25 July 1997

Atrazine still just as potent

REPORTS that some varieties of Italian ryegrass are resistant to the herbicide atrazine are unfounded, according to independent grassland consultant Mark Gillingham.

Some producers have undersown maize with the Italian ryegrass Bartissimo which survived, despite the application of atrazine at 2l/ha, and produced a crop of ryegrass.

Mr Gillingham says that the benefits of undersown grass would be to mop up any nitrogen unused by the maize crop, and to produce clean autumn grazing.

"We tried to reproduce the results in a trial run in conjunction with NIAB. Maize was undersown with 17 IRG varieties and atrazine applied at 3.5l/ha and 1.5l/ha. This killed all of the undersown grass in all plots," says Mr Gillingham.

"Maize was drilled at the beginning of May and atrazine applied on May 12. A month later the plots were undersown with ryegrass which emerged within two weeks. Within 10 days all grass was dead," reports Mr Gillingham.

NIABs Jim McVittie says that previous trials had shown that IRG could survive.

"It may be that other factors such as soil type, weather conditions or the application method used have an effect.

"In the first trial the crops were on light land and atrazine was applied to a well-established crop. This trial was on heavy clay soil and atrazine was applied soon after drilling maize.

"It could be that ryegrass was able to establish in the earlier trial under larger maize leaves which simply prevented atrazine reaching the ground," speculates Dr McVittie.n

Mark Gillingham (right) and maize grower David Lewis inspect a trial plot where atrazine wiped out undersown Italian Ryegrass.