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Feed preference in pigs: Effect of cereal sources at different inclusion rates¹

D. Solà-Oriol^*, E. Roura and D. Torrallardona^*,2

^* Animal Nutrition, Institut de Recerca i Tecnologia Agroalimentàries (IRTA), E-43120 Constantí, Tarragona, Spain; and R & D Feed Additives, Lucta SA, E-08170 Montornés del Vallès, Barcelona, Spain

	Abstract

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The palatability of different cereals was studied in 2 two-way choice (preference) experiments using pigs of 56 d of age and 17 kg of BW. In Exp. 1, the effect of 24 cereals vs. a common reference diet containing white rice on feed preference in pigs was studied. Pigs were offered free choice between the reference diet and a diet with the cereal under study for 4 d. Barley, corn (2 sources), wheat, cassava meal, biscuit meal, rye, sorghum, and 1 source of oats were tested at inclusion rates of 300 and 600 g·kg^–1. Short-grain rice (whole, brown, or extruded white), long-grain white rice (raw and cooked), extruded barley, extruded corn, extruded wheat, oats (2 sources), thick rolled oats, cooked oats, and naked oats (raw, extruded, or micronized) were tested at inclusion rates of 150, 300, and 600 g·kg^–1. Relative preference of cereals (% of total feed intake) was affected by type of cereal and by rate of inclusion. The diets containing extruded rice (150 g·kg^–1), extruded naked oats (150, 300, and 600 g·kg^–1), or naked oats (150 and 300 g·kg^–1) were preferred (P < 0.05) by pigs to the reference diet. However, the reference diet was preferred (P < 0.05) to the diets containing 150, 300, and 600 g·kg^–1 of cooked long-grain rice, oats, or cooked oats, 300 and 600 g·kg^–1 of extruded wheat, wheat, corn, sorghum, or unhulled short-grain rice, and 600 g·kg^–1 of thick rolled oats, extruded corn, rye, extruded barley, micronized naked oats, barley, cassava, or biscuit meal. Extrusion improved (P < 0.05) preference values for corn and naked oats by pigs, but had no effect on barley, rice, or wheat. In Exp. 2, the preferences of pigs for oats and barley were studied using mash and pelleted diets. Diet form did not affect preference in oats diets. However, for barley, greater preference values were obtained when measured in pelleted form compared with mash form. Additionally, direct 2-way choices were also performed between oats and barley diets and between diets presented in mash and pelleted forms. Pigs preferred barley to oats, and preferred diets presented in pelleted form to those presented in mash form. In conclusion, cereal type, inclusion rate, and diet form affected feed preference in pigs. Using cereals with greater preference values may contribute to the formulation of more palatable feeds, which enhance feed intake of piglets at critical stages such as weaning time.

Key Words: cereal • palatability • pig • preference • two-way choice

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Piglets at weaning have difficulty in initiating dry feed consumption. They are removed from the sow and offered dry feed and plain water, which, along with other stress factors (Lalles et al., 2007), results in what has been referred to as postweaning anorexia. Consequently, the integrity of the gastrointestinal tract may be compromised (Pluske et al., 1997). Thus, ensuring a fast initiation of feeding immediately after weaning is essential to maintain gut function and piglet growth. Animal taste and smell have evolved to associate beneficial (or nutritious) compounds and detrimental (or toxic) compounds with pleasant and unpleasant sensations, respectively (Goff and Klee, 2006). This may result in innate preferences or aversions for different feeds regardless of previous experiences or postingestive mechanisms. Therefore, the newly weaned pig mostly depends on its oro-nasal sensing systems to evaluate feed, and the most palatable feed ingredients should be used at this stage (Forbes, 1995).

Cereals constitute the main energy source in piglet diets and are chosen on the basis of their nutrient composition, ignoring possible differences in palatability. The quantification of pig preferences for different cereals should enable the formulation of highly palatable diets that improve feed intake at weaning. This study aimed to quantify the palatability of different cereals with the use of 2-way choice preference tests as proposed by Solà-Oriol (2008). The impact on feed preference of other factors that have been shown to affect feed intake, such as cereal extrusion and feed pelleting (Medel et al., 1999), has also been studied.

