Changes in growth and blood parameters in heatstressed broiler chicks in response to dietary ascorbic acid

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Elsevier Science Publishers B.V., Amsterdam

Changes in growth and blood parameters in heat-

stressed broiler chicks in response to dietary

ascorbic acid

H.R. Kutlu and J.M. Forbes

Department of Animal Physiology and Nutrition, University of Leeds, UK

(Accepted 15 January 1993 )

ABSTRACT

Three experiments were conducted to determine whether supplemental ascorbic acid (AA) would alleviate heat-induced deterioration in performance and metabolism of broiler chicks. Day-old female broiler chicks were fed ad lib. on a standard starter feed under either thermoneutral (UH) or heated (H, thermoneutral, heated to 36°C for 6-10 hours per day) conditions to 4 weeks of age. In Experi- ments 1 and 3 the diet was supplemented with 0, 250, 500 or 1000 mg/kg AA. In Experiment 2, AA was given at 200 mg/kg. In Experiment l, which started at 7 days of age, heating significantly reduced body growth, feed intake and feed efficiency. AA supplementation, particularly at 250 mg/kg, stimulated growth, intake and efficiency in the H birds but had no significant effect under UH conditions. At both temperatures AA supplementation increased adrenal weight and plasma AA concentration at the end of the experiment. Heating depressed growth, intake, efficiency and adrenal weight in Exper- iment 2, starting at 1 day of age, while plasma AA levels were increased. AA supplementation increased growth, intake and efficiency of H birds while having no effect under UH conditions. In the third Experiment, starting at 7 days of age, heating reduced growth, intake, efficiency, thyroid weight, and levels of plasma protein, potassium and calcium while increasing body temperature, water intake and plasma glucose, cholesterol and sodium. Under H conditions, AA supplementation alleviated many of these effects, particularly when included at 250 mg/kg. No significant influence was associated with AA supplementation under UH conditions.

These data demonstrate that AA, particularly at 250 mg/kg, ameliorates the heat-induced deterioration in performance and metabolism of broiler chicks. AA supplementation of non-heat-stressed birds tends to reduce performance.

Key words: Broiler: Ascorbic acid; Heat stress; Adrenal gland

INTRODUCTION

It is well documented that the environmental temperature to which birds are exposed affects performance. High ambient temperatures depress feed intake, body growth and feed efficiency (Donkoh, 1989 ). As it is expensive to cool animal buildings, several methods have been examined to alleviate the

Correspondence to: Prof. J.M. Forbes, Department of Animal Physiology and Nutrition, Uni- versity of Leeds, Leeds, LS2 9JT, UK.

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336 H.R. K U T L U AND J.M. FORBES effect of high environmental temperature on broiler performance. One such method is the supplementation of the diet with ascorbic acid, because it is thought that ascorbic acid synthesis in birds is reduced during periods of heat stress (Sykes, 1978; Coates, 1984; Hornig et al., 1984), with depletion of ascorbic acid from the adrenals (Siegel, 1971) and reduced plasma ascorbic acid concentrations (Nockels et al., 1973 ). Deficiency ofascorbic acid occurs in other stressful situations such as high humidity, high productive rate and parasitic infestation (Hornig et al., 1984). These reports suggest that conditions may exist in which endogenous ascorbic acid synthesis by birds is insuf- ficient to meet physiological needs and that under such conditions supplementation with ascorbic acid could be beneficial.

It has been generally accepted, however, that supplementary ascorbic acid is unnecessary for poultry. Early work showed that chicks maintained on ascorbic acid-free diets, which were otherwise nutritionally adequate, displayed no adverse effects (e.g. Mitchell et al., 1923) and more recently Sifri et al.

( 1977 ) did not demonstrate any significant growth stimulation when ascorbic acid was added to poultry diets. However, growth stimulation was reported when ascorbic acid was added to highly purified diets (Briggs et al., 1944), nutritionally inadequate diets (March and Biely, 1953) and tradi- tional corn/soya based rations for turkeys (Dorr and Balloun, 1976) and chicks (Schmeling and Nockels, 1978 ).

Several studies have been conducted to determine whether supplemental ascorbic acid can improve performance of broiler chicks experiencing heat stress but the results have been variable. Addition of 1000 mg ascorbic acid per kg of diet reduced weight loss and mortality in birds up to 4 weeks of age following acute heat exposure (Pardue et al., 1985a). A positive response in body growth to supplemental ascorbic acid was observed at 100 mg/kg (Kafri and Cherry, 1984) or 200 mg/kg (Njoku, 1986 ). However, no improvements in weight gain or livability were reported in other studies when ascorbic acid was provided at levels ranging from 66 to 1000 mg per kg of feed (Subaschan- dran and Balloun, 1967; Freeman et al., 1983; Pardue et al., 1985b; Stilborn et al., 1988).

