Among the trace metals in animal waste that are of environmental concern are arsenic, copper, selenium, zinc, cadmium, molybdenum, nickel, lead, iron, manganese, aluminum, and boron. In particular, arsenic, copper, selenium, and zinc are often used in poultry, swine and bovine diets as growth stimulants or to supplant the use of antibiotics. As examples, arsenic-containing feed supplements are added to poultry diets and copper-containing supplements are used for prevention of hoof rot in cattle. Pesticides used directly on livestock to control houseflies and other pests can also be a contributing source of trace metals. Nearly all of these additions end up in manure. USEPA (2003a) does not consider trace elements in agriculturally-applied manure to pose much of a risk to human health and the environment, though repeated application of manure above recommended rates could potentially result in cumulative metal loadings that could have potential effects on human health and the environment.
Trace metals tend to persist in the environment and can bioaccumulate in animal and plant tissues. Copper and zinc in some manures even exceed levels in biosolids. More consideration could be given to the potential increase in management costs from metals in manures compared to savings from supplementing diets with copper, zinc, or arsenic levels above nutritional requirements (Chaney, 2003). Arsenic in poultry litter is readily soluble, and this solubility can be enhanced by a transformation of arsenic to more soluble arsenate during composting (Garbarino et al., 2003).