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Evidence of a dietary selection for methionine by the piglet¹

Department of Animal Sciences, Division of Animal Nutrition and Production Physiology, Technical University of Munich-Weihenstephan, 85350, Germany

	Abstract

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The objective of the current study was to investigate if there is a preference of piglets for diets varying in Met content and whether these preferences change with time. For this purpose, a feeding trial was carried out over a period of 6 wk. Piglets (equal numbers of males and females) with an initial BW of 7.2 ± 0.1 kg were randomly subdivided into 4 groups of 12 pigs each. Two reference groups were fed either 0.19% Met (low-Met group) or 0.26% Met (high-Met group) diets. Two other groups had the choice between 2 diets containing 0.19 or 0.23% Met (Met-choice 1 group), or 0.19 or 0.26% Met (Met-choice 2 group). Compared with the low-Met group (397 g), daily feed intake was increased by 43%, 60%, and 82% (P < 0.05) in the Met-choice 1, Met-choice 2, and the high-Met group, respectively. Piglets on the Met-choice 1 and Met-choice 2 groups, respectively, selected 72 and 80% of the higher Met diet. In the first week, piglets on both Met-choice groups selected the diets at random, but they increased their preference for the diets higher in Met up to 81 and 89%, respectively, in the last experimental week. For the total experiment, mean Met contents of total diets were 0.22 and 0.25% in the Met-choice 1 and Met-choice 2 groups, respectively. Average daily gain of 195 g in the low-Met group was more than doubled (P < 0.05) in the high-Met group. Daily gains of 306 and 366 g in the Met-choice 1 and 2 groups, respectively, were (P < 0.05) improved compared with the low-Met group but lower (P < 0.05) than in the high-Met group. Plasma Met concentration of 46.3 µmol/mL in the Met-choice 2 group was greater than in the low-Met or the Met-choice 1 groups (25.3 and 32.8 µmol/mL, respectively) but lower (P < 0.05) compared with the high-Met group (59.6 µmol/mL). Conversely, the sum of essential AA showed a greater (P < 0.05) concentration in plasma from pigs on the low-Met and Met-choice 1 groups than in plasma from pigs on Met-choice 2 group and the high-Met group. In conclusion, the present data demonstrate that piglets are able to discriminate among diets of varying Met content. When given a choice, they prefer a diet better balanced for Met to a Met-deficient diet. As a result of the altered feeding pattern, piglets are able to partly redress the depressed performance and altered plasma AA pattern resulting from the ingestion of the diet more limiting in Met concentration.

Key Words: feeding preference • methionine • piglet

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Several studies have been conducted to investigate the ability of domesticated animals to discriminate among diets varying in nutrient concentration and to select a diet best suited to their needs. Among the farm animals, most studies have been performed in chickens. Those studies involved dietary preferences for a wide range of nutrients [e.g., protein (Steinruck and Kirchgessner, 1992a, 1993), different AA (Lys: Steinruck and Kirchgessner, 1992b; Met: Roth et al., 1990), vitamins (Steinruck et al., 1991), or minerals (Kirchgessner et al., 1990)]. In pigs, however, most studies have focused on the influence of overall dietary protein content (Bradford and Gous, 1991, 1992) or Lys content (Henry, 1987; Owen et al., 1994) on dietary selection.

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 to avoid and partly redress Lys deficiency. In a similar way, it has been shown that piglets select diets more adequate in Trp concentration over Trp-deficient diets (Ettle and Roth, 2004) with stronger responses than shown in the Lys experiments. Given that it has been shown in broiler chickens that the response in dietary selection varies depending on the deficient AA, namely Lys, Met, or Trp (Noble et al., 1993), dietary selection in pigs also may be influenced differently by diets varying in Met concentration than from diets varying in Lys or Trp. Thus, because there is a lack of data, the objective of the current study was to investigate if there is a preference of piglets for diets varying in Met content and whether these preferences change with time.

	MATERIALS AND METHODS

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A study using 48 crossbred piglets (German Land-race x Piétrain; equal numbers of males and females) with an initial BW of 7.2 x 0.1 kg was carried out over an experimental period of 42 d to examine the selection behavior for feed varying in Met content. Piglets were blocked by initial weight and sex and allotted at random to 12 replications of 4 dietary treatments to provide 6 replicate pens of barrows and 6 replicate pens of gilts per treatment. Ancestry (litter) was equalized among the 4 dietary treatments. The piglets were housed in individual pens (size: 60 x 100 cm) in a fully air-conditioned experimental piggery (initial room temperature of 29°C, stepwise reduction to 25°C, artificial light from 0700 to 1700 h) with free access to the diets and drinking water provided by nipple drinkers placed in one corner of the pen. Animal housing and care was conducted under supervision of the veterinary office of the Bavarian government. The handling protocol ensured proper care and treatment of all animals in conformity with the German law for animal protection.

