Iron Antagonism of MSMA Herbicide Applied to Bermudagrass: Characterization of the$Fe^{2+}-MAA$Complexation Reaction

• Published in: Weed science 2006-01, Vol.54 (1), p.23-30
• Main Authors: Joseph H. Massey, Taylor, James M., Nursen Binbuga, Kristen Chambers, G. Euel Coats, William P. Henry
• Format: Article
• Language: English
• Subjects: Arsenic compounds; Chelates; Herbicides; Ligands; Physiology, Chemistry and Biochemistry; Sulfates; Teeth; Titration; Weed control; Weed science
• 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 (FeSO4) are tank-mixed with MSMA, this discoloration is reduced. Experiments investigated the effect of tank-mixing organic arsenical herbicides with FeSO4or a chelated iron source (Sprint 330) in terms of southern crabgrass control and injury to bermudagrass. Tank-mixing MSMA with FeSO4reduced bermudagrass injury. However, southern crabgrass control was also reduced by at least 50% with the addition of$\geq 0.38 kg Fe^{2+} ha^{-1}$. Neither antagonism nor safening of bermudagrass was observed when the chelated Fe2+source was used. Applying FeSO4as 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 FeSO4concentration controlled the extent of complexation and level of antagonism observed in the field; inorganic Fe2+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 Fe2+to form a stable 1:1$Fe^{2+}-methylarsonic$acid chelate having two points of metal coordination and a stability constant$log_{10} (\beta) = 2.77 \pm 0.04$. Tank-mixing MSMA with FeSO4to protect against bermudagrass injury negates the benefit of applying the herbicide for weed control, and therefore is not a recommendable practice for turf managers.