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Resistant ryegrass major target in burn-down and wheat fields

In both burn-down applications and wheat fields, ryegrass is emerging as a very troublesome weed for Mid-South growers. I have a graduate student working on samples from all across Arkansas (over 225 sites) to determine levels of herbicide resistance to glyphosate as well as to our common wheat herbicides.

We hope to establish a baseline from which to work towards preventing the further development of resistance in ryegrass.

Approximately 25 of our samples came from fields where glyphosate failed in some way to control ryegrass in spring burn-down applications.

Mississippi has already determined that they have glyphosate-resistant ryegrass, and it is my understanding that they too have under done a massive screening program this spring. The Mid-South states are coming together at a meeting in Arkansas to try and better understand the threat level that resistant-ryegrass poses.

I am going to talk mostly about wheat weed control in this article, but I'll mention ryegrass in fallow fields that are going into soybeans, cotton or other crops next spring. Our data indicates that as the size of ryegrass increases, so does its tolerance to glyphosate.

Most of our fall-applied ryegrass treatments result in good control, but may need a re-spray prior to planting the next spring. In addition, January and February applications of glyphosate look much better than March applications.

The bottom line is that once ryegrass goes reproductive (March), it is pretty much impossible to kill without very high rates of glyphosate. In addition to glyphosate, Select MAX (clethodim) is also labeled for ryegrass control at these timings, however, it is similar in chemistry to Hoelon, which we already have a lot of resistance to.

Another good option is a fall or early spring application with a residual product like Dual. Others that work well on ryegrass are Valor XLT, Envive and Canopy EX. There are more and each company probably has one to recommend to you. Just make sure the rotational interval fits your intended crop!

The bottom line is that you cannot wait to control ryegrass that is more than 1 foot tall prior to planting; it must be controlled earlier for good results at reasonable rates.

In wheat, we now use Finesse and Osprey herbicides, and to a lesser degree, Axial XL and Prowl H2O herbicides, to battle ryegrass. Both Finesse and Osprey are ALS herbicides, and to date at least 12 populations of ryegrass that are resistant to ALS chemistry have turned up in Arkansas. In addition, these populations appear to also be resistant to Hoelon.

Axiom, a new product from Bayer CropSciences, is a premix containing the products metribuzin (Sencor) and flufenacet. It has provided good suppression of ryegrass as a very early-post at our Hoelon-resistant ryegrass location. It will control many other weeds in addition to ryegrass. When applied early it is effective on annual bluegrass or poa.

Axiom actually looks best under severe ryegrass pressure in a program as either a tank-mix with Osprey or followed by Osprey. One appealing thing about this approach is resistance management, because it provides three modes of action against ryegrass (although the metribuzin part is somewhat weak on this specific pest). This program has no plant-back restrictions that affect soybeans.

Glean herbicide is not labeled in Arkansas. However, Finesse herbicide (Glean + Ally) for some time has been the best pre-emergence product that I have looked at for ryegrass control in wheat. If you can get a rain to activate it, Finesse alone often provides season-long, acceptable ryegrass control. It also controls most other wheat weeds.

A common mistake that many have made is to apply Finesse post-emergence on already emerged ryegrass and hope for the same results. Control failures also often occur when no rainfall is received for activation.

A new formulation called Finesse Grass and Broadleaf (Glean + Everest (flucarbazone-sodium)) is now available and will work better on emerged ryegrass in this early postemergence timing. The product looked very good timed to three-leaf ryegrass in our plots this year. Finesse Grass and Broadleaf will control emerged ryegrass and provide the long residual that growers expect from Finesse.

With Glean-containing products you must rotate to STS soybeans the following year or unacceptable crop damage will occur. From a resistance management standpoint, both Finesse products contain ALS herbicides.

The new product from Dow AgroSciences called PowerFlex (pyroxsulam) looks very good in our program so far and performed well with only a limited launch last fall. Similar in spectrum to Osprey herbicide and also in the ALS group of chemistry, it has performed very well on ryegrass this year as well as on several other broad-leaf weeds. We observed excellent control of both ryegrass and Hoelon-resistant ryegrass, as well as control of henbit, curly dock, buttercup and red sorrel in our plots.

PowerFlex did not control wild garlic, primrose, cudweed or horseweed.

Currently there is a nine-month rotation to soybeans on the PowerFlex label; Dow is working to shorten this to 90 days, hopefully before wheat planting this fall. We did not see any soybean injury in our plots this year, but we only had two locations. One of the most exciting things about this product is that it may be priced very competitively.

The best plots in my program again this year were the two-shot programs of a pre (such as Finesse or Axiom) followed by a post (Osprey, Axial XL or PowerFlex).

The second-best plots combine a post herbicide with a residual herbicide (such as Osprey + Prowl H2O, or others) applied late in the fall.

I know most of you do not want to use a two-shot approach in wheat due to cost, but the hard fact is most one-shot programs will just not provide 100 percent control of heavy ryegrass infestations.

I do not know if you noticed or not, but we have gone from a heavy reliance on acc-ase chemistry (Hoelon and Axial XL) to heavy reliance on ALS chemistry (Finesse, Finesse G&B, Osprey, PowerFlex) for ryegrass control. It is only a matter of time before more ALS resistance shows up.

Currently, Axiom and Prowl H2O are the only options that are neither ALS nor acc-ase inhibitors. Incorporation of these two products into a program approach will help curb the development of ALS-resistant ryegrass.

For years, growers have used either tillage or Roundup (glyphosate) prior to wheat emergence to control the first early flush of ryegrass, which is often a very effective control method. This cultural practice could be used in a “two shot” approach and followed by PowerFlex or Osprey plus a residual.

If you do not plant wheat, consider a fall application of glyphosate or possibly glyphosate plus a residual material for ryegrass control. It is much easier to kill in the fall while it's small. We have not confirmed glyphosate-resistant ryegrass yet in Arkansas, but many think they have it, because it is so hard to kill once it is tillered out in the spring. We will provide an update on the distribution and confirmation of resistant populations once we have completed our screening program.

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