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New Zealand Plant Protection 64 (2011): 125-132

Factors affecting the persistence of some residual herbicides in maize silage fields

A. Rahman, T.K. James, M.R. Trolove and C. Dowsett

ABSTRACT

The persistence of residual herbicides used in maize (Zea mays) silage crops was investigated in three field trials located in Waikato, Taranaki and Canterbury. Herbicides used included atrazine and acetochlor applied pre-emergence and mesotrione and nicosulfuron applied 6-12 weeks after planting. Bioassay of soil samples collected about the time of silage harvest showed small but biologically toxic residues of only nicosulfuron and only at the Taranaki site. A subsequent glasshouse study investigated whether the differences in persistence of nicosulfuron were due principally to soil characteristics (four soil types) or rainfall (amount and timing). Heavy rainfall (50 mm) in the first week or two after application or for several consecutive weeks was more effective in leaching the herbicide and reducing the residues than light (10 mm) or moderate (25 mm) rain applied at similar times. Also, residues of nicosulfuron disappeared faster in soils with low pH and high organic matter.

Keywords: persistence, residues, bioassay, maize silage, nicosulfuron, atrazine, acetochlor, mesotrione.

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