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Crop management key in resistance build-up

Not just a question of which herbicide to use.

March 10, 2008  By Ralph Pearce

Early in 2006, Dr. Bill Johnson, a researcher at Purdue University, made a presentation on
developing resistance in Canada fleabane (horseweed in the US) and marked its rapid migration northward. In just a few years, the glyphosate resistant weed had advanced as far north as Fort Wayne, Indiana and Findlay, Ohio, and Johnson warned growers its appearance in Ontario was only a matter of time. Since then, the weed has been confirmed in Michigan.

Anecdotal evidence indicates other weeds, including ragweed, have shown signs of glyphosate resistance in the US, meaning growers need to rotate their chemistries more often.

As it turns out, the transfer of resistance in various weed species has been under the watchful eye of two other researchers since 2001. Dr. Micheal Owen of Iowa State University and Dr. Mark VanGessel at the University of Delaware have been trading insights and information on the recurring resistance within various weed species. In particular, they have been working with Conyza canadensis (Canada fleabane) and Conyza ramosissima (dwarf fleabane) and testing the relative ease with which the two can develop resistance.

In particular, the researchers wanted to study the variations in the plants’ abilities to adapt their appearance, development or behaviour relative to their respective genetic traits, also known as heritability. Owen started with forced and natural crosses to ensure a purely resistant plant, and compared it to the populations that were sensitive, then performed various crosses of those and went through the typical backcrossing procedure. “What we determined was that glyphosate resistance in Conyza is transferred through the pollen, in a ratio that strongly supports a partially dominant single gene trait,” explains Owen.


The importance here is the ease of transmission. Canada fleabane is a wind disseminated seed and is highly prolific, but it is the existence of the dominant single gene that may be the key. In the case of waterhemp, which likely has a polygenic controlled trait for glyphosate resistance, the spread has not been as extensive. “And it’s taken a lot more selection for populations to shift from that which are primarily sensitive to that which are primarily resistant,” says Owen. “In the case of horseweed, it just exploded and that makes sense when you have a single, semi-dominant gene trait; everything it crosses is going to be resistant,” says Owen.

The ‘plane’ truth about wind dissemination
To confirm the ease with which wind disseminated seed can spread, VanGessel collaborated on research with colleagues from Cornell University and Penn State University which tried to gauge the long distance movement of horseweed seed. Using remote controlled airplanes fitted with apparatus that could capture seeds, researchers flew the planes to set altitudes for about 30 minutes with the traps open.

“We were able to catch horseweed seed at about 125 to 150 metres above the ground,” says VanGessel. Based on aerodynamics, their altitude suggested they were ascending to cloud layer, where they would normally stay aloft until about one hour before sundown, when the earth’s surface begins to cool, at which point they would then begin to descend. “If you have a mild breeze of 15 to 25 kilometres per hour (10mph to 15mph) during that time of five to 10 hours, you can easily see how seeds could move in excess of 200 kilometres (120 miles) in a day. The fact that once that seed settles to the ground, we have so much no-till being adopted across the US and Canada, it’s finding a safe site to germinate and emerge and develop a plant that’s going to produce seed in that new location.” And then the process is repeated.

Glyphosate resistant Canada fleabane, also known as horseweed, has moved from Indiana and Ohio into Michigan, making Ontario the next likely destination.

Management practices also a factor
Recognizing Canada fleabane’s single genetic feature, and confirming the ease with which it spreads, is only part of the picture, states Owen. “There’s nothing magic about growing glyphosate resistant crops and weed population shifts,” he says. “The culture in which the crops are produced has a much more pervasive impact on the weed population and the weed community than the crop itself. The weeds have to be there initially and they have to be ecologically adapted before the herbicide can come in and select for the resistant individual weeds within the population.”

In that case, it is believed, the wide adoption of Roundup Ready technology and its corresponding impact on no-till farming has allowed the easy and fast spread of Canada fleabane across the US midwest and northward towards Ontario. VanGessel agrees with that assessment and points out that the biology of the weed is such that it germinates predominantly in the fall. Then there is often another flush of weeds in early spring just before growers begin tilling their ground for corn and soybean planting. “That’s not to say we should begin plowing to control Canada fleabane,” says VanGessel. “We have to look at the whole picture, and no-till just brings so many benefits to farming that tillage is the tool of last resort to control it.”

At the same time, adds Owen, there needs to be a mind-shift as to the rationale for using various herbicides. In the case of glyphosate, he says, too many growers view it as a means of just killing weeds, instead of trying to managing them. “Yet growers lose a lot of bushels because they spray glyphosate when it’s convenient and simple, but it’s often too late: they kill the weeds but they’ve already lost crop yield,” relates Owen. “What’s important is that by using alternative strategies, whatever they may be – but in all likelihood, those are older herbicides – they’re also providing ways to protect against the evolved resistance to glyphosate.”

One particular value statement that stands in the way of shifting mind-sets, notes VanGessel, is the relatively inexpensive pricing structures for newer trait technologies. Glyphosate tolerance is just one of many different genetic traits imparted into seed. “You’ve already invested in that technology when you buy the seed and whether you want it or not, it goes against human nature to use a different herbicide,” concedes VanGessel. “To do so, you have to pay more.” -end-

Take-home message
An analogy Dr. Micheal Owen likes to use to demonstrate the issues and development of resistance involves a partially filled balloon, which can represent a weed community or cropping system. Closing one’s hands around that balloon can represent a herbicide or cropping system being used to influence the inner system (the balloon).

“If you begin to squeeze the balloon, the balloon begins to push back and that’s nature’s way of saying, ‘I’m going to do what I have to do, regardless of what you do’,” relates Owen. “Because your hands are not able to entirely encompass the agro-eco system, which is analogous to imperfect weed control and the incredible genetic diversity in weeds, eventually a bubble will appear and that bubble is a weed that doesn’t respond to whatever it is that you’re doing.”

The weed may not respond to a particular herbicide or management practice. “Or it could be a weed that has been selected for that rare genetic event that provides resistance to herbicide brands X, Y or Z, or even glyphosate. And because it’s better adapted to all the other measures you’re using, it becomes the predominant weed in the field,” says Owen. “And it’s because growers continue to squeeze the balloon with the same control tactic; they generally have a very strong acceptance of anything that they perceive to be simple and easy.” -end-


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