Feeding Programs for Laying Hens: Heat Stress

An increasingly large proportion of the world’s laying hens are kept in areas where heat stress is likely to be a major management factor at some stage during egg production. Basically the problem relates to birds not consuming enough feed at this time, although there are also some subtle changes in the bird’s metabolism that affect both production and shell quality. While all types of poultry thrive in warm environments during the first few weeks of life, normal growth and development of older birds is often adversely affected. Obviously, the bird’s requirements for supplemental heat declines with age, because insulating feathers quickly develop and surface area in relation to body size is reduced. Heat stress is often used to describe bird status in hot environments, although it is obvious that more than just temperature is involved. Because birds must use evaporative cooling (as panting) to lose heat at high temperatures, humidity of inhaled air becomes critical. Thus high temperature and humidity together are much more stressful to birds than high temperature alone. Other environmental factors such as air speed and air movement also become important. It is also becoming clear that adaptation to heat stress can markedly influence bird response. For example, laying birds can tolerate constant environmental temperatures of 35°C and perform reasonably well. On the other hand, most birds are stressed at 35°C when fluctuating day/night temperatures are involved. In the following discussion, it is assumed that fluctuating conditions exist, since these are more common and certainly more stressful to the bird.

The main concern under hot weather conditions is the layer’s ability to consume feed. As poultry house temperature increases, then less heat is required to maintain body temperature and the birds consume less feed. In this situation, "environmental" energy is replacing feed energy and is economical. However, the relationship between body heat production and house temperature is not linear, since at a certain critical temperature, the bird’s energy demands are increased in order to initiate body cooling mechanisms. The following factors should be considered in attempting to accommodate heat stress.

A. Bird’s Response To Heat Stress
Figure 2 is a schematic representation of a heat stress effect. Minimal body heat production (and hence the most efficient situation) is seen at around 23°C. Below this temperature, (lower critical temperature) birds generally have to generate more body heat in order to keep warm.

However, there is only a narrow range of efficiency between 19-27°C, over which heat production is minimal. Above 27°C, birds start to use more energy in an attempt to stay cool. For example, at 27°C, birds will start to dilate certain blood vessels in order to get more blood to the comb, wattles, feet etc. in an attempt to increase cooling capacity. More easily observed by the egg producer is the characteristic panting and wing drooping that occurs at slightly higher temperatures. These activities at high environmental temperatures mean that the bird has an increased, rather than decreased, demand for energy. Unfortunately, the situation is not as clear cut as depicted in Figure 2, and this is likely the reason behind the variability seen in flock response to various environmental conditions. Rather than lower and upper critical temperature being rigidly fixed under all conditions, heat production is likely to fluctuate in response to a number of very practical on-farm conditions. Variation in response can be caused by such factors as a) increased feed intake, b) better feathering or c) increased bird activity. Such potential variability in bird response should be taken into account when interpreting the quantitative data discussed in Figures 3 and 4. The whole picture is further confused by the normal energy intake pattern of the bird (Figure 3). The base line shown in Figure 3 is a repeat of the temperature effect detailed in Figure 2.

The upper line of Figure 3 represents energy intake for a 1.5 kg layer. Again as environmental temperature increases, energy (feed) intake declines. However above 27-28°C, the decline becomes quite dramatic since the bird is changing its metabolic processes in response to the heat load, and actions such as panting, etc. adversely influence the feeding mechanisms in the brain and also reduce the time available for feeding. The shaded area between the lines in Figure 3, represents the energy available for production. As we approach and exceed the critical 28°C mark, then energy available for production is dramatically reduced and around 33°C actually becomes negative.

If the shaded area (available energy), in Figure 3 is itself plotted against temperature, then a clear pattern is seen with respect to potential for egg production (Figure 4). If we assume an average egg contains the equivalent of 80 kcal ME, then at 90% production, there is a daily need for around 70 kcal to meet needs for production alone. Our calculations from Figure 3 indicate total available energy at 90 kcal per day, and so we have a small positive difference that will likely go for growth or increased body weight. At 28°C, there is energy available only for eggs and none for growth. Above 28°C, available energy cannot meet energy demands for 90% egg production. Either egg production must be decreased, or other energy sources used. The bird’s body reserves (fat and muscle) could therefore be used at this time. These figures are not fixed and will likely vary with such factors as air speed, feathering etc. as previously detailed. However, for most flocks, these types of reactions, as depicted in Figure 4 are likely to occur at + 2°C of the values shown. In this scenario, the bird is in negative energy balance at 33°C (Figure 4). Various equations have been developed to relate energy intake to environmental temperature. For example, the equations given by NRC (1994) is ME(kcal/day) = W75 (173 - 1.95T) + 5.5 W + 2.07EE where W = body weight, kg; T = C, W = weight gain per day, g; and EE = daily egg mass, g. Solving this equation for environmental temperatures of 10-34°'a1C, shows an almost linear relationship for a 1.3 kg bird producing 50 g egg mass per day and gaining weight at 2 g per day (Fig. 5).

