Effect of Early Feed Restriction and Realimentation on Heat Production and Changes in Sizes of Digestive Organs of Male Broilers

Effect of Early Feed Restriction and Realimentation on Heat Production and Changes in Sizes of Digestive Organs of Male Broilers A. K. ZUBAIR and S. LEESON Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada, NIG 2W1

1994 Poultry Science 73:529-538

INTRODUCTION Increase in growth rate through genetic selection and improved nutrition in broiler chickens has been associated with high body fat deposition (Plavnik et ah, 1986; Yu and Robinson, 1992). This is particularly evident under ad libitum feeding that is normally practiced (Yu and Robinson,

Received for publication September 7, 1993. Accepted for publication December 9, 1993.

1992). Other problems associated with fast growth rate in broilers are skeletal and metabolic disorders (Robinson et ah, 1992). Studies of early feed restriction have demonstrated the potential for correction of these conditions (Plavnik and Hurwitz, 1985, 1988a,b, 1991; Jones and Farrell, 1992). Limiting the ME intake to only maintenance requirement for a short period early in the life of birds resulted in less carcass fat and a smaller abdominal fat pad (AFP), without retarding overall growth to 56 d of age (Plavnik and

529

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ABSTRACT Two experiments were conducted with broilers to determine the effect of early feed restriction and realimentation on metabolic heat production and changes in sizes of digestive organs. An indirect open circuit calorimeter was used. Treatments were a full-fed control (FF) and a feed-restricted group (FR). Feed during the restriction period (6 to 12 d) for the FR birds was limited to 50% of voluntary feed intake of the FF birds. This was followed by realimentation period when all birds were provided feed for ad libitum consumption. The purpose of Experiment 1 was to measure basal metabolic rate (BMR), and Experiment 2 was designed to measure 36-h fasting metabolic rate (FMR), in both cases during time of restriction and realimentation. At the end of the 36-h unfed period, birds were killed and their digestive organs excised, blotted, and weighed. The FR birds showed significantly (P < .01) lower BMR compared with the FF birds during the restriction period. This lower BMR did not carry over into the refeeding period, when there was no difference between the two treatments. Thirty-six-hour FMR, like the BMR was also lower for FR compared with FF birds, but only during the period of restriction. Weights of digestive organs (expressed as a percentage of BW) during restriction were generally heavier for FR compared with FF birds. Measurements of organ weights taken during realimentation show significantly (P < .05) heavier liver and pancreas for FR compared with FF birds. Results of these experiments suggest that lower MR of "restricted-refed" birds does not play a role in the ability of the birds to show improved feed efficiency and growth compensation. Greater feed intake relative to BW and its associated digestive adaptations seem to be contributing factors to any growth compensation. (Key words: broiler, metabolic rate, feed restriction, realimentation, digestive organs)

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There is no conclusive evidence in the literature of the relationship between energy metabolism, sizes of metabolically active tissues, and levels of feeding in the broiler chicken. Such information is necessary for the development of more effective feeding strategies. Experiments were conducted to examine changes in the rates of heat production by male broilers subjected to early feed restriction and realimentation. The influence of restricted feeding and subsequent refeeding on rates of FHP and changes in the sizes of digestive organs were also examined. MATERIALS AND METHODS Feed Restriction Protocol Throughout the experimental periods, all birds received a conventional broiler starter diet (Table 1), which was formulated to meet the NRC guidelines for ME:CP ratio (NRC, 1984). During the pre-experimental period (0 to 6 d of age), all birds consumed feed ad libitum. In Experiment 1, feed intake

TABLE 1. Percentage diet composition Ingredient

Composition

(%) Corn Soybean meal (49% CP) Limestone Calcium phosphate (20% P) Iodized salt Vitamin-mineral premix1 DL-methionine Animal-vegetable fat Nicarbazin Stafac (virginiamycin) Calculated nutrient content CP ME, kcal/kg Crude fat Calcium Available phosphorus Methionine Methionine + cystine Lysine

56.50 35.64 1.50 1.50 .30 .75 .08 3.63 .05 .05 22 3,073 6.96 .96 .42 .44 .78 1.25

iProvided per kilogram of diet: vitamin A, 800 IU (as retinyl acetate); cholecalciferol, 1,600 IU; vitamin E, 11.0 IU (as dl-a-tocopheryl acetate); riboflavin, 9.0 mg; pantothenic acid, 11.0 mg; vitamin B12,13 jtg; niacin, 26 mg; choline, 900 mg; vitamin K, 1.5 mg; folic acid, 1.5 mg; biotin, .25 mg; ethoxyquin (antioxidant), 125 mg; manganese, 55 mg; zinc, 50 mg; copper, 5 mg; iron, 30 mg; selenium, .1 mg.