	MATERIALS AND METHODS

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All the experiments were conducted at Animal Nutrition Unit, Institut de Recerca i Tecnologia Agroalimentàries (IRTA, Tarragona, Spain). The experimental procedures with animals described in this study were approved by the Ethical Committee on Animal Experimentation of IRTA.

Animals and Housing

A total of 912 pigs (Landrace x Pietrain) from 9 production batches in the IRTA sow herd were used to perform 9 groups of 2-way choice tests. Pigs (456 boars and 456 gilts) began the tests at 56 d of age (4 wk postweaning) with an average BW of 16.8 ± 4.2 kg. The results of our previous study (Solà-Oriol, 2008) indicated that feed preferences did not differ between piglets at weaning and 4 wk after weaning. Therefore, the results of the present study using 56-d-old pigs should be applicable to newly weaned pigs. Eight of the pig groups were used in Exp. 1 and the last group was used to conduct Exp. 2. For each group of pigs, there were 36 pens containing 2 to 3 pigs each in 2 weaning rooms (24 and 12 pens, respectively) with automatic, forced ventilation and completely slatted floors. Each pen was provided with 2 feeders with 2 feeding spaces placed beside each other and an equidistant independent water supply at the opposite wall. Within a pen, the 2 diets compared were offered in the 2 feeders according to a 2-way choice protocol.

Experimental Design

In Exp. 1, each of the 8 pig groups was used to conduct 12 two-way choice comparisons during 3 consecutive 4-d experimental periods (4 comparisons per period). All of the two-way choice comparisons were conducted using the same reference diet containing 600 g·kg^–1 of white broken rice as the only source of cereal. In each experimental period, 1 of the two-way comparisons consisted of a "reference diet vs. reference diet" comparison that was used as the control. The remaining 3 two-way comparisons were used to test 3 different cereals. In the first experimental period, the 3 cereals were tested at 300 g·kg^–1 for the first 3 groups of pigs and at 150 g·kg^–1 for the last 5 groups of pigs. In the second period, the inclusion rate of the cereals was increased to 600 and 300 g·kg^–1 for the first 3 and last 5 groups of pigs, respectively. Finally, in the third period, the cereals alone (at 1,000 g·kg^–1) were tested against rice on its own for the first 3 groups and at 600 g·kg^–1 for the last 5 groups of pigs. The same cereal was evaluated during the 3 periods for each of the 36 pens in each group of animals, and only the inclusion rate was increased with time. Therefore, in Exp. 1, a total of 24 different cereal ingredients were compared with the reference diet with white broken rice. Nine replications for each comparison were obtained. All the diets were presented in mash form.

The position of the feeders was not changed during the experiment, and the reference diet was always kept in the same position during the 3 periods. This eliminated the need for an adaptation period that would have been necessary to respond to the changes in feeder position and shortened the duration of the measurements. Although it could be argued that this approach would not correct for possible effects of feeder position on preference, a control "reference vs. reference" two-way comparison was included in every group of animals to detect such possible effects. For all the animals, it was confirmed that the prestarter and starter diets that they had been offered before the start of the trial did not contain rice (the reference cereal) or any of the cereals to be tested in each particular group.

The last group of animals (36 pens) was used to conduct Exp. 2, which was divided into 2 parts (2A and 2B). In Exp. 2A, 24 pens were used to test barley and oats vs. the reference diet, essentially as described for Exp. 1. Twelve pens were used to compare diets presented in mash form, and the other 12 were used to compare diets presented in pelleted form. Both cereals were tested at inclusion rates of 200, 400, and 600 g·kg^–1 in 3 consecutive 4-d periods. Six replications for each comparison were obtained. Experiment 2B was conducted with the last 12 pens. In the first 4-d period, the control comparison "reference diet vs. reference diet" was made with both mash and pelleted diets (6 pens each). In the second 4-d period, animals that had been offered pelleted diets during the first period were offered a direct two-way choice between pelleted diets containing 600 g·kg^–1 barley and 600 g·kg^–1 oats. The animals that had been offered mash diets were offered the same two-way choice between barley and oats diets, but presented as mash. Finally, in the third 4-d period, each group of 6 pens was divided into 2 groups of 3 pens. One was offered the choice between mash and pelleted barley diets (at 600 g·kg^–1), and the other was offered the choice between mash and pelleted oats diets (at 600 g·kg^–1). Six replications for each comparison were obtained, 3 of which had previous experience with pelleted and 3 with previous experience with mash diets.

