Self-selection of ascorbic acid in coloured foods by heat-stressed broiler chicks

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Physiology & Behavior,

Vol. 53, pp. 103-110, 1993 0031-9384/93 $6.00 + .00

Self-Selection of Ascorbic Acid in Coloured

Foods by Heat-Stressed Broiler Chicks

H. R. K U T L U A N D J. M. F O R B E S l

Department of Animal Physiology and Nutrition, University of Leeds, Leeds, LS2 9JT, UK

Received 24 F e b r u a r y 1992

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

Self-selection of ascorbic acid in coloured foods by heat-stressed broiler chicks.

PHYSIOL BEHAV 53(1) 103-110, 1993.--Female broiler chicks were given red and green supplemented (200 mg ascorbic acid (AA)/kg) and unsupplemented foods on alternate days for 8 days under heat-stressed and unstressed conditions. After this training period, the birds were given access to both foods, and the intake of each was monitored. In both heated treatments the intake of the supplemented food was significantly higher (p < 0.05) than of the unsupplemented food (800 vs. 518 g/14 days), although in the unheated groups significantly more of the unsupplemented food was eaten (756 vs. 639 g] 14 days). Birds were unable to select for AA when neither food was coloured, even when they were presented continuously in the same positions in the cage. It is concluded that chicks can learn to associate the colour of the food with its AA content and that they select proportions of supplemented and unsupplemented foods appropriate for their needs, as influenced by environmental temperature.

Diet selection Ascorbic a c i d Colour association Chickens Heat stress

IN birds, ascorbic acid (AA) synthesis has been reported to be inadequate in situations of stress such as high environmental temperature, humidity, high productive rate, and parasite infestation (6,10,23,27,29,30). Under heat stress conditions, dietary supplemental AA improves the depressed performance of broiler chicks (! 3,16,21,22). However, excessive supplementation of the diet can reduce performance, especially in the absence of stress (l 3,16), and the problem arises as to how to match the birds' requirements with the appropriate level of supplementation.

Several studies have shown that birds are able to choose a combination of the two foods which reflects their needs [see (24)]. The proposition as to whether chicks can differentiate between two foods varying in AA has not yet been tested. It may, therefore, be possible to offer chicks two foods, one of which is deficient and the other supplemented with AA, in order to allow them. to select proportions of the two to match their physiological reqmrements under stress or nonstress conditions. However, this selection would have to be related to the chicks' ability to associate some properties of the food with its AA supplementation, as AA included in the food at physiological levels is tasteless, odourless, and colourless.

Both young and mature chickens, in common with most birds, have a relatively well-developed colour vision [e.g., (14)]

but

this can be impaired by previously imposed discriminanda or environmental conditions (1,4,5,9). The fact that this ability does exist presents the opportunity to use colour as a natural stimulus for desirable responses in many behavioural patterns. For instance, by using coloured food or feeder or cage, food consumption could be increased (2,11,12,31). Food colour is

one of the properties which may be particularly useful in asso- ciating with AA supplementation.

If, following suitable training, chicks are able to associate supplemental AA with the colour of the food, that supplemented with AA would be expected to be consumed in greater amounts than the unsupplemented food under heat stress, and the reverse under nonstress conditions. Four experiments were carded out to determine whether broiler chicks, when receiving both AA supplemented and unsupplemented foods of different colours, associate AA supplementation with colour. The plasma corticosterone level was assessed in one experiment in order to determine whether supplemental ascorbic acid affects steroidogenesis.

GENERAL METHOD

Day-old female broiler chicks were obtained from Mayfield Chicks Ltd, Rosendale, Lanes. They were fed on standard starter crumbs (E. B. Bradshaw & Sons Ltd., Bell Mills, Driifield) ad lib. Coloured foods were prepared by mixing 10 ml of red or green food colouring (Gold Seal Liquid Colour, Clayton and Jowett Ltd., Runcorn, Cheshire) with 40 ml water and spraying onto 1 kg of the food which was then allowed to dry at room temperature. When 200 mg AA (L-AA BP, Roche Products Ltd., Welwyn Garden City, Herts) was added to 1 kg of the food, it was dissolved in 50 ml of tap water, sprayed onto and mixed with the food, in addition to the colouring agent. Thus, four foods were prepared: AA supplemented green food (G+), AA supplemented red food (R+), unsupplemented green food (G-),

Requests for reprints should be addressed to Prof. J. M. Forbes.