of birds in the feed-restricted group was limited to 50% of voluntary feed intake of the full-fed birds from 6 to 12 d of age. During this period, daily feed intake of the full-fed birds was recorded, and this was used to determine the daily feed allocation for feed-restricted birds. All broilers were fed ad libitum during the realimentation period of 12 to 21 d of age. Feed was withdrawn just before birds were put in the respiration chamber. Feed restriction procedure in Experiment 2 was similar to that described for Experiment 1. However, feeding for both treatments was terminated at 10 d of age when 36 h of feed deprivation commenced. All birds were returned to full-feeding at Day 12. This feed restriction procedure was repeated on Day 17 when all birds were subjected to another 36-h feed withdrawal followed again by ad libitum feeding. Experiment 1 The purpose of this experiment was to evaluate the effect of early-life feed restric-

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Hurwitz, 1985; Plavnik et al, 1986). Success in reducing fat deposition, however, was not achieved by several workers that used various feeding strategies (Griffiths et al, 1977; Summers et al, 1990; Yu et al, 1990). Improvement in feed efficiency through feed restriction has been attributed in part to higher metabolic efficiency associated with maintaining a smaller body and to lower metabolic rate during early growth (Dickerson, 1978). Calorimetric studies conducted with broilers so far have shown no conclusive evidence of the role of metabolic heat production during and after early feed restriction on body composition and growth rate (Jones and Farrell, 1992). Studies with other species showed higher fasting heat production (FHP) and maintenance energy (Em) during refeeding than previously recorded during feed restriction, which was associated with 50% heavier weights for metabolically active tissues such as small intestine, pancreas, and liver (Koong et al, 1982). In studies with broiler breeders, Spratt et al. (1990) reported that the gut, liver, and reproductive tracts, which make u p only 4% of body weight, account for 30% of total energy expenditure.

FEED RESTRICTION AND ENERGY METABOLISM IN BROILERS

Experiment 2 The purpose of this experiment was to evaluate the effect of early-life feed restriction on 36-h FHP of male broilers during restriction and subsequent realimentation. The same indirect open system calorimetry was used as in Experiment 1. Sixty male broiler chicks of commercial strain were hatched in three groups of 20 chicks each, as described in Experiment 1, although in this case hatches were separated by 48 h. Each h a t c h c o n s i s t e d of 20 chicks a n d represented a replicate for Treatments 1 and 2. The 2-d age difference between the replicates was designed to allow for measurements of 36-h FHP to be made for the three replicates of each treatment without introducing age as a factor. The housing and cage temperature were the same as in Experiment 1.

Respiration Chamber The respiration chamber consisted of a box (90 x 70 x 60 cm) with diametrically

JPetersime Incubator Co., Gettysburg, OH 45328. Fischer and Porter Co., Warminster, PA 18974. Gast MFG Corp., Benton Harbor, MI 49022. 4 Constant Temperature Control Ltd., Toronto, ON, Canada, M9L 1Y4. 2 3

opposed air inlet and outlet. The chamber was made air-tight by sealing all the joints with silicon caulking, followed by weather stripping tape. The top wall of the box was made of plexiglass, which allowed for general observation of the chicks inside the chamber without disturbing them or upsetting the equilibrium of the system. Outside air was supplied directly into the chamber through the inlet opening. A rotameter 2 was connected to the exit opening of the chamber to measure the air flow rate, which was regulated by a system p u m p 3 on the other side of the rotameter. The respiration chamber was situated inside a controlled environment chamber, 4 and this enabled the temperature to be maintained between 25 to 30 C when calorimetric measurements were being taken.

