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Physiological stress in broilers: Ramifications on nutrient digestibility and responses^1,2

W. S. Virden and M. T. Kidd³

Department of Poultry Science, Mississippi State University, Mississippi State 39762

³ Corresponding author: mkidd{at}poultry.msstate.edu

	SUMMARY

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
Physiological stress is one of many concerns facing the modern broiler producer. Research has yet to reveal mechanisms that would allow the producer to efficiently minimize the detrimental impacts of physiological stress on broiler performance. One possible approach for improvement in this area is to elucidate the effects of different nutrient regimens on stressed broilers. Sufficient prior knowledge of the effects of stress on metabolism and nutrient digestibility, as well as effective stress induction models, is essential to any researcher interested in delineating the effects of nutrients on stressed broilers. This review examines the physiological mechanisms that activate and control stress, as well as the effects of stress on immunity, metabolism, and broiler performance. Validation procedures for stress induction models are also examined. Additionally, this review examines available research focused on amino acid digest-libility in the presence of stress, digestibility assays necessary for this research, and nutrients that have been shown to have the potential for stress reduction or amelioration.

Key Words: physiological stress • broiler • amino acid digestibility

	DESCRIPTION OF PROBLEM

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
The broiler industry in the United States is primarily driven by the production of saleable white meat. One of the challenges the producer must overcome in the pursuit of this goal is potential stressors that the broiler may experience during life. Undoubtedly, the bird experiences various stressors each day in production. Short-term stress can be expected, and with the exception of situations such as acute heat stress, is typically of minimal concern. However, long-term stress can have far-reaching detrimental effects on poultry production.

With the exception of heat stress research, very little research has been conducted to aid in producers’ ability to combat the detrimental effects of stress in poultry through different nutritional regimens. One possible strategy for improvement in this area is to conduct nutritional research by using a model that induces physiological stress in broilers without using a specific stressor. If researchers had knowledge of a nutrient or combination of nutrients that offered the potential to ameliorate the detrimental effects of physiological stress, then future research involving nutrition and more specific stressors could be conducted more efficiently. Hence, research with more practical applications could be conducted more easily if a better foundation were available. Another strategy to combat stress nutritionally would be to establish amino acid requirements during stress conditions. If more were known about digestible amino acid requirements during stress conditions, industry nutritionists could possibly adjust amino acid minima or digestible coefficients in broiler diets to meet broiler nutritional needs more adequately when known stressors occur. The purpose of this literature review is 1) to examine the mechanism by which long-term stress negatively affects meat accretion in poultry, 2) to examine the various techniques for establishing digestible amino acid requirements for broilers during physiological stress, and 3) to examine the potential of nutrients or nutrient regimens to ameliorate the negative impact of stress.

	CLASSIFICATIONS OF STRESS

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
The term stress is very familiar to most researchers. However, there is no universal definition for stress. One definition states that stress is any situation that elicits the biological stress mechanisms of an animal [1]. Another broader definition states that stress is any biological response elicited when an animal perceives a threat to its homeostasis [2]. When a stressor is actually causing a negative impact on the well-being of an animal, this can be defined as distress [2]. To perceive how long-term stress can eventually cause a negative impact on the well-being of an animal, it is necessary to understand the physiological processes an animal undergoes when it is confronted with a stressor.

Stress responses can be categorized as specific or nonspecific [3]. Specific stressors are typically short-term, such as a sudden increase in environmental temperature [3]. Animals typically react to specific stressors by trying to combat the stressor [3]. Long-term or nonspecific stress, however, results in the animal taking measures to adapt the stressor, rather than dealing with it directly [3].

	ACTIVATION AND CONTROL OF THE STRESS RESPONSE

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
When an animal first encounters a stressor, the neurogenic system is activated [3]. The neurogenic system is composed of the sympathetic postganglionic neurons and adrenal medullary tissue [3]. This response was previously referred to as the "fight or flight" response [4]. Activation of the neurogenic system leads to marked increases in blood pressure, muscle tone, nerve sensitivity, blood sugar, and respiration [3]. This is brought about by secretion of the neurogenic amines epinephrine and norepinephrine [5].