None of these studies described above used a consistent low-humid-high temperature treatment characteristic of heat episodes during part of the day in summer months in some countries of the world. Because of the conflicting results on broiler growth performance observed in these studies, the objectives of the present study were to determine whether supplemental ascorbic acid would alleviate heat-induced depression in the performance of broiler chicks up to 4 weeks of age under our laboratory conditions, which repro- duced consistent low humid-high temperature environment for part of the day. In addition, the influence of heat stress and/or supplemental ascorbic acid on plasma and adrenal ascorbic acid concentrations, adrenal weight, body temperature, and other blood constituents were evaluated. This work was a necessary preliminary to studies on self-selection of ascorbic acid supplemented and unsupplemented feeds (Kutlu and Forbes, 1992, 1993 ).

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G E N E R A L M E T H O D S

Female broiler chicks were obtained from Mayfield Chicks Ltd, Rosendale, Lancs. They were fed on standard starter crumbs (E.B. Bradshaw and Sons Ltd, Bell Mills, Driffield) ad lib. When ascorbic acid (AA, Coated ascorbic acid BP, chemically stabilised to retard oxidation, Roche Products Ltd, PO Box 8, Welwyn Garden City, AL7 3AY, UK) was added to the diet it was dissolved in 50 ml water, sprayed onto and mixed with each kg of food. Un- supplemented birds had normal food into which 50 ml/kg of water was mixed. Each food was prepared fresh daily. At the end of the day food analysis for ascorbic acid (Schuep et al., 1984) showed that actual ascorbic acid were 246, 489, 993 mg/kg in the 250, 500 and 1000 ppm AA supplemented diets, respectively, and 4 mg/kg in the unsupplemented diet

At the beginning of the experiment the chicks were weighed, individually numbered and divided into groups of equal mean weight. The groups were housed in cages of the middle and top tiers of three-tier battery blocks. Birds in the top cages were destined to be heated and those in the middle cages served as unheated controls. The top and middle tiers of the blocks were as- sumed to be equal except in temperature, as they were equally well illumi- nated and no difference was observed between the performance of birds housed in these tiers under the same temperature regime in other experiments (H.R. Kutlu and J.M. Forbes, unpublished results). Heat was provided for part of the day by electric infrared brooders, which were suspended above the cages. During the experiment the chicks were reared at conventional ambient temperature (from 30 reducing to 24 °C by 3 ° C/week) and 60-70% relative humidity, except during periods of heating when the temperature of the heated groups during heating fluctuated between 35-37 °C with a 30-40% relative humidity. This caused mild panting and spreading of the wings. Light was provided for 23 h each day and water was continuously available from nipple drinkers. Each bird was weighed at 1, 2, 3 and 4 weeks of age. Weights of feed issued were recorded and feed residues were weighed daily per group.

The data were analysed statistically using the ANOVA procedure of SAS (SAS, 1985 ) and means were separated using Duncan's New Multiple Range Test.

E X P E R I M E N T 1

Materials and methods

Ninety-six chicks were used in this study, in 8 groups of 12 birds each. A 4 × 2 factorial arrangement was employed with four levels (0, 250, 500 and 1000 ppm) of AA and heating vs. unheating as main effects. The heated groups were heated for 6 hours (10.00 to 16.00) per day. At the start of the experiment, when the birds were 1 week old, their mean body weight was 111 g (SE 4.66 ). At 4 weeks of age the birds were lightly anaesthetised, killed by decap-

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338 H.R. K U T L U AND J.M. FORBES

itation with a guillotine after 3 hours exposure to heat stress and samples of blood were taken, coagulation being prevented using heparinised tubes. After centrifugation, the plasma was collected and stored at - 2 0 ° C pending anal- ysis. The adrenal glands were dissected from each bird and weighed immedi- ately. The plasma samples were analysed for total ascorbic acid (ascorbic acid plus dehydroascorbic acid) using High Performance Liquid Chromatography

(Schuep et al., 1984). Results

The results are summarized in Table 1. At 4 weeks of age, the main effects of heating ( P < 0 . 0 0 1 ) and AA ( P < 0 . 0 1 ) on body growth were significant. 250 ppm AA supplementation during heat-stress stimulated body growth sig- nificantly (P < 0.05 ) compared with the unsupplemented heated birds. The heated group receiving 0 mg/kg AA exhibited significantly lower weight gain than its unheated counterpart. Amongst the unheated groups, that receiving 1000 mg/kg of AA gained significantly less weight than the controls (P < 0.05 ). Feed intake was recorded per group and therefore could not be analysed statistically. However, it was observed that feed intake was affected in a sim- ilar manner to body growth. Heated groups always consumed less than un- heated birds on the same level of AA throughout the experiment. At 4 weeks

