Chris Preston, of the University of Adelaide, speaking at the recent GRDC update in Ballarat, said Australian growers, with a long experience with herbicide resistant weeds, were taking more care with chemical rotations than their American counterparts, where resistant weeds are a relatively new problem.

“While glyphosate resistance is happening in Australia, it is not to the extent it is going on in the US.”

There are large swathes of prime US farm land with perennial weeds resistant to glyphosate, particularly through southern areas.

Monsanto corporate affairs lead Keryn McLean said it was important to differentiate the way glyphosate was used in Australia, compared to the US.

“In Australia the risk of glyphosate resistance is from other parts of the farming system and from non agricultural uses of glyphosate."

She acknowledged the potential implications of resistance would be huge, and said it was important for all users of glyphosate, including non-farm users, to ensure they were not relying on glyphosate alone for weed control.

“We need growers and other glyphosate users to rotate herbicides and use non chemical control to manage problem weeds especially in these high risk situations."

She said it was critical to note that Australian glyphosate resistance was not occurring as a result of overuse of Roundup Ready GM crops.

“Only a very small proportion of glyphosate used in Australia, less than 5pc, is over the top of Roundup Ready crops.

“In Australia Crop Management Plans are required for Roundup Ready crops, which are developed in conjunction with industry weed scientists to protect the sustainable use of the technology.

“In the more than 11 years that Roundup Ready technology has been used in cotton in Australia there have been no incidences of glyphosate resistance found as a result of the Roundup Ready system."