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Posted 29 July 2014. PMN Crop News.

Herbicide-Resistant Weed Threatens No-Till Farming

Farmers increasingly resort to tillage to control Kochia

Source: Kansas State University Press Release.

Hays, Kansas (July 1, 2014)--Known by the name tumbleweed, it’s been romanticized in story and song. And when it’s called summer-cypress, it sounds downright exotic. No matter what you call it, the weed kochia, cuts into crop yields and farmers’ profits. And it’s become harder to control.


With roots that grow deep into the soil – as much as 16 feet during drought – the Kochia plant (Kochia scoparia) can grow up to 7 feet tall, with thousands of small individual flowers. And it saps much-needed moisture from crop land across the High Plains.

For 25 years, however, kochia along with other weeds was successfully controlled by glyphosate, a broad-spectrum herbicide, initially sold under the brand name Roundup and now available under a variety of names.

In 2007, however, Kansas State University weed scientists confirmed resistance to glyphosate in four separate kochia populations in western Kansas, according to Phil Stahlman, a weed scientist, based at K-State’s Agricultural Research Center in Hays.

“The problem really blew up and got out of control in 2010,” said Stahlman, who noted that testing of several populations by his graduate student, Amar Godar, “determined that resistance had become widespread in western Kansas and colleagues in Colorado, Nebraska, and South Dakota had found resistance in those states.

“By 2013, that the problem had developed from the Texas High Plains, all the way into the prairie provinces of Canada.”

Stahlman and Godar rated 1,500 wheat stubble fields throughout western Kansas in fall 2010, and found that about 30 percent were tilled in an effort to control the weed.

“That was higher than I expected,” he said, adding that there was evidence that the kochia in several fields had been sprayed first - unsuccessfully. “It was eye opening; they felt that they had to resort to tillage.”

The practice is counter to the common practice of no-till or minimum tillage. Leaving the soil mostly undisturbed leaves a layer of residue from previous crops, which boosts soil fertility, helps conserve soil moisture and makes it less prone to erosion from wind and rain. Many studies have shown that crop yields from no-till fields are higher than when fields are tilled.

In 2010, Stahlman and researchers in other states launched a regional effort to investigate alternatives to glyphosate for kochia control. They found that if an herbicide with residual properties was applied before the weed emerged in early spring, it cut emergence by at least 70 to 80 percent, often more than 90 percent. Then, the kochia that did emerge during the growing season was more easily managed.

In fall 2012, Stahlman and Godar asked crop consultants in western Kansas several questions to determine the impact of glyphosate-resistant kochia, evaluate growers’ response to the problem and measure their success in managing it.

Fifty-two crop consultants completed the survey, which covered 46 of Kansas’ 105 counties.

“Several important points came out,” he said. "The percentage of fields infested with kochia increased from 47 percent in 2007 to 70 percent in 2012. Over the same period of time, the average use rate of glyphosate went up from 0.75 pound per acre to 1.25 pound per acre. That in itself indicates that producers were having trouble controlling kochia.”

Many producers indicated that they had increased applications of glyphosate from two to three.

Stahlman and other scientists have been spreading the word at field days and other ways that applying herbicide prior to kochia emergence early in the spring can make a difference.

The good news, he said, was, in 2011-2012, the use of pre-emergent herbicide in addition to normal management began to be used effectively. Observations and reports from retailers indicate that many growers are now using a pre-emergent herbicide in early spring prior to kochia emergence and many more – perhaps a majority – are using a pre-emergent herbicide in addition to glyphosate in Roundup Ready corn and soybeans.

“If we till the soil to manage kochia, we’ve lost the conservation gains we made in the last decade or more of using no-till,” Stahlman said.

The researcher is optimistic that kochia is manageable by applying pre-emergent herbicide and dealing with what still comes up later in the growing season.

“Fortunately, the seed life of kochia is relatively short, not more than two to three years. But with uncontrolled kochia on roadsides and fence rows, and if we have one grower in an area who is not on board with how to manage it, it could be a problem.”

Stahlman and other researchers are studying the kochia plant’s mechanism of resistance as they look for even better ways to manage the weed and protect crops.

Comparing economics of tillage options for managing glyphosate-resistant Kochia

In 2013, then K-State agricultural economist Troy Dumler recognized that the growing resistance of kochia to glyphosate led many producers to consider returning to tillage options for weed control in western Kansas dryland crop rotations.

Long-term data from the K-State Research Center in Tribune, Kansas indicated there is a significant economic advantage to incorporating no-till practices in a wheat-sorghum-fallow (WSF) rotation, according to Dumler. From 2001-2011, no-till wheat and sorghum yields were approximately 8 bushels per acre and 43 bushels per acre higher, respectively, than when using conventional tillage.

Similarly, no-till wheat and sorghum yields were 5 bushels an acre and 30 bushels an acre higher, respectively, than a reduced-till rotation (conventional tillage prior to wheat and no-till prior to sorghum). The higher yields associated with no-till resulted in a $63 per acre advantage for no-till over reduced-till and an $83 per acre advantage for no-till over conventional-till.

“With the growing difficulty of controlling kochia with a glyphosate-oriented herbicide program, the natural question becomes: How much can be spent on herbicides for kochia control to maintain the economic advantage of no-till?” said Dumler, who now works in private industry.

With assistance from K-State’s weed scientists, he developed an example herbicide budget for kochia control to compare the relative profitability of tillage systems in a WSF rotation to that of a herbicide program that used Round-Up as the primary herbicide option. The results indicate that while herbicide costs nearly doubled for the kochia control program, returns for the no-till rotation were nearly $50 per acre greater than reduced-till and $55 per acre greater than conventional-till. However, the profitability of the no-till rotation decreased by $30 per acre.