With barnyardgrass already resistant to propanil and quinclorac, Mid-South rice farmers can ill afford the abundant weed to develop the ability to overcome more chemistries. Recent modeling work done by weed researchers shows there are ways to keep that from happening – or, at least, to slow the process down.

“In 2013, our rice acreage was down to around 55 percent Clearfield and it is likely to be 50 percent in 2014,” says Jason Norsworthy, University of Arkansas weed scientist. “However, in 2011 our Clearfield acreage was up to 70 percent.”  The decline in acreage is partly a result of ALS resistance in barnyardgrass beginning to become widespread.

How quickly can resistance move? The first ALS-resistant barnyardgrass was found in Arkansas three years ago. In 2013, Norsworthy and colleagues found that of 30 samples they screened, 13 tested positive for ALS resistance, 43percent of the tested samples.

“We want to understand how to best protect technologies and herbicides within rice and other crops. Towards that end, we’ve done a lot of modeling work, initially with Palmer amaranth in cotton.”

“Many of the strategies recommended today came out of the initial modeling work we did on glyphosate-resistant Palmer amaranth. For example, the overlaying of residual herbicides and making sure you provide season-long weed control and prevent escapes are a result of our modeling efforts.”

When he began to look closely at weeds in rice, Norsworthy focused on Newpath, an ALS herbicide used in Clearfield rice, and Ricestar, an ACCase herbicide.

The computer model used data from three locations in the Arkansas delta to create management scenarios across 1,000 hypothetical rice fields over a 30-year period. Three stages of growth were also in the mix: dormant seedbank, emerged seedlings, and mature plants.

“We looked for ways to protect both modes of action and to answer questions growers and consultants might have.”

Among those questions:

  • Are you better off using one mode of action until it no longer works?
  • Is there true value in utilizing multiple modes of action in a growing season?
  • Does using multiple modes of action increase the longevity of both modes of action?

“It’s interesting that no one has ever looked at development of resistance to two models of action simultaneously. What we found was we could, indeed, lengthen a herbicide’s longevity by about three years if we used the two together.”

One mode?

What if a producer does use just one mode of action until resistance builds up?

“The problem with that is the soil seedbank.  By the time resistance is confirmed, the seedbank is extremely high. What happens is resistance to the next herbicide used occurs quickly due to the high seedbank.

“BASF is developing Provisia rice (ACCase-resistant), which will likely be available in three to four years. If we let all this ALS resistance build up with the belief that the ACCase chemistry will be the answer, we’re mistaken. Once resistance builds to one mode of action the chances of more resistance developing is very high.”

And in some Mid-South fields, Provisio rice may not be an option even before it’s released. “Jason Bond, (Mississippi State University weed specialist), has been working with a population of barnyardgrass that is resistant to ACCase, ALS, propanil and Facet. That’s shocking and makes you wonder how the producer can control the weeds. Think about it: this population is already resistant to the herbicide to be used with Provisio rice and that hasn’t even hit the market yet.”  The selection pressure on this population likely came from applications of Ricestar HT or Clincher.

While working on the models, the researchers also found continuing confirmation of the value of diversity in weed control. But diversity only goes so far, warns Norsworthy. “It has to be done properly. You mustn’t use diverse herbicides at the wrong time, make applications to weeds that are too large. Otherwise, your program isn’t sustainable and resistance builds up.

“Even when using two modes of action, if you mistime the applications, the weeds will react in ways you don’t want. So, proper application timing is just as important as diversity.”

Clomazone under rice is essential at planting because “use of clomazone means the risk of evolving resistance to postemergence herbicides diminishes tremendously. So, you need a strong program at planting like clomazone or Command.”

Asked if the weed resistance message has gained full traction in the agriculture community, Norsworthy was positive. “Whether rice, cotton or soybean, I think it’s getting easier to get the message to stick. Farmers are more and more understanding and becoming part of the solution than the problem.

“But it wouldn’t be true to say all is well. Thirty percent of the soybean growers in the Mid-South are still using Liberty-only programs. Well, the good news is that 70 percent of the growers understand the real risks of weed resistance and are doing what they can to mitigate it.”

“Maybe it’s a mirror of society as a whole. Regardless of profession, some people are on top of their games and excel and push for better. Others barely get by.”

Will we ever get to the point where everyone is on board with preventing resistance? “No. One reason is that some producers seem overly optimistic that there will always be an answer provided when things get rough. Too often we’re reactive rather than proactive.”

Yellow nutsedge

In other work, Norsworthy has been working with yellow nutsedge.

“Several years ago, we found the first herbicide-resistant yellow nutsedge in the world. It’s a perennial weed and perennial sedges seldom reproduce by seed and rarely evolve herbicide resistance.”

While there is a bit of seed production by yellow nutsedge, “Overall, it reproduces by sending out runner with tubers – sort of like a peanut – below ground.

“The resistant yellow nutsedge we have has a prolific growth rate. It far exceeds that of just standard nutsedge. It’s absolutely amazing in its ability to grow, spread, and produce rhizomes.”

The resistant yellow nutsedge is “completely nonresponsive” to ALS herbicides like halosulfuron. The Mid-South continues to see an increase in ALS resistance among sedges, as a whole. “We have rice flatsedge that is also ALS-resistant.”

Further, “we have some growers throughout the Mid-South that are growing what I call ‘California rice’ -- zero-grade continuous rice. Some of those operations now have a weed common to California: smallflower umbrella sedge. It’s an annual sedge and is also ALS-resistant.

“To put it mildly, we really have our work cut out with resistance.”