TABLE 1

Effect of heating and supplemental ascorbic acid on performance, adrenal weight and plasma ascorbic acid concentrations of broiler chicks (Experiment 1 )

Supplemental ascorbic acid (mg/kg diet)

0 250 500 lO00

SED

Weight gain (g/21 days)

Heated 553b 640a 567b Unheated 688a 667a 665a Feed intake (g/21 days)*

Heated 1208 1334 1303

Unheated 1366 1333 1332

Efficiency (g weight gain/g food intake)

Heated 0.46 0.48 0.44 Unheated 0.50 0.50 0.50

Adrenal weight (mg/100 g body weight)

Heated 7.34b 7.96ab Unheated 7.43b 8.06ab

Plasma ascorbic acid (#g/ml)

Heated 12.8c 18.9bc 20.4bc Unheated 14.6bc 18.1 bc 23.2b 560b 558b 34.5 1241 1245 0.45 0.45 9.20ab 9.83a 8.38ab 8.65ab 0.86 35.1a 36.2a 4.52 SED: standard error of the difference between means.

a,b,c: means with different letters are significantly different (P < 0.05 ).

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of age, the heat-stressed group receiving 250 ppm AA consumed more than the other heated groups. However, under the unheated condition, the group that was fed with no supplemental AA showed greater feed intake than the groups receiving supplemental AA.

Feed efficiency, which was calculated by dividing body weight gain by feed intake was calculated per group due to the grouped determination of feed intake and therefore could not be analysed statistically. Feed efficiency was reduced by heating, the unheated groups always converting feed more effi- ciently than heated groups. Under heat stress, while 250 ppm AA supplementation increased feed efficiency, 500 or 1000 ppm had no positive effect. Un- der thermoneutral conditions AA supplementation at any level induced a small depression in feed efficiency.

There was a significant correlation between body weight at 4 weeks of age and adrenal weight ( r 2 = 0.97; P < 0.001 ) so relative adrenal weight (RADR

mg/100 g body weight) was calculated. Relative adrenal weight was significantly increased by AA supplementation (mg/kg) in a dose-related manner but there was no significant effect of heating.

Discussion

These results show that supplementation with AA provides significant al- leviation of the effects of heat stress on body growth, especially at 250 mg/kg. However, dietary supplementation with AA under thermoneutral conditions reduces weight gain, especially at the 1000 mg/kg level. Dietary supplemental AA increases plasma AA concentrations and the weight of the adrenal glands, relative to body weight, and these increases are dose dependent.

The objectives of this study were to determine whether 200 ppm supplemental AA would alleviate heat-induced depression in performance of broiler chicks and to evaluate the influence of heat stress and/or supplemental AA on adrenal weight and plasma and adrenal AA concentration. The experiment was replicated to enable statistical analyses of feed intakes.

Eighty chicks were used in 8 groups in an experiment of 2 × 2 factorial arrangement, with two replicates, including two levels (0 and 200 ppm) of AA and heating versus unheating as main effects. The mean weight at the start of the experiment at 1 day of age was 44.3 g (SE 1.64). The heated groups were heated for 6 hours ( 10.00hr to 16.00hr) per day. At the end of the experiment the birds were lightly anaesthetised with ether, killed by decapitation and samples of blood were taken. The adrenal glands were dissected from each bird and weighed and then immediately frozen in liquid nitrogen ( - 196 ° C ) .

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340 H.R. KUTLU AND J.M. FORBES

Plasma samples and adrenal glands were subsequently analysed for total ascorbic acid.

Results

The results are summarized in Table 2. At 4 weeks of age, there was a significant reduction in body growth due to heating (P < 0.05 ). AA supplementation significantly increased final body weight of the heated groups (P < 0.05 ) but had no effect on the unheated groups.

Feed intake was significantly affected by heating (negative, P < 0.001 ) and AA supplementation (positive, P < 0.001 ) and there was a significant heating by AA interaction ( ( P < 0 . 0 0 1 ) in which AA stimulated food intake under heat stress but had no significant effect in the unheated groups. Feed efficiency was not affected significantly by either heating or AA.

Heat stress reduced adrenal weight (P < 0.001 ) but there was no significant effect of AA supplementation. Heating significantly reduced adrenal ascorbic acid concentration in adrenal tissue ( P < 0 . 0 0 1 ) but supplementation increased the concentration under heat stress (P < 0.05 ). Plasma AA levels were increased by heating (P < 0.001 ) and by AA supplementation (P < 0.05 ).