Failed attempts to combat or flee from the stressor immediately result in the activation of the hypothalamic-pituitary-adrenal cortical system [3]. When this system is activated, the hypothalamus produces corticotrophin-releasing factor, which in turn stimulates the pituitary to release adrenocorticotropic hormone (ACTH) [6]. Secretion of ACTH causes the cells of the adrenal cortical tissue to proliferate and to secrete corticosteroids [6]. Cortisol is the primary corticosteroid in most mammals [7], whereas corticosterone (CS) is the primary corticosteroid in birds [8]. This hormonal cascade is probably facilitated by the action of catecholamines because catecholamines stimulate corticotrophin-releasing factor release from the hypothalamus [9], ACTH release from the pituitary [10, 11], and corticosteroid release from the adrenal cortex [11]. If corticosteroids remain at elevated levels in circulation, there are many possible effects, including, but not limited to, changes in glucose and mineral metabolism, cardiovascular diseases, hypercholesterolemia, gastrointestinal lesions, and alterations in immune system function [12]. The distribution of corticosteroids and the delivery of corticosteroids to the tissues are controlled, at least partially, by corticosteroid-binding globulins [12]. The role of corticosteroid-binding globulins is to restrict the absorption of CS by remaining bound to CS until in the presence of the specific target tissue because only unbound CS can pass through the cell membrane [13].

In mammals, corticosteroid secretion appears to be mediated largely by the hypothalamus and pituitary [3]. This is evident because in mammals, when the anterior pituitary is transplanted to other areas of the body or when the neurohumoral link from the hypothalamus to the pituitary is cut, circulating corticosteroid levels decline to levels similar to those when the anterior pituitary has been removed surgically [3]. However, in avian species, control of the hypothalamic-pituitary-adrenal cortical system appears to be less clear. Research involving quail, pigeons, and ducks has demonstrated that removal of the anterior pituitary results in a significantly larger decline in circulating CS levels than does anterior pituitary transplantation [14]. Furthermore, it has been demonstrated in vitro that avian leukocytes can be stimulated by corticotrophin-releasing factor [15] and are capable of ACTH production [16].

	PHYSIOLOGICAL CONSEQUENCES OF LONG-TERM STRESS

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
Before the effects of physiological stress on poultry are examined, it is important to understand the basic effects of long-term stress on animals. Predominantly, the effects of long-term physiological stress can be categorized as immunological or metabolic.

The amount of interest in the effects of stress on the immune system is evident, considering that nearly 200 reviews on this subject have been written in the last decade [17]. Corticosteroids have been shown to inhibit several immune system functions in various species, including lymphocyte proliferation, immunoglobulin production, cytokine production, cytotoxicity, and antiinflammatory agents [18]. Poultry treated with CS or ACTH have demonstrated depressed numbers of circulating lymphocytes [19–22]. The effect of this reduction in lymphocyte numbers is an increase in the ratio of circulating heterophils to lymphocytes, which is probably the most recognizable symptom of stress in poultry [12]. The cause of this decrease in lymphocyte numbers is probably due to the regression of lymphoid tissue caused by the presence of circulating CS for prolonged periods [5]. Regression of the thymus, bursa, and spleen has been demonstrated in chickens after CS or ACTH administration [23–29]. Glick [21] noted that lymphocytes were depleted at germinal centers after ACTH or CS injections, indicating that lymphocyte production is inhibited by lymphoid tissue atrophy.

The aforementioned alterations in immune system can result in immunosuppression. In vitro research has demonstrated that CS binds to specific receptors on lymphoid cells, resulting in alterations in enzyme function, formation of nucleic acid, and reduced protein synthesis [13, 30]. Glucocorticoid administration inhibits the production and secretion of the leukocytic cytokines interleukin-1, interleukin-2, -interferon, and MHC-II [31–33]. Reduction of these cytokines and MHC-II antigen presence can cause impaired immune responses and disease resistance because they play important roles in both cellular innate and humoral adaptive immunity [34].

Probably the most pronounced consequence of chronic stress is the alteration of metabolic function. Primarily, stress-induced metabolic alterations seem to be focused on the mobilization or production of glucose for energy needed to maintain homeostasis in the presence of the stressor. It has been suggested that during stress, animals adapt by creating a tissue priority hierarchy by which nutrients are devoted to certain tissues based on their order of importance [35, 36]. For example, one hypothesis states that stressed animals devote nutrients according to the following tissues, in order from greatest to least: neural, visceral, bone, muscle, and adipose [35, 36].

