Distribution and Bioavailability of Trace Elements in Livestock and Poultry Manure By-Products

Abstract

Intensive confined livestock and poultry production systems generate large quantities of manure by-products, which have the potential for being recycled on arable land. Protecting the quality of the environment is a major consideration when developing management practices to effectively use manure by-products as a nutrient resource and soil conditioner in agricultural production system. To date, most of the environmental problems associated with land application of manure by-products have centered on the contamination of groundwater and/or surface water with two major nutrients, nitrogen (N) and phosphorus (P). With increasing use of trace elements (metal is substituted for trace elements for brevity throughout the text) as nutritional supplement in the form of feed additive in intensive animal production industries, manure application has emerged as an important source of certain metals (e.g., As, Cu, and Zn) input in soils. Unlike application of sewage sludge, where application rate is limited based on allowable metal loadings, regulations governing livestock and poultry manure by-products are generally based on total N and/or P loading. Both sewage sludge and manure by-products are applied on land to primarily benefit from their N and/or P content but without regard to metals in the latter. The danger lies in accumulation of manure-borne metals since they virtually don't degrade with the potential of eventually becoming phytotoxic. In order to reduce the risk of offsite contamination, it is prudent to propose that land application guidelines for manure by-products be developed that consider their total composition rather than just only specific component (i.e., N and/or P). The present review aims to examine the impact of increased usage of certain metals, especially As, Cu, and Zn, in livestock and poultry production on the quality of manure by-products in relation to their distribution in soils and their subsequent bioavailability to plants. The review first discusses the various sources, concentration, and distribution of these metals in manure by-products. The beneficial effects of manure addition to overcome the deficiency of these metals in soils and the detrimental effects of manure-borne metals on plant growth and microbial functions are also examined. The practical implications of manure-borne metals on environmental contamination are discussed in relation to management guidelines for the safe and beneficial use of manure by-products in agricultural soils.