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Pest Patrol: Bluegrass becomes an emerging problem in field crops

#PestPatrol with Mike Cowbrough, OMAFRA

In Ontario, turf-like grassy weeds are being found in more cultivated fields. In herbicide-tolerant corn and soybean, control with glyphosate has been inconsistent. In cereal and forage crops, fast growth of these grasses in the spring makes them too big to be controlled with herbicides. Let’s review what is being found in fields and how to accurately identify the grassy weeds, and then look at how to manage them.

What “turf-like” grassy weeds are being found?

The most prominent species are annual bluegrass and roughstalk bluegrass. This article will focus on the management of these two species.

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Annual bluegrass grows in “clumps”; the leaf blades are smooth and light green (Figure 1 at top). You might assume, given the name, that this species has an annual life cycle. In southern climates a winter annual life cycle is most common. In northern climates, perennial biotypes exist which creep along the ground by way of stolons that root into the ground at each node. Both biotypes likely exist in Ontario and reproduce mainly by seed. The seed of winter annual biotypes tends to germinate between September and October (60 to 80 per cent) while the rest germinate in April through to June. The seed of perennial biotypes can germinate throughout the season.

Roughstalk bluegrass is a perennial that creeps along the ground by stolons that root at each node (Figure 2 below). The leaf blades are rough in texture. It reproduces by roots and seeds.

Figure 2: Roughstalk bluegrass.
photo: Supplied

Why does correct identification of the grassy weed matter?

It’s important to know which weedy grass species you’re dealing with. In Michigan, yield losses of up to 50 per cent have been observed when roughstalk bluegrass is competing with winter wheat. Spending upwards of $25 per acre on a herbicide that does not control the species is a costly mistake. For example, Dr. Christy Sprague (Michigan State University) has found that while “Axial” herbicide will control roughstalk bluegrass in winter wheat, it will not control annual bluegrass. By contrast, while annual bluegrass can be controlled with “Simplicity GODRI” in winter wheat, this treatment does not effectively control roughstalk bluegrass.

What’s the easiest way to identify an unknown grassy weed?

Grassy weeds are difficult to identify at the best of times, but turf-like grassy weeds take it to the next level. Fortunately, there is a $20 genetic test, called “DNA barcoding”, offered by Harvest Genomics Laboratory in Guelph. Although this test may not always be able to identify a weed at a species level, I have found it to be reliable when it comes to identifying roughstalk bluegrass and annual bluegrass.

How can we control these bluegrass species in cereals?

Dr. Sprague’s research out of Michigan offers guidance for Ontario. If you have roughstalk bluegrass, your only option is Axial herbicide. If you have annual bluegrass, experience in Ontario has shown that Focus herbicide applied prior to winter wheat planting provides good control of annual bluegrass. If both annual bluegrass and winter wheat are emerged, then Simplicity GODRI has been the most effective at controlling annual bluegrass. Emerged bluegrass species are more easily controlled when small (e.g. two to four inches tall) with 20 to 25 per cent reductions in control have been observed when bluegrass is bigger at application (e.g. 10 inches tall).

How can we control these bluegrass species in corn and soybean?

Annual bluegrass: Glyphosate typically does a good job of killing emerged plants, but new seedlings often 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 at preventing seedling emergence. Tank-mixing metribuzin (e.g. Sencor) has been shown to enhance control (Table 1 at left).

Recently, a post-harvest application timing was added to the Zidua SC label for the control of annual bluegrass. This fall application timing coincides with peak germination of winter annual biotypes. Zidua has also proven to be effective when applied in the spring (Figure 3 below). 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.

 

Figure 3: Annual bluegrass control at eight weeks after the application of Engenia + Zidua SC + glyphosate (left) compared to Engenia + glyphosate (right).
photo: Supplied

Roughstalk bluegrass: Control using glyphosate is affected by application timing. A Kansas study found that roughstalk bluegrass was controlled when glyphosate was applied in the spring (late April). Control declined when glyphosate applications were made in midsummer and late summer (Figure 4 below). Little research has evaluated control of seedlings with soil-applied herbicides. It is unknown whether pyroxasulfone, which controls annual bluegrass, would control rough stalk bluegrass. Past research has evaluated roughstalk bluegrass control. In corn, nicosulfuron (e.g. Accent) was most effective. In soybean, high rates of quizalofop (e.g. Yuma, Assure II) have provided the best control.

Figure 4: Effects of seasonal glyphosate timing on roughstalk bluegrass control. A) is untreated roughstalk bluegrass, B) was treated with glyphosate in the spring and C) was treated with glyphosate during midsummer (July).
photo: Taken from “Controlling Poa trivialis with glyphosate” by Cole Thompson, Ph.D.; Jack Fry, Ph.D.; Megan Kennelly, Ph.D.; Zac Reicher, Ph.D.; and Matt Sousek

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

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