When the bird is initially exposed to stress, the neurogenic system is activated, and the catecholamines, epinephrine and norepinephrine, are released [3]. Of these, epinephrine seems to have the largest role in altering metabolism [37]. Epinephrine binds to β-adrenergic receptors on the cell membrane [38]. When this occurs, the main result is enzyme activation, most notably the activation of certain protein kinases that signal the body to enact glycogenolysis and gluconeogenesis [38]. Probably the chief function of norepinephrine during stress is to induce nonshivering thermogenesis to provide energy from adipose tissue for skeletal muscles because the majority of energy from glucose is being devoted to other tissues [38, 39].

After the activation of the hypothalamic-pituitary-adrenal cortical system and the subsequent secretion of CS, further metabolic alterations occur, which focus on glucose production and mobilization. One of the chief metabolic functions of CS is to promote gluconeogenesis by causing the liberation of substrates from body tissues necessary for endogenous glucose production [40]. From a meat production standpoint, the most detrimental effect of this action is the catabolism of structural protein to free amino acids for use as gluconeogenic substrates [41]. There is evidence that this glucose production occurs at the expense of structural protein in that poultry treated with CS or ACTH have displayed increases in nonprotein nitrogen, decreases in the incorporation of glucose carbon into protein [42, 43], and increased uric acid excretion [44, 45].

In addition, the presence of CS causes what has been termed a "permissive effect" on the action of catecholamines and glucagon for the mobilization of glucose, as well as on the action of fatty acids for energy during stress [46]. Additionally, CS activates gluconeogenic enzymes. Research in adrenalectomized rats has demonstrated that treatment with glucocorticoids results in the stimulation of all necessary enzymes for gluconeogenesis in liver, both in vivo and in vitro [47]. All of the above actions of CS combined result in a large net increase in the rate of gluconeogenesis, suggested by some to be as much as 10-fold [39]. In addition to its effect on carbohydrate metabolism, CS influences the metabolism of protein. For example, administering corticosteroids to rats has resulted in decreases in DNA and RNA synthesis [48]. This result has also been noted in research involving the muscle tissue of rats [49] and chickens [50], in that protein synthesis in these tissues was reduced by administration of glucocorticoids. This reduction in protein synthesis appears to occur specifically at the expense of skeletal muscle tissue because corticosteroids have been shown to stimulate protein synthesis in the liver [51]. In addition, research has demonstrated that cardiac muscle is not susceptible to catabolism induced by glucocorticoids [52].

Glucocorticoids have an indirect role in lipid metabolism. Similar to the effects of CS on glycogen reserves mentioned previously, CS exerts a permissive effect on catecholamines to initiate lipolysis [53]. It is evident that this effect is at least partially under the control of glucocorticoid hormones because rats subjected to adrenalectomy have been shown to have decreased catecholamine-driven lipolysis [54]. Despite this, CS causes the rate of fat deposition to increase in poultry [43, 45, 55]. It has been demonstrated that mice treated with cortisol had an increased secretion of insulin, followed by an increase in abdominal fat accretion [56]. Thus, this phenomenon could be caused by an increase in insulin levels because of peaks in blood glucose during the stress response, or because of insensitivity to CS after prolonged exposure [53].

Glucocorticoid hormone secretion also has implications for mineral metabolism. Corticosteroids have been directly implicated in the development of osteoporosis in stressed animals [22, 57]. This is probably caused by interference with intestinal absorption of calcium through inhibition of the synthesis of carrier proteins [58]. Additionally, glucocorticoids support the action of catecholamines, which have been shown to increase urinary calcium [59] and sodium [60] excretion.

