Top Crop Manager

Features Herbicides Seed & Chemical
Beyond glyphosate

Think tank-mix, think timing.

December 17, 2007  By Top Crop Manager

20Using just one, single application of glyphosate might appear be the simplest and cheapest way to control weeds in glyphosate tolerant crops, but it likely costs money. Although it is a remarkable herbicide, glyphosate only kills the weeds that are already growing and, depending on the application rate used, control may not meet expectations. If spraying operations are timed to maximize yield, there might be a weedy mess to deal with at harvest. If spraying is delayed until all the weeds germinate, fields may be clean at harvest but there might be severe yield loss. A residual herbicide used on glyphosate tolerant soybeans or corn may cost a few dollars more but it may save a spray pass and put money in your pocket.

Tank mixing with glyphosate helps boost residual activity, but timing is critical, particularly at the three to eight leaf stage in corn.

“A glyphosate tank-mix addresses two issues,” says Mike Cowbrough,  weed specialist with OMAFRA. “One is a time management and minimization of yield loss issue and the other is resistance. I think the biggest value in mixing residual herbicides with glyphosate, in the Ontario marketplace, is in the area of efficiency and time management. One of the weaknesses with glyphosate is that it doesn’t have any residual activity so you’re more inclined to wait and apply that herbicide when all the weeds are up to have the best control possible. There have been a number of studies done in Ontario that show that if you take this strategy you’re leaving a lot of yield on the table.


“If you want to minimize yield loss from weeds you need to control them during the critical weed free period,” Cowbrough continues. “In corn, this is between the three to eight leaf stage. If you’re waiting and spraying corn at the seven to eight leaf stage, I will guarantee that you left some yield on the table. A tank-mix, with the right residual herbicide to control your broadleaf weeds, allows you to go in on the early side and minimize yield loss to weeds from early competition. It buys you time and, if you have to, you can go in with a second glyphosate application later on in the year.”

There are currently five glyphosate tank-mixes that are registered for use on glyphosate tolerant (GT) corn. Picking the right one boils down to basic weed science and the weed spectrum in a grower’s field. For example, Galaxy may be used in GT cornfields that have problems with grassy weeds like fall panicum.

“Guardian can be used very effectively as a burndown application in a no-till, Roundup Ready soybean system,” says Dave Kloppenburg, a technical sales representative with DuPont. “Guardian is a mixture of chlorimuron-ethyl and glyphosate, so it’s a burndown with residual. It |removes early season weed pressure and can be used as either a pre-emergent or in-crop application depending on your weed species. If you have problems with early emerging perennials like dandelion, you can use it as pre-emergence. If you are more concerned with later emerging weeds like nutsedge, sowthistle and velvet leaf, wait and apply it in-crop.”

One of the biggest advantages of combining glyphosate with other chemistries is for improved time management. Many growers struggle to get even one glyphosate application down on time let alone finding the time for two. For soybeans, a product like Guardian lets growers choose which application timing best suits their schedule. If they have more time during pre-plant, it can be applied pre-plant, and they can count on it to keep the field clean long enough that early weed pressure will not rob yield. Later, after the initial time crunch is past, they can go back in with a second glyphosate pass if necessary and not have to worry about any yield loss.

“A tank-mix (in-crop) won’t help you if you are struggling to get the first glyphosate application down at the right time.

Typically that’s at the three to four leaf stage with corn,” says OMAFRA corn specialist Greg Stewart. “What tank-mixing will do, perhaps, is eliminate any concern about having to go back for a second application.”

Tank-mixes also can be useful tools to slow down the development of glyphosate-resistant weeds. So far no resistant weeds have been confirmed in Canada but they have started to crop up south of the border. And, what is perhaps even a greater problem is expansion of weeds that are not well controlled by glyphosate like yellow nutsedge.

“In a perfect world, says Cowbrough, you would try to delay the development of resistant populations by adding another herbicide to have multi-modes of action on every individual weed species in the field. Glyphosate controls such a broad spectrum of weeds that we don’t have another chemical that is a perfect match to control resistance. Some tank-mixes provide a good resistance management tool for broadleaf weeds but aren’t effective on grassy weeds and so on. Tank-mixes aren’t the perfect tool for preventing resistance but it’s better than doing nothing at all.”

The broader the selection of weeds that the herbicide tank-mix controls, the better it will be for resistance management.

“Galaxy will control a broad cross section of weeds by itself,” Kloppenburg says. “Galaxy controls green foxtail, fall panicum and redroot pigweed.


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