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Glyphosate-resistant Italian ryegrass

Over the past five years, Dan Poston, Trey Koger, Tom Eubank, or I have sprayed glyphosate-resistant Italian ryegrass with just about every possible herbicide and combination of herbicides from the chemical building in Stoneville, Miss.

There were even some herbicides screened that are not commonly used in the Mid-South that I tracked down in hopes that they would offer us another option. Some of the herbicides controlled glyphosate-resistant Italian ryegrass in a few studies and not in others. Others that I thought were sure to work provided no control. Gramoxone Inteon and Select Max are the only two products that have consistently controlled glyphosate-resistant Italian ryegrass in spring burndown programs.

Gramoxone Inteon has worked well on glyphosate-resistant Italian ryegrass, but it certainly is not the answer to all problems. In general, no fields contain only Italian ryegrass in the spring, and Gramoxone Inteon is not that effective for controlling other problem species such as curly dock or Pennsylvania smartweed.

Gramoxone Inteon also has a tendency to drift and injure non-target plants. But it has performed well applied by ground when the application can be made more slowly utilizing a higher water volume than aerial applications.

If you are comfortable applying Gramoxone Inteon and can apply it with sufficient water volume (I prefer 10 to 15 gallons per acre), it is a good option for Italian ryegrass. If Italian ryegrass is just beginning to tiller (which is probably not the case this time of year), Gramoxone Inteon at 3 pints per acre plus a nonionic surfactant or crop oil concentrate should provide effective control. As the Italian ryegrass gets larger (more than three tillers), the rate should be increased to 4 pints per acre.

Regardless of the rate you use, monitor the field following application because a follow-up application will most likely be required.

Another idea to consider is adding either a triazine (atrazine, Sencor) or urea (Direx, Linex) herbicide with it. Combinations of Gramoxone Inteon and triazine or urea herbicides for burndown or for controlling failed corn stands were talked about quite a bit at the weed science meetings this winter.

The triazine or urea herbicide increases the uptake of Gramoxone Inteon into the target plant. Since Gramoxone Inteon is a contact herbicide, the more herbicide that enters the plant, the better the control will be. We evaluated some of these mixtures a couple of years ago and found that mixing Direx or Linex with Gramoxone Inteon improved control of Italian ryegrass 26 percent to 28 percent compared with Gramoxone Inteon alone.

Before choosing a triazine or urea herbicide to tank-mix with Gramoxone Inteon, make sure to consider the crop you will be planting in the treated field because preplant intervals will vary.

The second spring postemergence option for Italian ryegrass control is Select Max. As with Gramoxone Inteon, Select Max is not a complete solution. Select Max is a member of a group of herbicides referred to as graminicides, and it will only control grasses.

Other than Italian ryegrass and annual bluegrass, grass species are not a primary target of burndown herbicide programs. Because of its selectivity, an additional herbicide will be required with Select Max to control broadleaf weeds.

Antagonism, or reduced control, is often a problem when mixing a broadleaf herbicide with a graminicide. We have seen no problem with tank-mixtures of Select Max and glyphosate. But adding 2,4-D to combinations of Select Max and glyphosate has led to reduced control of Italian ryegrass in some cases. In a study we conducted last fall, 2,4-D at 1 quart per acre (4 pound active ingredient of 2,4-D) reduced Italian ryegrass control when the Select Max rate was 8 ounces per acre. We were able to overcome this problem by increasing the Select Max rate to 16 ounces per acre.

If your primary target is Italian ryegrass and horseweed or cutleaf evening-primrose is not a problem, then glyphosate plus Select Max at 12 ounces has performed well on Italian ryegrass. But in our plots, we needed a follow-up application of Select Max at 12 ounces per acre to get 90 percent Italian ryegrass control. Adding ammonium sulfate to Select Max will also improve performance on Italian ryegrass.

Other issues that may be encountered when choosing Select Max for Italian ryegrass control are preplant intervals and graminicide resistance. Select Max does possess some soil residual activity and it must be applied at least 30 days prior to planting grass crops (corn, rice, grain sorghum).

We have seen no problems to date in Mississippi with Italian ryegrass resistance to Select Max. Hoelon has the same mode of action as Select Max but is a member of a different herbicide family. Hoelon-resistant Italian ryegrass is widespread across the Mid-South, and herbicides with the mode of action of Hoelon or Select Max are highly susceptible to resistance development. So we need to be very judicious with Select Max applications to protect this herbicide.

The final problem with controlling Italian ryegrass with spring herbicide applications is the plant residue. Following March herbicide applications, fields with Italian ryegrass will have significant residue at planting. This residue will be difficult to plant through without planters or drills designed for no-till planting. Italian ryegrass residue will also compete with crop seedlings and hinder early-season herbicide programs due to inadequate spray coverage.

Glyphosate-resistant Italian ryegrass is difficult to control, but we can manage the problem. When we first discovered glyphosate-resistant horseweed, it had a big impact on the way we managed burndown programs. But there is a pretty good comfort level with managing horseweed now and hopefully in time we will get there with glyphosate-resistant Italian ryegrass, too.

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