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TABLE 1

THE DESIGN OF EXPERIMENT 1 Training Period

Foods

Group Treatment (Given on Alternate Days) Number of Birds

Heated 1 G+/R 16

Heated 2 R+/G 16

Unheated 3 G+/R 16

Unheated 4 R+/G 16

Testing Period

Origin and Number of Birds Transferred From Group Treatment Food(s) Training Period

Heated I P- Trt. 1, 2 (4 + 4) Heated 2 P+ Trt. 1, 2 (4 + 4) Heated 3 G + / R - Trt. 1 (8) Heated 4 R+/G Trt. 2 (8) Unheated 5 P Trt. 3, 4 (4 + 4) Unheated 6 P+ Trt. 3, 4 (4 + 4) Unheated 7 G + / R - Trt. 3 (8) Unheated 8 R+/G- Trt. 4 (8)

and unsupplemented red food ( R - ) . Each food was freshly prepared every day throughout the experiment.

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 designed to be heated, and those in the middle cages served as unheated controls. Heat was provided for 10 h per day (0900- 1900 h) by electric infrared brooders, which were suspended above the cages; mean house temperature was 26°C except during periods of heating when it fluctuated between 35 and 37°C. Light was provided for 23 h each day and water was continuously available from nipple drinkers. Each bird was weighed at 1 week of age and weekly thereafter. Weights of food issued were recorded, and food residues were weighed daily per group. Food efficiency was determined for each treatment. The first 8 days was a training period during which each food of the pair was given to chicks alternately every other day in order to allow them to become accustomed to the colour and physiological effect of the foods.

The data were analysed statistically using the ANOVA pro- cedure of SAS (25) and means were separated using Duncan's New Multiple Range Test.

EXPERIMENT l Method

Sixty-four, 7-day-old chicks were used for 21 days in this experiment. They were divided into eight groups with equal mean weight (119 g, SE 4.9). The first 14 days of the experiment was a training period in which half the birds were heated and half unheated, and within each temperature treatment half were given R+ and G - on alternate days, the other half G + and R - .

At the end of the training period the chicks were regrouped for the testing period in which a 2 X 4 factorial arrangement was employed with two levels of heating and four foods or food combinations:

1. unsupplemented uncoloured food (P-), 2. AA supplemented uncoloured food (P+), 3. a choice between G + and R - , and 4. a choice between R+ and G .

In treatments 1 and 2, equal numbers of birds were taken from the two training food regimes. Each chick stayed on the same heat treatment throughout. The design of the experiment is given in Table 1.

Results

At the end of the training period, when the birds were 3 weeks old, body weight was significantly different (p < 0.05) between the unheated (592 g) and heated (536 g) treatments (SE 35.1), with no significant effect of feeding treatment (Table 2). During the testing period, heating continued to have a significant effect (p < 0.01) on body weight while feeding treatment had no effect (Table 2).

During the training period, neither heat nor food treatments affected food intake significantly, the mean intake being 959 g (SE 41.8) over the 2-week period.

Food intake during the testing period were depressed by heating (Table 2), but as the groups were not replicated, it is not possible to assess the statistical significance of this difference. Supplementation of uncoloured food with AA did not affect food intake under either the heated or unheated condition.

Diet selection seemed to be affected by heating, AA supplementation, and colour. Under the heated condition, the group that was given a choice between G + and R - consumed 62% G + and 38% R - , although the group that was offered a choice between R+ and G - consumed similar amounts of each food (50%:50%, respectively). The opposite pattern of this food preference was observed in the unheated condition: although the group receiving G + and R - consumed similar amounts of each food (51% and 49%, respectively), the group that was given a choice between R + and G - consumed 39% R + and 61% G - .

