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ANIMAL NUTRITION |
Department of Animal Sciences, Division of Animal Nutrition and Production Physiology, Technical University of Munich-Weihenstephan, 85350 Freising, Germany
Abstract
The aim of the present study was to investigate whether pigs prefer diets varying in Trp content and whether these preferences change with time. To that end, a feeding trial was carried out over a 6-wk period. Piglets (equal proportion of males and females) with an initial BW of 8.20 ± 0.90 kg were randomly subdivided into four groups of 12 pigs each. Two reference groups were fed (as-fed basis) either 0.11% Trp (Trp-deficient) or 0.20% Trp (Trp-adequate) diets. Two other groups had a choice of two diets containing either 0.11 or 0.16% Trp (Trp-choice 1), or 0.11 or 0.20% Trp (Trp-choice 2). Average daily feed intake reached 335 and 366 g in pigs fed Trp-deficient and Trp-choice 1 diets, respectively. For Trp-choice 2 and Trp-adequate diets, a higher (P < 0.05) feed intake of 589 and 645 g/d, respectively, was observed. Piglets on Trp-choice 1 and Trp-choice 2, respectively, selected 87 and 93% of the higher Trp diet. Resulting Trp contents of total diets were 0.15 and 0.19% (as-fed basis) in Trp-choice 1 and Trp-choice 2, respectively. In wk 1, pigs on Trp-choice 2 chose lower proportions of the Trp-deficient feed (31% of total diet) than did pigs on Trp-choice 1 (44%), but at the end of the experiment, pigs of both groups almost exclusively chose the feed with the higher Trp content (96 and 98% for Trp-choice 1 and 2). Pigs on Trp-choice 1 had an ADG of 218 g, which was only slightly above the ADG of Trp-deficient pigs (198 g). Pigs on Trp-choice 2 and Trp-adequate diets had ADG of 404 and 458 g, respectively, which were higher (P < 0.05) than those observed for Trp-deficient and Trp-choice 1 groups. Plasma Trp concentrations in Trp-choice 2 and Trp-adequate groups (9.21 and 9.01 µmol/mL, respectively) were higher (P < 0.05) than in Trp-deficient and Trp-choice 1 groups (5.88 and 4.96 µmol/mL, respectively). Conversely, the sum of essential AA showed a higher (P < 0.05) concentration in plasma from pigs on the Trp-deficient and Trp-choice 1 diets than in plasma from pigs on Trp-choice 2 and Trp-adequate diets. Nutritional depletion of Trp influences the food selection behavior of piglets. Results of growth performance and the dietary preferences suggest that piglets are able to detect Trp-deficiency-induced metabolic changes and respond with an aversion against the Trp-deficient diet.
Key Words: Feeding Preferences • Piglets • Tryptophan
Introduction
In nature, a wide variety exists in available foods for most species. The ability of wild animals to select from among them a diet most suited to their needs might be an important factor in survival strategies. Observations on dietary selection behavior of chicks (Steinruck et al., 1991
; Steinruck and Kirchgessner, 1993) suggest that an innate ability to regulate the intake of macro- and micronutrients still exists in domesticated animals. A specific dietary selection behavior was observed in pigs, and there may be an economic advantage to choice feeding for commercial pig and poultry production systems (Rose and Kyriazakis, 1991). Several self-selection studies have demonstrated that growing pigs will select a diet that meets their requirements according to growth rate and feed efficiency when offered a choice between two feeds differing in their protein content (Kyriazakis et al., 1990; Bradford and Gous, 1991, 1992); therefore, self-selection of dietary protein has been suggested as an alternative feeding strategy that could minimize nitrogen output (Henry, 1993). Nevertheless, data regarding preferences for single AA in pigs are limited. Work of Edmonds et al. (1987) demonstrated the preference of pigs for a basal diet over diets containing Trp, Met, Arg, Lys, or Thr in excess. Studies by Henry (1987, 1993) and Kirchgessner et al. (1999) have shown that growing pigs given a choice of diets deficient or adequate in Lys content will show a specific preference for Lys in order to avoid and partly redress Lys deficiency. However, pig diets differing only in the contents of Trp have rarely been studied. Therefore, the objective of the present study was to investigate whether piglets have preferences regarding the amount of Trp in their diets and whether these preferences change with time.
