According to a recent survey of over 250 ryegrass samples herbicide, resistance is on the rise in Arkansas ryegrass populations. This survey involved both random samples and samples taken from fields where either burndown or post-applied wheat herbicides had failed. The survey also included samples from industrial sites, roadsides and commercially available seed.

Samples came from all over Arkansas, mainly in the row crop-producing counties. Ryegrass samples were tested for resistance to glyphosate (Roundup), Axial XL, Hoelon and PowerFlex (an ALS herbicide). Samples that came back resistant to PowerFlex are also likely to be resistant to other ALS herbicides, such as Osprey, Finesse and Beyond (Clearfield Wheat).

Of all the samples tested, 95 percent were found to be resistant to Hoelon. This was suspected because high levels of Hoelon resistance have been known to exist for some time. Even many of the commercial samples were Hoelon-resistant. This indicates that this resistance has infiltrated ryegrass production fields located across the South and in Oregon (where much of the ryegrass tested is grown).

In addition, 22 percent of the samples tested were resistant to glyphosate. This number is low compared to what we know is going on in Mississippi, and what we suspect is happening in Arkansas. In general, these results were somewhat expected. However, further findings of the survey were somewhat of a surprise.

Although Finesse herbicide has been used for ryegrass control in Arkansas for some time, the total number of acres treated has always been somewhat low due to the fact that you must rotate this ground to an STS soybean the following summer. It was not until a few years ago when Osprey and a bit later PowerFlex were introduced that widespread use of the ALS chemistry for ryegrass control in Arkansas started. This was a much-needed answer to Hoelon –resistance, which was becoming a bigger problem by the mid-2000s. So these ALS products have really not been used for that long.

That being said, it was surprising to find that at least 60 percent of the samples tested had some level of ALS-resistance. It is possible that this resistance — although mostly found in wheat fields — could have developed prior to the introduction of these herbicides in wheat. ALS-inhibitors like Oust and others have been used on roadsides and other areas for years. The fact remains that resistance has developed or is developing fast in Arkansas ryegrass populations.

Axial XL is an acc-ase inhibitor and is in the same family as Hoelon. Due to a slightly different chemical structure, however, it will control most Hoelon-resistant populations. Axial has become our “go-to” product for resistant fields, especially those where we have experienced a failure with either Osprey or PowerFlex. Of the 250 samples screened, only 22 were found to be resistant, mostly at low levels to Axial. Given that this is our last line of defense in wheat, even that number is troubling.

In addition, 18 populations had multiple resistances to Axial, Hoelon, and PowerFlex. Three populations were found to be resistant to Roundup, Axial, Hoelon and PowerFlex.

During testing we sampled about 160 plants per population. If only 5 percent survived a herbicide application, we said that population was resistant. So, some of the resistance did occur at low levels. However, if the same herbicide was continually used, these individuals would survive and produce seed, gradually leading to a completely resistant population.

All this resistance makes making herbicide recommendations difficult. You almost need to test a field to know what to apply. We are going to continuing our screening program here in Arkansas so that growers and consultants can monitor resistance levels on their individual farms.

Using herbicide programs with multiple modes of action and good crop rotation, possibly including fallow treatments with gramoxone (instead of Roundup) or fall applications of Dual, are good ways to prevent herbicide resistant ryegrass from developing on your farm.

The 2012 MP44 is now available online at www.uaex.eduand in print at county Extension offices in Arkansas. It contains many good recommendations for ryegrass control in wheat and burndown.