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SELF-SELECTION OF ASCORBIC ACID 105

TABLE 2

EFFECT OF HEATING, SUPPLEMENTAL AA AND COLOUR ON BODY WEIGHT AT THE BEGINNING AND END OF THE TESTING PERIOD

AND FOOD INTAKE (EXPERIMENT 1) Food or Food Combination

Group P- P+ G+/R- G-/R+ SED

Body weight (g/bird) at beginning of testing period

Heated 536 a 536 a 537 a 537 a

Unheated 591 ~ 592 ~ 59P 592 a

Body weight (g/bird) at end of testing period

Heated 823 b 825 b 840 b 836 "b

Unheated 945 ab 946 ~b 952 ~ 949 a

Food intake (g/bird/7 days) during testing period Heated Total 628 643 682 668 Sup/unsup 421/261 336/332 Percentage 62/38 50/50 Unheated Total 693 712 752 743 Sup/unsup 385/367 450/293 Percentage 51/49 61/39 52.4 79.9

ab Means in the same group with the same letter are not significantly different (p > 0.05). SED; standard error of the difference between means.

Discussion

These results suggested that birds might be able to associate AA supplementation with colour; although one of the heated groups preferred AA supplemented food to unsupplemented, one of the unheated groups showed a tendency to consume unsupplemented rather than supplemented.

EXPERIMENT 2

In the previous experiment, the association of AA with colour was strong when the food that the birds were expected to prefer was green, but not when it was red. It is, therefore, not clear whether it is AA or colour which is being preferred. In order to resolve this, the second experiment included additional treatments in which chicks were given pairs of foods differing only in colour. In addition, plasma corticosterone levels were mea-

TABLE 3

THE DESIGN OF EXPERIMENT 2

Group Treatment Foods

Heated 1 G - / R - Heated 2 G + / R - Heated 3 G - / R + Heated 4 R+/G+ Unheated 5 G - / R - Unheated 6 G + / R - Unheated 7 G - / R + Unheated 8 R+/G+

sured at the end of the experiment in view of the important role played by AA in corticosterone synthesis.

Method

Sixty-four, 7-day-old female broilers were used in this experiment which ended when the birds were 29 days old. At the beginning of the experiment the chicks were weighed and divided into eight groups of equal mean weight (128 g, SE 3.8). A 4 × 2 factorial arrangement was employed with four combinations of foods ( G - / R - ; G + / R - ; G - / R + ; G + / R + ) and heating vs. nonheating as main effects (Table 3).

The training stage lasted for 8 days, during which time each group received alternately AA supplemented and unsupplemented foods of different colours every other day. The testing stage lasted 14 days and both foods were offered simultaneously to examine food selection.

At the end of the experiment, birds were killed, after 3 h of heating, by decapitation with a guillotine between 1100 and 1300 h, and samples of blood for determination of corticosterone in plasma were taken. Plasma corticosterone levels were determined by radioimmunoassay (ICN Ltd, Lisle, IL 60532).

Results

During the training period, body weights of the chicks were not affected by either heating or supplemental AA and all groups achieved similar body weight (Table 4). During the testing period, heating had a significant negative influence on body growth (p < 0.001). The heated groups receiving both unsupplemented foods exhibited significantly lower body weight than their unheated counterparts (p < 0.05). The heated groups receiving one or both supplemented foods tended to have gained weight faster than the heated groups with both unsupplemented foods.

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"FABLE 4

EFFECT OF SUPPLEMENTAL ASCORBIC ACID AND COLOUR ON BODY WEIGH'l, FOOD INI AKE AND PREFERENCE IN THE TRAINING AND TESTING PERIODS (EXPERIMENT 2)

Freatment G- /R G + / R G /R ~ (i ~/R 4 SED

Body weight (g/bird) at end of training period

Heated 355.0" 350.8 ~ 367.9" 362.0 a

Unheated 367.3 a 354.4 ~ 374. I a 362.8 a 17.4

Heated 871.7 d 965.9 ~d 912.3 cd 984.2 ~b~

Unheated 1058.8 au 979.9 "b~a 1080.0 a 1009. Iab 47.9

Food intake (g/bird/8 days) during training period Heated Total 433 449 461 462 Sup/unsup 206/227 225/224 220/241 214/248 Percentage 48/52 50/50 48/52 46/54 Unheated Total 459 482 474 474 Sup/unsup 219/240 230/252 224/250 223/251 Percentage 48/52 48/48 47/53 47/53

