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4 Poultry INTRODUCTION With the exception of meat-type broiler breeders and a few strains of egg-laying hens, feeding systems for poultry are usually based on ad libitum feeding. The producer determines the nature and quality of the feed, but the bird itself governs feed intake. The birds are usually maintained in a controlled environment with re- gard to temperature and airflow, although systems dif- fer in complexity and the degree of control that is attainable. FEED INTAKE CONTROL MECHANISMS There is considerable disagreement about the mecha- nisms that control feed intake in poultry. (See Chapter 1 for an overall view of feed intake control mechanisms in animals.) In poultry, dietary energy, protein, weight, and volume all have significant effects upon feed con- sumption (reviewed by Gleaves et al., 1968~. Both pe- ripheral and central neural controls have been implicated in the regulation of feed intake. Peripheral receptors in the upper digestive tract serve as important regulators of feed intake and are presumed to be interre- lated and in contact with the hypothalamus via neurons (Polin and Wolford, 1973~. At least five theories, each based upon a variable monitored by the central nervous system, have been advanced to explain the control of feeding. These include the glucostatic, thermostatic, lipostatic, aminostatic, and ionostatic theories. Glucostatic Theory The glucostatic theory of feed intake regulation, as generally recognized in mammals, either is not present 42 in birds or is not readily detectable by normal protocols (Richardson, 1970; Smith and Bright-Taylor, 1974~. Food intake has not been augmented as it has in mam- mals after the implantation of gold thioglucose directly into several hypothalamic structures (Smith and Szper, 1976) or after peripheral injections (Gentle, 1976; Walker et al., 1981~. Feeding has not been altered after manipulation of blood glucose levels (Smith and Bright- Taylor,1974~. More recent studies have suggested pos- sible roles of central (Denbow et al., 1982) and peripheral (Shurlock and Forbes, 1981) glucoreceptors in affecting feed intake. However, when the glucose analog 2-deoxy-D-glucose (an inhibitor of glycolysis that causes a depression of intracellular glucose utilization) was injected intraperitoneally into fowl, it did not effect an increase in food intake (Smith et al., 1975~. Hence, the importance of blood glucose or a change in blood glucose in controlling feed intake in chickens is unre- solved at this time. Thermostatic Theory The thermostatic theory is based upon the output (loss) of body heat or energy that drives a bird to con- serve body heat and/or consume food. It requires a cen- tral controller (presumably in the brain) and several peripheral detectors to sense temperature changes. Progress has been made in poultry and other avian spe- cies to demonstrate that an anterior hypothalamic area contains thermally sensitive loci for induction of ther- moregulatory responses (Mills and Heath, 1970~. Ther- moreceptors have been identified in the bill (Necker, 1973), skin (Necker, 1977), and spinal cord (Rautenberg et al., 1972~. The relationships between environmental temperature and feed intake and between temperature and heat production are reasonably well known in birds (Carder and King, 1974; Sykes, 19791. However, more

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Poultry 43 data are needed to determine if a thermostatic mecha- nism is a primary controller of feed intake. Lipostatic Theory The lipostatic theory is based upon feedback from fat depots to the brain for long-term regulation of feed in- take. Based upon studies using the technique of forced feeding, Lepkovsky (1973) has suggested that a lipo- static mechanism for the control of feed intake regula- tion exists in poultry. The studies of Polin and Wolford (1973) have indicated that lipostatic systems show evi- dence of influence on feed intake, but the effects are not totally consistent with the proper functioning of a lipo- static mechanism. A recent study in which partial lipec- tomy was performed in both Leghorn and broiler chicks showed no compensatory feed intake response in either type of bird. The data suggest little, if any, negative feedback from adipose tissue mass in chickens (Maurice et al., 1983~. Aminostatic Theory It has been known for years that the balance of amino acids markedly influences food intake in both chicks and rats. Imbalanced diets cause rapid decreases in food intake and altered patterns of feeding (Rogers and Leung, 1973~. The addition