Weed shifts causing more concern than glyphosate resistance

Mark Twain once responded to inaccurate reports of his death as “greatly exaggerated.”

The same could be said for California about one of the hottest topics in U.S. agriculture today, the rapid growth of weeds resistant to the herbicide glyphosate. California has its share of glyphosate-resistance weeds (four of the nine identified so far); however, the problem is more hype than fact in the Golden State.

Although weed scientists have identified significantly more weeds resistant to far more herbicides that are now attributable to glyphosate, it is the rapid spread of Roundup-resistance that is the talk inside and outside of agriculture.

This “super weed” issue has captured headlines in California, Renee Pinel, president/CEO of Western Plant Health Association, told a meeting of the California Weed Science Society in Monterey.

“The perception of widespread resistance to glyphosate really isn’t an issue in California, even though we hear about it a lot,” she said. One reason is California’s different cropping systems allowing for more alternatives that other parts of the country. Although growers in the West are adopting minimum tillage systems more readily, cultivation is still more prevalent than in other areas of the U.S. And there is less reliance on a one-herbicide weed control strategy in California in most all crops.

Pinel said glyphosate resistance is “not a product issue” but more about growers not using sound, multiple weed control strategies. This has been compounded by overuse of cheap off-patent glyphosate, she added.

Nevertheless, the business of California weed control continues to evolve with shifts in dominant weed species and some weeds becoming more challenging to control at glyphosate label rates, says Tulare County Cooperative Extension Farm Advisor Steve Wright, who also spoke at CWSS.

“The Roundup resistance technology is outstanding, and California growers realize the economic benefit of it,” Wright said. “You hardly see fields with nutsedge any more, and it is due to the Roundup Ready technology.” It has also helped in controlling nightshade. Wright estimates California cotton growers have saved $200 per acre per year with this glyphosate-resistant technology.

Most cotton producing states except Texas and California grow virtually 100 percent herbicide-resistant cotton varieties. One reason California has not been that high is because there has not been a Roundup-Ready Pima, the dominant cotton planted in the San Joaquin Valley. That changed last year when the glyphosate-resistant technololgy was approved for California, and 50 percent of the crop was planted to Pima Flex. This year Pima Flex acreage will likely be a much higher percentage herbicide resistant.

About 85 percent of California upland varieties are glyphosate-resistant varieties.

Although documented weed resistance to glyphosate is spreading rapidly elsewhere, Wright is more concerned about weed shifts in California than resistance.

Weed shifts causing Palmer spread

Palmer amaranth has emerged as the No. 1 herbicide-resistant weed in many areas of the South and Southeast. The same weed is showing up more often in California. However, it is not resistance that is causing the spread. It is weed shifts, said Wright. As herbicides control one species of weed, another is likely to become problematic.

Palmer amaranth is a nasty weed, Wright warns. “A couple of years ago we were concerned about horseweed. Horseweed is easy to control compared to Palmer amaranth, which produces double or triple the seed of horseweed. ” Wind can carry Palmer amaranth seed a quarter of a mile. It is creeping into fields, orchards and vineyards from ditch banks and roadsides.

Wright is warning growers that left uncontrolled outside of fields invites an in-field problem with not only Palmer amaranth, but other weeds as well.

Herbicide-resistant Palmer amaranth now consumes 1.6 million acres of cropland, according to Wright. It grows at the rate of 2 inches per day. It is forcing no-till farmers to return to tillage systems and hand-weeding and putting residual herbicides back into cotton copping systems. Fortunately, Wright said California cotton growers only briefly abandoned preplant herbicides. They’re now back in the vegetation management strategy.

Wright also identified annual morningglory, pigweed, lambsquarter, sprangletop and barnyardgrass as becoming more problematic to control with Roundup over the past few years.

There are viable alternatives to glyphosate, said Wright. Most are tank mix compatible with glyphosate.

There are also herbicide-resistant traits like Liberty Link in some varieties and in development are cotton varieties resistant to 2, 4-D and Dicamba.

“There are a ton of herbicides that will work in cotton weed control. You do not have to rely on Roundup,” he said.