A major factor affecting this energy intake response to environmental temperature is feather cover, which represents insulating capacity for the bird. Coon and co-workers have developed equations that take into account degree of feathering. This equation is solved in Figure 6 for birds having 90, 75 or 60% feather cover. As expected, at low environmental temperatures, feather cover has a major effect on feed intake, while at 34°C which is close to body temperature; there is no effect of feather cover.


B. Energy Balance
Our main concern during heat stress therefore is the availability of energy for egg production. Optimizing such energy availability may be approached by either:
(i) Increasing diet energyspecifications
(ii) Stimulating feed intake or
(iii) Considering body energy reserves

(i) It is well known that birds consume less feed as the energy level of the feed increases. This is because the bird attempts to maintain a given energy intake each day. However, the mechanism is by no means perfect, and as energy level is increased, the expected decline in feed intake is seldom achieved. This obviously leads to "overconsumption" of energy. Also, as environmental temperature increases, the mechanism seems less perfect. The following results are seen when diet energy level is increased from 2860 kcal ME/kg to 3450 kcal ME/kg (Table 5, Payne, 1967).

At 18°C, there is fairly good adjustment by the bird, such that feed intake is markedly reduced with high energy diets; this in an attempt to normalize energy intake. At high temperatures, birds adjust feed intake less perfectly, such that "overconsumption" of energy occurs. It is not suggested that these extremes of diet energy be used, rather that energy intake will be maximized with as high a diet energy level as is possible. In order to increase diet energy level, the use of supplemental fat should be considered. Dietary fat has the advantage of increasing palatability and also reducing the amount of heat increment that is produced during its utilization in the body.

(ii) Various methods can be tried to stimulate feed intake. Feeding more times each day usually encourages feeding activity. Feeding at cooler times of the day, if possible, is also a useful method of increasing nutrient intake. If artificial lights are used, it may be useful, under extreme environmental conditions, to consider a so-called midnight feeding, when temperature will hopefully be lower and birds are more inclined to eat. Again, where conditions are extreme, making the diet more palatable may be advantageous. Such practices as pouring vegetable oil, molasses, or even water, directly onto the feed in the troughs may encourage intake. Whenever high levels of fat are used in a diet, or used as a top dressing as described here, care must be taken to ensure that rancidity does not occur. This can best be achieved by insisting on the incorporation of quality antioxidants in the feed and that feed is not allowed to "cake" in tanks, augers or troughs. Fresh feed becomes critical under these conditions.

Diet texture can also be used to advantage. Crumbles tend to stimulate intake while a sudden change from large to small crumbles also has a transitory effect on stimulating intake. It is interesting to observe that a sudden change from small to large crumbles seems to have a negative effect on intake (Table 6).

(iii) It is now realized that correct pullet rearing programs are essential for optimum economic return in the layer house. This becomes very critical under hot weather conditions as the bird may have to rely on its body reserves to supplement energy required to maintain egg production. In general, the larger the body weight at maturity, the larger the body weight throughout lay, and hence the larger the potential energy reserve and the greater the feed intake. It is not suggested that extremely fat pullets are desirable, but it is obvious that birds of optimum weight with a reasonable fat reserve will likely stand up better to heat stress situations. Pullets that are subjected to heat-stress and have less "available" energy than that required to sustain production, have no recourse but to reduce egg mass output in terms of egg weight and/or egg numbers.


C. Protein Nutrition
In the past, it has been common practice to increase protein levels during heat stress conditions. This has been done on the basis of reduced feed intake, and hence protein levels have been adjusted upwards in attempting to maintain intakes of around 17g crude protein/bird/day. It is now realized that such adjustments may be harmful. When any nutrient is metabolized in the body, the processes are not 100% efficient and as a result, some heat is produced. Unfortunately, protein is the most inefficiently utilized nutrient in this regard and so proportionately; more heat is evolved during metabolism of amino acids. The last thing that a heat stressed bird needs is additional waste heat being generated in the body. This extra heat production may well overload heat dissipation mechanisms (panting, blood circulation). We are therefore faced with a difficult problem of attempting to maintain "protein" intake in situations of reduced feed intake, yet we know that more crude protein may be detrimental. The answer to the problem is not to increase crude protein, but rather to increase the levels of essential amino acids. By feeding synthetic amino acids, we can therefore maintain the intake of these essential nutrients without loading up the body systems with excess crude protein (nitrogen). General recommendations are, therefore, to increase the use of synthetic methionine and lysine so as to maintain daily intakes of approximately 360 and 720mg respectively.