of single amino acids or groups of amino acids to the diet depresses growth rate (Fisher and Shapiro, 1961; Harper et al., 19701. Force feeding prevents the typical growth depression due to amino acid imbalance in chicks (Austic and Scott, 1975), indicating that the negative consequences of imbalance are due, ultimately, to reduced feed intake. The con- sumption of imbalanced diets leads to a rapid decrease in the concentration of the first limiting amino acid in the blood (Harper et al., 1970~. This may be the signal that triggers a reduction in appetite. Data suggest that the brain contains receptors for amino acids or their metab- olites. In studies by Tobin and Boorman (1979) young cockerels were fed a low-protein diet, imbalanced by the addition of an essential amino acid mixture lacking histi- dine. Infusion of histidine via the carotid artery signifi- cantly increased food intake, whereas infusion via the jugular vein did not. Thus, infusion via the carotid ar- tery rectifies the pattern of amino acids passing to the brain, causing the cockerel to respond as if the dietary amino acid pattern were balanced. Specific brain loci appear to be involved in giving mammals the ability to detect an amino acid-imbalanced diet (Leung and Rog- ers, 1971~. Since comparable brain areas have not been discovered in poultry to date, the aminostatic theory and the mechanism responsible for monitoring changes in the protein concentration of the diet remain the subject of speculation. Ionostatic Theory The extracellular role of Na+ and Ca+ + ions within the brain (more specifically the hypothalamus) has marked effects upon body temperature and feed intake. The set point for body temperature in mammals was proposed to be controlled by the ratio of Na+ and Ca++ ions. An excess of the latter caused hypothermia (Myers and Veale, 1970~. Further studies showed that an elevated concentration of brain calcium induced feeding in sati- ated rats (Myers et al., 1972~. To date, the ionostatic theory has not been adequately studied in poultry. Role of Brain Structure The theories presented above have all implied that specific brain structures are involved in the control of feed intake. A few brain loci have been identified that have clear effects on feed intake or body composition in poultry. Obesity without hyperphagia has been effected in the chicken after placement of large lesions within the medial basal hypothalamic region (Lepkovsky and Yasuda, 1966~. In no paper published to date has it been shown clearly which neural structures need to be de- stroyed to illicit hyperphagia and obesity in poultry. It was surmised that the ventromedial hypothalamic nu- cleus is a critical neural substrate (Kuenzel,1974~. How- ever, lesions were large and obviously encroached upon other structures such as the inferior hypothalamic nu- cleus, medial mammillary nucleus, ventral nora- drenergic bundle, dorsomedial hypothalamic nucleus, and periventricular hypothalamic nucleus. Transient aphagia or hypophagia has been produced in chickens (Feldman et al., 1957), broiler chickens (Kuenzel, 1982), and other avian species following bilat- eral lesions in and around the lateral hypothalamic- thalamic and midbrain areas. Structures that appear to be the most important for effecting aphagia/hypophagia in various avian species are the following: ansa lenticu- laris, posterior nucleus of the ansa lenticularis, and quinto-frontal tract (Kuenzel, 19821; stratum cellulare externum and lateral forebrain bundle (Wright, 1975~; and quinto-frontal tract and trigeminal structures (Ziegler and Karten, 1973~. Summary In summary, it is unclear at this time which of the five proposed theories of feed intake best applies to poultry. It is also unclear which brain structures are important in

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44 Pre~licting Feed Intake regulating feed and water intake. Biologists have not yet identified with assurance either the site or the nature of a mechanism which specifically regulates food intake in birds. A number of excellent reviews have been writ- ten on factors regulating feed intake in poultry (Gleaves et al., 1968; Polin and Wolford, 1973; Boorman and Freeman, 1979), and interested readers are directed to these reviews. It is generally recognized among poultry nutritionists that a primary determinant of feed intake is the energy concentration of the diet. These studies have been re- viewed in detail by Waldroup et al. (1976), Pesti (1982), and Pesti and Fletcher (1983~. As the level of metaboliz- able energy in the diet increases or decreases, food in- take changes