Grasses resistant to the most widely used herbicide have been found in New Zealand for the first time.
And Wanganui farmer Alan Taylor says the problem is likely to spread. He has listed ways to prevent it.
Last month the Foundation for Arable Research announced that ryegrass resistant to glyphosate, the active ingredient in Monsanto's Roundup sprays, has been found in a Marlborough vineyard. It's the first such incidence in New Zealand but glyphosate-resistant weeds have been developing in the United States and Australia for years.
When Roundup came out in the mid 1970s it was "almost a miracle". No other herbicide was as cheap or killed such a broad spectrum of plants right down into their roots. It also didn't leave residues in the soil to kill the next generation of plants.
Mr Taylor has been a cropping farmer for 40 years, and uses glyphosate to kill green matter off before he cultivates new seed beds, and also to brown off weeds in the final stages before harvesting a cereal crop, when the cereal plants themselves have already died.
He said pastoral farmers also used glyphosate to kill pasture before resowing with better species. Local authorities used it for killing roadside weeds, and it is one of the herbicides most often used by home gardeners.
Sheep farmers occasionally use it on pasture they don't intend to kill. It's sprayed at a very low rate, to stop grasses going to seed. The grasses then skip a seed cycle and carry on growing the low, green, leafy material that feeds animals better.
In the Whanganui region no plants with glyphosate resistance have been found so far, but Mr Taylor said that was probably just a matter of time.
"South Australia has had resistance in some ryegrasses for 10 to 15 years now. The agricultural management authorities ring-fence certain farms, saying they can't sell hay because it will be moving resistant seeds to other properties."
He listed several ways to avoid developing glyphosate-resistant plants. One was to use other herbicides from time to time, to kill the resistant plants.
But at present there wasn't another herbicide that was as good as glyphosate. "There's nothing I know of that's as broad a spectrum in such an environmentally friendly fashion and so cost-effective. We will need other chemicals coming on the market."
At the moment it might take two or three other sprays to kill the same range of weeds, and those sprays were unlikely to be as environmentally friendly or as cheap as glyphosate. Another way was to make sure there was a good kill every time glyphosate was used. "Don't be low in your application rates, because that can create partial resistance. Use effective rates to make sure the job gets done."
The third was to use alternative and non-chemical methods to kill the resistant plants. Fire was especially good at killing some. Or farmers could plough in the seeds of resistant plants so deeply that they could not germinate, or they could get stock to graze grasses so heavily that they died.
The constant use of glyphosate on roadsides was a special worry. "I don't like it being continually used on roadsides. That could lead to resistance because they don't have other management practices to deal with it," Mr Taylor said.