What are you doing about herbicide resistant weeds

As the results of the 2016 Manitoba weed survey are finalized and the 2017 Alberta survey begin to be tabulated, a familiar picture is starting to emerge: the incidence of herbicide resistance across the Prairies is very much on the rise…again.

Indeed, experts project that, once all the calculations are done, the total number of Prairie acres with resistant weeds will be in the vicinity of 38 million. “We already know from the 2015 Saskatchewan survey that there are 22 million acres with resistance,” says Hugh Beckie, research scientist with Agriculture and Agri-Food Canada (AAFC) in Saskatoon. “Once we finalize the 2016 Manitoba survey, and as we get the results from the 2017 Alberta survey, it’s easy to see it’s going to be at least 38 million acres, probably higher.”

It’s a staggering leap from the last round of Prairie weed surveys, completed between 2007 and 2009, where total acres with resistance was 24.4 million, which itself is more than double the previous survey period (2001 to 2003) where 10.9 million acres were identified as having resistance issues. 

Is Beckie surprised by the new projected total? “It’s what I expected, given what I was seeing in terms of current and historical herbicide use,” he says. “I knew selection pressure, particularly for Group 1 and 2 wild oats, was high.”

He sympathizes with farmers making herbicide choices – he knows they’re stuck between a rock and a hard place when it comes resistant wild oat control. “What else are they going to use in cereals?” he asks. “We’re at a stage where they’re going to use whatever works.”

Having said that, he’s also pleased to see that farmers are turning more and more to cultural weed control practices to keep resistance in check. With no new chemistries on the horizon, and the capacity of wild oats to survive just about everything thrown at them, a combination of herbicide tools and cultural practices is the only way to go forward.

 

What IS it with wild oats?

“Prairie-wide, wild oat is still the number two weed in abundance, despite all the new chemistries introduced since the 1970s,” says Beckie. It’s a testament to the tenacity of a weed that seems programmed to survive, no matter what.

Wild oats have a set of qualities that make it what he calls a “weedy” weed. It produces 150 to 200 seeds per plant and disperses those seeds through shattering before harvest. “The seeds can self-bury, too, working their way into the soil, and they stay in the seed bank for years,” he says. A staggered emergence pattern also gives wild oats a lot of chances throughout the season to get ahead of, or come in behind, a herbicide application and so live another day to set seed.

Is Beckie surprised by the new projected total? “It’s what I expected, given what I was seeing in terms of current and historical herbicide use,” he says. “I knew selection pressure, particularly for Group 1 and 2 wild oats, was high.”

He sympathizes with farmers making herbicide choices – he knows they’re stuck between a rock and a hard place when it comes resistant wild oat control. “What else are they going to use in cereals?” he asks. “We’re at a stage where they’re going to use whatever works.”

Having said that, he’s also pleased to see that farmers are turning more and more to cultural weed control practices to keep resistance in check. With no new chemistries on the horizon, and the capacity of wild oats to survive just about everything thrown at them, a combination of herbicide tools and cultural practices is the only way to go forward.

 

Get serious about IPM

The term “integrated pest management”, or IPM, is familiar to most Prairie growers and Beckie says it’s a particularly useful and valuable concept for growers trying to manage herbicide resistant wild oats.

He points to the Top 10 herbicide resistant weed management practices that he and fellow weed researcher, Neil Harker, put together last year.  They are:

10. Keep good field records; maintain a herbicide resistant weed database of your farm – know what’s where; know if and how patterns are changing.

9. Use strategic tillage when necessary.

8. Use field and site-specific control measures, such as patch management. There is no one-size-fits-all herbicide resistant weed management solution.

7. Watch weed sanitation at field borders to control and slow herbicide resistant weed dispersal.

6. Rotate in-crop wheat-selective herbicide to combat non-target site (e.g., metabolic) resistance.

5. Rotate herbicide groups – avoid back-to-back in-crop Group 1 or 2 applications.

4. Use herbicide mixtures and sequences.

3. Scout crop pre- and post- herbicide application to see what survives.

2. Use competitive crops and practices that promote competitiveness, to enable fast ground cover or canopy closure.

1. Practice crop rotation diversity.

 

“Growers are adopting more of these non-herbicide practices, and I think that’s positive,” says Beckie. Indeed, the 2016 Manitoba grower questionnaire showed fairly high adoption rates (80%+) of some practices, like pre- and post-application scouting, mixing herbicides, rotating herbicide groups and even growing competitive crops.

It sounds good, but drilling down a bit more, the study found that those experiencing resistant weed problems were more likely adopt these BMPs than those without, even though many of these practices are preventative in nature – in other words, growers should try to manage for resistant weeds before they have them.

This is particularly alarming when it comes to managing resistant wild oats. The 2016 Manitoba weed survey showed that 78% of fields with wild oats in that province have Group 1 resistance. “Manitoba is still the hot spot for wild oats,” says Beckie. “But Saskatchewan is closing the gap quickly and we’ll see what the Alberta numbers tell us.”

Indeed, the number one resistant weed problem in Manitoba and Saskatchewan right now is Group 1 resistant wild oats, with Group 2 rising fast. It means control options are increasingly limited for farmers with a Group 1 problem. Beckie says that, more and more, growers sending wild oat samples in for testing are asking to know what Group 1 herbicides are effective, so they can determine if there are any Group 1 options left to them.

Which brings us back to the Top 10 herbicide resistant weed management practices and the need for farmers to start incorporating more of them where possible. Some are happening naturally. In Manitoba, for example, crop diversity has shifted drastically with the recent explosion of glyphosate tolerant soybean acres. Beckie says that, along with high levels of glyphosate tolerant canola in rotation, are helping to manage Group 1 and Group 2 resistant wild oats. “But the trade-off could be more glyphosate tolerant weeds, which is a worse problem,” he says. For every action there is a reaction, and growers need to stay aware and make sure that solving one problem isn’t creating a new one.

Beckie says there are synergies that come with employing combinations of herbicide resistant weed management practices, and understands that most growers will only take on two or three at a time. He also knows that taking the long view to prevention is economically difficult – things like early silage cropping, adding forage crops to the rotation are not likely to take off any time soon, though they would be excellent ways to diversify crop rotation and break up weed cycles.

Given that, growers should be prepared to make some tough choices, if and when they have to, in order to effectively manage resistant weeds. “In Manitoba, we’re seeing an increase in the use of tillage,” he says. “These growers would probably rather do anything else, it’s a last resort for them.”

Take heart. Canadian farmers are not alone with this problem. “Grass weed resistance will always be more of a challenge to growers anywhere in the world,” says Beckie, adding that, globally, grassy weeds account for 25% of the worst weeds, but 40% of herbicide resistant weeds, so it’s a big problem everywhere.

His advice to Prairie farmers? “Keep plugging away at it. It’s going to be a constant battle with wild oats, and if you let up, they’ll come back with a vengeance.”