Assessment of Dairy Feeds for Heavy Metals

Heavy metals are by definition metals having densities higher than 5 g mL?1, for example, Fe, Cu, Pb, Cd, Hg, Ni, Zn, and Mn. Heavy metals can be classified into four major groups on their health importance: essential, non-essential, less toxic and highly toxic heavy metals. Approximately fifty three of the ninety naturally occurring elements are called heavy metals and many of these, such as Cu, Mn, Fe, and Zn, are essential micronutrients, but can become toxic at concentrations higher than the amount required for normal growth. Other heavy metals, such as Cd, Hg, and Pb, have so far unknown roles in living organisms, and are toxic even at very low concentrations. Zinc and Cu are essential trace minerals required for many biological processes, particularly enzyme functions, and they have a positive in?uence on livestock growth and reproduction. Due to the low Zn and Cu content in some homegrown feeds compared with recommendations and varying bioavailability, supplementation of these metals is necessary for most livestock species, and they are commonly added to dairy rations as mineral supplements. When these nutrients are added above requirements, however, the dairy cow may restrict undesired accumulation of Zn and Cu in tissues by adaptation of absorption and excretion leading to an increase in the Zn and Cu content of manure. In addition to Zn and Cu, heavy metals such as Cr, As, Cd, and Pb are generally considered contaminants of dairy feed that are imported into the ration involuntarily, generally via phosphate-containing concentrates and supplements. Most work concerning heavy metals in animal feed has focused on pig and poultry production where it is common practice to add Cu and Zn at rates much higher than animal requirements. Very little is known about the heavy metal concentration of dairy feeds and feed components. Typical concentrations of Cu and Zn have been published in the United States and Europe, but only a few studies have reported concentrations of contaminant heavy metals (e.g., Pb and Cd), which are of more concern for animal and human healt