D. Minerals And Vitamins
Calcium level should be adjusted according to anticipated level of feed intake, such that birds consume 3.5g per day. Under extreme conditions, this may be difficult since, as previously indicated, high energy diets are also desirable and these are difficult to achieve with the increased use of limestone or oyster shell. Table 7 shows the diet specifications needed to maintain intakes of Ca, P and vitamin D3, all of which are critical for eggshell quality.

Because it is also necessary to increase the energy level of the diet when feed intake is low, then it is counterproductive to add high levels of limestone and phosphates which effectively dilute the feed of all nutrients other than Ca and phosphorus. The problem of potential calcium deficiency is most often met by top dressing feed with oystershell or large particle limestone. The situation for phosphorus is more complex, and in fact it may be deleterious to use the high levels shown in Table 7. In practice, phosphorus levels are seldom increased to these extreme levels unless cage layer fatigue is an ongoing problem. The deficit of vitamin D3 is best met with use of D3 supplements in the drinking water.

There seems to be some benefit to adding sodium bicarbonate to the diet or drinking water. However, this must be done with care so as not to impose too high a load of sodium on the bird, and so salt levels may have to be altered. This should be done with great caution, taking into account sodium intake from the drinking water, which can be quite high during heat stress conditions. There is also an indication of beneficial effects of increasing the potassium levels in the diet, although again, this must be accomplished only after careful calculation, since high levels can be detrimental to electrolyte balance. While few reports indicate any improvement in adding supplemental B vitamins during heat stress, there are variable reports of the beneficial effects with the fat soluble vitamins. Although not always conclusive, increasing the levels of vitamins A, D3 and E have all been shown to be advantageous under certain conditions. While vitamin C (ascorbic acid) is not usually considered in poultry diets, there is evidence to support its use during hot weather conditions. Birds require vitamin C, but under most circumstances are able to synthesize enough in their own bodies. Under heat stress, such production may be inadequate and/or impaired. Adding up to 200 mg vitamin C/kg diet has proven beneficial for layers in terms of maintaining production.


E. Electrolyte Balance
As environmental temperature increases, birds increase their respiration rate in an attempt to increase the rate of evaporative cooling. As birds pant however, they tend to lose proportionally more CO2 and so changes in acid-base balance can quickly develop. With mild through to severe alkalosis, blood pH may change from 7.2 through 7.5 to 7.7 in extreme situations. This change in blood pH, together with loss of bicarbonate ions can influence eggshell quality and general bird health and metabolism. Under such heat stress conditions, it is the availability of bicarbonate per se which seems to be the major factor influencing eggshell thickness, and in turn, this is governed by acid-base balance, kidney function and respiration rate.

Under normal conditions, shell formation induces a renal acidosis related to the total resorption of filtered bicarbonate. At the same time, shell secretion induces a metabolic acidosis because the formation of insoluble CaCO3 from HCO3 - and Ca++ involves the liberation of H+ ions. Such H+ release would induce very acidic and physiologically destructive conditions, and is necessarily balanced by the bicarbonate buffer system in the fluid of the uterus. While a mild metabolic acidosis is therefore normal during shell synthesis, a more severe situation leads to reduced shell production because of intense competition for HCO3 -, as either a buffer or a shell component. A severe metabolic acidosis can be induced by feeding products such as NH4C1, and this results in reduced shell strength. In this scenario, it is likely that NH4 + rather than C1 is problematic because formation of urea in the liver (from NH4 +) again needs to be buffered with HCO3 + ions, creating more competition with uterine bicarbonate metabolism. Conversely, feeding sodium bicarbonate, especially when C1 levels are minimized, may well improve shell thickness. Under commercial conditions, the need to produce base excess in order to buffer any diet electrolytes must be avoided. Likewise it is important that birds not be subjected to severe respiratory excess, as occurs at high temperatures, because this lowers blood bicarbonate levels, and in extreme cases, causes a metabolic acidosis. Under practical conditions, replacement of part of the supplemental dietary NaC1 with NaHCO3 may be beneficial for shell production.

Acclimatization to heat stress is a confounding factor, because temporary acute conditions are more problematic. For example, pullets grown to 31 weeks under constant 35 versus 21°C conditions exhibit little difference in pattern of plasma electrolytes. If birds are allowed to acclimatize to high environmental temperatures, there is little correlation between plasma electrolytes and shell quality. Temporary acute heat stress and cyclic temperature conditions seem most stressful to the bird.

