DIET SELECTION FOR PROTEIN BY POULTRY

If birds can learn to associate different sensory properties of foods with different metabolic feelings caused by different protein contents, then they should be able to choose a balance of foods which makes them feel most comfortable. Broiler chickens given a choice between two foods, one containing a higher concentration of protein than required and the other a lower concentration, eat amounts of the two which will give a diet that is close to optimum for growth in terms of overall protein content. When both foods have excessive or insufficient protein the birds eat predominantly from that closer to their requirements. As broilers grow they choose a progressively lower protein diet in line with their reducing protein:energy requirements. Strains with higher potential rates of lean deposition voluntarily consume a higher ratio of high-protein to low-protein food compared with low-growth strains, while strains selected for high fat deposition eat less protein per unit of energy than lean birds. Males select higher protein diets than females. Although increases in environmental temperature would be predicted to reduce energy requirements and thus increase the protein:energy ratio in selected diets, heat-stressed birds do not increase the protein content of their diet, probably because of the high heat increment of protein metabolism. Short periods of access to one food alone are followed by a preference for the opposite food when access to both is restored. When given appropriate training, laying hens can balance their diet for protein but not as well as broilers. There is evidence that growing chicks can differentiate high-lysine from low-lysine foods but do not entirely balance their diet for lysine. Methionine-deficient broilers eat more of a methionine supplemented food than normal birds, while laying hens eat some methionine-fortified food when given a choice against a low-methionine food, but not enough to support normal egg production. It is concluded that poultry can show a great degree of nutritional wisdom' as far as protein is concerned but that they need to be allowed to differentiate between foods with different nutrient profiles by sensory means, and a lack of training might account for some of the poor results obtained.