Preference thresholds for four limiting essential amino acids in piglets

Promoting feed intake at weaning is important to prevent growth check. Thus, highly palatable and easily digestible ingredients including appetite enhancers are commonly used in piglet diets. For example, glutamate is known to stimulate taste and appetite in pigs. In addition, the long-term appetite for limiting essential amino acids has been well documented, particularly when piglets are fed deficient diets. However, little is known about the potential sensing of these amino acids Lysine (Lys), Methionine (Met), Tryptophan (Trp) and Threonine (Thr) by the peripheral system (i.e. taste and smell) and if this was related to preferences. It was hypothesised that pigs have a high preference for essential limiting amino acids unrelated to dietary deficiencies or other potential metabolic imbalances. The current experiment examined preference thresholds (defined as the lowest dose with a preference significantly higher than 50%) for Lys, Met, Trp and Thr solutions in pigs using a 2 min double-choice (DC) model consisting of two stainless steel bowls containing either water or the amino acid solution under evaluation following the method described previously by Roura et al. (2011).

Ninety-six piglets were selected and housed in 48 pairs of males or females in two environmentally controlled rooms. Pigs were trained on a DC procedure. The test solutions were either sugar (at 200 mM), a positive control, or: Lys (at 1.0, 2.5, 5.0, 7.5 and 10 mM), Met (at 0.25, 0.5 0.75, and 1 mM), Trp (at 1, 10, 15, 20, 25, 30 mM) and Thr (at 1, 10, 15, 20, 25, 30 mM). The dose range selected was based on unpublished preliminary data. Test solution preference, measured as a percentage ratio of test solution consumed over total consumed (test + water), was compared to the neutral no-preference value of 50%. Preference trends were analysed using a linear mixed model incorporating cubic smoothing splines. The package ASReml v3 for the statistical computing software R (VSN International, Hemel Hempstead, UK) was used to fit the models.

Preference results are shown in Fig. 1. Pigs had a significant preference of 74% for the sugar solution and an average intake of 230 g and is represented as a dotted line in the graphs. The highest significant preference values for Lys, Met and Trp were 56%, 60% and 62% at 10, 1 and 20 mM, respectively. Preference thresholds (P < 0.05) were set at 5, 0.25 and 10 nm, respectively. No significant preference was observed for Thr. In conclusion, the results showed that piglets have an accurate oral perception of Lys, Met and Trp but not Thr as shown by preference values compared to water. Also, the preference threshold differed among the amino acids tested, with Met having the lowest value.

Fig. 1.  Dose-dependent preference values for Lysine, Methionine, Tryptophan and Threonine solutions in pigs. Preference is shown as a fraction of 1 (equivalent to 100%). Shaded regions in the figures represent approximate 95% confidence bands (P < 0.05). The dashed lines depict the preference value for the positive control (200 mM sugar solution).