Features Herbicides Seed & Chemical
Getting the most from glyphosate tolerant production systems

Glyphosate has become the cornerstone of Canadian corn and soybean production during the past decade. While farmers have come to depend on the herbicide for its inexpensive, broad spectrum weed control, it is becoming increasingly obvious that glyphosate alone will not solve all weed control problems. Efforts to broaden and sharpen weed control and take proactive measures to reduce the
likelihood of resistance are seeing an increase in tank-mixes of glyphosate and other herbicides.

“Adding a herbicide like Marksman, for example, is a really good way to improve control of lamb’s-quarters and broadleaf control in corn,” says Paul Sullivan, an eastern Ontario crop consultant. “The combination is particularly strong on lamb’s quarters. If the weeds get to any size, the tank-mix is much better than just straight glyphosate.”

Tank-mixing glyphosate with a residual herbicide, like Marksman, will strengthen weed control, help maximize the returns from other inputs and reduce the potential for the development of resistance.

“Adding Marksman to glyphosate is definitely a good way to get sharper weed control in corn,” concurs Trevor Kraus with BASF. “It has very good activity on weeds like common ragweed and Canada fleabane too, and that’s important because these weeds were the first to develop resistance to glyphosate in the US. If you’re tank-mixing with Marksman, the combination will provide control if the glyphosate alone doesn’t.”

Glyphosate is very effective at killing many weeds that are already out of the ground but it has no effect on weeds that germinate afterwards. Rather than spray twice, many producers are
tempted to wait for all the weeds to come up, then deal with them.

“The trend nowadays is for growers to apply glyphosate a bit later,” says Gilles Leroux, professor of weed science at the University of Laval in Quebec City. “We recommend not waiting too long. Glyphosate can kill big weeds but, if you wait until all the weeds germinate, those early weeds compete with the crop and can cause crop losses.”

Early season weed control maximizes value of other inputs
Kraus says an early season application of Marksman can help growers using the Roundup Ready system reduce these early season weed pressures. It will also help maximize the value of other inputs. Studies done in the US back this up. For example, a 2002 University of Missouri study by Hellwig, Johnson and Scharf, published in Weed Science, showed that early season weeds can tie up as much as 34 pounds of nitrogen and reduce corn yields by 17 bushels per acre. With nitrogen peaking at nearly $600 per tonne in 2007, robbing yield is no longer just an accident.

There is similar evidence in soybeans. According to a University of Nebraska-Lincoln study by Knezevic and Evans in 2000, every leaf stage a grower delays weed control in soybeans costs the grower two to three percent in lost yield. In Roundup Ready beans, reducing weed pressure early with glyphosate and tank-mixing with a residual herbicide, will give growers the benefits of early weed removal, more effective weed control, and the ability to capture more yield potential.

“It’s basic weed science,” Sullivan says. “Weeds are easy to kill when they are small. In no-till corn for example, we found the most effective way to keep the crop weed free was by applying a mixture (glyphosate with a residual herbicide) at planting. This combination gave us good early weed control. Then, depending on the weeds that were there, we could come back when the corn was in the five, six, seven or eight leaf stage with a second glyphosate application. That would be enough to keep the field clean until the canopy would take over weed control.”

Tank-mixes deliver other benefits too
Tank-mixing with a residual herbicide also provides an expanded window in case the weather turns after the first application. If it turns wet for weeks after planting, the residual is often enough. Even in worst case scenarios, it slows the weeds down long enough that they can be controlled with a second glyphosate application before they start to rob yield.

“Another approach we’re looking at, for fields where the weed pressure is known to be very high, is to start with a pre-emergence or an early post-emergence residual herbicide,” Leroux says. “Later on, when the corn is at about the seven to eight leaf stage, they can spray with a glyphosate at the lower rate if needed. That way you have a good weed control program and use different modes of action.”
Using an example of a glyphosate and atrazine + dicamba (Marksman) tank-mix also brings other benefits to the table – including three different modes of action (Groups 4, 5 and 9). This is important because glyphosate is such a key herbicide. Growers should always keep a resistance management strategy in mind when planning their weed control program.

“There is starting to be more concern of weed resistance spreading from the US,” says Leroux. “Common ragweed and Canada fleabane have developed resistance in the US. Since these weeds are also widespread in eastern Canada, weed scientists, like myself, are concerned.

“Weed resistance is certainly something that is a concern,” says Sullivan. “Most farmers recognize the potential of weed changes taking place. Producers have seen resistance develop with other products so they are certainly thinking about the possibility of glyphosate resistance happening too.”

To-date the biggest glyphosate resistance headaches in Canada have been caused by volunteer glyphosate tolerant canola, corn and soybeans germinating in subsequent crop years. For the most part, problems have been minor and easily controlled in-crop.

Mixing two products, with different modes of action, dramatically lowers the potential for a weed to develop herbicide resistance, Leroux states. It would need to develop resistance to multiple modes of action at once to survive. It is still possible, but the odds become considerably reduced. -end-