Prevention of electrolyte imbalance should obviously be approached through incorporation of appropriate cations and anions in diet formulations. However it must be accepted that diet is only one factor influencing potential imbalance, and so general bird management and welfare also become of prime importance. Electrolyte balance is most usually accommodated by consideration of Na+KC1 balance in the diet, and under most dietary situations this seems a reasonable simplification. Electrolyte balance is usually expressed in terms of mEq of the various electrolytes, and for an individual electrolyte this is calculated as Mwt ÷ 1000. This unit is used on the basis that most minerals are present at a relatively low level in feeds. As an example calculation, the mEq for a diet containing 0.17% Na, 0.80%K and 0.22% C1 can be developed as follows:

A balance of around 250 mEq/kg is usual, and so for this diet there needs to be either an increase in Na or K level of the diet, or a decrease in C1 level.

Under practical conditions, electrolyte balance seems to be more problematic when chloride levels are high. On the other hand, use of NaHCO3 to replace NaC1, as is sometimes recommended during heat stress, can lead to a deficiency of chloride. Changes in diet electrolyte balance most commonly occur when there is a major change in ingredient usage, and especially when animal protein sources replace soybean meal and vice versa. Table 8 outlines electrolyte content and electrolyte balance of some major feed ingredients.

Within the cereals, electrolyte balance for milo is low, while wheat is high relative to corn. Major differences occur in the protein-rich ingredients, and relative to soy, all sources are low in electrolyte balance. As shown in Table 8, this situation develops due to the very high potassium content of soybean meal. Careful consideration to electrolyte balance must therefore be given when changes are made in protein sources used in formulation. For example, the overall balance for a diet containing 60% milo and 25% soy is 210 mEq/kg, while for a diet containing 75% milo and 10% fish meal the balance is only 75 mEq/kg. The milo-fish diet would perhaps need to be supplemented with NaHCO3.

Assuming that heat stress cannot be tempered by normal management techniques, then electrolyte manipulation of the diet may be beneficial. However, the technique should be different for immature birds compared to egg layers. With adult female birds, there is a need to maintain the bicarbonate buffer system as it relates to eggshell quality. As such, diet or water treatment with sodium bicarbonate may be beneficial again emphasizing the necessity to meet minimal chloride requirements. On the other hand, treatment of respiratory alkalosis in layers, with acidifiers such as NH4C1 while relieving respiratory distress, may well result in reduced shell quality. For immature pullets treatment with electrolytes is often beneficial and there is less need for caution related to bicarbonate buffering. Up to 0.3% dietary NH4C1 may improve the growth rate of heat stressed birds, although as detailed previously, it is not clear if this beneficial effect is via electrolyte balance/blood pH or simply via the indirect effect of stimulating water intake. Under commercial conditions, adding salt to the drinking water of young birds has been reported to alleviate bird distress and to stimulate growth.


F. Water
A nutritional factor often overlooked during heat stress is the metabolism of water. It is well known that birds in hot environments drink more water, yet this has not been capitalized upon to any degree. It would seem logical to provide nutrients in the water, because water intake is increased at times when feed intake is depressed. Unfortunately, we have only met with very limited success to date with this type of management. What does seem more advantageous is to cool the water of laying hens. In studies with small numbers of birds, we have shown a distinct advantage to cooling the drinking water of birds housed in very warm environments. In a more large scale study in a commercial unit in California, Bell (personal communication) indicates improved feed intake and egg production in response to the cooling of water by just 5°C at an environmental temperature of 32°C (Table 9). Another factor to consider with water intake, is the possible effect of dissolved minerals and contaminants, etc., the effects of which may be greatly accentuated with increase drinking activity. In terms of mineral content, sodium concentration is the one most likely to cause problems.

G. General Recommendations Concerning Heat Stress
Under normal conditions, birds should be fed so as to attain optimum daily intakes of essential nutrients. Regardless of environment, the correct decisions cannot be made without knowledge gained from the monitoring of feed intake, body weight and egg weight. With heat stress situations (28- 40°C), the following points should be considered:

1. Never place underweight pullets in the laying house. They will always remain small with low feed intake and have little body fat reserve to sustain optimum egg production.

2. Increase the energy level of the diet (2850 Kcal ME/kg minimum) ideally by incorporation of fats or oils. Limit the use of fibrous ingredients if possible.

3. Reduce crude protein component (17% CP maximum) while maintaining daily intakes of methionine (360 mg) and lysine (720mg).

4. Increase mineral-vitamin premix in accord with anticipated change in feed intake. Maintain daily intakes of calcium (3.5g) and available phosphorus (400 mg).

5. Where shell quality is a problem, consider the incorporation of sodium bicarbonate. At this time, monitor total sodium intake, and ensure adequate chloride levels in the diet.

6. Use supplemental vitamin C (150 g/tonne) when heat stress occurs.

7. Increase the number of feedings per day and try to feed at cooler times of the day.

8. Keep drinking water as cool as possible. Analyze sodium content of water so as to calculate "salt" contribution from the water.

9. Use crumbled feed or large particle mash feed if available.

10. Do not make any diet changes when sudden short term (3 - 5 days) heat stress occurs.

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