Discussion

These results, as those for Experiment 1, show that supplemental AA has the potential to alleviate heat stress effects on body growth, while having no positive effect under thermoneutral conditions. Heat stress resulted in de-

TABLE 2

Effect of heating and supplemental ascorbic acid on performance adrenal weight, and adrenal and plasma ascorbic acid concentrations ( g / b i r d ) (Experiment 2)

Unheated Heated

Supplemental AA 0 200 0 200 SED Body weight gain (g/28 days)

875a 858ab 80 lb 869a 31.0 Feed intake (g/28 days)

1714a 1728a 1613b 1732a 37.5 Feed conversion efficiency (g weight gain/g food intake)

0.51a 0.49a 0.49a 0.50a 0.06 Adrenal weight (mg/100g body weight)

8.32a 8.34a 6.80b 6.99b 0.56 Adrenal ascorbic acid concentration (~g/g adrenal gland)

429a 442a 351 c 393b 13.1 Plasma ascorbic acid (/~g/ml)

13.8c 14.8bc 16.3b 19.2a 1.09 SED: standard error of the difference between means.

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creased adrenal gland weight and AA concentration. The reduction in adrenal weight can be attributed to exhaustion of the adrenals as a result of long-term heat stress and to depletion of cholesterol (Siegel, 1971 ) and AA (this experiment). The results also show that dietary AA increased plasma AA concentration as well as the AA concentration of the adrenal glands, suggesting reduced glucocorticoid synthesis as Kitabchi and West (1975) reported a negative correlation between adrenal AA concentration and the rate of steroidogenesis. Providing supplemental AA under stress conditions increases adrenal AA - concentration and reduces plasma corticosterone levels (Pardue et al., 1985a; Satterlee et al., 1989; Kutlu and Forbes, 1993 ). The reduced rate of corticosterone synthesis during heat stress in AA supplemented birds can result in improved performance, as observed in this experiment.

Heat stress increased plasma AA levels but this disagrees with the results of Experiment l, in which plasma AA concentration was slightly lowered by heat treatment. The difference may be due to the fact that in this experiment the birds were killed after half an hour of exposure to heat whereas in Experiment 1 three hours had elapsed since the onset of the daily heat period. Presumably there is a sudden increase in plasma AA in the initial period of heat stress due to AA depletion from the adrenals. In the later period the level drops below the pre-stress level, probably due to increased demand for AA or reduced bio- synthesis, because of heat stress.

In the first two experiments birds were subjected to a daily period of six hours heat stress and it was observed that supplemental AA had the potential to relieve the heat-induced depression in growth. The present study was ini- tiated (i) to determine whether dietary supplemental AA would alleviate heat- induced deterioration in performance and metabolism of broiler chicks exposed to longer periods (10 hours per day) of heat and (ii) to characterise the changes in body temperature, thyroid weight and blood constituents as affected by heating and/or supplemental AA.

Sixty-four chicks were used for a 3-week-period in this study, starting at 1 week of age. A 4 × 2 factorial arrangement was employed with four levels (0, 250, 500 and 1000 ppm (mg/kg)) of AA and heating vs. unheating as main effects.

The birds were divided into 16 groups, with equal mean weight ( 142 g, SE 4.1 ), so that each treatment had two replicates of 4 birds each. Heating was applied for ten hours (0900-1900 ) per day.

Water consumption was determined daily. Body temperature was measured weekly, between 1400 and 1600 hours, by inserting a digital clinical

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342 H.R. K U T L U AND J.M. FORBES thermometer 20 mm into the cloaca for 60 s and by placing on the skin of the abdomen for skin temperature.

The birds were killed after 3 h of heat exposure. After blood collection at the end of the experiment, the thyroid glands were dissected from each bird and weighed immediately. Plasma samples were analysed for total protein, glucose, cholesterol, calcium and phosphate using an automated spectropho- tometer (Cobas-Mira, Roche Diagnostic Systems, Welwyn Garden City, Herts). Sodium and potassium concentrations were determined using a flame photometer (Coming 400, Coming Ltd. Halstead, Essex).

Results

The results are summarized in Table 3. Body weight gain was significantly (P < 0.001 ) reduced by heating. Although increasing levels of AA in the diet induced a small depression in body growth of unheated chicks, under conditions of heat stress the group that received 250 mg/kg AA showed significantly (P < 0.05 ) greater body weight than the group receiving no supplemental AA.

Feed intake was also depressed significantly (P < 0.001 ) by heating and there was no effect of AA supplementation. Feed efficiency was significantly reduced by heating (P<0.01) and enhanced by AA supplementation

(P<0.05). A heating by AA interaction (P<0.01) was observed in which AA improved feed conversion under heat stress but reduced it in the unheated condition. The heated groups receiving no supplementary AA exhibited significantly lower feed efficiency than their unheated counterparts.