	VALIDATION OF PHYSIOLOGICAL STRESS MODELS FOR POULTRY

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
Before we examine the stress response in poultry, it is important to point out that there are many suitable methods to induce stress in birds for the purpose of research. Some of these include various environmental stressors, such as heat or cold stress, injection of stress-inducing pharmacological preparations, alterations in social groupings, and feeding of toxic substances [27]. Other methods include administration of CS or ACTH to induce controlled stress at the adrenal level [27]. Although all of these methods have produced signs of physiological stress [61], the adaptive processes the birds exhibit have seldom been consistent or predictable. For this reason and for the sake of brevity, this review focuses on research using CS or ACTH administration to induce stress. Although not all avian research using CS or ACTH administration has produced consistent results [27], there seems to be more similarity in some of the responses that are typically demonstrated when stress is induced at the adrenal level.

One common measurement of stress in poultry is to measure plasma CS concentration in birds after treatment with CS or ACTH [12]. Elevations in circulating levels of CS in poultry after treatment with ACTH have been demonstrated repeatedly [27–29, 41, 62, 63,]. Nagra and Meyer [43] measured effluent blood from the adrenal vein after i.v. injections of ACTH in several avian species and found that the plasma CS level increased by as much as 250% postinjection. Because ACTH, whether produced endogenously or administered exogenously, causes avian plasma CS levels to increase, it can be surmised that stress responses are fairly similar when treatment with either ACTH or CS is used.

Liver weight has been consistently shown to increase in broilers treated with CS or ACTH [25–29, 45, 64, 65]. This is probably due to an increase in liver fat because liver lipids have been shown to increase significantly in broilers treated with ACTH [27, 28]. This may also be partially attributable to liver muscle enlargement because corticosteroids have been shown to stimulate protein synthesis in the liver [51].

Other common validation procedures for the occurrence of physiological stress after CS or ACTH treatment involve the measurement of immune system parameters. One such example is to measure the lymphoid organs (i.e., bursa, thymus, and spleen) because regression of these organs has been shown repeatedly to occur as a result of CS or ACTH treatment [21, 23, 24, 27, 66–68]. Probably the most common immunity-related stress validation reference is an increase in the heterophil-to-lymphocyte ratio [27]. This parameter has been observed repeatedly in poultry treated with CS or ACTH [26–29, 69, 70].

Several other stress validation signals exist, ranging from increases in blood glucose [27, 29] to cholesterol [24, 27, 28, 66, 69, 71, 72]. However, for broiler nutritional research, many of these procedures are impractical owing to the time and labor that must be devoted because much of this research is conducted by examining pens as opposed to examining individual birds. For this reason, it is more practical to focus on stress validation procedures that are focused on poultry live performance and carcass characteristics.

	EFFECTS OF PHYSIOLOGICAL STRESS ON BROILER PERFORMANCE AND CARCASS DATA

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
Physiological stress causes several detrimental effects on broiler performance and carcass characteristics. These are most likely due to the aforementioned effects of glucocorticoids on glucose metabolism.

Probably the most recognizable effect of CS or ACTH treatment on live performance is a sharp reduction in BW gain [27–29, 41, 45, 50, 55, 67, 68, 72–76]. This reduction in BW gain often occurs despite significant increases in feed intake [41, 45, 55]. The potential for elevations in feed conversion, which have been observed in broilers given injections of ACTH, is fairly obvious [41]. Stressed broilers also typically display increases in abdominal fat deposition [43, 45, 55]. Carcass data further reflect that a reduction in muscle accretion, as well as an increase in abdominal fat deposition, occurs in poultry treated with CS or ACTH [43, 45, 55]. These effects most likely occur because of the action of glucocorticoids on glucose metabolism. As mentioned previously, corticosteroids promote gluconeogenesis by causing the liberation of substrates from body tissues necessary for endogenous glucose production [40]. This action, combined with CS-inhibited protein synthesis [50] as well as insulin-induced increases in adipocytes [56], is a likely explanation for the diminishing effects of CS and ACTH on broiler performance.

	EFFECT OF PHYSIOLOGICAL STRESS ON AMINO ACID DIGESTIBILITY

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
By considering the many effects of physiological stress on metabolism and broiler performance, it could be surmised that the potential exists for a reduction in the detrimental effects of stress through nutritional regimens that better meet the nutrient requirements of birds during stress. To accomplish this goal, the effects of stress on nutrient digestion must be elucidated. Of these nutrients, acquiring knowledge of the effects of stress on protein or amino acid digestion is very important, given the known effects of glucocorticoids on protein synthesis and catabolism. Unfortunately, very little research of this type has been conducted.

