It was not difficult to pick up on the topic du jour at the annual California Weed Science Society (CWSS) meeting in Monterey in late January. Growers and researchers continue to be stymied by the proliferation of herbicide resistance issues and the equally fast pace at which weeds themselves are spreading.

Palmer amaranth is a prolific weed impacting growers across the United States. It’s not the only one, but it is dominant.  In many areas researchers report it has developed resistance to glyphosate and other herbicides. According to Brad Hanson, weed specialist with the University of California, Davis, discovering glyphosate-resistant Palmer amaranth in California is a matter of time.

Even the chemical companies recognize the problem. At a recent company-sponsored meeting highlighting their new chemical portfolio, representatives with one company told growers they realize resistance issues will force them to use other company products with differing modes of action. That said, they assured growers and the professional crop advisers in the audience that their line of products are an effective part of a good herbicide rotation.

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Hanson said the proliferation of Round Up-ready technology has led to the bulk of herbicide resistant weed (HRW) issues. Lately, it seems the cases of glyphosate-resistant weeds seem to be growing almost exponentially. Resistance could expand to other modes of action, he said.

According to Hanson, the number of HRW species confirmed in California jumped from one in 1981 – Common groundsel in Asparagus was the first species confirmed resistant to a specific mode of action known as photosystem II inhibitor – to more than 20 by 2008.

Rice

Rice in California is the most affected crop in terms of weed pressure and resistance issues, though glyphosate resistance is not one of them. Multiple resistance issues in rice are very concerning to researchers and growers alike, he said. Hanson showed a slide with 14 different weeds within rice systems that are confirmed to be resistant to various modes of action.

HRW issues in rice have been around since the 1990’s, he said. Currently about 75 percent of California rice fields have some sort of HRW issue, according to Hanson.

The problem in rice is that it is a very herbicide-reliant system with only a few modes of action available, Hanson said. Moreover, rice is part of a monoculture system that is not rotated into other crops, which only aids the problem.

Hanson expects more HRW issues to pop up in the coming years as researchers identify new weed species that can survive multiple chemistries.

Stanley Culpepper, professor and extension agronomist with the University of Georgia, also sees confirmed cases of HRW on the rise, but non-confirmed cases seem to be equally troubling as research suggests more cases will be confirmed in the near future.

“The dominant resistant weed in the Southeast is Palmer amaranth,” Culpepper told the CWSS audience. “It is very drought resistant and will grow two-to-three inches a day.”

Culpepper said growers in his region seem to be making headway in attacking the problem, but not without cost.

“They’re still getting the production, but it’s more costly to get that production,” Culpepper said.

In one example, Culpepper said a grower in the Southeast was forced to hand weed his fields in order to remove the weeds and control the seed bank these weeds create when they go to seed. Photos showed workers stacking weeds on large flatbed trailers to be hauled out of the field.

Sustainable practices

Culpepper used a chart with three conjoined circles to explain how growers in the Southeast and other parts of the country are addressing weed resistance – not just in the Palmer amaranth, but in other prolific weeds such as Horseweed, common Ragweed and Ryegrass.

Citing three types of control methods: mechanical, cultural and chemical, Culpepper said growers in his region are not relying heavily on any one method, but are employing strategies in all three methods as equally as possible to be sustainable. Where herbicides once dominated agronomic practices to control weeds, with much less reliance on cultural and even less upon mechanical means of weed control, Culpepper said growers are working hard to equally apply all three methods.

“These practices must become more equalized,” he told his California audience. “Georgia is there now.”

Aside from hand-weeding, Culpepper said growers have also applied mechanical practices to manage weeds that seem to work. The catch there is these types of practices do not fit well with the reduced-tillage methods promoted by the National Resource Conservation Service.

These types of labor-intensive practices are not economically sustainable, Culpepper said. The only reason growers in the Southeast remain economically viable in certain crops is because the price paid for some of the commodities they grow have increased sufficiently to cover these costs, at least for now.

“This has worked for cotton,” he said. “But if we return to 60-cent cotton these guys are done.”

Aside from the triple-pronged approach Culpepper suggests, many researchers during the meeting repeated the same general ideas throughout the conference. Growers need to better understand the weeds growing in their fields and be willing to devote the labor and the resources to reduce the seed banks that these weeds create.

According to Hanson, gone are the days of the $10-per-acre cost of weed control. Once growers cross that hurdle in their minds they can begin to see the value of what Culpepper recommends.

Along that note, Culpepper emphasized the success growers are having in states where there is considerable cooperation between growers and university extension offices. Much success has been seen through the cooperative efforts of growers and researchers through the various land grant universities, he said.

For Albert Fischer, weed science professor in rice from UC Davis, the message is one of label rates. He recommends a strict adherence to the upper limit of label-rate applications, and no more.

Fischer discovered that using not enough herbicides to control weeds leads to the same HRW issues that an over-application will yield.

“This is a catch-22,” Fisher said. “The Europeans discovered this with lower application rates.”

Gene mutations at the molecular level suggest that weeds can more effectively become resistant to chemical applications than when a proper label rate is applied.

Fisher also recommends rotating various modes of action, which means a careful inspection of group numbers when deciding which herbicides to use.

He also recommends staying away from late applications and proper maintenance of spray rigs and sprayer calibrations in order to achieve proper application rates.

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