A field of rolling green hills and tidy crop plants in rows. There are trees in the distance, and the sun is shining down on the field.

By Jennifer Barber

Simply put, early weed removal is the best practice when it comes to protecting yield. It comes with other benefits too, like easier harvest and keeping the seed bed clean. But when weather is challenging and you’re dealing with multiple resistant weeds, you may have to tweak your weed removal strategy to achieve the best control and the best return on your crop investment.

A Redwood pigweed plant which looks like a sturdy tree in miniature, with rough leaves, and clustered colorful flowers.

Redroot pigweed is a common annual weed found on the Prairies.

“Controlling weeds when they are at their smallest is always the best option,” says Jason Voogt, an agronomist and owner of Field2Field Agronomy in Carman, MB. “Certain weeds, like pigweed species, have multiple growing points so growers need to know their weeds and their biology and be aware of the critical weed-free period for their crop because controlling for those slight differences will help keep them on top of their weed control early on.”

Voogt says that pre-emergent control is becoming more popular as growers have seen empirical evidence of the impact of weeds during the critical weed-free period of their crop. “For crops like corn, soybeans and canola, that critical weed-free period is especially important as these are less competitive crops in their really early stages and even low weed pressure can have a big impact on yield.”

Voogt says that in the 2022 season, some of his growers were concerned because they had high rainfall soon after they applied their pre-emergent weed control. While these herbicides require moisture to activate, they questioned how much moisture is too much.

“We received four inches when these products only need ¾ to 1 inch,” he says. “And no, that is not ideal. But because of how the chemical works it will still bind to the soil and, therefore, you will still get weed control — but perhaps not as high as expected. It’s still better than none at all, and that’s when you need to consider managing weed escapes with an in-season application.”


“If we look at the major commodities, most are highly affected by early season (weed) competition,” says Mike Cowbrough, weed specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs. “You can’t forecast for the weather in a growing season, so you have to consider your logistics and assess whether you can go in as soon as weeds start to emerge during the critical weed-free period. More and more, we are seeing growers choosing two-pass strategies to make sure their crop isn’t competing with early weeds.”

Peter Sikkema, professor of field crop weed management at the University of Guelph discusses a small plot corn trial while standing in a field.

Peter Sikkema, professor of field crop weed management at the University of Guelph discusses a small plot corn trial. For over 40 years, the university has run trials looking into all aspects of corn production to help farmers get more from their crops.

Cowbrough says there are two main strategies when it comes to minimizing early season weed competition. While both strategies have the same goal — get the crop out of the ground without weed competition — one uses cultural methods to give the grower the upper hand.

“If tillage is used to remove early season weed emergence, then it’s better to delay planting until soil conditions are warm and the crop can emerge out of the ground quickly,” he says. “This provides the opportunity for you to make multiple tillage passes to remove multiple flushes of weeds before the crop is planted. The hope with this approach is that you’ve exhausted the majority of weed seedlings that will emerge in the spring. Since delayed planting can result in reduced yield potential, the other approach is to plant the crop as early as possible and then use other means to remove early weed flushes.”

The second strategy involves herbicides. “Even in a really wet or really dry year, the return on investment from getting even some control during the critical weed-free period has a significant impact on yield,” he says. “Growers have to forget about what they could get if they had optimal moisture for the (pre-emergent) herbicide because even if the herbicide does not perform at its best, minimizing weed pressure will pay off in the long run.”

After crops like soybean and wheat are harvested, ground cover can be a very effective solution to prevent weed emergence and minimize seed return to the seed bank. Cover crops like winter triticale can out-compete weeds and provide a secondary income as a forage crop harvested the following spring.

“Early weed control is driven by yield,” says Cowbrough. “Controlling weeds that emerge after the critical weed-free period is driven more by harvesting ease, maintaining crop quality and minimizing weed seed return to the soil.”


A photo of Jason Voogt, Agronomist, and owner of Field2Field Agronomy. He is standing among tall corn plants.

“Overall, utilizing a two-pass herbicide application will result in the best economic return for growers over the long term”


“Overall, utilizing a two-pass herbicide application will result in the best economic return for growers over the long term,” says Peter Sikkema, professor of field crop weed management at the University of Guelph. “If you look at a 10-year horizon for maximum profitability, our research indicates that farm profitability is maximized with a planned, two-pass weed control program in corn, soybean and dry bean. I do acknowledge that there are some fields in some years you could get away with only a single herbicide application, but I can’t tell you as we head into any given field-season which fields that will be.”

When weeds aren’t controlled prior to emergence, they can still be managed early in the season with post-emergent herbicides — but the bigger the weeds get, the greater the impact on crop yield. Sikkema and his research team looked at the effect of delayed application of postemergence herbicides on corn yield. Yield loss varied from field-to-field based on relative time of weed and crop emergence, weed species composition, weed density, soil texture, nutrients and weather.

Based on 1,000 acres of corn priced at $8/bushel, if a grower waited until weeds were two inches high in fields with heavy weed pressure, there was a yield loss of 5 bu/ac, or $40,000. If the first post-emergent herbicide application was delayed until weeds were four inches high, the yield loss was 17.4 bu/ac, or $139,000. At six inches that loss jumped to 30.4 bu/ac, or $243,000. The tallest weeds they measured in the study were eight inches, which caused a 40.6 bu/ac loss, or $325,000 in crop value.

Taking that same experimental approach to soybeans, again over 1,000 acres and $18/bushel soybeans, delaying application until weeds were two inches high, with heavy weed pressure resulted in a yield loss of 1.8 bu/ac, or $33,000. Four inches was a loss of 5.1 bu/ac, or $92,000, six inches was 8.3 bu/ac, or $149,000 and eight inches was 10.9 bu/ac, or $197,000.

“My advice is to put down the best soilapplied herbicide and then manage for weed escapes,” says Sikkema. “Once you have put down a soil-applied herbicide you should scout the field repeatedly and manage any weed escapes.”


A portrait of Peter Sikkema, Professor of Field Crop Weed Management at the University of Guelph. He is wearing a grey button up short and a colourful tie.

“Overall, utilizing a two-pass herbicide application will result in the best economic return for growers over the long term”


“The use of multiple herbicide modes of action will delay the evolution of herbicideresistant weeds,” says Sikkema. “You need to know the resistance profile in each field and choose a soil-applied herbicide that will control those weed biotypes. There is 5-way resistant waterhemp in Ontario, and growers can still get near perfect control with a diversified, long-term integrated weed management program. It may require careful planning but it can be done.”

Recent advances in herbicide-resistant crop traits are helping growers manage better for early weed control, but even then, Cowbrough says, growers need to make sure they are rotating their chemistries to make sure they have preplant and pre-emergent control that will manage for resistance going forward.

“Those who invest time in scouting and know the abundance and types of weed species in their fields are better able to select the right herbicide program and will not worry so much about overspending up front for early weed control,” says Cowbrough. “If you don’t have a good handle on what weed species are in your field and want to get good early season control, there is no sense nickel and diming up front because you won’t get top notch control unless you can match the herbicide program to the weed spectrum. It’s an investment that will almost always pay off.”