Food intake (g/bird/t4 days) during testing period Heated Total 1254 1324 1309 1430 Sup/unsup 816/438 781/543 432/877 678/752 Percentage 65/35 59/41 33/67 47/53 Unheated Total 1502 1351 1535 1370 Sup/unsup 656/846 356/995 942/593 758/612 Percentage 44/56 26/74 61/39 55/45

Plasma corticosterone level (nm/ml)

Heated 2.99" 2.32 ab 2.11"~ 1.30 b~

Unheated 2.20 ~ 2.10 a~ 2.06 "~ 1.10 ¢ 0.48

a~ Means in the same group with the same letter are not significantly different (p > 0.05). SED, standard error of the difference between means.

Food intake could not be analysed statistically by reason of the grouped determination of food intake. In the training period there was no effect o f heat treatment (Table 4). In the heated groups, that which received both AA supplemented foods ( G + / R + ) tended to consume more food than the group offered both unsupplemented foods ( G - / R - ) .

Food preference was affected by heating, AA supplementation, and colour. While the heated group receiving G - / R - preferred green food to red food, its unheated counterpart ate m o r e red food than green food. The results, with respect to food preference, also showed that these natural preferences were altered by AA supplementation: heated treatment G + / R - preferred green food to red, although G - / R + chose red food in contrast to green food; both the chosen foods were AA supplemented.

In the nonheated groups, G + / R - preferred red food to green, whereas R + / G - ate more green food (Table 4). G + / R + ate more than G - / R - . These results suggest that the supplemented and unsupplemented foods were differentiated by the nonheated groups but that their AA requirement was lower than the heated groups.

Heat stress tended to elevate plasma corticostcrone levels (Table 4), although they were significantly reduced, compared

to control ( G - / R - ) , when both foods contained AA supplementation.

Discussion

Although AA supplementation increased body weight in the heated groups, no positive effect of AA on body weight was observed in the nonheated groups, in agreement with previous work under the same conditions (16).

It is apparent that under heat stress the chicks can differentiate AA supplemented and unsupplemented foods by means of their colour, as AA itself is tasteless at the low levels used here. How- ever, it will be necessary to replicate such experiments in order to demonstrate whether or not these trends are significant.

Heating tended to increase the plasma levels of corticosterone; a more definite result might have been obtained had the birds been killed closer to the start o f the heating period, as it has been shown that plasma AA levels are significantly increased o n e - h a l f h after the onset o f heating, but not after 3 h (16). W h e n both foods contained AA there was significant reduction in plasma corticosterone concentration, compared with levels in birds receiving no supplementation, even when these controls

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SELF-SELECTION OF ASCORBIC ACID 107

were not heat stressed. Despite this large difference in corticosterone level, there was no significant improvement in weight gain when both foods were supplemented, compared to one food being supplemented.

EXPERIMENT 3

In the previous experiment, chicks were given AA supplemented and unsupplemented foods of different colours. There was a tendency that, while the heated groups preferred AA supplemented food to unsupplemented food, the nonheated group selected unsupplemented food more than AA supplemented food. This suggests that the chicks were able to learn AA supplementation by means of colour, but this could not be subjected to statistical analysis due to lack of replication.

The present study used only choices between supplemented and unsupplemented foods but provided five replicates for each treatment in order to allow food intake data to be analysed statistically to find out with more certainty whether broiler chicks are able to associate AA supplementation and colour significantly under heat stress or thermoneutral conditions.

Method

Eighty, 7-day-old female broiler chicks were used in this experiment which ended when the birds reached 29 days of age. They were weighed and divided into 20 groups of similar mean weight (124 g SE 2.2). A 2 × 2 factorial arrangement was employed with two different food combinations ( G + / R - and G - / R + ) and heating vs. nonheating as main effects. Each treatment was replicated five times, each subgroup comprising four chicks. The AA supplemented foods contained 200 mg/kg.

The training period, in which each group received AA supplemented and unsupplemented food with different colours on alternate days, ran from 7 to 15 days of age. The testing period lasted 14 days, during which both supplemented and unsupplemented foods were offered. Each group of birds received the same foods and heat treatments throughout, the foods being alternated during the training period and offered free choice in the testing period. The design of the experiment is given in Ta- ble 5.

