metabolic heat production is 20-70% less in starved birds than in fed birds. Therefore, during hot weather, birds should be deprived of feed while the temperature is reaching and at its peak. Feeding during early and late hours of the day will help to minimize growth checks and mortality in broilers. Intermittent feeding, i.e. providing the light for 30 minutes followed by 3 hours dark, may also reduce the activity (heat production) of the bird but 20-30% more feeder and waterier space will be required. For layers, feeding during later part of the day will ensure sufficient calcium is available for optimum shell calcification,
Low feed intake is the main cause of poor performance at high temperatures. The following practices can help to raise feed consumption and may be worthwhile considering:
• Wet mash feeding
• pellet or crumble form of feed
• Low-calcium diets with choice feeding of calcium sources
• Frequent feeding and stirring of feed in the feeder• Addition of fat or molasses so to increase feed palatability.