Diets and Feeding

A basal diet (Table 1) was formulated to meet or exceed the nutrient requirements for swine (NRC, 1998). All of the cereal ingredients were ground with the same hammer mill using a 3-mm screen. Different cereals were evaluated in diets prepared by replacing the corresponding amount of broken rice from the reference diet with the cereal of interest. To avoid possible interferences on feed palatability caused by ingredients other than the cereals, energy, protein, and AA composition were not corrected. The addition of fat or free AA themselves may affect palatability (Mori et al., 1991a,b; Tabuchi et al., 1991; Mizushige et al., 2007). It was decided not to correct for nutrient composition because it would not have been possible to assess the palatability of the cereals per se if fat or AA were added to the feed.

The different cereal ingredients tested were analyzed according to AOAC (2000) for DM (method 925.09), ash (method 923.03), crude fiber (method 962.09), ether extract (method 920.39), and CP (method 968.06). The results of the analysis are shown in Table 2.

Measurements and Calculations

In each pen, feed disappearance from the 2 feeders was measured between d 0 to 4, d 4 to 8, and d 8 to 12. Preference was measured as intake of the test diet expressed as percentage of total feed intake. The following mathematical equation was used:

Statistical Analysis

Exp. 1. Preference values were compared (between cereals by inclusion rate or between inclusion rates by cereal) by ANOVA using the GLM procedure (SAS Inst. Inc., Cary NC). The mathematical model used was Y_i = µ + _i + _i, where Y_i is the preference value for the observations of cereal ingredient (or inclusion rate) i, µ is the general mean of all observations, _i is the effect of the feed cereal source (or of inclusion rate), and is the unexplained random error. Means from the ANOVA models were compared using Tukey’s Studentized Range (Honestly Significant Difference) test. Additionally, the preference for each cereal at each inclusion rate was compared with the neutral value of 50% using a Student’s t-test.

The preferences for diets containing either 300 or 600 g·kg^–1 extruded barley, corn, naked oats, white rice, or wheat were compared with that of the corresponding diets with raw cereal using a Student’s t-test. The level used for the determination of significance for all the analysis was 0.05.

Exp. 2. Preference values for barley and oats at each inclusion rate (200, 400, and 600 g·kg^–1) were analyzed with ANOVA using the GLM procedure of SAS by considering the main effects of diet form, cereal type, and their interaction. The mathematical model used was Y_ij = µ + _i + β_j + (β)_ij + _ij, where Y_ij is the percentage of preference for the observations of diet form i and cereal j, µ is the general mean of all observations, _i is the effect of the diet form (mash or pelleted), β_j is the cereal effect (barley or oats), (β)_ij is the interaction between diet form and cereal, and _ij is the unexplained random error.

Additionally, Student t-tests were performed to compare 1) the preference for each cereal at each inclusion rate and each diet form with the neutral value of 50%, 2) ADFI of barley- and oats-based diets offered as two-way choice either in mash or pelleted form, and 3) ADFI between mash and pelleted diets offered as two-way choice according to cereal composition (barley or oats) and previous diet form experience of the pigs (mash or pelleted).

	RESULTS

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Exp. 1

The preference values obtained in the first 3 groups of animals using pure cereals were erratic (i.e., not always agreeing well with those obtained at lesser inclusion rates). Taking this into account and the fact that cereals are always included in a balanced diet under practical conditions, in groups 4 to 8, cereals were not tested in pure form; instead, the inclusion rate of 150 g·kg^–1 was evaluated. The percentage preference measured for the double reference control did not differ from 50% (overall mean = 50.4; SE = 1.22) in any of the 8 groups of pigs (P-values ranging from 0.29 to 0.94), indicating that the observed preference values may be attributed to dietary changes in cereal ingredients.

