Earlier this year, University of Arkansas weed scientists confirmed barnyardgrass in at least one Arkansas rice field had developed resistance to Newpath herbicide. Barnyardgrass from this particular field was also resistant to propanil and Facet.

This was a problem that had been on the horizon. It was just a matter of when it would be identified and how fast it would spread.

In the five years I have been in Mississippi, dozens of barnyardgrass samples have been collected from rice fields throughout the Delta. We have sprayed these plants in the greenhouse with propanil, Facet, Newpath, and Clincher.

Ordinarily, most of the samples screened in the last few years have been controlled with these herbicides under greenhouse conditions. But there are always a few samples every year where some plants are killed but others collected from the same field are not.

In 2009, two samples were collected from Mississippi rice fields that Newpath and Beyond completely failed to control. Over 70 percent of rice acres in Mississippi were planted to Clearfield varieties or hybrids in 2010. Considering the Clearfield acres in Mississippi along with the control failures observed in the greenhouse last year, I was afraid that 2010 would be the year that more problems with barnyardgrass in Clearfield rice started to emerge.

I received a call on April 30 from a rice consultant who said he had just checked a field where barnyardgrass had survived an application of Newpath plus Command. We went through the usual checklist of questions and answers:

“How big was the barnyardgrass when it was sprayed?”

“One- to two-leaf.”

“How wet was the field when it was sprayed?”

“Wet.”

“What was the water volume for the spray application?”

“Ten gallons per acre.”

Everything checked out. Also, this field was directly across the road from where we had collected one of the two samples I mentioned earlier that were not controlled in the greenhouse in 2009. Over the next six weeks, the field was treated with Ricestar HT at 21 ounces per acre, Clearpath at a half pound per acre plus Regiment at 0.6 ounce per acre, and finally Clincher at 15 ounces per acre. The grower will harvest a crop from the field, but even today (Aug. 12) there is an alarming amount of barnyardgrass.

On May 3, my crew and I collected about 100 samples of barnyardgrass from the field. We brought these samples to the greenhouse in Stoneville and sprayed 30 plants with 6 ounces of Newpath and 30 different plants with 12 ounces of Newpath. Most plants had four leaves when we made our greenhouse applications.

No plants died in the trays treated with the 6-ounce rate and only two plants died in the trays treated with the 12-ounce rate. Around the end of May, some of these barnyardgrass plants were sent to a lab for molecular testing. The molecular tests determined these were most likely resistant to Newpath.

Confirmation of Newpath-resistant barnyardgrass in Mississippi will not be based solely on the molecular testing. Additional research will be conducted this fall to verify what we have seen in the field. However, if it walks like a duck and quacks like a duck, then it is more than likely a duck. In this case, if you spray the same herbicide on the same weed multiple times, and it does not die, then the potential for that weed to be resistant to that herbicide is pretty high.

What happens now?

So what happens now? I tell people to manage barnyardgrass in rice just like Palmer amaranth in row crops. Like Palmer amaranth, barnyardgrass is much easier to control before it comes out of the ground. The Clearfield technology, similar to Roundup Ready, has let us become somewhat careless with herbicide timing. Too many times, barnyardgrass is sprayed in the three- to four-leaf stage (or tillering) rather than the one- to two-leaf stage. This may work some of the time with some herbicides, but it will not work all of the time. You cannot spray barnyardgrass too early. Get on it early and stay on it until the field is flooded. That is the only way to ensure you will have a clean crop in August.

Whether you are growing Clearfield or conventional rice, I think any barnyardgrass control program should begin with Command at planting. Over-the-top programs should contain tank-mixtures of postemergence and residual herbicides with multiple modes of action.

For instance, the first over-the-top application could include propanil plus Bolero (or RiceBeaux) or Facet. If you are in an area with a history of propanil resistance, then Ricestar HT plus Facet or Prowl H2O would be an alternative.

Every year I do a study on barnyardgrass control without using ALS (Newpath, Regiment, Grasp) or ACCase (Ricestar HT, Clincher) herbicides. Our best treatments have at least two applications of a residual herbicide, one of which goes out either pre-emergence or delayed-pre-emergence. The barnyardgrass pressure was severe in my rice plots at Stoneville this year. The only plots that I have right now that are completely free of barnyardgrass received at least two postemergence applications following a full rate of Command (1.33 pints per acre for my soil type).

Barnyardgrass can be managed in rice, even if it is resistant to Newpath. However, programs must be built around residual herbicides. When barnyardgrass does emerge, the timing of the postemergence herbicide is critical. With these postemergence applications, you should always use premixes or tank-mixtures of herbicides with multiple modes of action to reduce the pressure on the few chemistries available that are active against barnyardgrass.

At every rice meeting, I remind growers that there are no new barnyardgrass herbicides on the way. Since I have been at Stoneville, no company has asked me to test a new herbicide with activity on barnyardgrass. The current herbicides with barnyardgrass activity are the herbicides we will rely on in 2015 and 2016.

So, with that in mind, protect the herbicides and technologies that are in place in 2010. There are still some effective herbicides for barnyardgrass, but it is everyone’s responsibility to help prolong the effectiveness of these products.

e-mail: Jbond@drec.msstate.edu