The only published research delineating the effects of physiological stress on nutrient digestibility is that by Puvadolpirod and Thaxton [41], in which the nutrient digestibility of broilers treated with ACTH-dispensing implants was examined. These researchers concluded that broilers given ACTH had significantly lower protein and carbohydrate digestibility than broilers in the nonstressed control group [41]. These researchers concluded that this reduction in digestion of protein was most likely due to an increase in feed passage time in the presence of the stressor because birds treated with ACTH displayed polydipsia and polyuria during and after stress [41]. Puvadolpirod and Thaxton [41] calculated nutrient digestibility by dividing nutrient intake per day by nutrients excreted per day, multiplying the quotient by 100, and then dividing the quotient by nutrient intake per day. Excreta protein contents were obtained by determining total nitrogen by the Kjeldahl procedure, and then adjusting for protein content [41].

	CURRENT AMINO ACID DIGESTIBLITY ASSAYS

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
Before the effects of physiological stress on amino acid digestibility can be elucidated, an assay must be chosen by which to measure amino acid digestibility during stress both accurately and practically. Of the amino acid digestibility assays currently available, the method that best meets these two criteria is the apparent ileal amino acid digestibility assay.

Before the last several years, probably the most widely accepted amino acid digestibility assay was the precision feeding assay, which is based on excreta measurements [77]. This assay requires that adult cockerels be fasted for 24 to 48 h and then force-fed a premeasured amount of test ingredient directly into the crop [77]. One disadvantage of this model is that measurements are made from excreta, and urinary amino acid contributions cannot be subtracted [78]. Additionally, this assay requires the use of fully mature birds; thus, estimates may not clearly represent amino acid digestibility for younger birds [78]. Another disadvantage of this assay, which should not be neglected in any discussion of its use in a stress model, is that force-feeding is required. It could be argued that this assay could induce stress.

More recently, the apparent ileal amino acid digestibility assay has gained acceptance [79]. This assay entails the removal of the ileum from the bird, followed by the removal of the ileal contents [80]. The ileal contents can then be dried, digested, and analyzed for nutrient content by HPLC [81]. The advantages of this assay are that it is easy to obtain ileal contents (i.e., kill the bird and remove its ileal contents), it can be performed on birds of any age, and it is very precise, with low variability [78]. Furthermore, it is not necessary to force-feed birds because this assay uses the inclusion of an indigestible marker (i.e., chromic oxide, titanium dioxide, or ash-insoluble ash) in the treatment diet [81]. Additionally, this allows for digestibility measurements in a complete feed, as opposed to the individual feedstuffs required for the precision feeding assay [78]. One of the most important advantages of this assay is that the digesta sample is obtained from the ileum, and variation is reduced because contamination from hindgut fermentation and urinary waste does not occur [78]. Because uric acid excretion has been shown to increase during stress in poultry owing to CS-driven gluconeogenesis [43], the usefulness of this assay for nutritional research involving stress appears invaluable.

Recently, research conducted in this laboratory examined the effects of CS-induced stress on amino acid digestibility in broilers [82]. This research involved feeding broilers CS (15 mg/ kg of diet) blended into the diet for 10 d and then measuring amino acid digestibility by using the apparent ileal amino acid digestibility assay. Virden et al. [82] determined that physiological stress induced by feeding this level of CS for this duration had no effect on amino acid digestibility.

	POTENTIAL OF VARIOUS NUTRIENTS TO AFFECT THE STRESS RESPONSE OF BROILERS

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 
As mentioned previously, very little research exists delineating the effects of specific nutritional regimens on the stress response of broilers. However, based on the effects of long-term stress on metabolism, as well as nutritional research conducted on heat-stressed broilers, it could be argued that the following examples have the potential to ameliorate stress, at least partially.

Considering the fact that stressed broilers prefer to metabolize glucose, increased dietary or supplemental glucose, or inclusion of dietary carbohydrate or amino acids that can be converted to glucose would seem to have potential for stress alleviation. For example, supplementing glucose in the drinking water of broilers subjected to heat stress resulted in increased BW gain compared with heat-stressed broilers fed no supplemental glucose [83, 84].