Preference values for the different cereal ingredients studied relative to the common reference diet are shown in Table 3. Differences among cereals were observed at each of the inclusion rates tested (P < 0.05). Among the cereals tested at inclusion rates of 150 g·kg^–1, the greatest preference values were observed for extruded rice, extruded naked oats, and naked oats, and having a preference value greater than 50% indicated that they were preferred to the reference diet (P < 0.05). However, the preference values observed for extruded barley, brown rice, extruded corn, thick rolled oats, extruded wheat, micronized naked oats, long-grain white rice, and unhulled short-grain rice were not different from the reference diet. Finally, the least preference values (<50%) were observed for cooked white long-grain rice, oats (sources 2 and 3), and cooked oats (P < 0.05).

At the greatest inclusion rate of 600 g·kg^–1, a greater preference value than the neutral value of 50% was only observed for extruded naked oats (P < 0.05). The preference values for extruded rice, brown rice, long-grain white rice, and naked oats were not different from 50%; whereas thick rolled oats, extruded corn, rye, cooked long-grain rice, extruded barley, micronized naked oats, extruded wheat, barley, cassava, wheat, biscuit meal, corn (sources 1 and 2), sorghum, unhulled rice, oats (sources 1 to 3), and cooked oats had preference values below 50% (P < 0.05). When the cereals were tested as pure ingredients (1,000 g·kg^–1) using pure white short-grain rice as a reference, the preference values for cassava and barley were greater than 50% (P < 0.05), whereas corn (source 1), oats (source 1), and sorghum were less than 50% (P < 0.05).

The effect of inclusion rate (P < 0.05) was observed regardless of the cereal ingredients. When pure ingredient results were discarded, cereal preference decreased as the rate of inclusion increased, with the exception of extruded naked oats for which a greater preference value was observed at the inclusion rate of 300 g·kg^–1 than at 150 g·kg^–1 (P < 0.05). As mentioned before, the preference values for the pure cereals did not always agree with those obtained at lesser inclusion rates. Although preference generally decreased with increasing inclusion rates, greater preference values were obtained with barley, corn (source 2), cassava, biscuit meal, and oats (source 1) at 1,000 g·kg^–1 than those cereals included at 600 g·kg^–1 (P < 0.05). However, it must be taken into account that pure cereals were compared with pure rice instead of the reference diet, which may explain these observations.

The effect of extrusion on the preference of different cereals is shown in Figure 1. Extrusion improved the preference value for corn (P = 0.06 and P < 0.05 at 300 and 600 g·kg^–1, respectively) and naked oats (P < 0.05), but had no effect on barley, rice, or wheat.

View larger version (18K): [in this window] [in a new window]   Figure 1. Percentage of preference of raw vs. extruded for different cereals tested included at 300 (A) and 600 g·kg–1 (B; Exp. 1). Differences in preference between diets with raw and extruded cereal: *P < 0.05 and P < 0.10. Values are means (n = 9) with SE represented by vertical bars.

The preferences for the double reference control in mash and pelleted form (Exp. 2B, first period) were 44.62 (SE, 3.93) and 46.65% (SE, 4.58%) respectively, which were not different from 50% (P = 0.293 and 0.645, respectively). Preference values for barley and oats by diet form (mash or pellet; Exp. 2A) are presented in Table 4. Greater preference values were observed for barley than for oats at all inclusion rates (P < 0.01), confirming the findings of Exp. 1. A tendency (P < 0.10) for greater preference values was observed in pelleted diets for the inclusion rates of 200 and 400 g·kg^–1. At these inclusion rates, interactions between diet form and cereal type were observed (P = 0.10 and P < 0.05, respectively), indicating that preference improved when measured in pelleted form for barley but not for oats.

View larger version (12K): [in this window] [in a new window]   Figure 2. Individual ADFI of barley- and oats-based diets (600 g·kg–1) offered in a 2-way choice: presented either in mash or pellet form (Exp. 2B, second period). Differences between barley and oats diets: *P < 0.05. Values are means (n = 6) with SE represented by vertical bars.

