Your Reading List

Pest Patrol: Don’t be cheap with weed control

#PestPatrol with Mike Cowbrough, OMAFRA

Drought stress symptoms in corn leaves where no soil-applied herbicides had been applied and weed control was done with a late post-emergence application.

“Weed control is an investment into the current and future crops on every field.”

The above quote was parting advice from Norfolk and Oxford County farmer Ann Vermeersch at the 2019 FarmSmart Conference in Guelph. In front of 80 farmers and agronomists, I moderated a session with Vermeersch and Steph Kowalski, a certified crop advisor and lead agronomist with the Agronomy Company of Canada. Both shared their thoughts on minimizing the impact of weeds on crop production. Here is a recap of their key points:

1. Use a soil-applied herbicide in corn and soybeans, even if growing Roundup Ready cultivars. The soil-applied herbicide pass eliminates early season weed competition (which has the greatest effect on crop yield loss), and is particularly impactful during dry years (see photo at top). Soil-applied herbicide will improve control of any post-emergence herbicides because if there are weed escapes, you are typically targeting fewer weeds and they are all usually at a similar stage of growth (see photos below).

Lamb’s-quarters escapes after a soil-applied herbicide treatment. Note there are fewer and smaller ( photo: Supplied
Lamb’s-quarters in a soybean crop where no soil-applied herbicide had been used. The population is higher and weed size varies from five to 15 cm (two to four inches) in height. photo: Supplied

2. Make fall weed control a priority. On this point, Vermeersch shared with the audience that on their farm, they aren’t afraid to do this job in December provided the target weeds are still green and they can get a couple of days after application with temperatures above 0 C. You won’t see much benefit that fall, but the benefits become obvious the next spring, especially with perennial weeds. Although they typically use glyphosate for fall weed control, both Kowalski and Vermeersch like tank mixing other modes of action, like 2,4-D ester.

3. Explore the use of cover crops. Both presenters have seen weed-suppression benefits with cover crops. Vermeersch shared photos of a corn field where Canada fleabane was almost non-existent in the area that was under cereal rye cover the previous fall compared to a considerable amount of Canada fleabane where no cereal rye cover existed (see photo further down).


This interactive session provided an opportunity for the audience to ask many questions and share their experiences. A comment from this session stood out as an item that needs to be explored more through applied research and farmer innovation.

The concern was late weed emergence. Whether it is eastern black nightshade, pigweed species or crab grass, several producers expressed frustration with species that emerge well past any reasonable window for herbicide control. Although unlikely to affect crop yield in that season, there is a concern that it is increasing weed populations in subsequent seasons.

Canada fleabane pressure found in corn where no cereal rye cover existed the previous fall (right) compared to where cereal rye was present the previous fall (left). photo: Courtesy of Ann Vermeersch

Have a question you want answered? Hashtag #PestPatrol on Twitter to @cowbrough or email Mike at [email protected].

About the author



Stories from our other publications