inversely, although the rate of adjustment is not always sufficient to keep energy intake constant. As the energy content of the diet increases, there is almost always a greater daily intake of calories by the bird (Morris, 19681. Since poultry are generally offered diets containing low-moisture ingredients such as cereal grains and ani- mal or vegetable protein supplements, there has been little emphasis upon estimating dry matter intake in this species. Rather, the emphasis has been upon estimating either total feed intake, by assuming a constant dietary energy, or upon estimating an energy requirement, thus allowing for an estimation of feed intake from knowl- edge of the dietary energy content. As more studies are conducted on the estimation of true metabolizable en- ergy, amino acid digestion, and carcass retention of nu- trients, more emphasis likely will be given to dry matter intake. ESTIMATING FEED INTAKE OF LAYING HENS Laying hens attain a virtual steady state of feed con- sumption once peak production has been attained, thus allowing for more consistent estimates of feed intake as influenced by various production and environmental factors. Nutrient requirements of laying hens are in- creasingly being expressed on a daily intake basis, thus implying the importance of knowledge of daily intake of total feed. Because of this, a greater emphasis has been placed upon estimating feed intake in laying hens than in other poultry species. Byerly (1941) reported an equation for estimating the daily feed intake of laying hens varying in body weight from about 1 to 3 kg. The 0.653 power of body weight gave the best estimate of feed requirement for mainte- nance from these data. This equation was later modified by Combs (1968) to express the nutrient needs in terms of apparent metabolizable energy (AME) adjustable to various ambient temperatures. Byerly et al. (1980) have developed partition equa- tions that describe the feed intake for five genetically different groups of hens, including small Leghorns, white egg hybrids, brown egg hybrids, female line broiler breeders, and broiler-cross pullets. For the first four groups, equations that assume a 70 percent effi- ciency of use of metabolizable energy (ME) for mainte- - nance, tissue formation, and egg formation gave the best fit to the observed data. The equations were as follows: F = (0.534 - 0.0047) ~''i3:3 + 2.76^ W + O.SOEM (I) and F = (0.259 - 0.00259T)W 75 + 2.76^ W + O.SOEM (2) where F is feed/hen/day (g), T is ambient temperature (C), Wis live weight (g), i\Wis daily change in live weight (g), and EM is egg mass/hen/day (g). For broiler-cross pullets, an energetic efficiency of 65 percent gave the best fit to the data. Corresponding equations for this type of pullet were as follows: F= (0.589 - 0.0044~75 + 2.9AW+ 0.85EM F= (0.275 - 0.002757)W75 + 2.9AW+ 0.85EM. (3) (4) These equations were developed using the diets con- taining 2,890 kcal of AME/kg. Morris (1968) observed an effect of dietary energy level on voluntary feed con- sumption, and Byerly et al. (1980) noted that this effect should be considered when dietary energy levels differ markedly from 2,890 kcal of AME/kg. Morris (1968) analyzed the data from 34 published reports and ob- served that groups of hens offered diets with different energy levels tend to adjust consumption so as to main- tain a similar caloric intake; however, the adjustment is imperfect in the majority of cases. This is especially true when high-energy diets containing supplemental fats are fed to hens. Under such circumstances hens tend to overconsume calories. This does not necessarily result in higher egg production; hens fed such diets usually have slightly larger eggs and usually show increased body weight gains that reflect greater stores of adipose tissue and not protein (muscle tissue). Morris (1968) noted that the degree of energy over- consumption observed when a particular strain of hen is offered a range of diets, each with a different energy content, is correlated with the characteristic caloric in- take of that strain. Strains with characteristically high caloric intakes adjust their food consumption to com- pensate for differences in energy content of the diet less efficiently than small strains which have characteristi- cally low caloric intakes. From the observed relation- ships, Morris (1968) developed a formula that predicts

