CONQUER WEEDS through best management practices

Weed experts encourage farmers to heed the warnings of weed surveys and the growing problem of resistance to make better weed control decisions

Although some weeds grow in abundance, expand their range or develop resistance to certain herbicides, those that are the most troublesome today, are the same weeds that caused problems decades ago.

Take wild oats, green foxtail and wild buckwheat in western Canada. “Those top three have stayed the same for the last 50 years,” says Hugh Beckie, research scientist with Agriculture and Agri-Food Canada in Saskatoon. “Wild oats are everywhere and have stayed everywhere.”

Mike Cowbrough is seeing the same thing in Ontario. He’s the weed management field crops program lead with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) in Guelph. In 2014, he conducted a weed survey across four crops in six counties in that province — a follow up to a similar survey done by OMAFRA in 1988. He found that, in general, weeds identified as most abundant in 1988 were also most abundant in 2014, with green foxtail and lamb’s quarters topping both lists.

“When I show that survey to people, they’re surprised that Canada fleabane isn’t on it,” says Cowbrough. Indeed, Canada fleabane shows up only once in his survey — as the fourth most abundant weed in soybeans in Wellington County. That’s it.

So what gives? Both Beckie and Cowbrough say there’s a distinction to be made between incidence and importance when it comes to weeds, and Canada fleabane is a good example. While it doesn’t top any list in terms of incidence (it’s not as widespread as green foxtail, for instance), it is rising on many lists as an important weed because some populations have developed resistance to glyphosate.

What does all this mean to farmers trying to devise an effective weed control program? It doesn’t mean a huge change in some respects — herbicide resistant weed populations is still the most pressing problem in all crop-growing regions, and producers still need to maintain best management practices to slow the spread of established resistant weed patches, and delay the onset of new resistance.

Having said that, farmers need to be aware that weed abundance can — not always, but can — indicate resistance problems to come.

“The top five or 10 weeds will be the same over the next 100 years,” says Cowbrough. “It’s the nuance within that ranking that is important. Resistance is, by and large, a numbers game and if you have a weed that’s really abundant, chances are greater there’s a resistant biotype in that population, so let’s focus on using multiple modes of action for those top abundant species.” In other words, don’t wait for resistance to develop before employing resistance management tools.

Hugh Beckie’s Top 10 best management practices for resistant weed management

1. Crop rotation diversity
2. Use competitive crops and practices that promote competitiveness
3. Pre- and post-herbicide scouting: find out what’s surviving
4. Herbicide mixtures/sequences (pre, in-crop and post)
5. Herbicide group rotation: avoid back-to-back Group 1 or 2
6. Rotate in-crop, wheat-selective herbicides
7. Weed sanitation: field immigration and dispersal
8. Field- and site-specific (patch) weed management
9. Strategic tillage
10. Sound record keeping

Beckie agrees and says the problem is magnified where weed populations are both abundant and resistant. “On the 2014 Saskatchewan weed survey, cleavers is the seventh most abundant weed in the province, and in the 1970s, it was in fiftieth place,” he says. “Resistance is becoming a very serious issue where populations are staying highly abundant.”

Cowbrough says the warnings these surveys offer are clear. “Take giant ragweed,” he says. “It wasn’t on anyone’s radar in historical surveys. Now it’s up in the middle of the pack in the crop advisors’ survey, and we have the first glyphosate resistant species in Ontario, so that elevates it somewhat as a weed to watch, even though it’s not as abundant as other weeds.”

Beckie notes the same trends in the west where some resistant weeds have become so ubiquitous he fears producers are almost acclimatizing to the situation. “Wild oats will always be our number one resistance problem,” he says. “It’s sort of creeping forward slowly, getting worse and worse every year so crop yield losses get disguised — people just accept that there will be some loss. So it’s chronic, but it’s also acute for some growers because of the increasing complexity of resistance. We’re seeing some wild oat populations with five-way (Group) resistance.”

Beckie says green foxtail, the most abundant weed across the prairies (just slightly higher than wild oats), is flying under the radar a bit because it’s not a very competitive weed if the crop is doing well. Cowbrough says it’s the same with purslane speedwell, which, according to his 2014 weed survey, ranked as the second and third most abundant weed in winter wheat in two Ontario counties, but is usually finished its lifecycle by the time the crop gets going.

Both weed specialists say that farmers need to use weed abundance to take stock of how they approach weed control using the herbicide tools they have. “There’s been a whole generation of people not knowing anything but spraying glyphosate,” says Cowbrough.

He’s struggled in the past to convince students and farmers of the importance of knowing the differences between specific weeds and when and how to spray them for maximum control. The one-pass approach, regardless of weed stage, should not be a grower’s first choice, he says. “Why would I want to know the different leaf stages and product options between fall panicum and green foxtail when glyphosate gets it all?” laughs Cowbrough.

Beckie and Cowbrough think farmers are doing as good a job as they can when it comes to effective weed control, and both accept the financial imperatives that sometimes lead farmers to make decisions that go against best management practices.

But they also urge growers to heed the warnings of weed surveys and the ever-widening spread of resistance to make better weed control decisions when they can, even if they’re tough to make.

The situation is only going to get more intense as the years go by, says Beckie, pointing to current herbicide resistance issues with the pigweed complex (waterhemp, palmer amaranth, red root and smooth pigweed) in the U.S. Midwest and Southern regions. “It’s probably the number one global focal point for resistance now,” he says. Canada is not at that stage yet, which is good, because recognizing the problem is halfway toward fixing it.