Heating significantly increased water consumption (P < 0.01 ). There was a significant heating by AA interaction (P < 0.001 ) whereby increasing AA in the diet reduced water intake under heat stress but increased water intake in the thermoneutral condition.

Heat exposure significantly increased rectal and skin temperature (P < 0.001 ), compared with unheated birds. Supplementation for heated birds significantly reduced their body temperatures.

Thyroid weight is expressed as rag/100 g body weight. Relative thyroid weight was reduced significantly (P < 0.001 ) by heating.

Total plasma protein was significantly depressed (P<0.01) by heating, particularly in the absence of supplemental AA. Blood glucose was significantly (P > 0.001 ) increased by heating, particularly in the absence of AA supplementation. Plasma cholesterol was significantly increased (P<0.05) by heating, especially in the absence of AA supplementation.

Heating significantly increased plasma sodium concentration (P < 0.001 ) while AA supplementation reduced it (P<0.01) in a dose-related manner. Plasma potassium and calcium concentrations were significantly (P>0.05) depressed by heating, only in the absence of dietary AA supplementation. There was a significant interaction between the effects of heating and supplementary AA on plasma calcium concentration (P < 0.01 ). Increasing levels

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TABLE 3

Effect of heating and supplemental ascorbic acid on performance and blood composition of broilers (Experiment 3 )

Supplemental ascorbic acid ( m g / k g diet )

0 250 500 1000

SED

Weight gain (g/bird/21 days)

Heated 717d 817abc 768bed 751 cd

Unheated 864a 850a 825ab 852a 34.00 Feed intake (g/bird/21 days)

Heated 1516c 1549bc 1493c 1484c

Unheated 1618ab 1614ab 1610ab 1643a 35.50 Feed conversion efficiency (g gain/g intake)

Heated 0.47c 0.53ab 0.52ab 0.51 b

Unheated 0.53a 0.52ab 0.52ab 0.52ab 0.01 Water Intake ( m l / b i r d / 2 1 days)

Heated 3417a 3038b 2914b 2874b Unheated 2563c 2853b 2860b 3104b 113.5 Rectal Temperature ( ° C) Heated 42.46a 42.23b 42.3 lb 42.32b Unheated 41.26c 41.32c 41.30c 41.34c 0.06 Skin Temperature ( ° C) Heated 42.46a 42.24b 42.32b 42.33b Unheated 41.24c 41.32c 41.30c 41.33c 0.06 Thyroid Weight (mg/100 g body weight)

Heated 8.09b 8.58b 8.68b 8.33b

Unheated 14.43a 15.27a 14.08a 14.05a 0.64 Plasma protein (g/100 ml)

Heated 2.98b 3.21 ab 3.21 ab 3.20ab

Unheated 3.30a 3.3 l a 3.30a 3.34a 0.13 Plasma glucose (mg/100 ml ) Heated 224.4a 209.9b 211.6b 208.4b Unheated 201.8b 199.7b 203.9b 205.3b 6.17 Plasma cholesterol (mg/100 ml) Heated 206.7a 188.2b 187.7b 188.7b Unheated 183.2b 180.6b 184.0b 183.9b 8.65 Plasma sodium ( m M )

Heated 146.9a 144.4ab 143.lab 140.9bc

Heated 141.0bc 138.5cd 137.0cd 134.6d 2.06 Plasma potassium ( m M )

Heated 8.13b 8.74a 8.66a 8.66a

Unheated 8.84a 8.76a 8.79a 8.67a 0.20 Plasma calcium ( m M )

Heated 2.17b 2.36ab 2.39a 2.4 l a

Unheated 2.46a 2.30ab 2.26ab 2.16b 0.09 Plasma phosphate ( m M )

Heated 2.45bc 2.75a 2.64ab 2.49bc

Unheated 2.51 bc 2.48bc 2.45bc 2.42c 0. l 0 SED: standard error of the difference between means.

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344 H.R. K U T L U AND J.M. FORBES of AA resulted in elevated calcium during heat stress, while calcium levels fell in the non-heated environment. Plasma phosphorus concentration was significantly increased by heating (P < 0.05 ) and AA supplementation ( P < 0.05 ) at 250 mg/kg in birds exposed to elevated ambient temperature.

Discussion

Body growth, feed intake and feed efficiency were depressed by heat exposure in a similar manner to Experiments 1 and 2 and were alleviated by AA supplementation, especially at 250 mg/kg.

