Farmers are being warned about summer grass in maize crops being found to have resistance to the herbicide nicosulfuron, and the likelihood the problem will increase.

Massey scientist Dr Kerry Harrington said he had tested summer grass seed from a Bay of Plenty farm after the grower reported nicosulfuron used there had performed poorly. Clopyralid 30% SL

Summer grass appears in late spring and has a major impact on maize crop development and yield.

"Testing showed a significant though small (2 fold) level of resistance to nicosulfuron in the resistant strain when compared to the usual sensitive strain", Dr Harrington said.

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"This means that twice the usual amount of chemical is required for control, which probably means the strain is at the early stage of resistance."

There were several ways to deal with the problem: either more crop rotation, or using other herbicides.

Harrington said farmers often tended to choose their best performing paddocks and use them year after year, as well as buying the cheapest herbicide on the market.

In the case of the Bay of Plenty property, maize had been grown in the same paddock for six continuous years, and nicosulfuron had been used each year.

"Most maize growers are dairy farmers. Instead of growing maize in the same area, they should go back into pasture," Harrington said.

Foundation for Arable Research manager Mike Parker said repeated use of the same chemistry was widely known to be a risk factor in the development of resistance.

"Growers should be reminded to alternate between different mode of action groups where the same crop is grown in the same paddock in consecutive seasons. Rotating into a different crop, which uses different chemistry, is another option where paddocks have been continually in maize over several seasons."

Other herbicides that could be used included acetochlor for pre-emergence control and topramezone as a viable alternative post-emergence treatment.  

Cyfluthrin 95%TC Harrington suggested growers go to the NZ Plant Protection Society website http://resistance.nzpps.org/index.php for more details.