The Ability of Chickens to Select Nutritive and Avoid Toxic Concentrations of Heavy Metals in Feeds

Abstract

ABSTRACT The ability of free range chickens to detect and select feeds with varying heavy metal concentrations is important in relation to their ability to avoid toxic heavy metals in polluted regions and to self-supplement their diet with nutritive metals. An operant test was conducted to determine whether chickens could detect a range of heavy metals mixed with feed pellets, with seven birds rewarded for selecting feeds with added metals and seven birds rewarded for selecting feeds without added metals. The chickens demonstrated an ability to detect very high levels of iron, copper, and zinc in feed, but they could not detect concentrations typical of polluted regions, that is, 1,500–2,000 mg/kg feed. They also could not detect the potentially toxic elements cadmium, lead, or selenium at concentrations just below the level likely to induce toxicity. The chickens were successfully trained to discriminate between three different pairs of colored plates (red/green, black/yellow, and blue/white) and to associate the different pairs of colored plates with food contaminated and uncontaminated with copper, zinc, and iron, respectively, at the maximum concentration which they could detect. Their preference for the contaminated or uncontaminated food samples was then investigated in a Y maze, using the association with plate color. Although there was no significant selection for or against copper in feed, they exhibited a partial preference for feed contaminated with iron, compared with feed without iron. There was limited evidence of selection against feed with added zinc. It is concluded that chickens can detect some nutritive heavy metals in their feed when they are present at high concentrations, but that they are unlikely to be able to detect typical levels of either nutritive or potentially toxic heavy metals when their food is obtained in free range situations.