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Swiss Federal Institute of Technology, Institute for Animal Sciences, Nutrition Biology, ETH-Zentrum, 8092 Zurich, Switzerland
1To whom correspondence should be addressed. E-mail: christine.zuberbuehler{at}inw.agrl.ethz.ch.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • selenium • deficiency • laying hens • choice feeding • taste aversion
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Both Se and vitamin E are important components of the antioxidant defense system that helps protect PUFA in cell membranes from peroxidative damage (1
). Therefore, nutritional Se deficiency and its physiologic effect must be considered in terms of Se and vitamin E status.
Diets low in Se and vitamin E cause serious Se-vitamin E deficiency disorders in many species. But in animals receiving normal allowances of vitamin E, there was little evidence of Se-responsive disease. However, Nesheim and Scott (2
) provided strong evidence for the indispensability of Se when they found that chicks required Se for growth and survival even when their diet contained high amounts of vitamin E. Nevertheless, a strong controversy about the status of Se as an essential nutrient persisted for many years (3
). Thompson and Scott (4
) showed that the requirement for Se in chicks can vary over a wide range because it increases when the dietary level of vitamin E is reduced. Furthermore, they found that diets low in vitamin E and moderately low in Se resulted in exudative diathesis with otherwise good growth. On the other hand, diets containing substantial amounts of vitamin E and extremely small amounts of Se affected mainly growth and feathering, whereas signs of exudative diathesis appeared only during the terminal stages of the deficiency disease, if at all. Later, it was demonstrated that chicks have a specific requirement for Se for growth and maintenance of pancreatic exocrine function (5
).
Adult animals generally exhibit appropriate food selection behavior. Apart from innate sensory preferences and aversions to foods, animals are able to select foods in response to certain physiologic signals exerted or relieved by these foods. Food preferences and aversions of animals and humans are often related to the taste or appearance of the food. In other words, humans and animals can learn to associate specific sensory attributes of foods with pleasant or noxious postprandial effects and thereby discriminate among foods [e.g., (6
,7
)]. For example, to cover their nutrient requirements for growth and reproduction, many species are able to select appropriate portions from several mixed diets differing in protein or energy content [e.g., (8
–12
)]. Certain animals, including laying hens, have this ability to some extent even when the diets differ only in a single nutrient, particularly when the animals are deficient in this nutrient [e.g., (13
–20
)] or when diets contain toxic amounts of a nutrient (6
,21
). The craving for one or the other nutrient changes with the variations of the reserves still available in the body (22
).
To our knowledge, only one study to date has examined Se in the context of food choice. Heinz and Sanderson (21
) observed that mallard ducks clearly selected a low Se diet when they were offered a control feed together with a feed containing either 10 or 20 mg/kg Se as selenomethionine, possibly to avoid a feeling of discomfort, indicating an evolving intoxication. The authors attributed the ducks’ response to a learned aversion for either the taste or the placement of the Se-treated diet.
Marginal-to-moderate Se deficiency could develop slowly and not be noticed for some time, for example, in a flock kept on an open range grassland providing large amounts of highly peroxidizable PUFA and inadequate amounts of Se.
For these reasons as well as the essential role of nutritional Se and the devastating effects of Se deficiency, it seemed pertinent to design a food choice experiment at the lower end of the scale of Se intake. Therefore, we investigated whether marginally Se-deficient laying hens would preferentially select diets higher in Se. After consumption of a Se-deficient diet, a vague discomfort or sickness might be perceived before clear deficiency symptoms become apparent. With the transition from marginal to severe deficiency, the discomfort would gradually exacerbate and result in deficiency disorders. It seems plausible that Se-deficient laying hens would try to relieve discomfort due to Se deficiency.
We investigated whether marginally Se-deficient laying hens preferentially select a diet containing more Se when offered a choice of two feeds with different Se concentrations.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The protocol described below was approved by the Veterinary Office of the Canton of Zurich (Approval no. 160/97). A basal diet was formulated using ingredients common in practical diets (Table 1
). The diet was calculated to meet or moderately exceed recommendations (25
) for Leghorn-Type laying hens with respect to all nutrients other than Se. The components were carefully selected for a low natural Se content. Selenium was analyzed in the main ingredients and diets as described by Haldimann and Zimmerli (23
). The Se concentration of the basal diet originated mainly from soybean meal (52%), corn (27%), wheat (17%) and dried grass (4%). No particular effort was made to select ingredients with low vitamin E content.
