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Canadian farmers understand the plight of herbicide-resistant weeds. After Australia and the United States, Canada has the largest population of weeds resistant to one or more herbicides. Tank-mixing herbicides is one way to tackle weed control while lowering risk of resistance development and improving yield.

Liz Simpson, canola agronomic systems manager with Bayer Crop Science, discussed the issue of herbicide resistance and how tank-mixing addresses it in a webinar hosted by Top Crop Manager and presented by Mix It Up, Bayer Crop Science.

Don’t delay, spray today

It’s easy to justify delaying early season spraying: it’s better to wait until later emerging weeds show up; it’s silly to spray when there’s nothing there; the crop hasn’t sprouted yet, so it isn’t competing for nutrients, sun and water yet; making another pass so soon after planting risks compaction. However, weeds are much easier to control when they’re small – four inches or less, ideally.

“The smaller the weed, the more efficacy we’re going to get out of that herbicide,” Simpson says.

Younger, smaller weeds are more susceptible to herbicide control, too. Contact herbicides don’t translocate well, so they’re less effective on larger weeds where coverage is spread over more leaves and foliage. This is especially true with herbicide-resistant weeds – by killing them early, they can’t set seeds and increase herbicide-resistant populations.

The yield benefit of a solid pre-season spray program is more than two bushels per acre. This relates to the agronomics of spraying early and having a residual, rather than the chemistry involved. Crops are vulnerable at much earlier stages than expected, and they set their yield potential based on environmental signals received early on.

“I like to think of it as the plant basically deciding what it has the capacity to do,” Simpson says. “So, not even competing necessarily for these resources, but the potential to have to compete for these resources is what actually can cause our yield damage.”

It’s important to identify a crop’s critical period of weed control and keep the field clean during it to maximize yield. This period is crop-dependent: for corn, it’s emergence to four-leaf; for soybeans, it’s emergence to second trifoliate. Canola is most vulnerable between emergence and six-leaf stage, and wheat is most susceptible between one- and three-leaf stage. When the crop matures to a certain point and is well-established, yield is no longer affected by weeds.

Work smarter

Simpson identifies three steps in outsmarting weeds: understanding what resistance is and how it develops, tank-mixing with multiple effective modes of action, and diversifying crop rotation.

The weeds in a field may all be the same, but some might have biological differences that make them resistant to a herbicide that would otherwise kill them. These weeds survive the application and, if they set seed, populate the field with more resistant weeds. Repeated use of that one herbicide allows the resistant population to grow, until eventually, most weeds in that field are resistant.

“This timeframe can happen very, very quickly or very, very slowly, depending on our situation,” Simpson says. “But you can see how that one weed that we let by – slowly, over time – is going to overtake that population.”

Once resistance has developed, those weeds will never be affected by that herbicide again.

“Relying on any one herbicide to be your go-to weed control for whatever weed population you’re looking at, or at any single timing, can put a lot of pressure on that herbicide, and eventually [it] will become ineffective,” Simpson says.

Glyphosate runs a serious risk of this. In 2019, 71 per cent of glyphosate applied as pre-seed weed control in Western Canada was applied alone. While many weeds are controlled with glyphosate, they’re much more likely to develop resistance without another mode of action to diversify control.

“I see this as a huge opportunity, or a place that we can add basically a single action into our farm management practice by simply adding that tank mix, where we can bring down that vulnerability of creating herbicide resistance for glyphosate in the springtime,” Simpson says.

Multiple effective modes of action

Understanding the purpose of multiple effective modes of action is important in delaying resistance development, too.

“When we say [multiple effective modes of action], we’re talking about [mixing] two modes of action in a herbicide that can kill the same weed on its own,” Simpson says. This can be achieved by either tank-mixing herbicides with different effective modes of action or using these herbicides sequentially throughout the season.

Simpson cautions against trait and herbicide rotation alone. There is a solid body of evidence demonstrating that mixtures work better than herbicide rotations, including farm surveys, modelling simulations and field research. Effective weed control means using herbicide mixtures or herbicide layering every year within a diverse crop rotation.

Weeds develop a foothold through repetition and Simpson says weeds outsmart habits, not systems. Crop rotation is an effective strategy to help manage diseases, insects and weeds, and also provides an opportunity to diversify a herbicide program.

“Weeds are pretty clever,” Simpson adds. “The more opportunities that we have to break up the timing, or to not be doing everything the same every year, the less likely it is for something to be able to overcome that, because it’s not predictable.”

When determining which products work best as tank-mix partners and the order in which they should be added, Simpson says provincial crop protection guides are a reliable resource. For situation-specific advice, she recommends talking to a company representative to determine the best options based on crops, conditions and region.

Editor’s note: During the original webinar, Liz Simpson mentioned there aren’t many tank-mixing options for wild oat but that Groups 1, 9 and 11 are starting points. Unfortunately Group 11 will not help with wild oat control.

For information on resistance management, visit www.mixitup.ca