Measurement of Metabolic Heat Production by Indirect Calorimetry In both experiments, each replicate group of chicks was placed into the respiration chamber for 3 h at 5 d of age to familiarize the chicks with the chamber. In Experiment 1, chicks were put into the chamber for measurements of metabolic heat production on Days 8 and 12 during feed restriction and on Days 15 and 19 during realimentation. N o data were collected during the first 1 h when chicks were allowed to acclimate and the system to equilibrate. During the following 3 h, percentage oxygen and carbon dioxide of the exit air were measured for 10 min at 1-h intervals. Body weight was measured each time chicks were put into or taken out of the respiration chamber. Chicks in the feed restriction treatment were given feed after coming out of the chamber on Days 8 and 12. In Experiment 2, chicks were put into the respiration chamber for measurement of their 36 h FHP at 10 and 17 d of age. Calorimetry for Experiment 2 was done as described for Experiment 1. Percentage oxygen and carbon dioxide, however, were measured for 2 h following the 1-h acclimation period. These observations were repeated for the 6, 12, 24, and 36 h of feed withdrawal for Replicate 1 of both Treatments 1 and 2. The same processes were carried out for Replicates 2 and 3 of both

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tion on heat production during restriction and subsequent refeeding. An indirect open circuit calorimeter was used as described below. Sixty male broiler chicks of commercial strain were hatched on 3 successive d. Each hatch consisted of 20 chicks with 10 chicks randomly allocated to each of two treatment groups. The three hatches represented three replicates. The 1-d age difference between the replicates was designed to allow measurements of metabolic heat production to be made for the three replicates of each treatment without introducing age as a factor. Each replicate of 10 chicks was housed in a cage in a Petersime brooder. 1 The cage temperature was maintained at 32.5 C from 0 to 5 d of age and then gradually reduced approximately .by .5 C / d down to about 24 C by 3 wk of age. Birds were weighed in groups every other day during the experiment.

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RESULTS Metabolizable Energy Intake, Body Weight Gain, and Digestive Organ Weights

Metabolizable energy intake, body weight gain, and efficiency of utilization of ME for weight gain by broiler chicks during restriction and realimentation are shown in Table 2. As anticipated, chicks that were subjected to 50% physical feed restriction gained less weight, at only 77 g per bird during the 6 d of restriction. Body weight

sServomex Ltd., Crowborough, Sussex, TN6 3DU, England.

gain over the 9 d of realimentation was similar for the two treatment groups. The efficiency of utilization of ME for weight gain was significantly better (P < .01) for the restricted birds compared with the control birds during realimentation. Weights of the digestive organs expressed as a percentage of body weight on 11 d of age were generally higher for the restricted birds compared with the full-fed birds, and especially the crop, proventriculus, and gizzard, for which the differences were significant (P < .01, Table 3). The weights of liver and pancreas, which were not different between the treatments during the time of feed restriction, subsequently showed significance after 5 d of realimentation (Table 3). Metabolic Heat Production

Metabolic heat production and respiratory quotients obtained from Experiment 1 are presented in Table 4. Daily heat production per bird during the restriction period was significantly (P < .01) higher for the full-fed birds compared to the feedrestricted birds. This difference was significant (P < .05) even on the 1st d that measurements were taken, when the feedrestricted birds had been on feed restriction for only 2 d. The metabolic measurements taken during the realimentation period also showed significantly higher heat production by the full-fed control birds compared with the feed-restricted birds. The results of the heat production per unit metabolic body weight (BW-67) showed lower heat production by the feed-restricted birds only during the restriction period. After 3 d of realimentation (15 d), the metabolic rates of the feed-restricted birds had risen to the same level as their full-fed counterparts. Measurements taken during the remaining periods of realimentation also showed no difference between the two treatments. Daily FHP per bird (Table 5) and per unit of metabolic weight (Figure la) during the restriction period are in both cases significantly (P < .01) higher for the full-fed birds than for the restricted-fed birds. Heat production, however, decreased with duration of feed deprivation in both full-fed and feed-restricted birds (Table 5). The decrease in heat production after 36 h feed depriva-

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treatments at their respective 10 and 17 d of age. At the end of the 36 h of feed withdrawal and measurements of oxygen and carbon dioxide contents of the exit air on both 11 and 18 d of age, five sample chicks from each replicate were killed by cervical dislocation. The different digestive organs were excised, blotted with filter paper, and weighed separately for each bird. The rates of oxygen consumption and carbon dioxide production were calculated from the air flow rate, and mean percentage oxygen, and carbon dioxide of the exit air as described by McLean and Tobin (1987). Oxygen and carbon dioxide contents of exit air were measured with paramagnetic5 and infrared analyzers,5 respectively. The air flow rate used ranged from 20 to 35 L/min, depending on the chicks' weight. An adequate air flow rate through the chamber was maintained to ensure that the percentage oxygen in the exit air did not drop by more than 1% from the composition of the inlet (outside) air. The equation of Brouwer (1965) was used to calculate metabolic heat production from oxygen consumed and carbon dioxide produced. No correction was made for methane production or uric acid nitrogen metabolism, because the error in not including these two terms in the calculation of heat production in birds is less than .2% (Romijn and Lokhorst, 1961). Statistical analysis of the data was carried out by Student's t test (SAS Institute, 1991).

