MISSISSIPPI STATE – Weeds that have developed resistance to the commonly used herbicide glyphosate are forcing row crop farmers to change their production methods to battle the problem.

Five weeds found in Mississippi have developed resistance to glyphosate, the active ingredient found in Roundup herbicide. Since 1996, this broad spectrum herbicide has been used extensively as an easy and effective way to control weeds in row crops that have been genetically modified to withstand the chemical.

But glyphosate’s near-exclusive use led to significant problems with glyphosate-resistant weeds in Mississippi and the nation. Now producers are scrambling to once again control weeds efficiently.

Darrin Dodds, Extension cotton specialist, said glyphosate-resistant weeds have become a significant problem, especially for cotton growers in the Delta.

“Infestation with glyphosate-resistant weeds will increase the cost of weed control and ultimately the cost of cotton production,” Dodds said.

Horseweed has been a problem for several years, and nearly all of the Delta is infested. Whenever Dodds makes weed control recommendations, he assumes the horseweed is glyphosate resistant, but it is not the only resistant weed that cotton farmers deal with.

“Glyphosate-resistant Italian ryegrass has been confirmed in 12 counties in Mississippi, and it can present serious problems during the early portion of the growing season,” Dodds said. “Once it gets some size on it in the spring, control becomes difficult and expensive.”

Glyphosate-resistant Palmer amaranth has been confirmed in nine Mississippi counties, with confirmed cases expected in several more counties by year’s end.

“Glyphosate-resistant Palmer amaranth is especially problematic because it grows aggressively, is tolerant of difficult growing conditions and produces seeds prolifically,” Dodds said. “It is difficult to grasp the magnitude of the problem until you have dealt with it firsthand. This weed will change the way we currently practice weed control in cotton.”

Glyphosate-resistant water hemp, a close relative of Palmer amaranth, has been confirmed in two Delta counties.

Many growers have already changed their weed control practices in response to glyphosate-resistant weeds. Dicamba, or 2,4-D, is typically included with glyphosate burn-down applications in the spring. The timing of herbicide applications has become critical.

“In some cases, growers who are facing glyphosate-resistant weeds, primarily glyphosate-resistant Italian ryegrass, are considering fall herbicide applications to prevent problems in the spring,” Dodds said.

Other factors making cotton’s battle with glyphosate-resistant weeds more difficult is the fact that fewer herbicides can be used on cotton compared to other row crops, and cotton is slow to form a canopy over the rows. It takes nearly two months for cotton plants on neighboring rows to grow so they touch each other, forming a canopy that naturally limits weeds’ growth.

Trey Koger, Extension soybean specialist, said managing glyphosate resistance is going to become producers’ biggest soybean production issue.

“In the north Delta, our growers are either already very conscious or quickly becoming aware of the growing problem with glyphosate resistance. They’re the ones dealing with it more than anyone else,” Koger said. “In the south Delta, they’re becoming more aware of it and concerned about it. Their awareness might mean we can cut off the problem quicker.”

Before glyphosate-resistant weeds developed, producers growing Roundup Resistant soybeans could scout about once every two weeks for weeds and spray when they grew large. This wasn’t always the best practice but often was done because glyphosate was effective overall at controlling most weeds in the field, even large weeds.

“That really allowed all of us to become complacent,” Koger said. “Now with Palmer pigweed, we have to scout just about every other day and apply residual herbicides in a very timely manner.”

Glyphosate-resistant Palmer pigweed can be controlled in soybeans, but it requires timely application of different chemicals, excellent spray coverage and often a slower application speed.

“We can achieve 100 percent control of glyphosate-resistant Palmer pigweed when our primary post-emergence herbicide is applied to 1- to 2-inch tall Palmer pigweed,” Koger said. “However, waiting even three to four days and applying the same herbicide to 4- to 6-inch-tall Palmer pigweed typically only provides about 75 percent control.”

Koger said growing corn is a great tool to manage weed resistance, but the solution is not as simple as switching to corn production for a few years.

“There are tremendous herbicide options for corn that we don’t have for cotton and soybeans. Corn growers don’t have nearly the resistance issues right now,” Koger said. “But it takes a lot of equipment and a lot of capital to get into corn production. It’s a huge shift and very costly to switch to corn.”

Erick Larson, Extension corn specialist, agreed that corn can play a large role in the battle against some glyphosate-resistant weeds.

“Growing corn allows producers much better herbicide options to control Palmer amaranth and horseweed, so using corn in a crop rotation system in areas where these weeds are predominant is the best strategy available,” Larson said.

Many of the weed resistance issues have developed in areas where corn planted in rotation with other crops is less common.

“Corn is planted earlier in the spring than other summer crops, giving growers an opportunity to control these troublesome weeds before their growth quickens with higher temperatures,” Larson said. “Corn also shades the soil earlier than other crops, naturally enhancing weed control.”

The battle against Italian ryegrass is still difficult, as this weed competes with corn and cannot be readily controlled once corn has emerged.

“Ryegrass must be aggressively managed by using residual and contact herbicides to control it during the fall to early spring,” Larson said. “This will require a major change in producer management strategy.”