Water intake was markedly affected by heating. This is simply a reflection of the use of vaporization of water from the body surfaces to increase heat loss by insensible means when sensible heat loss becomes more difficult in the hot environment. AA reduced water intake which might suggest that evapo- ration was less in supplemented heat-stressed birds when compared with those on the unsupplemented diet. However, increasing AA for unheated birds re- suited in increased water consumption suggesting that it may cause a thermogenic effect.

Rectal and skin temperatures were markedly increased by heating and AA alleviated this. Panting was observed in heated birds during the heating period of the day, but it was also noted that heated birds fed with supplemental AA exhibited relatively less panting than birds receiving no supplemental AA. Thyroid weight was depressed by heat treatment, as expected, in view of its role in the control of heat production via thyroxine and triiodothyronine

(Haddad and Mashaly, 1989). In the present study, AA supplementation had no significant effect on thyroid weight. However, Abdel-Wahab et al. ( 1975 ) reported that adding 50 or 100 mg/kg AA to the diet of chicks under high environmental temperature increased thyroid activity as measured by uptake of ~25I. Increased thyroid weights from feeding AA to unheated chicks have been reported by March and Biely ( 1953 ) and Schmeling and Nockels ( 1978 ). However, May and McNaughton (1980) reported that supplemental AA (1000 mg/kg diet) did not affect thyroid hormone secretion and thyroid weight. The influence of AA on thyroid weight or activity under either hot or thermoneutral conditions therefore remains uncertain.

Heating reduced plasma protein concentration while markedly increasing blood glucose level, confirming the observations of Donkoh ( 1989 ). The el- evation in glucose may be attributed to increased glucocorticoid secretion which increases glucogenesis (Webster, 1983 ). Supplementation with AA reversed these changes, presumably by reducing the secretion of glucocorticoids. In further work under the same conditions as those used in the experiments reported here, we have found increased plasma corticosterone concentrations due to heat stress to be reversed by AA supplementation at 200 ppm (Kutlu and Forbes, 1993).

A significant rise in plasma cholesterol was also detected in heated chicks receiving no supplemental AA. A deficiency of AA, which is necessary for the

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oxidation step in the conversion of cholesterol to bile acids, leads to a build- up of cholesterol in the blood (Ginter, 1974, 1978, 1979 ).

Elevated plasma sodium and reduced potassium concentrations were also associated with heating, possibly in response to changes in mineralocorticoid secretion in parallel with the elevated glucocorticoids. AA supplementation reduced sodium levels, perhaps due to increased cation excretion by increased urinary excretion ofAA (Lewin, 1976) and/or reduced mineralocorticoid synthesis associated with the AA-induced reduction in glucocorticoid secretion. Potassium is excreted in greater quantities during the alkalosis induced by panting (Smitt and Teeter, 1987), and again the effect of heating on plasma potassium was reversed by AA.

Conrad (1939) demonstrated that the calcium-carrying capacity of the hen's blood was decreased by high environmental temperature and the present re- suits show that this also occurs in broilers. It is likely that organic acids pro- duced during thermal panting complex with calcium (Allen and Somjen, 1983) and the reversal with AA supplementation may be attributed to the reduction in panting.

Plasma phosphate concentration was not affected by heating, but AA supplementation at the 250 mg/kg level increased it under heat stress while 1000 mg/kg depressed it under thermoneutral conditions.

In summary, the results of this experiment showed that supplemental AA completely or partly reversed the effects of heat stress on most of the parameters measured.

G E N E R A L D I S C U S S I O N

The effects of high temperature obtained in this study, with respect to body growth, feed intake and feed efficiency, are in agreement with those reported by many authors (e.g. Donkoh, 1989). Modem fast-growing broiler chicks must consume large quantities of feed in order to attain maximal growth rate. However, the intake and metabolism of feed have a thermogenic effect and at high ambient temperatures this heat increment aggravates the problem by adding more heat to an already-heat-stressed system. The bird, therefore, reacts by lowering its voluntary feed intake and its metabolic rate. This resuRs in poorer feed conversion efficiency and body growth. Under these circum- stances, supplemental AA offers the potential to stimulate body growth and feed efficiency.

In our experimental conditions, significant stimulation of growth and im- provement in feed efficiency were observed in heat-stress but not thermoneutral conditions with the 250 mg/kg AA supplemented diet.

Our results with respect to the optimum level of dietary AA supplementation under heat stress are not fully in agreement with the findings of Pardue et al. (1985a), but similar observations in terms of better efficiency with a low level of AA supplementation under heat stress have been made by Kafri

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3 4 6 H.R. KUTLU AND J.M. FORBES

and Cherry (1984) and Njoku ( 1986 ). It is apparent that dietary AA supplementation offers a feasible way to reduce the losses in body growth and feed efficiency due to heat stress.

