Evidence of a specific dietary selection for lysine by the piglet

The ability of animals to appropriate sources of food and select among them to obtain a nutritionally adequate diet that is best suited for their physiologic status has been examined in numerous species, but considerable research has addressed self-selection trials with chicks. Observations on diet preference have led to the assumption that birds have an innate ability to regulate their intake of macro- and micronutrients, and that they can do so with a considerable degree of precision (e.g. Holcombe et al. 1975, 1976; Elkin et al. 1985; Harper and Peters 1989; Emmerson et al. 1991; Steinruck et al. 1991a,b; Steinruck and Kirchgessner 1993a,b,c; Shariatmadari and Forbes 1993; Forbes and Shariatmadari 1994, 1996). Experimental evidence also shows that domesticated pigs which are offered a choice between two diets will select a diet containing an adequate amount of protein in preference to one that is deficient in protein, or will select specific proteins among various protein sources (Robinson 1974; Kyriazakis et al. 1990; Kyriazakis and Emmans 1992, 1993; Bradford and Gous 1992; Roberts and Azain 1997). The characteristic amount of a specific protein selected by the animals appeared to be related to the quality of the protein source. This was obvious from a few amino acid selection studies showing that growing-finishing pigs have the ability to select preferentially amino acid-adequate diets over amino acid-deficient diets within a few days (Devilat et al. 1970; Robinson 1975a,b; Davey 1978; Henry 1987, 1993). In contrast, there are also some exceptions to the hypothesis that selection of diet is related to the nutritional quality of the protein fed (Ashley and Anderson 1975). However, some of those studies are not conclusive due to changes in preferences by the animals over time and differences in protein source, palatability and amino acid pattern between the test diets. Since, an investigation of a self-selection behaviour by piglets has rarely been studied in detail, the purpose of this research was to determine whether piglets that are given a choice of diets with high or low lysine contents can regulate the intake of each to optimize lysine intake and performance. The objectives of this study were to determine the preference of piglets for diets varying in lysine content and to determine whether the preferences for these diets change with time. This indispensable amino acid was selected for study because it is first limiting in conventional diets for growing pigs.