Why the first weeks matter for soybean weed control

Getting weeds under control is important. Getting them under control before they affect your soybeans is what helps protect yield.

A field can look clean after an in-crop spray, but that does not tell the whole story. Weeds may already have competed with the crop and removing them later cannot necessarily recover the yield potential lost.

That is where pre-emergence weed control can help. Used as part of a planned program, an herbicide with residual activity can control susceptible weeds as they emerge, helping protect soybeans through an important early stage.

Small weeds can still affect yield

Weeds do not have to tower over soybeans to cause problems. Those emerging alongside the crop can be especially competitive. Ontario’s agriculture ministry explains that weed competition during the critical weed-free period can cause irreversible yield loss.¹

Ontario research shows how much weed pressure can change the timing.

In eight field experiments conducted from 2017 to 2019, University of Guelph researchers Nader Soltani, Christy Shropshire and Peter Sikkema studied how delaying the first in-crop herbicide application affected soybean yield.

Under high weed pressure, their model estimated 5% yield loss when weeds averaged just 6 cm tall at application. Under lower pressure, the same estimated loss occurred at an average weed height of 20 cm.²

Those numbers describe the conditions in the study, rather than a spray threshold for every field. But the takeaway is straightforward: even small weeds can put yield at risk. Waiting until the problem looks serious may mean waiting too long.

Keep working beyond the first spray

Starting with a clean field takes care of weeds already present. Residual control helps manage what comes next.

A residual herbicide remains active after application, controlling susceptible weeds as they germinate and emerge. Applied before soybean emergence, it can help reduce early competition ahead of an in-crop treatment.

Think of the first pass as having two jobs: address the weeds that are there and help manage those still to come. Depending on the field and products selected, that may involve tillage, a burndown treatment and residual weed control.

Ontario guidance recommends starting clean at planting and identifies soil-applied residual herbicides as one way to help manage weeds emerging with soybeans.¹

For the grower, the value is in planning that early protection rather than relying entirely on a later spray to catch up.

Build a program that fits your field

Pre-emergence herbicides work best when selected for the weeds you need to control.

In 10 southwestern Ontario trials, researchers evaluated programs that started with a pre-emergence herbicide and followed with in-crop glyphosate. Eight weeks after the in-crop application, those programs provided 96 to 100% control of the weed species assessed.³

The results support using pre-emergence control as part of a complete program. They also showed that different pre-emergence treatments had different strengths against individual weeds.³

Start with what you know about your fields. Which weeds caused trouble last year? Where were the heaviest patches? Have any survived previous treatments?

Those details help guide a more useful decision than choosing a treatment based on price alone.

The economics still matter. In the same trials, returns from pre-emergence followed by in-crop programs were generally comparable to two in-crop glyphosate applications, with some returning less.³ The aim is to choose a program that balances effective control, resistance management, yield protection and total treatment costs for your conditions.

Check that your first pass is working

A residual herbicide needs the right conditions to do its job.

Purdue University extension specialists explain that soil-applied residual herbicides need precipitation for activation. Requirements vary by product, and weeds may emerge if activating moisture does not arrive in time.⁴

Keep scouting after application. Watch for new weeds and check whether the treatment has received the moisture it needs. Follow the Canadian product label and use what you find to guide any follow-up treatment.

Give your soybeans a clean start

Pre-emergence weed control is about protecting the crop early, when competition can have lasting consequences.

Start clean, choose residual control suited to your weeds and keep scouting through the season. A well-planned first pass can help protect soybean yield potential and give the rest of your weed-control program a stronger foundation.

¹https://fieldcropnews.com/2022/06/crop-report-june-15-2022/
²https://www.cambridge.org/core/services/aop-cambridge-core/content/view/B757A1B0AFE431967301CCE28AFD6C75/S0890037X22000367a.pdf/soybean_yield_loss_from_delayed_postemergence_herbicide_application_based_on_weed_height_days_after_emergence_accumulated_crop_heat_units_and_soybean_growth_stage.pdf
³https://file.scirp.org/Html/5-3000767_46018.htm
⁴https://extension.entm.purdue.edu/newsletters/pestandcrop/article/residual-herbicides-precipitation-requirements-for-activation-and-the-likelihood-to-receive-it/

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