Each year at the Extension Service, we receive a number of samples of various weeds to test for herbicide resistance. Consultants, growers and county agents have utilized this testing service to help make decisions about herbicides to use in rice and wheat production.

Primarily, the testing has been for propanil-resistant barnyardgrass and for Hoelon-resistant ryegrass. Recently, this program revealed that we had more than 10 populations of ryegrass in the state resistant to both Hoelon (Accase inhibitor) and Osprey (ALS inhibitor) type herbicides.

The herbicide screen now has been expanded to include about every mode of action of herbicide that we use today.

Unfortunately, the number of barnyardgrass samples that are resistant to at least one mode of action of herbicide continues to increase. Roughly half the samples of barnyardgrass that we received in 2008 have been determined to be resistant to propanil.

About half of those are also resistant to post applications of quinclorac or Facet. While there is some evidence that populations of barnyardgrass resistant to Facet post are susceptible to pre-emergence applications, the expansion of Facet-resistant barnyardgrass is disheartening.

To date, two populations of barnyardgrass have been confirmed to be resistant to clomazone, the active ingredient in Command herbicide. These populations appear to be isolated at this time. Command still represents our best first line of defense against resistant barnyardgrass, but concern grows as we continue to utilize this herbicide on a large percentage of rice acres.

Be aware that escapes or “misses” from a Command pre-emergence application could be resistant off-types and should be controlled post, if possible.

So far, of all the samples screened, no populations have been confirmed to be resistant to the Accase inhibitors for rice (RiceStar and Clincher). We do see failures with these products from time to time due to moisture, grass size and coverage, and that may be why some of these samples were sent in.

We are aware of two populations of sprangletop in Louisiana that are resistant to both RiceStar and Clincher, but no barnyardgrass.

Jason Norsworthy, my research counterpart, is running our herbicide screening program up in Fayetteville, Ark. In my opinion, the most significant discovery he has made from the 2008 samples is finding one population of barnyardgrass from northeast Arkansas that is resistant to both Newpath and Grasp herbicides (ALS inhibitors).

He has not yet screened other ALS inhibitors like Regiment and Beyond, but we suspect they will also be ineffective on this weed biotype.

This should serve as a warning to Clearfield rice growers. We cannot rely solely on Newpath and Beyond for weed control on these acres, especially with the projected acres of Clearfield being well over 50 percent for 2009. That’s at least 750,000 acres of Newpath selection pressure in Arkansas alone.

I used to be concerned primarily with red rice resistance to Newpath, but barnyardgrass will hurt the usefulness of the technology as well.

Many of you are already incorporating Command pre, Prowl or tank-mixtures with Facet into your Clearfield programs. There are also some new pre-mixes available that contain two modes of action against barnyardgrass, such as, RicePro (Propanil + Facet) and RiceBeaux (Propanil + Bolero).

These are good steps toward preventing the development of widespread resistance to Newpath if incorporated into a Clearfield weed control program.

I hope that BASF, Horizon AG, and RiceTec, the companies that benefit most from the Clearfield technology, will take note of this discovery and develop programs to offset the cost and development of resistant weeds.

I will be promoting resistance management in both rice and soybeans in my county talks this spring, and will support any company efforts along those lines.

Our herbicide resistance screening programs are paid for in part with funds from the Arkansas rice, soybean and wheat promotion boards. Without their support, this service to the growers of Arkansas would not be possible.

e-mail: bscott@uaex.edu