218 ± 10 77 ± 3

6 to 12 d 636 ± 505 ±

(g per bird) 418 ± 12 428 ± 13 NS

6 to 21 d

12 to 21 d

Pancreas

Liver

*P < .05. **P < .01.

.93 ± .06 .97 ± .04 4.40 ± .23 6.19 ± .14 1.09 ± .04 .52 ± .03 4.10 ± .12 1.32 ± .09 1.34 ± .06 5.40 ± .25 6.19 ± .20 1.21 ± .08 .53 ± .03 4.13 ± .12 NS NS NS NS »* *

Large intestine

1. Full-fed 2. Restricted (6 to 12 d)

Gizzard

11 d

Small intestine

Crop

Proventriculus

Treatment

*P < .05. **P < .01.

6 to 21 d

(kcal ME per bird) 2,077 ± 19 2,737 ± 11 1,869 ± 26 2,189 ± 26

12 to 21 d

Body weight gain

.84 ± .02 1.08 ± .03

Crop

.80 ± .02 .81 ± .03 NS

Proventriculus

TABLE 3. Digestive organ weights of male broilers following a 36-h unfed period, pe

660 ± 7 320 ± 0

1. 2.

Full-fed Restricted (6 to 12 d)

6 to 12 d

Feed treatment

Energy intake

TABLE 2. Metabolizable energy intake and body weight gain of male broilers during and after early

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*

*

.72 ± .04

.86 ± .04

Respiratory quotient

76 + 6 53 ± 5

84 ± 4 57 + 4

**

90 + 5 59 ± 5

*

101 + 8 64 + 6

* if

24 h

Hours of deprivation starting on 10 d 12 h

1. 2.

*P < .05. **P < .01.

*

1.93 ± .03

2.68 ± .04

6 h

0 h

Full-fed Restricted (6 to 12 d)

*

71 + 10

132 ± 7

Heat Metabolic production 1 rate 2

12 d

*

128 ± 7

160 ± 7 NS

2.49 ± .0

2.60 ± .0

Heat Metaboli production 1 rate 2

15 d

*

70 + 4 49 ± 4

36 h

NS

165 + 13 147 + 7

0 h

TABLE 5. Fasting heat production (kilocalories per bird per day) by male 36-h feed deprivation during periods of early feed restriction and realim

treatment

**P < .01.

*

*

*

*

Kilocalories per gram of metabolic body weight.

•P < .05.

2

iKilocalories per bird per day.

(6 to 12 d)

.74 ± .03

.93 ± .03

Respiratory quotient

1.62 + .09

2.54 + .15

88 ± 8

Full-fed

Restricted

1.

2.

48 ± 4

Heat Metabolic production 1 rate 2

Feed treatment

8 d

TABLE 4. Mean + SD daily heat production, metabolic rate, and respiratory qu broilers during early feed restriction (6 to 12 d) and realimentation (12

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535

FEED RESTRICTION AND ENERGY METABOLISM IN BROILERS

supply during feed restriction. Results of Experiment 2 show a decline in RQ values during the feed restriction period in the case of full-fed birds from a high of .94 measured at the beginning of feed deprivation to a much lower value of .71 after 36 h of deprivation (Figure lc). This decline in RQ was not observed in case of feed-restricted birds whose RQ was consistently low, again likely related to fat catabolism. The RQ following 36 h feed deprivation during the realimentation period were, however, similar for the two treatments (Figure Id).

tion was greater in the full-fed birds at about 30%, compared with 24% for the restricted birds. The FHP per unit of metabolic weight during the restriction period also shows the same trend to decrease over 36 h of feed deprivation (Figure la). There was no difference in FHP per unit metabolic weight between the treatments over the 36 h of feed deprivation during the realimentation period when birds were 18 d of age (Figure lb). After 36 h of feed deprivation the heat production per bird decreased by about 20% from the initial level (at 0 h) in both treatments during realimentation. Respiratory quotients (RQ) measured during the feed restriction period in Experiment 1 show higher values for full-fed birds relative to their restricted counterparts, whereas the values for the two treatments were generally high during realimentation (Table 4). The full-fed birds had free access to feed; therefore dietary carbohydrate was likely their predominant energy source, whereas the feed-restricted birds likely metabolized more body fats for their energy

DISCUSSION

11 d

1 d

f\ IV V,

\\ Y

-^

~T"~T 12

18

24

30

36

6

12

18

24

30

36

HOURS OF

HOURS OF FASTING

FIGURE 1. Metabolic rate and respiratory quotients after 36-h feed removal of male broilers either full-fed (•) feed-restricted (A) from 6 to 12 d of age. Data represent mean ± SE of 10 broilers.