Materials and Methods
A study using 48 crossbred piglets (German Landrace x Piétrain) with an initial BW of 8.2 ± 0.9 kg was carried out over an experimental period of 42 d to examine the selection behavior for feed varying in Trp content. Pigs were divided into four treatment groups of 12 piglets each. Their allotment to dietary treatments was made at random from blocks based on litter, sex, and initial weight to provide six replicate pens of barrows and six replicate pens of gilts per treatment.
Piglets of two of the four treatment groups were given a choice between a pair of diets with differing Trp contents (Trp-choice groups), whereas the other two treatments were used as reference groups. The Trp-choice was allowed to choose from a pair of diets containing either 0.11 or 0.16% Trp; the Trp-choice 2 was allowed to choose from a pair of diets containing either 0.11 or 0.20% Trp. The reference groups were fed either 0.11% Trp (Trp-deficient) or 0.20% Trp (Trp-adequate) with no choice for selection (as-fed basis).
Diets were mainly based on corn, corn gluten feed, wheat, and field bean. For diets containing 0.16 or 0.20% Trp, L-Trp was added to the Trp-deficient diet at 0.51 or 0.92 g/kg, respectively, at the expense of corn and soybean oil. The 0.20% Trp level was chosen to meet NRC (1998) recommendations for piglets of the 8- to 30-kg BW range and to give a ratio of Lys:Trp of 1:0.19, whereas the level of 0.16% Trp represents an intermediate Trp supply. The composition of the basal diet (0.11% Trp diet) is given in Table 1. The ratio of Lys:Met+Cys:Thr was at least 1:0.6:0.65.
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Leftover feed was removed from the feeders and weighed back twice weekly. After removing the feed, piglets had no access to feed for about 30 min. At refilling the feeders, the position of the two diets of the Trp-choice groups was altered in the feeders. With the beginning of wk 3 of the experiment, spontaneous food selection behavior was observed for approximately 5 min after refilling the feeders. Possible observations within this time period were by definition: 1) spontaneous preference for the 0.16% or 0.20% Trp diet, respectively; 2) spontaneous preference for the 0.11% Trp diet; 3) change to the 0.16% or 0.20% Trp diet, respectively, after testing the 0.11% Trp diet; and 4) change to the 0.11% Trp diet after testing the 0.16% or 0.20% Trp diet, respectively. The term "testing" means ingestion of a diet over a time period of 2 to 3 min. The term "spontaneous" includes a short visit of a feeder and a quick sensory test of the respective alternative diet.
Live weight and feed intake were determined at 1-wk intervals before feeding in the morning. At the end of the experiment, blood from the jugular vein of seven pigs per treatment was collected into EDTA-treated tubes (9 mL). Blood samples of one pig in each of the Trp-choice 2 and Trp-adequate groups were not analyzed for plasma amino acids because of hemolysis.
Tryptophan content of feed was determined by reversed-phase HPLC (FL Detector L-7480; Interface D-7000; Autosampler L-7200, Merck, Darmstadt, Germany; Hitachi, Tokyo, Japan) following alkaline hydrolysis with barium hydroxide (Fontaine et al., 1998).
Plasma for determination of AA was obtained by centrifugation for 20 min at 800 x g. The plasma proteins were precipitated with salicylsulfonic acid and centrifuged at 11,000 x g for 10 min. After dilution with a lithium acetate solution, the protein-free supernatant was analyzed by ion exchange chromatography (Naumann and Bassler, 1988) on an automatic AA analyzer (LC 3000, Biotronik, Hamburg, Germany). All samples were analyzed in duplicate.
The SAS software (SAS Inst., Inc., Cary, NC) was used in all statistical evaluations. Data were treated by ANOVA, with treatment as the main effect and litter as covariate. Mean values for the effect of treatment on feed intake, growth performance, and plasma AA pattern were compared using the PDIFF statement of the GLM procedure and the Bonferroni adjustment. Ingested amounts of the two diets offered for choice in the Trp-choice treatments were compared by the Student’s t-test, and the effect of time was evaluated by a mixed model with repeated measures, with treatment and week as fixed effects. The spontaneous food selection behavior was evaluated by the 
2 test.