A likely mechanism for the protective effects of AA is its inhibitory effect on adrenal steroidogenesis, as supplemental AA reduces plasma glucocorticoids (Pardue et al., 1985a; Sattedee et al., 1989; Kutlu and Forbes, 1993). Heat stress increases steroidogenesis which adversely affects tissues and metabolism (Webster, 1983 ). The adrenal glands synthesize the adrenal-related steroid hormones and contain the highest concentration of AA amongst body tissues (McDowell, 1989). The two adrenal factors, corticosterone which is considered to be major steroid hormone of the adrenal glands, and AA have always been closely associated. During stress, AA is secreted into the periph- eral circulation by the adrenals before release of corticosterone (Frankel, 1970). This led to speculation that AA may regulate steroidogenesis. Shi- mazu (1970) reported that a high concentration of AA inhibits adrenal corticosterone synthesis whereas a low concentration of AA stimulates synthesis. Earlier Kitabchi (1967) provided some understanding of the regulation of steroidogenesis by AA in beef adrenals by demonstrating that AA inhibits the

11-hydroxylation

and 21-hydroxylation steps of the steroid pathway. Pardue and Thaxton (1984) postulated that the amelioration of steroid-mediated immunosuppression by AA was partially due to protection of immunobiol- ogical tissues from the cytotoxic effects of adrenal steroids. So, it seems that under heat stress, increasing AA in the diet prevents the negative influence of corticosteroid hormones by reducing their synthesis and this results in improved performance of heat-stressed birds.

Our finding of the influence of AA on body temperature confirms other studies; Thornton ( 1962 ) reported that supplementing the diet with 44 nag/ kg AA did not affect body temperature of laying hens within the thermoneutral range, but it induced significantly lower body temperature when the birds were maintained under 37.8°C. Similar results under high environmental temperature were observed by Abroad et al. ( 1967 ), Lyle and Moreng ( 1968 ) and Atria (1976). The beneficial effect of supplemental AA in the maintenance of body temperature under cold stress conditions has also been reported; Thornton (1962) demonstrated that the body temperature of birds receiving supplemental AA was not affected significantly by low environmental temperature (7.8 °C), while a significant reduction in the body temperature of unsupplemented hens was observed. The results of these studies suggest that AA is of benefit in the maintenance of body temperature in birds exposed to either high or low environmental temperatures.

As presented in the Introduction, previous studies described variable results in terms of growth stimulation when AA was added to diets of broiler chicks under heat stress; one variable to take into account is the degree of stress in the control group.

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cording to the exact conditions under which the trials are conducted, and the deleterious effect of over-supplementation, makes it difficult to advise what levels should be used in any particular practical situation. In view of the abil- ity of chickens to select proportions of two feeds, one of which is deficient, the other over-abundant in an essential nutrient (Rose and Kyriazakis, 1991 ), we have trained broiler chicks to recognise the difference between feeds with and without AA supplementation and found that they choose more of the supplemented food when heat-stressed and more of the unsupplemented food when kept within their thermoneutral zone (Kutlu and Forbes, 1993 ). Thus, it may be possible to let the birds regulate their own AA intake in order to achieve optimum growth.

In conclusion, under the conditions of these experiments heat stress, applied for 6-10 hours per day, such as might occur in hot countries, depressed growth and food intake of broiler chicks and this was alleviated by AA supplementation of the diet, especially at 200-250 mg/kg of food. The changes in adrenal weight, adrenal AA concentration and various blood constituents are consistent with an inhibitory effect of AA on adrenal steroidogenesis. However, adding AA to the diet of broilers under thermoneutral conditions induces a small depression in performance in our experiments and those of Kafri and Cherry (1984). These results paved the way for studies of the abil- ity of chicks to select suitable amounts of AA when given a choice between supplemented and unsupplemented feeds (Kutlu and Forbes, 1992, 1993 ).

ACKNOWLEDGEMENTS

The authors are grateful to Mr. M. Putnam of Roche Products Ltd for gifts of protected ascorbic acid, to Mrs. S. Eaton and her staff for assistance with the management of the chicks and to Mrs. R. Prentice for technical assistance. HRK was on study leave from the University of Cukurova, Turkey.

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Ahmad, M.M., Moreng, R.E. and Muller, H.D., 1967. Breed responses in body temperature to elevated environmental temperature and ascorbic acid. Poultry Sci., 46: 6-15.

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Kutlu, H.R. and Forbes, J.M., 1993 Influence de l'acide ascorbique alimentaire sur la croissance et les param~tres sanguins chez le poulet de chair soumis tt un stress thermique; Livest. Prod.

Sci., 36: 335-350.