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Body weight gain by the feed-restricted birds during the restriction period was 77 g per bird, which represents only 35% of the weight gained by their full-fed counterparts over the same time period. Even though weight gain over the 9-d realimentation period was similar for the two treatments, it was about 170% of the body weight of the feed-restricted birds at the beginning of the realimentation (Day 13), but it represented only a 100% increase of

536

ZUBAIR AND LEESON

k

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the body weight of the full-fed birds at the minimal, also confirms lowered metabolic same age. Figure 2 shows the ME intake as rate due to feed restriction by birds in a unit of metabolic body weight during Treatment 2. A number of workers have both restriction and realimentation. Birds speculated on possible mechanisms to in Treatment 2 that were subjected to 50% explain the metabolic adaptation that alfeed restriction received enough ME to lows animals on reduced feed supply to meet their maintenance requirement dur- exhibit lowered metabolic rates, based ing restriction, based on maintenance mostly on evidence from studies with rats energy requirement of 1.5 kcal/g BW-*7 and humans. It has been shown that estimated for male broilers by Plavnik and energy restriction reduces serum triioHurwitz (1989). The ME intake by the dothyronine (T3) concentrations (Spauldrestricted birds above maintenance re- ing et al., 1976) and the activity of the quirement is minimal during the restric- sympathetic nervous system (Jung et al., tion period, leading to very little weight 1980). The relationship between energy gain during this period. This is also expenditure and these changes in symdemonstrated by their poor efficiency of pathetic nervous system and T3 concentrautilization (P < .01) of ME intake for tions have not been adequately explained. weight gain compared with the full-fed Results of daily heat production from birds during feed restriction (Table 2). Experiment 1 (Table 4), showed no differRestriction of feed intake by 50% ence between the two groups of birds resulted in about 40% lowered daily heat during realimentation, indicating that the production compared with the ad libitum lowered metabolic rates exhibited by the broilers. Similar results were reported by birds in Treatment 2 when undergoing Forsum et al. (1981) in rats and by Jones feed restriction did not carry over into the and Farrell (1992) in broilers. Other wor- refeeding period. This is in agreement kers reported that energy expenditure with the results of Jones and Farrell (1992), decreases during both short- and long- who also reported no difference in metaterm underfeeding in humans (Duncey, bolic rate between full-fed and feed1980; Webb and Abrams, 1983). Results of restricted broilers after only a few days of FHP during the feed restriction period refeeding. This is, however contrary to the obtained in Experiment 2 also show low- suggestion given by a number workers ered metabolic rate as a result of 50% feed that the energy and nutrients, which restriction. The FHP measured over 36 h support growth compensation come from of feed deprivation, where specific dy- reduction of maintenance requirements namic action (SDA) is expected to be during refeeding after a period of undernutrition (Wilson and Osbourn, 1960; Meyer and Clawson, 1964; Ashworth, 1969; Alden, 1970; Graham and Searle, 1975). Some workers have reported that the adaptation to energy restriction continues ^ ~ ~ _ ^ ^ even upon refeeding, which explains the m en 3 high efficiency of energy retention during refeeding (Harris and Martin, 1984; De Boer et al., 1986). Lower metabolic rate V-""* was not observed in the restricted-refed -._. — — — broilers in these experiments. Dickerson (1978) suggested that the improvement in feed efficiency during realimentation is Days due partly to improved metabolic efficiency associated with maintaining a FIGURE 2. Metabolizable energy intake (MEI) of male broilers either full-fed or feed-restricted from 6 smaller body. This mechanism is demonto 12 d of age. Data represent mean ± SE of 10 strated in these experiments by the ability broilers. • full-fed control; A feed-restricted (6 to 12 of the feed restricted birds to exhibit d); • maintenance energy requirement (Plavnik and higher ME intake/BW 67 during the Hurwitz, 1989).