Results
As shown in Table 2, pigs on both Trp-choice groups showed a clear preference for the diet that was higher in Trp content. Over the whole experimental period, pigs on Trp-choice 1 ate 1.8 kg of the 0.11% Trp diet and 13.6 kg of the 0.16% Trp diet. Pigs on Trp-choice 2 consumed 1.9 kg of the 0.11% Trp diet and 22.8 kg of the 0.20% Trp diet. The development of food selection behavior over the time course of the experimental period was comparable at both Trp-choice groups and no time x treatment interaction was observed. Pigs on both treatments reduced the weekly ingested amount of food containing 0.11% Trp from an average of 0.57 kg for both groups in wk 1 to 0.15 kg at the end of the trial. Conversely, the weekly ingested amount of feed with the higher Trp content increased in the Trp-choice 1 group from a starting value of 0.87 kg to 3.65 kg at the end of the trial, and in the Trp-choice 2 group from 1.27 to 6.11 kg. Differences in ingested amounts of Trp-deficient and higher-Trp feed were significant (P < 0.05) in each experimental week in Trp-choice 2, whereas pigs on Trp-choice 1 chose the diets in the first experimental week at random. As a mean of the whole experimental period, chosen diets of Trp-choice 1 and 2 contained on average 13 and 7%, respectively, of the 0.11% Trp diet. Therefore, the resulting Trp concentrations of the total diets chosen by pigs of both groups were just 0.01% below the respective maximal possible Trp content (0.15 and 0.19% Trp in Trp-choice 1 and 2, respectively). In wk 1, the chosen proportions of the 0.11% Trp diet by pigs on Trp-choice 1 (44% of total diet) were numerically higher than by pigs on Trp-choice 2 (31%), but at the end of the experiment pigs of both groups chose nearly exclusively the feed with the higher Trp content (96 and 98% for Trp-choice 1 and 2; Figure 1).
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. Total observations were 87 and 101 for Trp-choice 1 and 2, respectively. In 72 and 66% of observations for spontaneous feeding behavior, pigs on Trp-choice 1 and 2 chose the diets higher in Trp content immediately after refilling the feeders. In 6% of the observations, pigs on Trp-choice 1 spontaneously chose the 0.11% Trp diet, whereas this was not observed in Trp-choice 2. In 22 and 34% of the observations, pigs on Trp-choice 1 and 2 tested the 0.11% Trp diet after the troughs were refilled, but subsequently changed to and consumed the 0.16 or 0.20% Trp diet, respectively, for the remaining period of observation. No observations were recorded in which pigs consumed the diet higher in Trp and then subsequently changed to the 0.11% Trp diet.
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). Varying Trp contents of diets, food selection pattern of Trp-choice groups and the differences in feed intake resulted in marked differences (P < 0.05) in daily Trp intake among the four treatments (0.38, 0.55, 1.14, and 1.31 g Trp/pig daily in Trp-deficient, Trp-choice 1 and 2, and Trp-adequate, respectively).
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Plasma AA pattern was markedly influenced by the varying dietary Trp supply (Table 5). As expected, the higher Trp intake in the Trp-choice 2 and Trp-adequate groups led to higher (P < 0.05) plasma Trp concentrations (9.21 and 9.01 µmol/mL, respectively) than in Trp-deficient and Trp-choice 1 groups (5.88 and 4.96 µmol/mL, respectively). On the other hand, the lower Trp supply in Trp-deficient and in Trp-choice 1 pigs led to higher (P < 0.05) plasma Thr concentrations (961 and 1,047 µmol/mL, respectively) than in pigs on Trp-choice 2 and Trp-adequate diets (503 and 548 µmol/mL, respectively). The same tendency was observed for some of the other essential AA, such as Lys, Val, and Ile. The sum of essential AA showed a higher (P < 0.05) concentration in plasma from the Trp-deficient and Trp-choice 1 pigs than in plasma from pigs fed Trp-choice 2 and Trp-adequate. The plasma concentration of urea was 5.09 mmol/L in Trp-choice 1 pigs. The Trp-choice 2 and Trp-adequate pigs had lower (P < 0.05) plasma urea concentrations of 3.11 and 2.99 mmol/L, whereas Trp-deficient pigs showed an intermediate value of 4.24 mmol/L.