Trois exp6riences ont 6t6 r6alis6es pour d6terminer si la suppl6mentation en acide ascorbique (AA, 200/l 1000 mg/kg) permettait de r6duire la baisse des performances et les troubles du m6tabllisme induits par la chaleur chez le poulet de chair. Des poulettes de chair ~tg6es d'un jour 6taient nourries ad libitum avec un aliment de d6marrage standard. Elles 6taient plac6es dans des conditions de thermoneutralit6 (UH) oud de chauffage (H, thermoneutralit6, chauff6es 36 °C pendant 6-10 heures/jour) j usqu'~ l'~ge de 4 semaines. Dans l'exp6riences 1, qui d6butait

7 jours d'~ge, le chauffage diminuait la vitesse de croissance, la consommation d'aliment et l'efficacit6 alimentaire. La suppl6mentation en AA, an particulair it 250 mg/kg, am61iorait la croissance, la consommation et l'efficacit6 alimentaire chez les poulets H, mais n'avait aucun effet darts les conditions UH. Dans l'exp6rience 2 quie d6butait ~ 1 jour d'~ge, le chauffage

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r~duisait la croissance, la consommation, l'efficacit6 alimentaire et le poids des surr6nales, tan- dis que les niveaux plasmatiques d'AA augmentaient. La suppl6mentation en AA am61iorait la croissance, la consommation et l'efficacit6 alimentaire chez les poulets H, mais n'avait aucun effet dans les conditions UH. Dans la troisi~me exp6rience qui d6butait ~ 7 jours d'fige, le chauffage r6duisait la croissance, la consommation, l'efficacit6 alimentaire, le poids de la thyro'/de et les taux plasmatiques de prot6ines, de potassium et de calcium, mais augmentait la temp6rature corporelle, la consommation d'eau et les niveaux plasmatiques de glucose, de cholest6rol et de sodium. Dans les conditions H, la suppl6mentation en AA att6nuait plusieurs de ces cons6- quences. Ces r6sultats d6montrent que I'AA, en particular lorsqu'il est introduit au taux de 250 mg/kg, r6duit les baisses de performances et les troubles du m6tabolisme induits par la chaleur chez le poulet de chair.

KURZFASSUNG

Kutlu, H.R. und Forbes, J.M., 1993. Ver~inderungen des Wachstums und der Blutparameter nach Erglinzung des Futters mit Ascorbins~ure bei Broilerkiiken unter Hitzestress. Livest. Prod.

Sci., 36: 335-350.

Um festzustellen, ob Ascorbins~iure (AS, 200-1000 mg/kg) durch Hitzestress beeintr~ichi- tigte Leistung und StoffwechselvorgS.nge von Broilerkiiken aufheben kann, wurden drie Ver- suche durchgeftihrt. Weibliche Eintagskiiken wurden his zu einem Alter von 4 Wochen ad libi- turn mit einem Standard-Startefutter gef'tittert, und zwar, sowohl bei thermoneutraler (TN) als auch bei heiBer (T; Temperatur bis 36°C iiber 6-10 Stunden pro Tag) Umbegung. Im ersten Versuch, welcher bei einem Alter von 1 Woche anfing, fief der Hitzestress eine signifikant Beeintr~ichtigung yon K6rpergewicht, Futteraufnahme und Futteraufwand hervor. Die Erg~ng- zung mit AS, insbesondere von 250 mg/kg, erh/~hte Wachstum, Aufnahme und Effizienz in der T-Gruppe, zeigte aber keine signifikante Auswirkung in der TN-Gruppe. Unter heiBen Bedin- gungen waren im Versuch 2 (Beginn bei Eintagskiiken) Wachstum, Futteraufnahme und -verwertung sowie das Gewicht der Nebenniere reduziert, w~ihrend der Plasmaspiegel der AS zugen- ommen hatte. Die AS-Supplementierung verbesserte das Wachstum, die Futteraufnhame und den Futteraufwand bei heiBer Umgebung (T-Gruppe), nicht aber bei Thermoneutralitiit (TN- Gruppe). Im dritten Versuch (Ktiken bei Versuchsbeginn 1 Woche alt) reduzierte der Hitzes- tress Wachstum, Futteraufnahme und -verwertung, das Schilddriisengewicht und die Plasma- konzentrationen an Protein, Kalium und Kalzium, wiihrend die K/Srpertemperature, die Was- seraufnahme und die Plasmaspiegle an Glukose, Cholesterin und Natrium erh~ht wurden. In der T-Gruppe konnte die As-Supplementierung vide dieser Effekte aufheben. Diese Ergebnisse zeigen, dab Ascorbinsiiure (AS), insbesondere die 250 mg/kg Dosis, die durch Hitzestress aus- gelSste Verschlechterung der Leistung und des Metabolismus yon Broilerkiiken herabsetzten kann.