Heavy metal occurrence and risk assessment in dairy feeds and manures from the typical intensive dairy farms in China

Modern farming practice features extensive overuse of additives in animal feed. Subsequent use of manure as a fertilizer has resulted in significant heavy metal accumulation in agricultural soil, which is particularly apparent in areas of intensive farming. Here, samples of dairy feed, manure, water, and soil were collected from four intensive dairy farms in China and analyzed to assess selected heavy metal concentrations (Cu, Zn, Cr, Ni, Pb, and Cd). Results revealed that all feed samples contained the selected heavy metals, attesting to the wide use of additives during intensive dairy farming. The average Cr and Pb concentrations were 6.1 to 17.1 times greater than their recommended guidelines. Overall, average heavy metal concentrations in manure decreased in the following order: Zn > Cu > Cr > Ni > Pb > Cd. Using data obtained from the sequential extraction procedure, proposed by the Community Bureau of Reference (BCR), metal bioavailability also decreased according to the following order: Pb (69.4%) > Cr (63.7%) > Ni (60.8%) > Cu (53.4%) > Zn (50.0%) > Cd (34.5%). Heavy metal levels in sampled wastewater were also relatively high; however, surface and well water levels were relatively low. Although use of manure in dairy farming has not resulted in serious pollution until now, Zn, Cu, and Cd are all known to pose significant risk to soil quality. Finally, principal component analysis (PCA) results indicated that heavy metal levels in soil originated predominantly from parent soil materials and were then enhanced by anthropogenic sources.