Iron antagonism of MSMA herbicide applied to bermudagrass: characterization of the Fe 2+ -MAA complexation reaction

• Published in: Weed science 2006-02, Vol.54 (1), p.23-30
• Main Authors: Massey, Joseph H., Taylor, James M., Binbuga, Nursen, Chambers, Kristen, Coats, G. Euel, Henry, William P.
• Format: Article
• Language: English
• ISSN: 0043-1745; 1550-2759
• DOI: 10.1614/WS-05-089R.1

Discoloration of bermudagrass often results from application of MSMA herbicide used to control southern crabgrass and other weeds. However, when products containing iron sulfate (FeSO 4 ) are tank-mixed with MSMA, this discoloration is reduced. Experiments investigated the effect of tank-mixing organic arsenical herbicides with FeSO 4 or a chelated iron source (Sprint 330) in terms of southern crabgrass control and injury to bermudagrass. Tank-mixing MSMA with FeSO 4 reduced bermudagrass injury. However, southern crabgrass control was also reduced by at least 50% with the addition of ≥0.38 kg Fe 2+ ha −1 . Neither antagonism nor safening of bermudagrass was observed when the chelated Fe 2+ source was used. Applying FeSO 4 as a separate treatment 1 to 4 d before or after MSMA application did not reduce visual burmudagrass injury 1 wk after treatment. Solution pH and FeSO 4 concentration controlled the extent of complexation and level of antagonism observed in the field; inorganic Fe 2+ reacted with MSMA to form a complex having reduced herbicidal activity. Potentiometric and spectrophotometric investigations found that methylarsonate, the parent acid of MSMA and other organic arsenical herbicides, reacts with inorganic Fe 2+ to form a stable 1:1 Fe 2+ -methylarsonic acid chelate having two points of metal coordination and a stability constant log 10 (β) = 2.77 ± 0.04. Tank-mixing MSMA with FeSO 4 to protect against bermudagrass injury negates the benefit of applying the herbicide for weed control, and therefore is not a recommendable practice for turf managers.