Piglets of 2 of the 4 treatment groups were given the choice between a pair of diets with differing Met content (Met-choice groups), whereas the other 2 treatments were used as reference groups. The reference groups were fed either 0.19% Met (low-Met group) or 0.26% Met (high-Met group) with no option for diet selection. The Met-choice 1 group was allowed to choose between a pair of diets containing either 0.19 or 0.23% Met, and the Met-choice 2 group was allowed to choose between a pair of diets containing either 0.19 or 0.26% Met.

Diets were mainly based on wheat, peas, field bean, and barley. To create 3 isonitrogenous diets, for diets containing 0.23 or 0.26% Met, DL-Met was added to the low-Met diet at the expense of glutamic acid. The composition of the diets is given in Table 1. The ratio of Lys:Thr:Trp was at least 1:0.6:0.19.

At the beginning of wk 3 of the experiment, spontaneous feed selection behavior was observed for about 5 min after refilling the feeders. For the Met-choice 1 and Met-choice 2 groups, possible observations within this time period were a) spontaneous preference for the 0.23% or 0.26% Met diet, respectively; b) spontaneous preference for the 0.19% Met diet; c) change to the 0.23% or 0.26% Met diet, respectively, after testing the 0.19% Met diet; and d) change to the 0.19% Met diet after testing the 0.23% or 0.26% Met diet, respectively. "Testing" means ingestion of a diet over a time period of 2 to 3 min. "Spontaneous" includes a short visit to a feeder and a quick sensory test of the respective alternative diet.

Live weight and feed intake were determined at weekly intervals before feeding in the morning. The end weight was determined after fasting the piglets for approximately 15 h. At the end of the fifth wk, blood from the jugular vein of each pig was collected into an EDTA-treated tube (9 mL) after fasting the piglets for about 3 h.

Met and Cys content of feed was analyzed by ion exchange chromatography (Naumann and Bassler, 1988) on an automatic AA analyzer (LC 2000, Biotronik, Hamburg, Germany). After oxidation with performic acid (Naumann and Bassler, 1988), Met and Cys were determined as methionine sulphone and cysteic acid, respectively.

Plasma was obtained for determination of AA 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 data were analyzed as a randomized complete block design using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). The statistical model included the effects of treatment, initial body weight, sex, and litter. 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. To evaluate differences in ingested amounts of the 2 feeds on offer within a dietary choice group, a 1-sample t-test was used to test whether the difference between the amounts of the 2 feeds consumed was different from zero, a value that represents equal amounts consumed of each feed (Roth et al., 1990). The effect of time was evaluated by mixed model with repeated measures with treatment and week as fixed effects, and a week x treatment interaction term. The spontaneous feed selection behavior was evaluated by the ² test. For all evaluations, differences between means were considered significant at = 0.05.

	RESULTS

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As shown in Table 2, pigs on both Met-choice groups showed a clear preference for the diet that was higher in Met content. Over the whole experimental period, pigs on the Met-choice 1 group ate 6.0 kg (SE = 1.3) of the 0.19% Met diet and 18.6 kg (SE = 2.34) of the 0.23% Met diet. Pigs on the Met-choice 2 group consumed 5.5 kg (SE = 0.9) of the 0.19% Met diet and 22.0 kg (SE = 1.1) of the 0.26% Met diet.

Differences in ingested amounts of low-Met feed and feed higher in Met concentration were significant (P < 0.05) in wk 4 to 6 in the Met-choice 1 group and in wk 2 to 6 in the Met-choice 2 group. As shown in Figure 1, the chosen proportions of the diets higher in Met were close to 50% at the onset of the experiment but increased (P < 0.05) in the course of the experiment to values of 81 and 89% for the Met-choice 1 and 2 groups, respectively, in the last experimental week. As a mean of the whole experimental period, chosen diets of the Met-choice 1 and 2 groups contained on average 28 and 20%, respectively, of the 0.19% Met diet. The resulting Met concentrations of the total diets chosen by the piglets were 0.22 and 0.25% Met in the Met-choice 1 and 2 groups, respectively.

View larger version (14K): [in this window] [in a new window]   Figure 1. Selection (portion of total diet) of 0.23 or 0.26% Met diet by piglets given the choice of the 0.19 or 0.23% Met diet (Met-choice 1 group) or the choice of the 0.19 or 0.26% Met diet (Met-choice 2 group). Each data point represents a mean of 12 observations. Error bars indicate SE.

