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.

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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.”