In a separate case, Monsanto and specialists at Mississippi State University confirmed a case of Johnson grass resistance to glyphosate near Clarksdale, Mississippi.

The two cases were investigated during the past several months. In initial greenhouse trials conducted by the University of Arkansas and Monsanto, Johnson grass was not controlled with labelled rates of glyphosate. Additional trials will be conducted in the field this season.

"We're looking at Johnson grass populations in a field where there has been a history of control issues," Dr Bob Scott, a University of Arkansas extension weed scientist.

The field in question is owned by a grower near Crittenden County, Arkansas, and has been in continuous Roundup Ready soybeans.

"Our greenhouse trials show differing levels of response including some plants that survive following application above labelled rates of glyphosate.

"Additional populations suspected to be resistant, were also tested but shown susceptible to Roundup in testing," Dr Scott says.

"The resistant populations are being controlled well with selective chemistry. We will continue working with the grower on control methods and recommendations."

Monsanto's technology development team has been working with a farmer and his dealer outside of Clarksdale, Mississippi on a field with Johnson grass control issues as well.

* From Farm Progress, Fairfax Media's farm publishing arm in the US.