The effect of protein source on the diets selected by pigs given a choice between a low and high protein food.

To test the effect of the protein source on the diet selection of growing pigs, four foods of high protein concentration were made. Each food was based on a different protein meal: fish, soya bean, and two types of rape seed (with low and high glucosinolate contents), and was offered to growing pigs together with a food of low protein concentration (L), as a choice. Pigs given a choice between the fish or the soya bean meal-based food and L, selected a diet that met their protein requirements, as judged by their growth rates and food efficiency: the protein content of the selected diet declined systematically with pig weight. Pigs given a choice between the rape seed meal-based foods and L selected diets with a high proportion of L and, consequently, failed to meet their protein requirements. Given that the preference for L was more marked when the animals were given access to the rape seed meal higher in glucosinolates, it is suggested that their diet selection on these foodpairs was a tradeoff between minimising glucosinolate intake, which has potentially goitrogenic properties, and achieving a protein intake sufficient for maximum performance. This suggestion is supported by the fact that the fish meal-based food was preferred over the ones containing rape seed meal, and that the rape seed meal lower in glucosinolate content was preferred over the other when pigs were given a choice between them.