Long‐term effects of copper rich swine manure application on continuous corn production

Three long‐term field experiments were established in the spring of 1978 and continued through 1988 to evaluate corn (Zea mays L.) response to high Cu levels from Cu rich swine manure and CuSO4 applications. The field research was conducted on soils with diverse properties, i.e., on a Bertie fine sandy loam (fine‐loamy, mixed, thermic, Aquic Hapludults), a Guernsey silt loam (fine, mixed, mesic, Aquic Hapludalfs), and a Starr‐Dyke clay loam (fine‐loamy, mixed, thermic Fluventic Dystrochrepts‐clayey, mixed, mesic, Typic Rhodudults). Three treatments in the long‐term field experiments on the three soils were i) a control, ii) annual applications of Cu as Cu rich swine manure, and iii) annual applications of Cu as CuSO4 equivalent to that in the manure. After the 11 years, 1109 mt ha‐1 of wet Cu rich swine manure were applied for the manure treatment. The Cu rich manure, which contained an average of 1316 mg Cu kg‐1, was produced by swine fed diets supplemented with an average of 251 mg Cu kg‐1 as CuSO4. An average of 325 kg Cu ha‐1 added to the soils from the manure application over the 11 years exceeded U.S.E.P.A. guidelines for safe copper loading levels for cropland by 45 kg ha‐1. Copper concentrations in corn ear leaves were within the normal range of 3 to 15 mg kg‐1 where the 325 kg Cu ha‐1 were applied to the soils as either Cu rich manure or CuSO4 over the 11 years. Concentrations of Cu in the grain also were in the normal range of 1 to 5 mg kg‐1 where the high level of the two Cu sources were applied to the soils. There was no decrease in corn yield on the three soils from application of either Cu source. The lack of yield decrease from the Cu rich manure and CuSO4 applications may reflect decreased Cu availability at the near neutral pH in the soils under study.