Andrew Swallow

Don’t wait until you have a resistance problem before you change your ways with agrichemicals, a leading weed scientist from the US warned growers at FAR’s Arable Research In Action event earlier this summer.

“From what I’ve seen here, you have an opportunity to be very proactive,” Professor Carol Mallory-Smith of Oregon State University said.

It was human nature to reach for herbicides as the first line of defence against weeds because they were easy to use and, initially at least, very effective, she said. Even if problems did arise, in the past there had usually been a new product that saved the day, however, it was now nearly 30 years since a herbicide with a new mode of action had been launched and it didn’t look likely that would change.

In the meantime, in countries such as Australia and the United States, including Oregon which has similar farming systems to much of Canterbury, over-reliance on herbicides had seen resistant weeds of one species or another arise to nearly every mode of action.

‘I’ve heard people talk about watching a patch of weeds get bigger and bigger, season by season. Don’t let that happen. Pull them in the first year.’

“We’ve been able to get resistance to just about any product that’s out there,” she noted.

New Zealand growers had a chance to heed that history and integrate alternative weed controls before similar problems developed here, she suggested.

“Yes, you have some resistance issues emerging but they are still at a much lower and simpler level than we have in Oregon.”

As resistance issues multiply, control becomes increasingly complex with some species developing “cross-resistance” meaning they can no longer be controlled by two or more modes of action that were once effective.

“In Oregon we have ryegrass which has become resistant to six different classes of herbicide including glyphosate and glufosinate, so if you think you’ll be able to save yourself with a clean-up of glyphosate, well yes, you can for a while, but you’ll end up with resistance to that as well,” she warned.

To prevent such a downward spiral, be proactive and plan herbicide use integrated with cultural controls over the course of the rotation, not just annually.

“In the next crop, with the same weeds, are there options to do things differently?”

Don’t ignore control failures: try to work out what went wrong and ensure there’s no seed-set from survivors.

“I’ve heard people talk about watching a patch of weeds get bigger and bigger, season by season. Don’t let that happen. Pull them in the first year,” she stressed.

If resistance is suspected, get it diagnosed by the likes of FAR and don’t use the same herbicide again, even at a higher rate, in an attempt to clean up survivors. “You’ll just select for the most resistant types in that population.”

Ironically, it is often the most effective herbicides that weeds develop resistance to first because growers over-use them most.

Besides integrating cultural controls to minimise the weed population before a herbicide is applied – “it is a numbers game: the more numbers you have of a weed species exposed to the herbicide, the more likely you are to find the resistant individuals” – bolster biosecurity to prevent movement of weed seeds between farms and within farms, she advised.

“Seed is the most important source of movement,” she said, in response to a question about whether pollen could transmit resistance.

“In ryegrass we have seen pollen move [resistance] traits around. In oats, not so much. It is very species dependent.”

Once a population was resistant to a mode of action it was very unlikely growers would ever be able to go back to using that mode of action against that species of weed again, even if it was avoided for years in the hope of running down the resistant seedbank, she added.

“You might get a year or two, but pretty soon it will go right back up again.”

Weed control

  • Be proactive to preserve herbicide efficacy.
  • Integrate cultural controls with chemical.
  • Avoid repeat use of same MOA*.
  • Think five years out, not just one.
  • Analyse causes of poor control.
  • Get possible resistance cases checked.