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Poultry 45 the expected daily caloric intake for any given level of dietary energy, given a knowledge of the caloric intake of the strain when fed some standard diet: Y= Y2 700 + (0.0005465) - 0.~466)(x- 2,700), where Y is predicted energy (kcal/bird/day), Y2 700 is characteristic energy intake when the diet contains 2,700 kcal of ME/k~ ~nr1 Kit in nct~1 c1iet~rv ener~v (kcal/kg). Leeson et al. (1973) observed that the equation of Byerly (1941) predicts a feed intake about 15 percent higher than that observed for many commercial hybrid laying flocks. They reported a modified partition equa- tion describing feed intake. Balnave et al. (1978) re- ported equations for minimum AME intakes for laying hens based on an assumed efficiency of 75 percent for the use of ME in egg formation. They reported the fol- lowing equations for use in estimating daily ME require- ments. For egg-producing strains: 0, w _ _ ~ ~ ~,,, O ,, . MEmin = 388 W-75 e0 027(22 - T) + 8.67E. (6) For meat-producing strains: ME i = 450 W0.75eO.027~22 - T) + 8 67E (7 where MEmin is the minimum ME requirement (k T/day), Wis body weight (kg), Eis egg product (g/day), and Tis environmental temperature (C). These formulas do not include any requirement for body weight gain. Pullets entering production or on the verge of entering the peak laying period will require an additional allowance for weight gain, however. Baloave and associates (1978) suggest that an allowance for gain may be calculated by assuming an ME requirement of 8 k T/g of body weight gain. The National Research Council (NRC; 1981) exam- ined the results of several published studies and derived an equation to estimate the AME intake of laying hens: ME = 130W75 + (1.015)(~t) + 5.50AW+ 2.07EE, (8) where W is body weight (kg); ~t is the difference be- tween 25C and the ambient temperature, AW is growth rate or rate of loss (g/day), EE is egg product (g/day). The equation has since been modified (NRC, 1984) to read: ME = W 75(173 - 1.951) + 5.5^ W + 2.07EE, (9) where T is ambient temperature (C). ESTIMATING FEED INTAKE OF BROILERS Nutrition studies designed to directly examine daily feed or dry matter intake of broilers have been very limited in number. Since broilers are generally fed ad libitum the primary objectives in research have been to examine the response to different diets at some fixed point associated with final market weights, rather than to characterize feed intake. However, there have been some excellent reviews that have examined the litera- ture and made some estimates of the effect of dietary energy level on rate of food intake. Fisher and Wilson (1974) reviewed more than 160 published research reports on the influence of ME con- tent of the diet on body weight gain and feed consump- tion. From these data they derived response re- lationships that allow an estimate of consumption from the dietary energy content, assuming that other nutri- ents are not limiting. Waldroup et al. (1976) developed equations to predict changes in feed consumption asso- ciated with changes in dietary nutrient density level. In these and other, similar papers, the effect of energy on food consumption has been examined in an indirect manner. With the growing interest in modeling of poul- try growth and performance as a means of predicting nutrient requirements, emphasis has been directed to- ward estimating the energy required to support a given rate of gain established by the genetic potential of the bird. These estimates have generally been divided into three major areas: (l) energy needed for maintenance of body weight; (2) energy needed per gram of tissue syn- thesized; and (3) modifications due to environmental temperature. Hurwitz et al. (1978) calculated the maintenance en- ergy of the male broiler-type chick to be 1.91 cal/g0 66, with an energy requirement for growth of 2.05 cal/g of gain. Robbins and Ballew (1984) found the maintenance needs of the broiler chick from 8 to 22 days posthatching - to be 153 kcal of ME/kg0 75 and 190 kcal of ME/kg0 75 from 28 to 42 days posthatching. Using these data, the daily energy needs of a growing broiler chick main- tained in a thermoneutral environment can be esti- mated. From this estimate, a daily dry matter intake can be calculated based upon knowledge of the ME and dry matter content of the diet. Hurwitz et al. (1980) have confirmed the results of their estimates in feeding trials. CALCUATIONS OF DRY MATTER INTAKE Using equations 1, 2, 8, and 9 given above and typical production standards, estimates of daily dry matter con- sumption have been made for laying hens (Table 4-1) and broilers (Table 4-2~. These are based on the use of a diet containing 2,890 kcal of ME/kg for laying hens and a diet containing 3,200 kcal of ME/kg for broilers, each