In the last week of the testing period, all the chicks were subjected individually to a preference test in which each bird was fasted for 2 hours and then placed in a mesh-floored cage (250 m m high, 350 m m wide, and 450 m m deep) at the opposite end to two small containers, one containing green food, the other red. The time to first peck was recorded; if a chick had not approached either food container within 2 min, no result was recorded. I f a choice was towards the colour which, for that particular bird was associated with AA supplementation, the time was recorded as positive; if the choice was for the unsupplemented colour, the time was recorded as negative. The re- ciprocal of the time was then calculated to give a value repre- senting the speed with which a choice was made and the resulting data were subjected to t-test.

Results

During the training period, body weight gains were not affected by heat or dietary treatment (Table 6). During the testing period, heating had a negative effect on weight gain, and at the end of the experiment the heated birds were significantly lighter than those that were not heated (p < 0.001). Food intake was reduced by heating by 9% (NS).

During the training period there was no difference between the intakes of the two foods given on alternate days. However,

TABLE 5

THE DESIGN OF EXPERIMENT 3 Groups Treatments Foods Heated 1 G + / R - Heated 2 G - / R + Unheated 3 G + / R - Unheated 4 G - / R +

during the testing period there was significant selection, heated birds eating more of the AA-supplemented food, irrespective of its colour (p < 0.05), and unheated birds eating more of the unsupplemented food (significant in the case of R + / G - , p < 0.05).

In the short observation tests carried out in the last week of the testing period, 26 of the heated chicks selected the colour associated with AA supplementation, 11 chose the other colour, this being significantly different from random (p < 0.05), and 3 made no choice within the 2 min allowed. Of the unheated birds, 13 chose the colour associated with the supplemented food, 23 the other colour (not significant), and 4 made no choice. During the training period, food conversion efficiency was not significantly affected by treatment (Table 6). During the testing period, heating reduced efficiency (p < 0.001) compared with the nonheated groups.

Discussion

The results obtained in this study confirm statistically, in three comparisons out of four, the findings of the previous experiment, i.e., that chicks can associate AA supplementation with colour and prefer AA supplemented food to unsupplemented food when heat stressed, and vice versa when not heated.

EXPERIMENT 4

The previous experiments showed that chicks can differentiate between foods of different colours in such a way as to suggest that they have learned to associate colour with AA supplementation. However, it is possible that they could do this in the absence of the colour difference, by means of taste and/or smell and/or location, i.e., left or right. This is tested in this experiment by the inclusion of treatments in which choices of supplemented and unsupplemented foods, both uncoloured, were offered, as well as coloured choices as in the previous experiments. Method

Sixty-four, 7-day-old female broiler chicks, weighing 142 g (SE 5.0) were divided into 16 groups and used for 22 days in this experiment. A 4 × 2 factorial arrangement was employed with four food pairs, which were AA supplemented uncoloured food (P+) and unsupplemented uncoloured food ( P - ) in either a) fixed position or b) alternated position, c) G + R - , and d) R + / G - , with heating vs. nonheating as main effects (Ta- ble 7).

The experiment was divided into two stages. The first stage was an 8-day training period in which each food of the pair was given to chicks on alternate days in order to allow them to become accustomed to the position or colour and physiological effect of the foods. The chicks were then given access to both foods of a pair, which were offered in two troughs side by side (right and left) at the front of the cage. In order to determine a

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T A B L E 6

EFFECT OF FOOD COMBINATION AND HEAT S'I RESS ON BODY WEIGH1. FOOD INTAKE, FOOD CHOICE, AND FOOD CONVERSION

EFFICIENCY (EXPERIMENT 3) I~oods Heated Unheated

Treatment G + / R - R + / G G + / R - R + / G SED

Body weight (g/bird) at end

of training period 323 a 335 a 328 a 314 ~ 1 1.9 Body weight (g/bird) at end

of testing period 825 b 808 ~ 930" 903 a 29.8 Food intake (g/bird/8 days)

during training period

Total 407 a 413 a 397 a 378 a I 1.4

Sup/unsup 202/205 205/208 197/200 185/194

Percentage 50/50 50/50 50/50 49/51

Food intake (g/bird/14 days) during testing period

Total 1114 a 1055 a 1202 a 1163" 69.9

Sup/unsup 658/456* 591/465" 575/627 520/643*

Percentage 59/41 56/44 48/52 45/55

Food conversion efficiency

during training period 0.49 ~ 0.51 a 0.52 a 0.50 a 0.017 Food conversion efficiency

during testing period 0.46 b 0.46 ~ 0.50 a 0.50 a 0.012 ab Means with the same letter in the same row are not significantly different (p > 0.05). SED; standard error of the difference between means.