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Two functions of diet selection, especially for wild animals, are avoidance of ingesting harmful substances or the necessity to exploit resources efficiently. A third function is the need to select from the available foods a diet, which can satisfy the animal’s nutritional requirements (Rogers and Blundell, 1991). Kirchgessner et al. (1999) concluded from results of a choice feeding experiment with Lys, that food selection is still evident for domesticated piglets, and that these animals are able to detect metabolic changes caused by Lys deficiency and attempt to partly redress them by modifying their food pattern. This concept is clearly supported by the present findings. Piglets on Trp-choice 1 showed a weight gain and feed efficiency comparable to those realized on Trp-deficient and therefore a poorer performance (P < 0.05) than shown in Trp-adequate. Given that pigs chose a diet containing nearly the maximum possible Trp concentration (0.16%), it can be concluded that dietary self-selection and the avoidance of the 0.11% Trp diet was an attempt to avoid Trp deficiency and to compensate for the physiological defects. On the other hand, performance of pigs on Trp-choice 2 was numerically inferior compared with Trp-adequate because of the high preference for the 0.20% Trp diet and the rejection of the 0.11% Trp diet. Piglets on Trp-choice 1 and Trp-choice 2 had deficient Trp intake at the beginning of the study but met Trp requirements (NRC, 1998) at the end of the study. Therefore, to reach maximum possible growth, piglets had to select exclusively the high-Trp diet through to the end of experimental period.
Besides the observed decrease in feed intake and growth rate in pigs on Trp-deficient and Trp-choice 1, physiological defects are reflected in marked differences in plasma AA pattern. Concentration of plasma Trp was higher (P < 0.05) in Trp-choice 2 and in Trp-adequate compared with Trp-deficient and Trp-choice 1. On the other hand, in spite of the higher absolute protein supply as a consequence of the higher feed intake, the sum of essential AA, as well as plasma urea content of pigs on Trp-choice 2 and Trp-adequate groups, was lower than in pigs on Trp-deficient and Trp-choice 1. These data indicate that AA were used more efficiently for growth at a greater extent in Trp-choice 2 and Trp-adequate group.
Decreased plasma Trp concentration as a result of ingestion of the 0.11% Trp diet may have played an important role in the development of the high preference for Trp in the present study. Gietzen (1993) found from work with AA-imbalanced diets in rats that there seem to be different phases in the response to AA deficiency. For the first phase of recognition of the deficiency, a decrease in the concentration of the limiting AA in a specific brain area (the prepyriform cortex) is a necessary (but not the only) step. Subsequent to this recognition of deficiency, a conditioned taste aversion develops, mediated in part by serotonin at the level of the vagus (Gietzen, 1993), and in a situation of choice feeding, this aversion must result in a preference for the alternative feed more adequate in AA composition.
Results of Ashley and Anderson (1975) indicate that the selection among different protein sources depended on food taste rather than on the protein quality. Given that in the present study different diets had exactly the same composition except for added L-Trp at amounts of 0.51 and 0.92 g/kg diet, it seems unlikely that pigs selected for Trp because of a "pleasant palatability" of diets high in Trp content. On the contrary, Trp has an extremely bitter taste and a very unpleasant odor (Edmonds et al., 1987), and these organoleptic properties of Trp are unlikely to cause a selection for Trp because of taste cues. Furthermore, if rejection of the 0.11% Trp diet occurred because of its unpalatability, it can be assumed that even in the first week of the experiment, this diet would have been rejected at an extent close to 100%. On the other hand, some sensory recognition for Trp in piglets must be assumed, because pigs in the present study were able to detect the feed rich in Trp even if the position of the feed in the troughs were changed. Our observations of spontaneous selection behavior have further shown that the decision for these diets occurred within the first 5 min after the feeding, a time interval in which a decisive change in the momentary metabolic status seems to be unlikely. It has been shown that several AA taste sweet or delicious to humans and are attractive to animals (Iwasaki et al., 1985). Recently, a heteromer of some taste-specific G-protein-coupled receptors was demonstrated to respond to most of the 20 standard L-AA in mammalians, whereby sequence differences in specific receptors within and between species can significantly influence the specificity and selectivity of taste responses (Nelson et al., 2002). Therefore, it is concluded that the piglets in the present study were able to differentiate between diets with differing Trp content because of taste cues, but the rejection of the 0.11% Trp diet was a result of the recognition of an AA deficiency by physiological mechanisms and of an association of the organoleptic properties of the feed with its physiological consequences. According to a review by Gietzen (1993), palatability does not contribute to the initial recognition of an AA-imbalanced diet, but in rats, it has been demonstrated that satisfying the nutrient requirements is more important in diet selection than an aversive taste (Yamamoto et al., 1985; Leung et al., 1986).
