With the new Roundup Ready Flex cotton varieties on the market, growers are wondering if the full-season weed control benefit of the new-line glyphosate system will be enough to keep weeds down.

Flex cotton extends the over-the-top application period past the fourth leaf stage and is designed to provide season-wide weed control. Some growers, like Jerry Stuckey, Moscow, KS, are even going with a straight preplant glyphosate burndown and foregoing yellow herbicide application. But if weeds persist, he's ready to come back with a lay-by herbicide program to counter late-season weeds.

Other areas, like much of Mississippi, are seeing “widespread” resistance to glyphosate, meaning extra control measures are necessary, says Tom Barber, Mississippi State University Extension cotton agronomist. “It's a real problem if you don't burndown early enough,” he stresses.

Stuckey, one of a new breed of cotton producers in southwest Kansas, far northwest Texas and the Oklahoma Panhandle, had good results with an all-glyphosate program on new Flex varieties for 2006. But he intends to use rotation with corn to help head off weed resistance down the road.

The glyphosate works well in his no-till cotton program. “With no-till, if cotton is planted after corn or grain sorghum, incorporating a preplant yellow herbicide into the soil can be difficult,” he says.

“For 2006, we went with Flex and received good season-wide control except for areas where sprayer tracks in dry fields didn't receive good coverage,” says Stuckey, who also manages the local co-op gin in Moscow.

He's used more than one mode of action in past years. And he knows that going exclusively with the glyphosate cotton could lead to weed resistance problems like those seen in other parts of the Cotton Belt.

“We haven't had much talk about resistance yet in our area,” he says. “But if it happens, it will probably be with pigweed, as fast as it can grow. So our plan is to go with two years of Flex, then rotate those acres into corn for the third year. If needed, we will also use a lay-by program of applying either Lasso or Dual later in the year.”

Crop rotation to offset resistance is a key recommendation by university agronomists, crop consultants and others. Trey Bullock, a Hattiesburg, MS, consultant, says some growers are finding a cotton rotation with peanuts works well for both crops.

“I hope our rotation with peanuts will help some,” says Bullock, adding that a yellow herbicide application is always part of his weed control programs. “Where we are putting out Roundup, we are also putting out other herbicides, such as Dual. We rarely go with straight Roundup in our burndown program.”

Bullock has several application formulas that work for his grower clients. “During burndown, we go out with a 2,4-D application far ahead of planting,” he says. “Or we might also go with a Clarity type of herbicide. We'll also use Valor on some of the hard-to-control weeds.”

Even though the southern Mississippi region has not seen resistance problems, there are major resistance issues with marestail along the Delta, says Bullock. Growers in Georgia and other states are also coping with glyphosate-resistant pigweed. There has been resistance to ragweed in Arkansas and ryegrass resistance elsewhere.

Article contined on next page >

“Weed resistance to glyphosate is a very serious threat,” says Steve Brown, University of Georgia cotton weed specialist. “A key component of a resistance management strategy is to utilize multiple herbicide modes of action. The use of pre-emergence herbicides is strongly encouraged. They will not only aid in resistance management, but also make timing of the first glyphosate application much less critical.”

Brown says that complete dependence on Flex varieties could expedite resistance problems. “Because it allows an almost unrestricted range of application times, Flex could increase grower reliance upon glyphosate and ultimately speed the development of resistance,” he says.

Barber says some growers are on the verge of fighting a losing battle against marestail resistance if they fail to get an early burndown to kill overwintered weeds.

“They need to begin burndown in February and have it completed by March 1,” says Barber. “We recommend a tankmix of Clarity with glyphosate at burndown. In some cases you can get by with soil residual herbicides in the tankmix. But horseweed can have wide germination during the growing season.

“If there is in-season Roundup Ready resistance, I really don't have a (control) plan for it. You can go with an in-hood spray in the middle, but it may not control weeds in the row. Some growers who have resistance may have to put down a residual herbicide in the fall,” Barber says.

There have been some shifts to Liberty Link cotton in heavy resistance areas of Mississippi. Barber says he expects the resistance to move southward into southern Delta production areas.

Again, managing glyphosate resistance usually requires using several modes of action. “An effective resistant management strategy will incorporate herbicides having three or more modes of action,” says Brown.

“Growers can incorporate the recommended diversity in modes of action into a glyphosate-based management program by using soil-applied residual herbicides or tankmixing another herbicide with glyphosate-applied postemergence. They may also use alternatives to glyphosate or at least a glyphosate tank mix at lay-by.”

Use of full rates of glyphosate is encouraged, even in tankmixes, he says, and crop rotation can aid in resistance management if herbicide modes of action for the rotational crop are wisely selected.

“Where practical, cultivation would also be a very effective component of a resistance management strategy,” says Brown. “Additionally, growers are encouraged to minimize their reliance on ALS-inhibiting herbicides, since these chemistries are extremely vulnerable to resistance.” (ALS is the acetolactate synthase class of herbicides.)

Brown reminds growers that glyphosate may be applied over the top or directed to Roundup Ready Flex varieties any time from cotton emergence until seven days prior to harvest.

“The maximum rate for any single application between crop emergence and the 60% open boll stage is 1.13 lbs./acre, or 32 fluid oz. of Original (Roundup) Max or WeatherMax,” he says. “A total of 4.5 lbs./acre, or 4 qt. Original Max or WeatherMax, can be applied during this time frame.

“Depending upon application rate, four to six applications can be made over the top or directed. An additional 1.55 lbs./acre can be applied from the 60% open boll stage until seven days prior to harvest.”

Meanwhile, Stuckey likes what he sees from the Flex varieties and weed control. But he isn't afraid to switch back to several modes in a season.

“We know a good burndown is essential,” he says. “Glyphosate does a good job there. But if needed, we will come back with the lay-by program.”

He adds that in this type of program, he's tried applying half the labeled rate of Lasso and half the rate or Dual through the planter, then the other half of the applications in the lay-by program.

“I couldn't tell the difference in their effectiveness,” he says.

Barber and Brown encourage growers to consult with their regional Extension cotton agronomists, weed specialists or crop consultants to determine the best control measures for their regions.