Additionally, Met, Thr, Arg, Val, Ile, Trp, Cys, Gly, Ser, Tyr, Phe, Ala, His, Pro, and Asp are glucogenic [85], meaning that these amino acids can be converted to glucose. Although gluconeogenesis is a metabolically expensive process [85], it is well established that this process occurs during stress because of the actions of CS [47]. Hence, providing increased glucogenic amino acids in the diet could lessen the glucose production burden placed on skeletal muscle during stress, thereby decreasing CS-driven catabolism. Furthermore, the potential exists to reduce the detrimental effects of this muscle catabolism simply by feeding diets with increased amino acid density. Additionally, as noted previously, physiological stress has been shown to reduce the protein digestibility of broilers [41]. This could increase the requirements for amino acids during stress.

The amino acid Trp is worth consideration because research in various species exposed to myriad stressors has shown that Trp has the potential to alleviate stress [86–93]. Furthermore, although no direct references to physiological stress reduction in broilers attributable to Trp supplementation could be found, increasing dietary Trp has been shown to reduce hysteria in layers [87], as well as aggressiveness in broiler breeders [88, 90]. Tryptophan has long been thought to have the potential for stress reduction through its role as a substrate for serotonin and melatonin production [94]. Melatonin has been shown to have the potential to reduce ACTH secretion in various species [95–97].

Heat stress research has often implicated high-electrolyte diets or electrolyte supplementation as having a potential role in stress reduction. Researchers have demonstrated that dietary supplements that increase dietary electrolyte balance have the potential to improve broiler survival during heat stress [98–103]. Although much of this research was conducted in attempts to reduce respiratory alkalosis, a condition that occurs specifically during heat stress [98, 100, 103], the potential still exists for electrolytes to play a role in physiological stress reduction. Because glucocorticoid and catecholamine function cause increases in urinary electrolyte excretion [58, 59], perhaps electrolyte deficiencies occur during physiological stress.

Another group of nutrients worth exploring is vitamins. Research has demonstrated that supplemental A, D, E, and B-complex vitamins in the drinking water of heat-stressed broilers result in improved performance [92]. In addition, broilers given ascorbic acid supplements during heat stress had improved performance as well as reduced plasma CS levels [104–106].

	CONCLUSIONS AND APPLICATIONS

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 

1. Short-term stress is typically of minimal consequence to broiler production.
   
2. Exposure to stressors for extended durations results in the secretion of the stress hormone CS. If this hormone remains in circulation for long periods, impaired live performance can be expected because of CS-induced gluconeogenesis.
   
3. Corticosterone causes impaired immune system function and regression of the lymphoid tissues, often accompanied by an increased heterophil-to-lymphocyte ratio.
   
4. Stress causes broilers to preferentially metabolize glucose. This results in alterations in carbohydrate, protein, lipid, and mineral metabolism, which in turn causes the depletion of structural protein and the deposition of abdominal fat.
   
5. Research models using exogenous CS or ACTH for stress induction have been validated, and are probably better suited for conducting studies aimed at affecting the physiological stress response because specific effects of specific stressors can be avoided.
   
6. More research should be conducted to examine the effects of physiological stress on nutrient digestibility. Probably the best assay for this research is the apparent ileal digestibility assay.
   
7. More research is needed to examine the effects of different nutrient regimens on the physiological stress response of broilers.
   

	FOOTNOTES

 
¹ This is Journal Article Number J-11143 from the Mississippi Agricultural and Forestry Experiment Station, supported by MIS-322230.

² Use of trade names in this publication does not imply endorsement by the Mississippi Agricultural and Forestry Experiment Station of the products, or of similar ones not mentioned.

	REFERENCES AND NOTES

TOP SUMMARY DESCRIPTION OF PROBLEM CLASSIFICATIONS OF STRESS ACTIVATION AND CONTROL OF... PHYSIOLOGICAL CONSEQUENCES OF... VALIDATION OF PHYSIOLOGICAL... EFFECTS OF PHYSIOLOGICAL STRESS... EFFECT OF PHYSIOLOGICAL STRESS... CURRENT AMINO ACID DIGESTIBLITY... POTENTIAL OF VARIOUS NUTRIENTS... CONCLUSIONS AND APPLICATIONS REFERENCES AND NOTES

 

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