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46 Predicting Feed Intake TABLE 4-1 Estimated Dry Matter Intake of Laying Hens at Different Stages of Egg Production Egg Age Production (weeks) (%) 20 5.0 24 62.0 28 91.0 32 89.0 36 87.0 40 85.0 Egg Weight BW (g) (g) 47.7 50.7 55.0 57.6 59.3 60.4 Daily Gain (g, approx.) 1,317 7 1,513 6 1,653 6 1,737 3 1,821 2 1,877 0 Dry Matter Intake (g/day) at 25C Estimated by the Following Equations:a _ 1 2 8 9 2 60.2 82.2 98.0 93.2 92.6 88.5 56.0 78.2 94.1 89.4 88.8 84.9 59.7 81.5 96.7 94.6 95.1 93.0 61.9 83.9 99.3 97.2 97.9 95.8 aBased on the following equations for estimating food or energy intake, using a diet with 2.89 ME kcal/g and 12 percent moisture: Equation 1, F = (0.534 - 0.0041) We 653 + 2.76~ W + 0.80EM (Byerly et al., 1980); Equation 2, F = (0.259 - 0.002597)W~ 75 + 2.76/` W + 0.80EM (Byerly et al., 1980); Equation 8, ME = 130 W0 75 + (1.015)(/~) + 5.50 ~ W + 2.07EM (NCR, 1981); Equation 9, ME = W75(173 - 1.9571 + 5.5A W+ 2.07EM(NRC, 1984),whereFisfeed/hen/day(ing); Tis the ambient temperature (in C); AT = difference between 25C and ambient temperature; Wis liveweight (in g); ~ Wis daily change in liveweight (in g); and EM is egg mass/hen/day (in g). TABLE 4-2 Estimated Dry Matter Intake of Broilers at Different Ages . Age BW Daily Gain (days) (g) (g, approx) Estimated Energy Needsa (ME kcal/day) Maintenance Gain Daily Consumption Daily Dry of Air-Dry Feed Total (3.2 ME kcal/g) Matter Intake (g) (12% Moisture) 28.3 42.8 58.4 77.0 93.6 104.2 115.6 7 14 21 28 35 42 49 130 27 320 34 560 43 860 56 1,250 63 1,690 59 2,100 60 47.4 85.9 124.4 165.1 211.3 257.8 297.6 55.3 102.7 32.2 69.7 155.6 48.6 88.1 212.5 66.4 114.8 279.9 87.5 129.2 340.5 106.4 120.9 378.7 118.4 123.0 420.6 131.4 aBased on the following equation of Hurwitz et al. (1978): ME (kcal/day) = 1.91 BW 0.66 + 2.05^ where BW is body weight (in g) and ~ W is daily gain (in g). with a 12 percent moisture content. These values are typical of the majority of feeds used for broilers and laying hens in the United States and other major poultry-producing countries. SUMMARY It is apparent that poultry researchers have not been as concerned about dry matter intake as those working with other animal species. With the growing interest in modeling as a means of estimating nutrient require- ments, additional data will undoubtedly be developed with regard to ME needs. Since it is likely that poultry will continue to be fed diets based largely on low- moisture ingredients, accurate estimates of dry matter intakes for these animals will be obtained primarily through knowledge of energy consumption. REFERENCES Austic, R. E., and R. L. Scott. 1975. Involvement of food intake in the lysine-arginine antagonism in chicks. J. Nutr. 105:1122. Balnave, D., D. J. Farrell, and R. B. Cumming. 1978. The minimum metabolizable energy requirement of laying hens. World's Poult. Sci.34:149. Boorman, K. N., and B. M. Freeman. 1979. Food Intake Regulation in Poultry. Edinburgh: British Poultry Science Ltd. Byerly, T. C. 1941. Feed and other costs of producing market eggs. Md. Agric. Exp. Stn. Bull. A-1. Byerly, T. C., J. W. Kessler, R. M. Gous, and O. P. Thomas. 1980. Feed requirements for egg production. Poult. Sci. 59:2500. Calder, W. A., and J. R. King. 1974. Thermal and caloric relations of birds. Pp. 260-413 in Avian Biology, Vol. IV, D. S. Farner and J. R. King, eds. New York: Academic Press. Combs, G. F. 1968. Amino acid requirements of broilers and laying hens. Pp. 86-96 in Proceedings of the Maryland Nutrition Confer- ence. Washington, D.C. Denbow, D. M., J. A. Cherry, H. P. Van Krey, and P. B. Siegel. 1982. Food and water intake following injection of glucose into the lateral ventricle of the brain of broiler type chicks. Poult. Sci. 61:1713. Feldman, S. E., S. Larsson, M. Dimick, and S. Lepkovsky. 1957. Aphagia in chickens. Am. J. Physiol. 191:259. Fisher, C., and B. J. Wilson. 1974. Response to energy concentration in diets for growing chickens. Pp. 151-184 in Energy Requirements of Poultry. K. N. Boorman and B. J. Wilson, eds. Edinburgh: British Poultry Science Ltd. Fisher, H., and R. Shapiro. 1961. Amino acid imbalance: Rations low in tryptophan, methionine, or lysine and the efficiency of utilization of nitrogen in imbalanced rations. J. Nutr. 75:395. _ J