FEED RESTRICTION AND ENERGY METABOLISM IN BROILERS

show higher feed efficiency and growth compensation, as proposed by many workers. Some of the nutrients that support growth compensation could possibly come from improved metabolic efficiency associated with maintaining a smaller body. The higher feed intake relative to body weight and its associated digestive adaptations exhibited by "restricted-refed" broilers indicates that these are contributing factors to growth compensation. ACKNOWLEDGMENTS This work was supported by the Ontario Ministry of Agriculture and Food, Toronto, ON and the Natural Sciences and Engineering Research Council of Canada; A. K. Zubair is supported by a Canadian Commonwealth Scholarship. REFERENCES Alden, W. G., 1970. The effect of nutritional deprivation on the subsequent productivity of sheep and cattle. Nutr. Abstr. Rev. 40:1167-1184. Ashworth, A., 1969. Growth rates in children recovering from protein-calorie malnutrition. Br. J. Nutr. 23:835-845. Ashworth, A., and D. J. Millward, 1986. Catch-up growth in children. Nutr. Rev. 44:157-163. Brouwer, E., 1965. Report of sub-committee on constants and factors. Pages 441-443 in: Symposium on Energy Metabolism, Troon, Scotland. European Association of Animal Production. Publication Number 11. Academic Press, London, England. Dibner, J. J., and F. J. Ivey, 1990. Hepatic protein and amino-acid metabolism in poultry. Poultry Sci. 69:1188-1194. Dickerson, G. E., 1978. Animal size and efficiency: basic concepts. Anim. Prod. 27:367-379. Donaldson, W. E., 1990. Lipid metabolism in liver of chicks: Response to feeding. Poultry Sci. 69: 1183-1187. Duncey, M. J., 1980. Metabolic effect of altering the 24-h energy intake in man, using direct and indirect calorimetry. Br. J. Nutr. 43:257-269. De Boer, J. O., A.J.H. Van Es, L. A. Roovers, J.M.A. Van Raaij, and G.A.J. Hautvast, 1986. Adaptation of energy metabolism of overweight women to low-energy intake, studied with whole-body calorimeters. Am. J. Clin. Nutr. 44: 585-595. Forsum, E., P. E. Hillman, and M. C. Nesheim, 1981. Effect of energy restriction on total heat production, basal metabolic rate, and specific dynamic action of food in rats. J. Nutr. 111:1691-1697. Graham, N. McC, and T. W. Searle, 1975. Studies of weaned sheep during and after a period of weight stasis. I. Energy and nitrogen utilization. Aust. J. Agric. Res. 26:343-353.

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realimentation period (Figure 2). These data support the concept that energy required to support growth compensation during refeeding following a period of undernutrition can be attributed to increased feed intake relative to body size (Ashworth, 1969; Ashworth and Millward, 1986). The weights of pancreas and liver that were not different between treatment groups during the feed restriction period became significantly higher for the restricted birds compared with their ad libitum counterparts during realimentation. Rosebrough et al. (1986) feedrestricted male broiler chicks from 6 to 12 d of age and reported heavier weights of liver for the restricted birds compared with the controls on Days 14, 16, and 18. Male broilers subjected to feed restriction, as used in these current studies, exhibited very high rates of adipocyte hypertrophy (Zubair and Leeson, 1993). Apart from its very high rates of protein turnover (Dibner and Ivey, 1990), the liver is also the main site for lipid synthesis in avian species (Donaldson, 1990). Enlargement of the liver observed during refeeding in these experiments was probably an adaptation to enable the birds to exhibit a high rate of fat deposition. This is in agreement with the results of Rosebrough et al. (1986) and McMurtry et al. (1988), who showed over-shoot in the activities of hepatic lipogenic enzymes during refeeding, following their being suppressed by early feed restriction. After about 2 wk of refeeding, the activities of the enzymes declined to levels lower than seen in full-fed control birds. This lower lipid metabolism in the older "restrictedrefed" birds probably explains why early nutrient restriction could result in lower carcass fat deposition at 7 or 8 wk (Plavnik and Hurwitz, 1985; Plavnik et al, 1986). Enlargement in the size of the pancreas observed during realimentation was probably a response to higher need for digestive enzymes due to increased feed intake relative to body size (Table 2). Results of these experiments showed lower metabolic rate by broilers subjected to early feed restriction. Lower metabolic rate during realimentation, however, does not play a role in the ability of the birds to

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