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–28
). Selenomethionine is the predominant form of Se identified in Se-yeast and in plants (29
,30
).
The diets were analyzed by standard methods (31
). The content of dry matter and ash was analyzed using an automated muffle furnace (TGA-500, Leco, St. Joseph, MI). Nitrogen (N) content was determined by means of an automated nitrogen analyzer (CN Analyzer Type FP-2000, Leco) and crude protein (CP) content was calculated (CP = N x 6.25). The dry matter content was 90.1% containing 12.4% ash, 15.7% CP, 4.1% crude fat, and 2.9% crude fiber, consistent with the calculated concentrations. The three experimental diets were indistinguishable to the human eye and sense of taste.
Animals and experimental design.
ISA Brown laying hens (n = 56), 18 wk of age, were selected at UFA-Buehl (Hendschiken, AG Switzerland) for a body weight between 1500 and 1600 g. The hens were randomly distributed to 7 treatment groups of 8 hens each and individually housed in wire cages, approved by the regulations of the Swiss Animal Welfare Law. Gray plastic walls were inserted between the cages to prevent visual or bodily contact among the hens during the experiment. Each cage was equipped with dust-bath, nest, perch and two fresh water nipple drinkers. The dust-bath was cleaned twice a week and refilled with fresh sawdust (484 µg Se/kg). Selenium was not detectable in drinking water. Two identical feed troughs were attached side by side to each cage front. All hens were fed the M diet in both feed troughs after their arrival (3 wk before the start of the experiment). Feed and water were available ad libitum. The lighting phase was extended gradually from 11 h initially to 14 h within 4 wk and was maintained at this level throughout the trial.
The start of the experiment was set at wk 21 of age (1 wk after all hens had begun to lay). The study design is outlined in detail in Table 2
. The trial consisted of two identical consecutive parts (I and II), each beginning with a 6-wk baseline period (A), followed by a 9-wk depletion period (B), and concluding with a 6-wk choice-feeding period (C). During the baseline observation period, all hens were adapted to the M diet from both feed troughs. During the depletion period treatment groups 1, 2 and 3 continued to receive the M diet, whereas treatment groups 4, 5 and 6 were fed the L diet. During the choice feeding period each group was offered two different feeds simultaneously. Groups 1 and 4 were offered a choice between the M and L diets (ML), groups 2 and 5 were offered a choice between the M and H diets (MH), and groups 3 and 6 could choose between the L and H diets (LH). Group 7 (control) was fed the M diet exclusively throughout the experiment. The treatment groups are designated according to the diets offered during the depletion and choice periods, e.g., M-LM for group 1, M-MH for group 2, M-LH for group 3, L-LM for group 4, L-MH for group 5 and L-LH for group 6.
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Body weight was recorded weekly. Eggs were collected and weighed daily. Laying rate, egg mass and the feed efficiency ratio (feed needed for egg mass production) were calculated for each period of both parts of the study. Food consumption was recorded weekly from each trough separately. From these data, total daily feed intake (DFI) and daily Se intake (DSI) were calculated as well as a factor for the exhibited "choice interest for Se" (CIS), which is the amount of the feed consumed with the higher Se content, expressed as a percentage of DFI. During the choice feeding period, the left-right position of the feed troughs was determined alternately for each hen within treatment to compensate for positional effects. The position of the feed troughs was inverted only after 3 wk, to allow the hens to associate postingestional effects of their food choice with the location of the diet. Because in all probability there were neither taste nor visual cues present, postingestional reinforcement could be associated solely with the position of the diets. In the configuration used in this experiment, we assumed that if both feed troughs contained the same diet, then the birds would eat equal amounts from both troughs. Accordingly, the mean CIS was expected to be close to 50% during the baseline and depletion periods. For the choice periods, we argued that if the hens’ choice were a real preference for a particular feed and not simply a side preference, CIS would not change with the inversion of the feed troughs or change only for a short period of time before returning close to the value of before the inversion. On the other hand, if the hens’ choice represents a preference for a particular feed location, CIS would change with the inversion of the feed troughs because the feed in the preferred location had changed.