	DISCUSSION

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The results of the current study clearly demonstrate that such a feeding behavior is not only evoked by diets varying in their overall protein (Bradford and Gous, 1991, 1992; Kyriazakis et al., 1990), Lys, or Trp concentration, but also that dietary choice in piglets is altered by the dietary Met concentration. The ingestion of the 0.19% Met diet in the low-Met group resulted in a 2-fold lower growth rate, in a reduction of daily feed intake of >300 g, and a decrease of G:F of about 20% compared with the high-Met group. Moreover, the markedly reduced plasma Met and Cys concentrations clearly demonstrate the inadequacy of the Met-deficient diet.

When given a choice, piglets on both Met-choice groups selected a total diet based mainly on the diet higher in Met concentration. As a mean of the whole experimental period, chosen diets of the Met-choice 1 and 2 groups contained on average 72 and 80%, respectively, of the respective diet higher in Met. As a result of this selection pattern, the mean chosen dietary Met concentrations of 0.22 and 0.25% in the Met-choice 1 and 2 groups, respectively, were above the Met concentration of the low-Met diet, and because of this preference for the diets higher in Met, performance of the piglets on the dietary choice groups was improved (P < 0.05) compared with the piglets on the Met-deficient diet. Therefore, piglets on the Met-choice groups became aware of the deficiency provided by the 0.19% Met diet and responded successfully with an altered feed selection pattern, and therefore the signs of deficiency were at least partly redressed.

It has been suggested that animals are able to detect nutrient deficiencies and to abolish them because of an innate specific appetite for the nutrient (Richter, 1943). However, because it has been demonstrated that rats prefer a novel diet to a nutrient-deficient diet, even if this novel diet is also deficient in the specific nutrient on test (Rozin, 1965, 1969), the theory of the existence of a specific appetite has been questioned (Langhans, 1986). More likely, the anorectic response to diets deficient in an essential nutrient seems to be caused by an aversion to the deficient diet resulting from the animal’s association of the displeasing postingestive consequences caused by the ingestion of this diet and its organoleptic properties (Langhans, 1986).

As a result of the dietary-induced Met deficiency, in the low-Met group, the plasma concentration of Met (and Cys) was decreased compared with the high-Met group, whereas the sum of essential AA was increased (Table 5). The low Met concentration of the diets may have restricted body protein synthesis as reflected by poor growth rates, and therefore other essential AA were present in abundance because they were not utilized by the animal. Thus, higher total essential AA concentration in plasma of the piglets of the low-Met group and the Met-choice 1 group may be a result of the relative dietary Met deficiency compared with other AA. A relative surplus of other AA compared with Met (and Cys) is reflected by increased plasma urea concentration, which may indicate an increased oxidation and energetic use of AA in Met deficiency.

Such an excess of other essential AA may compete with the limiting AA for uptake at the blood brain barrier. Consequently, the concentration of the limiting AA in the brain declines (Peng et al., 1972; Tews et al., 1978), and this decrease may serve as a signal for recognition of the deficiency (Hrupka et al., 1997). The recognition of deficiency seems to occur in the anterior pyriform cortex, but other brain areas such as the lateral hypothalamus and surrounding areas also seem to be involved in the responses because of ingestion of AA-deficient diets (Blevins et al., 2003). Subsequent to the 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 can result in a preference for the alternative feed more adequate in AA composition.

Thus, dietary selection seems to be based on learning processes, which may explain the development of dietary selection over the time course of the present experiment. As shown in Figure 1, the chosen proportions of the diets higher in Met were close to 50% at the onset of the experiment but increased in the course of the experiment to values of 81 and 89% for the Met-choice 1 and 2 groups, respectively, in the last experimental week. As shown by Ettle and Roth (2004), when 2 feeds are offered for a choice, a period of time is necessary for the piglet to associate the feeds with their physiological consequences. Therefore, the model of a learning process, which has been found to be accelerated by giving access to only 1 of each of the 2 feeds on alternate days before the choice feeding regimen is begun (Kyriazakis et al., 1990), may explain at least in part the lower performance of animals in the Met-choice 2 group compared with the high-Met group. The Met deficiency resulting from the inability to select for the diets higher in Met at the onset of the experiment may have lead to a depressed performance at the beginning of the trial. Despite the greater Met intake due to the preferences for the diets higher in Met in the latter part of the experiment, the animals were not able to compensate for the deficiency at the beginning of the experiment. This would argue against introduction of a choice feeding system as an alternative feeding strategy for possible economic and ecologic advantages in commercial pig production. However, further studies using diets with a broader range of Met concentrations are required to determine whether a choice feeding system is suitable to optimize Met supply in swine production.