View larger version (14K): [in this window] [in a new window]   Figure 3. Individual ADFI of mash and pelleted diets offered in a 2-way choice according to previous experience with either mash or pelleted diets. Diets contained 600 g·kg–1 of barley (A) or oats (B; Exp. 2B, third period). Differences between mash and pelleted diets: *P < 0.05. Values are means (n = 3) with SE represented by vertical bars.

	DISCUSSION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Cereals are the main source of energy in piglet diets, and they may play an important role in the palatability of feed and its recognition by the weaned piglet. The measurement of 2-way choice preferences against a common reference diet has been proposed as a way of quantifying feed palatability (Solà-Oriol, 2008). The results of our previous study indicated that feed preferences did not differ between newly weaned piglets and pigs at 4 wk postweaning if the animals from either age group did not have previous experience with either of the 2 ingredients tested. Although we used pigs at 4 wk postweaning, previous experience with the cereals tested was avoided, and therefore, the preference values observed could be applicable to newly weaned pigs.

Our results showed an increased palatability for cereals not commonly used in piglet feeding such as white rice and naked oats, whereas a widely used cereal such as corn had relatively poor preference values. This is in agreement with the work of Mateos et al. (2006) who showed greater feed intake and BW gain for rice-based diets than for corn-based diets.

We observed that pigs had a particularly reduced preference for oats. This is probably because of its increased fiber content, as husk removal (in thick rolled oats) clearly improved the preference. This may also explain the decreased preference of pigs for the unhulled short-grain rice compared with polished white rice.

Cereals with increased fiber content may increase the bulkiness of the diet and affect intake because of the physical capacity of the gastrointestinal tract (Kyriazakis and Emmans, 1995; Whittemore et al., 2001). Mateos et al. (2006) observed that oat hulls reduced feed intake when added to corn-based diets, but had the opposite effect when added to rice-based diets, indicating that fiber only affects palatability above certain rates of inclusion.

However, crude fiber content cannot explain the differences observed between other cereal sources. Many plant volatile compounds are produced in plant tissues at specific developmental stages. Although hundreds of volatiles are synthesized at different stages of cereal grain development, only a small subset helps animals recognize the appropriate feeds and avoid poor or dangerous feeds (Goff and Klee, 2006). Studies of flavor preferences and aversions indicate that flavor perception may be linked to the nutritional value associated with the perceived feeds (Gilbertson et al., 1997; Myers et al., 2005). Some of the preference values observed in our study may be explained by the sensory evaluation of the feed, as each cereal ingredient is characterized by a particular volatile profile that can be distinguished by the olfactory system of mammals (Grosch and Schieberle, 1997; Zhou et al., 1999; Sides et al., 2001; Schuh and Schieberle, 2005).

Innate preferences may be due to the evolution of taste sensory receptors to recognize specific nutrients or toxic constituents (Mueller et al., 2005). Nutritive characteristics are usually associated with receptors of pleasant tastes: sweet receptors with carbohydrate energy sources, umami with AA, and salty with mineral sources. In contrast, the stimulation of sour and bitter receptors is unpleasant and warns the animal against deteriorated feed or toxic compounds, respectively. This is a potentially important factor when evaluating feed preferences in naive weanling pigs.

Some plant volatiles or their precursors may also have antimicrobial or health-promoting activities (Villalba and Provenza, 2005; Goff and Klee, 2006), and as a result, animals may have developed a preference for them. For example, it is well accepted that grazing herbivores are able to self-medicate by selecting plants with medicinal properties (Villalba and Provenza, 2007).

It is possible that the increased preference values observed in our work for white rice could be related to its health-promoting properties. Pluske et al. (2002, 2007) observed that a diet based on white rice improved the resistance of piglets to an Escherichia coli infection and proposed the use of this cereal as a means of controlling postweaning diarrhea in piglets. Similarly, Mateos et al. (2001) indicated improved growth in piglets fed cooked rice, but they attributed their results to a greater digestibility and decreased content of nonstarch polysaccharides.