Statistical analyses.
Statistical analyses were conducted using the GLM procedure of the SYSTAT statistical package (version 10.0 for Windows 2000 Statistics. SPSS, Chicago, IL). To determine changes in CIS over time, repeated-measures ANOVA was carried out and significant within-subject (time) effects were further assessed. The effect of treatment was examined in each time period separately. One-way ANOVA was performed for the depletion periods. Two-way ANOVA was conducted for the choice periods, allowing examination of the main effects of both factor B (Se treatment during depletion periods B) and factor C (feed choice during choice periods C), as well as interactions between the two factors. Tukey’s multiple comparisons test was applied to compare means. Means were considered significantly different at P < 0.05 for all tests used. The laying rate and DSI data were log transformed before analysis. Results presented in tables are expressed as means ± SEM.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Body weight increased significantly with age in all groups (P = 0.003). At the end of baseline period IA, the hens had reached a body weight of 1815 ± 107 g. The hens in the L-LM, L-MH, and L-LH groups generally grew faster than the M-LM, M-MH, and M-LH hens (Table 3
). For periods IC2, IIC1 and IIC2, choice effects were found (P = 0.033, P = 0.011 and P = 0.022, respectively). The hens were heavier when offered the H diet in combination with either the M or L diet than when they had the choice between the L and M diets. Interestingly, hens offered this latter diet combination, LM (groups M-LM and L-LM), tended to eat less during IC2 (C: P = 0.102) and consequently stopped growing, whereas all other hens, including the controls, continued to grow during part II up to the end of the study.
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During the baseline period IA, the DSI was 21.1 ± 1.5 µg (Table 4
). During IB, hens fed the L diet had a lower DSI (P < 0.001) of 8.1 ± 0.5 µg, whereas those that continued to consume the M diet had an unchanged DSI of 22.3 ± 1.7 µg. During IC1, the previously Se-depleted hens tended to have a higher DSI (B: P = 0.099) than those fed adequate amounts of Se. In addition, the DSI was affected by the diets offered for choice (C: P < 0.001), which resulted in an interaction between the two factors (B x C: P = 0.030). Group L-LH had a higher DSI than the M-LM, M-LH, L-LM, control (P < 0.005), and M-MH groups (P < 0.05). In fact, the DSI increased in all groups but M-LM in which the DSI decreased to the same level as it rose in its comparison group, L-LM. During IC2, after the inversion of the feed troughs, the diets offered for choice continued to affect the DSI (C: P < 0.001). Group M-LM had a further decrease in the DSI, whereas group L-LM remained stable. The DSI increased slightly in the M-MH, M-LH, and L-MH groups, whereas it decreased sharply in group L-LH. In part II of the study, the DSI did not differ from that in part I. There was, however, no interaction of the treatment factors during IIC1.
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Because both feed troughs contained the M diet during the baseline periods (IA and IIA), a CIS of 
50% was expected for all hens, and a CIS of 49.1 ± 9.6% was observed (Table 5
). For period IB, CIS of 46.9 ± 9.6% and 49.8 ± 9.4% were calculated for the hens fed the M and L diets, respectively. During the choice period IC1, a significant effect was found for factor B (P = 0.037), indicating that the hens previously depleted of Se ate more of the diet containing more of the essential trace element. In particular, group L-LH hens had a higher CIS than the M-MH, M-LH (P < 0.01), and control groups (P < 0.05). No Se preference was apparent in any group after the inversion of the feed troughs in IC2. When examined at weekly intervals rather than every 3 wk or more, the CIS of group L-LH was found to change dramatically over time (P = 0.032). This was observed in wk 16 with a CIS of 61.4%, which rose to 65.1% in wk 17 at and declined again in wk 18 to 60.7%. This did not occur in part II of the study.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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); consuming this amount would result in a DSI of 15–400 µg (based on a feed intake of 100 g/d). In the present study, the L diet was expected to produce mild-to-moderate Se deficiency. It was intended to contain enough Se and vitamin E to prevent irreversible damage to tissues and organs but to result in mild, reversible deficiency symptoms. The M diet was estimated to supply Se amounts adequate for normal growth and egg production (25
). The Se content of the H diet, on the other hand, was chosen so that Se intake would never reach a toxic level, even if the hens chose to eat the H diet exclusively. These expectations were confirmed by the DSI calculated from the DFI and the actual choice made by the hens. During the depletion periods, the DSI was never <5.8 µg/d, and during the choice periods, the DSI was never >161.1 µg/d for any hen.