* Significant difference between intakes of the two foods on offer.

possible effect of food position on the association of AA supplementation, while food treatment a) had AA supplemented uncoloured food always on the right side, and unsupplemented uncoioured food on the left, in treatment b) the location of AA supplemented and unsupplemented uncoloured foods were ex- changed on alternate days.

Results

During the training period, heating and feeding treatments did not significantly affect body weight. In the testing period, heating had a significant (p < 0.001) depressing effect on body growth. Food pair did not significantly influence body growth under either heated or unheated condition (Table 8).

During the training period there was no effect on total food intake and food preference. Neither the heated groups nor the

T A B L E 7

THE DESIGN OF EXPERIMENT 4 Group Treatment Foods

Heated 1 P + / P - (fixed) Heated 2 P - / P + (alternate) Heated 3 G + / R - Heated 4 R + / G - Unheated 5 P + / P - (fixed) Unheated 6 P - / P + (alternate) Unheated 7 G + / R - Unheated 8 R + / G -

unheated groups exhibited a choice between AA supplemented food and unsupplemented food. In the testing period, heating reduced total food intake significantly (p < 0.001). Type of food pair (uncoloured or coloured) did not affect total food intake.

During the testing period there was no significant preference observed between AA supplemented and unsupplemented uncoloured food with either fixed location or alternated location in either the heated or unheated condition (Table 8). However, when the foods were labelled with colour, the heated groups selected AA supplemented food against unsupplemented food significantly (p < 0.05), while the unheated groups preferred unsupplemented food to AA supplemented food (p < 0.05). Discussion

U n d e r the conditions of this experiment, AA supplementation was not associated with food location, even when this was fixed throughout the experiment, showing that the taste or other char- acteristics of the supplementary AA were not detected by the birds. W h e n the foods were coloured they were able to differentiate between supplemented and unsupplemented foods whether in the heated or unheated condition, as in the previous experiments. The results, again, suggest that chicks under heat stress require supplemental AA, but that they avoid it under unheated conditions.

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

It is well d o c u m e n t e d that high environmental temperature depresses broiler performance [e.g., (7)]. The depression under heat stress is mostly attributed to lowered voluntary feed intake and metabolic rate o f birds. It has also been reported that high

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S E L F - S E L E C T I O N O F A S C O R B I C A C I D 109

T A B L E 8

EFFECT OF HEATING AND TYPE OF FOOD PAIR ON BODY GROWTH AND FOOD INTAKE (EXPERIMENT 4)

P+/P- P-/P+

Food Pairs (fixed) (alternate) G+/R- R+/G- SED Body weight (g/bird) at end

of training period

Heated 356 a 372 a 352 a 364a

Unheated 374 a 382 a 376 a 377 a 14.1

Heated 997 b 995 b 999 b 998 b

Unheated 1076 ab 1121 a 1071 ab 1102 a 42.0

Food intake (g/bird/8 days) in the training period Heated Total 375 a 368 a 386 a 365 ~ Sup/unsup 190/185 188/180 195/191 184/181 Percentage 51/49 51/49 51/49 50/50 Unheated Total 377 a 359 ~ 367 a 390 a 20.8 Sup/unsup 188/189 180/179 185/182 196/194 Percentage 50/50 50/50 50/50 50/50

Food intake (g/bird/14 days) in the testing period Heated

Total 1281 de 1263 e 1316 ~de 1319 °de

Sup/unsup 647/634 667/596 799/517* 800/519" Percentage 51/49 53/47 61/39 61/39 Unheated Total 1344 t~ 1396 ab 1371 a~ 1419 a 30.5 Sup/unsup 656/688 694/702 637/734* 640/779* Percentage 49/51 50/50 46/54 45/55

a.b,c,d,e Means with the different letter are significantly different (p < .05). SED; standard error of the difference between means.

