Effect of Long-Term Application of Biosolids on Molybdenum Content and Quality of Winter Wheat Forage

Abstract

Forages that accumulate excessive molybdenum (Mo) from excessive land application of Mo can cause molybdenosis, a copper (Cu) induced deficiency in ruminants. Limited information is available on the effect of land-applied biosolids, Mo content, and quality of winter wheat forage (Triticum aestivum L.). The objectives of this study were (1) to determine the effect of biosolids application on tissue molybdenum and tissue copper (Cu):Mo of winter wheat forage and, (2) to measure the Mo uptake coefficient value (UC) for winter wheat under field conditions and compare it with the U.S. EPA Part 503 risk-based UC value of 0.42. Two nitrogen sources, anaerobically digested biosolids and ammonium nitrate, were applied annually from 1993–2001 to continuous winter wheat. The experimental design was a complete factorial arrangement of treatments composed of six nitrogen (N) rates (0, 45, 90, 180, 269, and 539 kg N ha−1 yr−1) and two N sources (anaerobically-digested biosolids and ammonium nitrate, 34-0-0). Application of biosolids did not significantly alter soil pH, which ranged from 6.6 to 7.2. Biosolids application increased soil Cu from 7.19 to 19.6 mg kg-1 and soil Mo from 0.44 to 1.02 mg/kg. Forage uptake of Mo and Cu showed temporal variation between years but increased with biosolids application rate. The Cu:Mo ratio of washed forage was > 2.9 and was inversely related to biosolids application rate. Forage Mo was < 2 mg kg-1. Unwashed forage had a larger Cu:Mo ratio than washed forage. Forage Mo increased with biosolids application for unwashed forage. However, the Cu:Mo ratio of unwashed forage decreased with increased biosolids application and the unwashed forage Cu:Mo ratio was > 10. Biosolids application had no effect on forage sulfur (S) content that was < 3000 mg kg-1. The calculated UC of Mo for winter wheat in Oklahoma of 0.24 (washed forage) and 0.36 (unwashed forage) is less than the UC of 0.42 used by U.S. EPA in its risk assessment governing land application of biosolids. These results suggest winter wheat forage produced using biosolids presents minimal risk of molybdenosis to livestock. Ingestion of soil on unwashed forage increases forage Cu:Mo and offers more protection from the risk of molybdenosis.