Many studies have found a beneficial influence of Se supplementation on feed consumption, body weight, weight gain and the prevention of Se deficiency symptoms and mortality in poultry (32
–38
).
Taste aversion has been reported extensively to be a principal cause of anorexia in deficiency disorders as well as in poisonings [for review see (39
)]. In fact, depression of food intake results from deficiency of any of the essential minerals (40
). Indeed, even deficiencies of trace minerals, in particular zinc, cause anorexia in rats and broilers, probably due to learned taste aversion (14
,41
).
It has been demonstrated that the growth depression in Se-deficient rats is due primarily to reduced feed consumption (42
). The supplementation of 0.1 mg Se/kg diet as sodium selenite significantly increased feed intake of hens fed a corn-soy diet containing 0.038 mg/kg naturally occurring Se and 13.6 mg/kg vitamin E (43
). A single oral dose of Se stimulated short-term feed intake in Se-deficient chicks (34
). However, the residual growth response, which is independent of feed intake, likely resulted from metabolic effects of Se deficiency on fat digestion, absorption and/or postabsorptive utilization of nutrients, in particular, energy and nitrogen (34
,42
).
As a consequence of reduced feed intake (and possibly additional reasons) hens offered a choice between the L and M diets ceased growing by the end of part I, irrespective of the treatment during the depletion period. This suggests that the Se supply might have been suboptimal at a critical point in time, whereas the continuing vigorous growth of the other hens implies that Se supplementation might be particularly beneficial as the hens grow older. In other words, Se requirements may increase with age.
A deficiency or excess of dietary Se may have deleterious effects on egg production in laying hens (44
–49
) and turkeys (50
), whereas egg weight (47
–49
), egg shell thickness (44
), and egg shell breaking strength (43
,46
) are unaffected.
As little as 0.1 mg Se/kg diet in the form of sodium selenite or selenomethionine improved egg production when added to a corn-soy diet containing 0.027 mg naturally occurring Se per kg diet and 16 mg/kg vitamin E (46
). Moreover, even as little as 0.01 mg Se/kg diet in the form of sodium selenite effectively increased egg production when added to a diet containing 0.04 mg naturally occurring Se/kg diet and no supplemental vitamin E (48
). Hassan (49
) observed a significant decrease in the laying rate of hens fed an unsupplemented diet containing 0.03 mg Se/kg and 33.37 mg vitamin E/kg after 12 wk of consuming this diet. A supplementation of 0.2 mg Se/kg diet either as sodium selenite or as high Se barley was adequate to maintain egg production. In the present study, Se depletion was probably not severe enough to affect laying rate in a negative manner, indicating that either the depletion periods were too short or the Se content of the L diet was too high or both. In contrast to other studies (49
,51
), we recorded much higher laying rates, which could be attributed to the breed used or, even more likely, to the solitary caging, which excluded any social competition. Interestingly, the M-LM, M-MH and M-LH hens reached maximal laying rates during baseline IA, whereas all others reached this plateau only later in IB. It appears that the lower laying rate of L-LM, L-MH and L-LH hens during IA was compensated by faster growth.
Animals, including chickens, exhibit specific appetites for a number of single nutrients, in particular when they suffer from deficiency of this nutrient. Most appetites for essential nutrients are supposed to be the result of learned associations of sensory properties of diets containing or lacking the nutrients with postingestional consequences (39
).