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Poultry 47 Gentle, M. J. 1976. The effects of gold thioglucose on the central nervous system of chicks. Toxicol. Appl. Pharmacol. 35:223. Gleaves, E. W., L. V. Tonkinson, J. D. Wolf, C. K. Harman, R. H. Thayer, and R. D. Morrison. 1968. The action and interaction of physiological food intake regulators in the laying hen. 1. Effects of dietary factors upon feed consumption and production responses. Poult. Sci. 47:38. Harper, A. E., N. J. Benevenga, and R. M. Wohlheuter. 1970. Effects of ingestion of disproportionate amounts of amino acids. Physiol. Rev. 50:428. Hurwitz, S., D. Sklan, and I. Bartov. 1978. New formal approaches to the determination of energy and amino acid requirements of chicks. Poult. Sci. 57:197. Hurwitz, S., I. Plavnik, I. Bartov, and S. Bornstein. 1980. The amino acid requirements of chicks: Experimental validation of model-cal- culated requirements. Poult. Sci. 59:2470. Kuenz~, W. J. 1974. 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Poult. Sci. 62:1466. Mills, S. H., and J. E. Heath. 1970. Thermoresponsiveness of the preoptic region of the brain in house sparrows. Science 168:1008. Morris, T. R.1968. The effect of dietary energy level on the voluntary calorie intake of laying hens. Br. Poult. Sci. 9:285. Myers, R. D., and W. L. Veale.1970. Body temperature: Possible ionic mechanism in the hypothalamus controlling the set point. Science 170:95. Myers, R. D., S. A. Bender, M. Krstic, and P. D. Brophy. 1972. Feed- ing produced in the satiated rat by elevating the concentration of calcium in the brain. Science 176:1124. National Research Council. 1981. Effect of Environment on Nutrient Requirements of Domestic Animals. Washington, D.C.: National Academy Press. National Research Council. 1984. Nutrient Requirements of Poultry, 8th rev. ed. Washington, D.C.: National Academy Press. Necker, R. 1973. Temperature sensitivity of thermoreceptors and mechanoreceptors on the beak of pigeons. J. Comp. Physiol.87:379. Necker, R. 1977. Thermal sensitivity of different skin areas in pi- geons. J. Comp. Physiol. 116:239. Pesti, G. M. 1982. Characterisation of the response of male broiler chickens to diets of various protein and energy contents. Br. Poult. Sci. 23:527. Pesti, G. M., and D. L. Fletcher. 1983. The response of male broiler chickens to diets with various protein and energy contents during the growing phase. Br. Poult. Sci. 24:91. Polin, D., and J. H. Wolford.1973. Factors influencing food intake and caloric balance in chickens. Fed. Proc. 32:1720. Rautenberg, W., R. Necker, and B. May. 1972. Thermoregulatory responses of the pigeon to changes of the brain and spinal cord temperatures. Pfluegers Arch. Physiol. 338:31. Richardson, A. J. 1970. Blood glucose levels and food intake in the domestic chicken. Br. Poult. Sci. 11:501. Robbins, K. R., and J. E. Ballew.1984. Utilization of energy for main- tenance and gain in broilers and Leghorns at two ages. Poult. Sci. 63:1419. Rogers, Q. R., and P. M. B. Leung.1973. The influence of amino acids on the neuroregulation of food intake. Fed. Proc. 32:1709. Shurlock, T. G. H., and J. M. Forbes. 1981. Evidence for hepatic glucostatic regulation of food intake in the domestic chicken and its interaction with gastrointestinal control. Br. Poult. Sci. 22:333. Smith, C. J. V., and B. Bright-Taylor. 1974. Does a glucostatic mecha- nism for food intake control exist in chickens? Poult. Sci. 53:1720. Smith, C. J. V., and I. Szper.1976. The influence of direct implantation of gold thioglucose into the brain of chickens on food consumption and weight gain. Poult. Sci. 55:2421. Smith, C. J. V., J. Hatfield, S. Fowler, and B. Bright-Taylor. 1975. Changes in food consumption and blood glucose levels in the domes- tic chicken, Gallus domestic?vs in response to the administration of 2- deoxy-D-glucose. Comp. Biochem. Physiol. 51A:811. Sykes, A. H.1979. Environmental temperature and energy balance in the laying hen. Pp.207-229 in Food Intake Regulation in Poultry, K. N. Boorman and B. M. Freeman, eds. Edinburgh: British Poultry Science Ltd. Tobin, G., and K. N. Boorman. 1979. Carotid or jugular amino acid infusions and food intake in the cockerel. Br. J. Nutr. 41:157. Waldroup, P. W., R. J. Mitchell, J. R. Payne, and Z. B. Johnson. 1976. Characterization of the response of broiler chickens to diets varying in nutrient density content. Poult. Sci. 55:130. Walker, P. S., H. P. Van Krey, J. A. Cherry, and P. B. Siegel. 1981. The effect of gold thioglucose on feed consumption in domestic fowl. Poult. Sci. 60:1325. Wright, P. 1975. The neural substrates of feeding behaviour in birds. Pp. 319-349 in Neural and Endocrine Aspects of Behaviour in Birds, P. Wright, P. G. Caryl, and D. M. Vowles, eds. Amsterdam: Elsevier. Ziegler, H. P., and H. J. Karten. 1973. Brain mechanisms and feeding behavior in the pigeon (Columba livia). I. Quinto-frontal structures. J. Comp. Neurol. 152:59.