Food intake of birds can be affected by particle size as well as diet composition. In order to investigate whether food intake of diets including excessive amount of phenylalanine (Phe) was influenced by diet types, a series of experiments were conducted in growing chicks and laying hens. Growing chicks significantly decreased food intake in a semipurified excessive Phe diet compared with a semipurified control diet, while laying hens fed a practical diet including excess Phe did not significantly reduce their food intake over a corn starch-substituted control diet. In an attempt to find out whether diet type affects food intake in layers, the semipurified type diet with excess Phe greatly reduced food ingestion, but the effect was delayed in the practical type diet. Moreover, under choice feeding regimes between the Phe and either starch, tyrosine (Tyr) or fiber diets in order to investigate whether the decreased food intake in the presence of an excess of dietary Phe in laying hens is involved in the payability for the diet, there was no significant difference between Phe and starch diet while a preference for the Phe diet tended to be increased when birds were offered Tyr. Laying hens ingested significantly more die Phe diet than the fiber diet within 1 h after feeding. For supporting the idea that preference for the Phe diet may be affected by manipulating taste sense, an anaesthetic or saline was intramuscularly administered under the tongue just before a choice feeding. Preference for the Phe diet was not significantly different from that for the fiber reference diet within 1 h in the anaesthetized birds while the birds preferred the Phe diet in the saline treated group. It is suggested that because birds are able to select a diet, the decreased food intake induced by dietary excess Phe may be due to the repulsive effect of Phe after ingestion but not the taste of Phe.
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