* Significant difference between intakes of the two foods on offer.

environmental temperature is associated with increased levels of plasma corticosterone, which induces adverse changes in metabolism (28). These result in poorer feed conversion efficiency and body growth. Several c o m p o u n d s have been examined to alleviate the effect o f high environmental temperature on broiler performance, one such being ascorbic acid, and it is thought that ascorbic acid synthesis in birds is reduced during periods of heat stress (6). However, the literature concerning supplemental ascorbic acid and alleviation of heat stress effects on broiler performance has elicited inconsistent and conflicting findings. In parallel work carried out under the same conditions as the experiments whose results are reported here, AA supplementation at levels similar to those used here alleviated m u c h o f the deleterious effects o f heating on growth, food intake, and metabolism, while the same level o f inclusion of A A for nonheated birds depresses growth and intake and increases body temperature and water intake (16). Thus, heating induces a re- q u i r e m e n t for dietary A A in chicks which are normally thought o f as not requiring supplemental A A (6,20). The experiments currently reported show that this requirement for AA, in order to achieve optimal growth, can be expressed as an appetite for AA, as long as the birds are given appropriate training to associate cues, in this case colours, with metabolic effects o f ingestion. Thus, the uncertainty of how m u c h A A to put into foods for

stressed birds can be o v e r c o m e by allowing t h e m to select their own intake.

Several other appetites have been demonstrated in poultry, and it is likely that these would be exhibited more quickly and more consistently by prior training using alternate exposure to the two foods which are to be given to choice (18). As far as self- selection of ascorbic acid is concerned, a further study under the same conditions as the experiments reported here showed that broiler chicks significantly associate colour cues with the ascorbic acid content of the diet within 3 days o f a change in environmental temperature (17). However, it is likely that the alteration to current practice involved in feeding, firstly, different foods on alternate days and secondly, a choice o f two foods, will prevent choice feeding from being widely used, at least in the near future.

The mechanisms whereby AA alleviates heat stress are dis- cussed by Kutlu and Forbes (l 6), who supported the hypothesis that was made by Kitabchi and West (15) that ascorbic acid inhibits steroidogenesis. Heat stress tended to increase corticosterone levels in Experiment 2, although the effect was small in this case, reflecting the relatively mild nature o f the stressor ap- plied. AA supplementation tended to reduce plasma corticosterone levels, and at the highest level o f supplementation they were significantly lower than in unsupplemented birds under

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b o t h t e m p e r a t u r e regimes, it m a y seem paradoxical t h a t AA seemed to alleviate stress, a l t h o u g h at the same t i m e reducing corticosterone levels when corticosterone is considered to protect a n i m a l s from stress. However, a l t h o u g h s h o r t - t e r m rises in corticosterone secretion are protective, w h e n the levels are c o n t i n - uously high they a p p a r e n t l y b e c o m e deleterious, a n d p l a s m a corticosterone c a n n o t be used as a reliable indicator o f the degree of stress to which birds are exposed.

As the responses to stressors o t h e r t h a n heat are t h o u g h t to have a similar basis, there is every reason to believe t h a t AA supplementation, the level being d e t e r m i n e d by choice-fed birds, could be useful in a wide range o f stressful situations, e.g., high h u m i d i t y , high p r o d u c t i v e rate, a n d parasitic infestation (see the I n t r o d u c t i o n section) as well as highly purified diets (19), nu-

tritionally inadequate diets (16), and traditional corn/soya based rations for turkeys (8) a n d chicks (26).

in conclusion, broiler chicks can be t a u g h t to distinguish between different levels o f AA in foods by m e a n s of their colour a n d t h e n to adjust the p r o p o r t i o n of s u p p l e m e n t e d a n d unsup- p l e m e n t e d foods eaten to meet the r e q u i r e m e n t s for AA ac- cording to e n v i r o n m e n t a l t e m p e r a t u r e .

~CKNOWI.EDGEMENTS

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. H.R.K. was on study leave from the University of Cukurova, Turkey.

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