GROWERS of GM crops are urged to learn from the US, writes PETER HEMPHILL

Grain growers moving to Roundup Ready genetically modified crops should keep a careful eye on weed management.

While there is a temptation to move to continuous cropping systems with herbicide-resistant GM crops, growers should not fall into the same trap their American counterparts did when the revolutionary crops came on to the market in the 1990s.

University of Adelaide school of agriculture lecturer Chris Preston said US farmers were now experiencing major problems with glyphosate resistance in the pervasive weed Palmer amaranth in cotton and soybean crops and Kochia species in western plains crops.

Dr Preston has been working with American colleagues on glyphosate resistance in weeds and visited the US earlier this year.

He said the US now had a problem with glyphosate resistance in a number of weed species due to an over-reliance on the herbicide to control weeds in a number of Roundup Ready crops, particularly cotton.

He said it was also exacerbated by having many types of Roundup Ready crops in the US.

Dr Preston said that, in south-eastern American states, 95 per cent of cotton country was continuously cropped to Roundup Ready varieties.

"The problem in the US arose because growers and commercial players said glyphosate resistance would not be a problem," he said.

"But it was no surprise glyphosate resistance would build up in weeds.

"It has now left them with a problem in how to manage their cropping systems.

"It is starting to cause a few headaches for growers."

Dr Preston said American farmers were now having to work out whether they were better off going back to the way they used to crop or move to a new type of cropping system.

He said Australian growers needed to learn from the mistakes of US farmers to minimise herbicide resistance problems.

"They need to have a diverse strategy which has a variety of crop options," he said.

Dr Preston said Australian growers needed to ensure they continued to rotate their crops, rotate herbicides and use tactics to reduce weed seed numbers in paddocks.

"They have got to have a rotation," he said.

"They have got to have a strategy to keep weed numbers down.

"And they need to know when they should take action on weed problems in that rotation."