Take a Proactive Approach to Glyphosate Resistance

Glyphosate is a post-emergent, nonselective herbicide used in weed control programs around the world since its commercialization in 1974.

At the current time, the scientific community in North America is well aware that widespread glyphosate resistance is a looming possibility. Symposiums on the subject are packed, and weed scientists are publishing a multitude of scientific papers on glyphosate resistance every year.

Considering what the loss of this tool would mean to producers, it’s necessary to take a proactive approach to save this important chemical from extinction.

Recent resistance updates include glyphosate resistant marestail in the Del Mar, Virginia, area, west Tennessee, Kentucky, southern Indiana and southern Ohio. Marestail (horseweed) is a major concern because it is a prolific seed producer and spreads like wildfire. Marestail is the first row crop weed to develop glyphosate resistance in the U.S.

In Iowa and Missouri, weed scientists have tested waterhemp from several fields. It appears to have developed more tolerance to glyphosate. Producers are also observing and reporting marginal control of velvetleaf, ivy leaf, morning glory and lamb’s-quarters with glyphosate.

Two weed species so far have confirmed glyphosate resistance. Resistant goose grass was reported in Malaysia. Glyphosate resistant ryegrass has been confirmed in Australia. No glyphosate resistant weed species have been reported in Canada to date. Researchers at Agriculture and Agri-Food Canada (AAFC) in Saskatoon initiated a survey of 50 high-risk fields across Manitoba, Saskatchewan and Alberta. "Field selection included direct-seeded fields with at least 10 consecutive years of glyphosate use and some fields with multiple annual applications," explains Dr. Hugh Beckie, weed scientist at AAFC in Saskatoon. "We tested both grassy and broadleaf weed samples, and none of the fields revealed any resistance."

Conservation tillage farmers and glyphosate: use it wisely.

However, there was some evidence of weeds with enhanced tolerance to glyphosate. "This could be a warning sign that continued used of glyphosate may cause the level of tolerance of weeds species to increase to glyphosate," says Beckie.

USE INCREASE

Glyphosate use has increased substantially over the past decade. "The number of conservation tillage acres has gone up, and the use of glyphosate in pre-harvest applications has increased substantially," explains Dr. Doug Derksen, research scientist, Weed-Crop Ecology with the AAFC Brandon Research Centre in Manitoba. "The introduction of Roundup Ready™ canola is also a big factor in increased use of glyphosate. This factor will likely have as big an impact as any because now growers are hitting peak emergence curves of a lot of annual weeds."

With a Roundup Ready canola system, glyphosate is usually applied early as a preseed burn-off application and then again in-crop.

"This increases the selection pressure. Plus the population density from which we’re selecting is more often at its peak," explains Derksen. "Fighting resistance is all about number. It may be a one in a million change or with Roundup, for example, it may be one in a billion."

Canola growers have other choices. For example, they can rotate into a herbicide tolerant canola system that doesn’t rely on glyphosate. The LibertyLink™ canola system is one example. Liberty™, the herbicide in this canola system, is in Group 10 rather than glyphosate’s Group 2.

The use of glyphosate in a herbicide tolerant wheat system would add more pressure on the weed selection process. "This could result in a doubling of selection pressure for weeds, which have seeds in high number and are being hit at peak populations," says Derksen. "It could also mean a move up the log curve of probability of resistance."

CONTINUED USE

Glyphosate is popular because it works so well. A future without glyphosate is difficult to imagine.

"First, don’t stop using glyphosate, use it only when it is needed or appropriate," says Derksen. "Consider other preseeding burn-off herbicide options such as 2,4-D. Other options are not necessarily effective or economical in all situations, but they can be useful if a broad-spectrum herbicide such as glyphosate is not needed.

"Growers should also strive to keep weed densities low through the use of diverse cropping systems and well planned rotations," says Derksen. "If the selection pressure is one in a million, then the farms most likely to have resistance occur are those farms with higher weeds densities."

Adding rotations such as winter wheat and forages will be an advantage in changing target weeds and reducing weed densities. "Not only can weed densities be reduced in these rotations, but the number of years in a row requiring a wild oat or green foxtail herbicide application are also reduced," he says.

Taking a proactive approach is the key to preserving the future use of this chemistry. "We don’t have to wait for a confirmed population of glyphosate resistance before we make some changes," says Derksen. "Although resistance may be less likely, we’re dealing with biological systems and we should be careful."

There aren’t a lot of new herbicides coming down the road that will bail out farmers needing broad-spectrum weed control. The best advice is rotate crops and herbicide Groups, and use glyphosate only when you really need it.

*All products are trademarks of their respective companies.

Return to Spraying Articles