In broiler chickens it has been demonstrated that the birds used the localization of feeds in different feeders in addition to gustatory responses to find a diet more balanced in Met concentration (Steinruck et al., 1990). The more frequently the position of a Met-deficient and a Met-adequate diet offered simultaneously in 2 troughs was changed, the less pronounced the selection for the Met-adequate diet. Accordingly, it can be speculated that a choice feeding regimen without changing the position of the diets in the current study would have resulted in a more pronounced preference for the diets higher in Met in both choice feeding groups.

We used the chosen experimental design to determine whether the piglets select for the better balanced diets immediately after the change in position of the diets in the troughs. These observations revealed that the piglets chose the diets higher in Met concentration within 5 min after refilling the feeders in 68 and 79% of observations in the Met-choice 1 and 2 groups, respectively. As discussed earlier (Ettle and Roth, 2004), a decisive change in the piglets momentary metabolic status because of ingestion of a diet over 5 min seems to be unlikely, and therefore it is concluded that the piglets were able to discriminate among diets with differing Met content because of taste or odor cues.

It has been demonstrated that piglets show gustatory responses to various AA (Tinti et al., 2000) and 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 mammals, whereby sequence differences in specific receptors within and among species can significantly influence the specificity and selectivity of taste responses (Nelson et al., 2002). The number of fungiform taste buds found in humans is about 1,600 (Miller, 1986), but about 5,000 are found in pigs (Chamorro et al., 1993). Moreover, in humans there is a positive correlation between the number of fungiform taste buds and the ability to taste (Miller and Reedy, 1990). Thus, provided a similar relationship exists among species, it is likely that pigs’ sense of taste is superior to that of humans (Danilova et al., 1999), and it seems possible that piglets were able to differentiate among the diets even if the difference in Met-concentration was only 0.07%. Thus, it is concluded that piglets were able to recognize the physiological consequences of ingestion of the 0.19% Met diet, to associate those consequences with the organoleptic properties of the diet, and to alter their feeding behavior with the result that the deficiency was partly redressed.

The apparent preference for a diet with crystalline Met is interesting. In contrast to other AA such as Lys or Thr, Met is bitter tasting and has a distinctive cabbage-like odor and taste that humans tend to reject, even with very low concentrations. In the current study, however, piglets of both Met-choice groups showed no tendency to reject a Met-supplemented diet, but rather the higher the supplementation, the higher the preference for the Met-supplemented diet. Similar responses were shown in piglets given the choice between diets deficient in Lys (Kirchgessner et al., 1999), Trp (Ettle and Roth, 2004), and Thr (Ettle and Roth, 2005), or supplemented with increasing amounts of crystalline AA up to the range of requirement. Therefore, within this range of supplementation, it appears that independent of the tested AA, piglets do not reject a diet because of the inclusion of crystalline AA when this diet is more adequate in AA composition than the alternative diet.

In the current study, diets offered in both dietary choice groups were apparently below the Met requirements for 10-kg pigs (NRC, 1998). Forbes and Kyriazakis (1995) demonstrated that in instances where animals have been given a choice of feeds, neither of which fulfill their requirements, animals appear to consume appreciable amounts of the more limiting feed and in some cases approached random choice. Based on Kyriazakis et al. (1990), the consumption of the low-Met diet in the choice groups in the current study may be interpreted as an amplified sampling behavior. Such a feeding behavior can help the animal to be aware of any changes in their nutritional environment (Forbes and Kyriazakis, 1995), which may be necessary when none of the offered feeds fulfills the requirements. However, in the Met-choice 2 group, the daily consumption of the low-Met diet approximates to only 100 g in the last experimental week, and this amount seems to be negligible at a BW of about 23 kg. Moreover, in a comparable experiment, piglets given the choice of 2 Trp-deficient diets (0.11 and 0.16%, respectively) showed a 96% preference for the diet higher in Trp in the sixth wk after introduction of the choice feeding regimen (Ettle and Roth, 2004). Thus, it appears that piglets are able to dispose of such a proposed amplified feeding behavior by learning about a given nutritional environment, but the necessary time span seems to be influenced by the AA on test.

	Footnotes

 
¹ The financial support of Arbeitskreis Tierernährungsforschung Weihenstephan (ATW) e.V. is gratefully acknowledged.

² Corresponding author: roth_fx{at}wzw.tum.de

Received for publication April 12, 2005. Accepted for publication September 21, 2005.

	LITERATURE CITED

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 


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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Roth, F. X. Articles by Ettle, T. Search for Related Content PubMed PubMed Citation Articles by Roth, F. X. Articles by Ettle, T.