Tryptophan is a precursor of serotonin (5-hydroxytryptamine, 5-HT) in the brain, and is controversially thought to influence nutrient selection behavior (Fernstrom, 1977, 1985; Harper and Peters, 1989). In spite of its role in influencing selection behavior, total feed intake in piglets in the present study was highly related to the Trp supply and similar results were also shown by Eder et al. (2001). Data from some authors (Meunier-Salaün et al., 1991; Séve et al., 1991; Henry and Séve, 1993) indicate that an increased feed intake with increasing dietary Trp content is accompanied by an increased brain 5-HT concentration. A strong positive correlation between brain serotonin synthesis and plasma Trp concentration, or more precisely, the ratio of Trp to large neutral amino acids (LNAA; isoleucine, leucine, valine, phenylalanine, tyrosine) in the plasma of pigs seems to exist (Meunier-Salaün et al., 1991; Henry et al., 1992). These results indicate, that in the pig, as in rats (Fernstrom and Wurtman, 1972), plasma Trp must compete with other plasma LNAA for access to carrier binding sites, which allow the specific transport of LNAA from blood to the brain. Serotonin concentration in the brain was not measured in the present study, but because the plasma Trp:LNAA ratio was higher (P < 0.05) for pigs on the Trp-choice 2 and Trp-adequate diets (Trp:LNAA ratio: 1.58 and 1.45%, respectively) than for those on Trp-deficient and Trp-choice 1 diets (Trp:LNAA ratio: 0.87 and 0.74%, respectively), a higher brain serotonin synthesis can be assumed for pigs given a Trp-adequate diet. It is known that the activation of brain 5-HT function may depress feed intake under distinct circumstances (Blundell, 1984; Morris et al., 1987), and therefore a possibly higher brain 5-HT synthesis after ingestion of the Trp-adequate diets seems contradictory with the observed higher feed intake. An explanation may be that changes in the hypothalamic 5-HT concentration is of secondary importance relative to plasma and brain concentrations of the limiting AA (Tackman et al., 1990; Henry et al., 1992) for regulation of feed intake. Therefore, irrespective of plasma Trp concentration as a precursor for brain 5-HT synthesis, the highly depressed intake of Trp-deficient and Trp-choice 1 diets must be set in the context of a diet deficient in any essential AA. Comparable to the situation with low-protein diets, an animal cannot compensate for the lack of an AA in a diet by increasing its intake of this diet, and therefore, feed intake decreases (Henry, 1985; Langhans, 2001).
Implications
When given a choice between a tryptophan-deficient diet and a diet with a higher tryptophan level, piglets show a clear preference for the higher tryptophan diet. Ingestion of the 0.11% tryptophan diet resulted in decreased growth performance and changes in plasma amino acid pattern. Piglets responded with a conditioned aversion against the tryptophan-deficient diet offered for a choice. Given that pigs detected the diet more adequate in tryptophan even after the location of the diets in the feeder was changed, it seems that pigs are able to distinguish between the different diets on offer because of some sensory recognition for tryptophan; therefore, dietary selection of piglets is highly influenced by the tryptophan concentration of the diet.
Footnotes
1 The authors acknowledge A. Böhm for technical support. 
2 Correspondence: Fachgebiet Tierernährung und Leistungsphysiologie, Technische Universität München, Hochfeldweg 6, 85350 Freising- Weihenstephan, Germany (phone: +49(0)8161713551; fax: +49(0)8161715367; e-mail: roth_fx{at}wzw.tum.de).
Received for publication July 14, 2003. Accepted for publication December 16, 2003.
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