Pest Patrol: What was that grassy weed in my winter wheat at harvest time?

#PestPatrol with Mike Cowbrough, OMAFRA

Rough-stalk bluegrass in wheat, and a closeup (inset) of the open panicle of rough-stalk bluegrass.

Three different types of grassy weeds were found above the winter wheat canopy at harvest this year. To prevent them from becoming a problem, they need to be identified so that the appropriate management actions can be taken. Here is a breakdown of each grass species and its control options: 

Chess (Bromus secalinus)

Chess grass is a winter annual species that emerges primarily in the fall and will affect fall-planted crops the most. Spring tillage or burn-down herbicides will control seedlings which is why it is usually not an issue in corn, soybeans or dry beans. It is important to make note of fields which have this weed, so that the next time they are planted to winter cereals, the species can be controlled in the fall or early spring with herbicides that are effective.
Herbicide options:

  • After crop emergence (fall): Simplicity GoDRI 
  • After crop emergence (spring): Simplicity GoDRI 

Annual/Italian ryegrass (Lolium multiflorum)

Although ryegrass can certainly be a valuable forage grass for livestock production, it is a difficult-to-control weed in other crops. Ryegrass can express as an annual, biennial or perennial. In the U.S., populations that are resistant to Groups 1, 2, 9, 10 and 15 have been documented. This presents several challenges for successful control. 

Management: The U.S. based “take action” initiative has a very good resource on management of Italian ryegrass at Research by Dr. Peter Sikkema and Dr. Darren Robinson (University of Guelph, Ridgetown) on spring control ahead of soybeans and corn has demonstrated that air temperatures affect herbicide performance so applications should be made when there is a good stretch of temperatures above 10 C. 

Ryegrass is more sensitive to herbicides when it is less than 30 cm (12 inches) in height.

The “spike” seed head with distinctive short awns of annual/Italian ryegrass poking through a winter wheat canopy. photo: Supplied

A rate of glyphosate 540 g/L at 1.87 L/ acre should be used. Tank mixing this rate of glyphosate with Assure II (quizalofop-pethyl) ahead of soybeans or Steadfast (nico-sulfuron/rimsulfuron) ahead of corn will improve control. 

If you see ryegrass in your cereal stubble, it would be worthwhile to spot spray plants with a two per cent glyphosate solution to see if they will die. If they survive, contact either [email protected] or [email protected] so resistance testing can be pursued. 

Bluegrass (POA spp: Annual, rough-stalk, Canada)

Bluegrass species are becoming more common on Ontario farms. Three species are consistently being found in field crops: annual bluegrass, roughstalk bluegrass and Canada bluegrass, the latter two being perennial species. 

Management: Glyphosate (540 g/L) at 1.34 L/acre typically does a good job of killing emerged plants, but new seedlings will emerge later. The inclusion of soil-applied herbicides is a useful tactic to reduce later-emerging seedlings and seed dispersal. Ontario research has demonstrated that the soil-applied active ingredient called “pyroxasulfone” (found in Fierce EZ, Focus, Zidua SC) does the best job of preventing seedling emergence.

The seed head of chess grass extending beyond the winter wheat canopy. photo: Supplied

A post-harvest application of glyphosate + Zidua SC has been effective at providing control of annual bluegrass into the next season. This fall application timing coincides with peak germination of winter annual biotypes. Zidua SC has also proven to be effective when applied in the spring. Trifluralin (e.g. Treflan, Rival) is the only other herbicide available in Eastern Canada that lists annual bluegrass on its label as being controlled. It can be used in soybean, dry bean and canola. 

In winter wheat, spring applications of either Simplicity GoDRI, Axial or Varro to bluegrass 10 cm tall or less provided excellent control of roughstalk bluegrass in a 2021 Ontario trial.

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

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