If the hens’ choice was simply a preference for a feed location, then CIS would change with the inversion of the feed troughs due to the change of the feed in the preferred location; this did not occur in our study. Furthermore, novelty seemed not to be involved in the Se-deficient hens’ choice because they displayed a preference for the H diet only when it was offered in combination with the L but not the M diet. In both choice situations, the H diet was novel to them.
In mammals, taste is considered the most important factor in nutrient selection, whereas in most birds, taste usually plays only a minor role in choice among solid feeds. It is involved in the assessment of fluids, however (22
,52
). On the other hand, it has been demonstrated repeatedly in humans and animals that deficiency of specific nutrients can alter the gustatory perception of taste. For instance, iodine deficiency and hypothyroidism were reported to affect taste perception in rats and in patients (53
,54
). This effect is of special interest in the context of our study. Because Se, as a functional constituent of deiodinases, plays an important role in the metabolism of the thyroid hormones (55
,56
), the possibility of an involvement of Se deficiency in alterations of taste perception must be considered, especially because thyroid hormones are involved in learning and memory.
Moreover, the H diet was chosen predominantly by Se-deficient hens only when offered in combination with the L diet but not when offered with the M diet. On the other hand, animals can learn to associate specific attributes of feeds with pleasant or noxious postingestional effects and thereby discriminate among feeds (6
,7
). Thus, the Se-deficient hens may have developed an aversion to the L diet by associating the consequences of its ingestion, presumably some sort of sickness due to the developing Se deficiency, either with the location of the L diet or possibly with its taste, if taste perception is altered by Se deficiency. Such a conditioned behavior was suggested to be responsible in the study of Heinz and Sanderson (21
); the ducks preferred a Low-Se diet when offered a control diet together with a diet containing high amounts (5, 10 or 20 mg/kg) of Se.
In our study, only Se-deficient hens with a choice between the L and H diets had a preference for the H diet or, indeed, an aversion to the L diet. We assumed that Se-deficient hens did not choose between the M and H diets because they did not need to; both diets alleviated Se deficiency. When the hens were offered the L and M diets, only a complete preference for the M diet would have alleviated Se deficiency. Hence, they were possibly not able to make a choice because reinforcement from beneficial feedback after ingestion was lacking, indicating that the differences between the diets might have been too small for the hens to discern, at least in the time available.
We assumed that the Se stores of the deficient hens would be replenished after 1–3 wk of feeding the H diet in combination with either the L or M diet. Thus, the hens would be very likely to cease any specific discrimination of the diets within this period of time because the postulated Se deficiency discomfort would gradually abate and be cured by regaining full function of Se-dependent processes. Such a change in behavior was demonstrated clearly in our study with the CIS of group L-LH increasing in wk 1 of choice feeding, then reaching the peak in the second and decreasing again in the third. This observation strongly suggests that the diet selection of Se-deficient hens is repletion dependent. Similarly, it has been reported that zinc-deficient rats progressively lost the ability to discriminate between zinc-deficient and zinc-supplemented diets as they recovered from the deficiency (15
).
Extensive research revealed that only the appetite for sodium and possibly that for calcium have innate properties; all other appetites for single nutrients are assumed to be learned (57
,58
). However, it is not possible to determine definitively whether the diet choice exhibited by the Se-deficient hens occurred because the hens could actually detect Se in the diet by its taste, or whether they were simply avoiding the diet responsible for their Se deficiency-induced discomfort. The fact that the diets were designed to be identical, except for the presence or absence of Se-yeast, implies that this dietary component played a role in the ability of the hens to recognize the diets as different. In particular, if taste perception is altered by Se deficiency, the concept of learned taste aversion offers a plausible explanation for our hens’ feed choice. In part II of the study, no such aversion developed, possibly due to the age-dependent decrease in the ability for taste discrimination (59
) or because the depletion period was not long enough to deplete the larger Se stores of older hens, thereby not producing a need for Se at all.
We conclude that young Se-deficient laying hens reduce their Se deficit if they have a choice between a L diet and a H diet, possibly due to a learned taste aversion toward the L diet, presumably in response to a feeling of discomfort caused by Se deficiency.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Manuscript received 19 April 2002. Initial review completed 24 June 2002. Revision accepted 13 August 2002.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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