Feed intake and preference may be regulated by postingestive factors as well as sensorial perception. The nutrient composition of the diet has been reported to affect feed intake (Kyriazakis and Emmans, 1995; Whittemore et al., 2001) probably via feedback signals from the gastrointestinal tract that respond to quality and quantity of ingested nutrients. In our studies, the different cereals replaced the white rice from the reference diet on a weight basis without adjusting the nutritional value. Correcting for energy and AA composition was not made as this may affect feed palatability by itself. Fat (Mizushige et al., 2007) and free AA (Mori et al., 1991a,b; Tabuchi et al., 1991) have been shown to have an effect on palatability. In addition, under our experimental conditions (Solà-Oriol, 2008), the preference responses are evident as early as the first day of testing, and do not change for at least 10 to 12 d. This indicates a sensorial rather than a postingestive (nutritional) response. Nevertheless, feed preference responses to nutritional composition seem to require a period of adaptation longer than the 4 d used in our experiments (Black et al., 1986). Kyriazakis et al. (1991) also reported that although pigs are able to make appropriate choices between diets differing in protein composition to meet their nutritional needs, they can only do so if they had previous experience with the feeds being offered and had been able to learn about their nutritional consequences. In the current study, pigs did not have previous experience with any of the diets under evaluation, further refuting a possible nutritionally driven choice response.

It could be hypothesized that the improved preference observed with extrusion cooking of corn and naked oats was due to an improvement in the availability of their starch and faster glucose release (Medel et al., 1999; Piao et al., 1999) that resulted in improved palatability. The improvement of feed intake and performance of piglets due to the extrusion of maize supports this hypothesis (Hongtrakul et al., 1998). However, Medel et al. (1999) observed that extrusion of corn improved BW gain of piglets without affecting feed intake, which indicates an effect on nutrient digestibility rather than palatability. Similarly, in our study, the micronization of naked oats had a negative effect on preference, despite an expected improvement in nutrient digestibility (Huang et al., 1998). Therefore, there seems to be no clear relationship between nutrient availability and feed preference. Differences in orosensorial perception cannot be discarded as it is well known that heat treatments can cause a dramatic change in the profile of volatiles released.

Our study on cereal preference was performed using diets in mash form. Considering the fact that diets are often presented as pellets in practice, we performed a 2-way choice to test whether diet form could affect the preference values in oats- and barley-based diets. Although no effect of diet form on the measurement of preference was observed for oats, the preference values for barley were increased in pelleted diets. It seems that pelleting affected the palatability of barley in a different way compared with white rice. Therefore, possible interactions between cereal type and technological processing of feed must be considered.

Because of the impossibility of testing all cereals against each other, they were compared with the same reference diet containing rice. It could be assumed for a given pair of cereals that the observed relative preferences (vs. rice in the reference diet) would be indicative of which of the 2 cereals would be preferred in a direct comparison against each other. In this way, in Exp. 1, it can be expected that barley should be preferred to oats because the preference value for barley was 23% at 600 g·kg^–1, whereas the preference value for oats ranged between 3 and 8%. The greater intake observed in Exp. 2 for the barley- compared with the oat-based diets when offered in a direct 2-way choice supports this approach.

The greater preferences observed for pellet over mash feed form for both barley- and oat-based diets are in good agreement with the results of Skoch et al. (1983). They observed that when pigs were offered a free choice between corn-based pelleted and mash diets, pigs preferred pelleted diets (85.5 vs. 14.5%, respectively).

The nutritional value of feedstuffs is the main criteria for feed formulation and some aspects such as palatability are often ignored. Our work showed marked differences in pig preference for different cereals. This information may allow nutritionists to choose cereals with an increased preference by the piglets, which may contribute to more efficient feed formulation at critical stages such as immediately after weaning.

	Footnotes

 
¹ This study was supported by CDTI (Spanish Innovation Agency, Ministry of Industry Tourism and Trade), Madrid, Spain; project 050369. The authors thank the technical support received from the IRTA laboratory and farm staff.

² Corresponding author: David.Torrallardona{at}irta.es

Received for publication February 12, 2008. Accepted for publication October 13, 2008.

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