Carolina cotton growers learn to deal with glyphosate resistant pigweed

Most cotton farmers in South Carolina know first hand what a problem a few pigweed escapes can cause, and fighting these pesky weeds is nothing new.

When Roundup Ready cotton came on the scene in 1997, the fight seemed to be won. However, with the onset of glyphosate resistant pigweed, it seems the battle is just now beginning.

Former Clemson University Weed Scientist Ed Murdock and others worked with South Carolina farmers for over 20 years to develop herbicide systems to kill pigweed in cotton. None of these combinations were foolproof, so when a few escapes started popping up in cotton fields around the state in the past few years, farmers didn't pay much attention.

In 2006, the escapes became parts of whole fields and the same farmers found that double and triple rates of glyphosate had no effect on these pigweed. Herbicide resistant pigweed now scares them to death, and the heavy dependence on glyphosate and ALS herbicides has created a situation that offers only a few alternatives, and none of them are cheap.

To make a profit — and stay in business — many farmers in the Southeast have taken advantage of lower cost production systems based on Roundup Ready technology to greatly expand their farming operations. Many farmers have more acres, less labor, higher fuel costs and expensive, specialized equipment to help them manage large acreages.

There is no doubt glyphosate resistance is more widespread than previously reported. Alan York, a weed scientist at North Carolina State University began a wide-scale program in North Carolina in 2005 and found pockets of resistant Palmer pigweed around the state. Similar, but less formal studies in Georgia and South Carolina indicate the widespread existence of resistant pigweed in those states.

Though not scientific in the least, a ride through the countryside from Florence, S.C., to Fayetteville, N.C., will provide a simple, but undisputable scenario. Two or three fields may be pristine, then there is a field with clumps of distinctive Palmer pigweed heads rising high above the cotton and soybeans, and sporadically fields that have been abandoned because 4-6 foot pigweed have taken charge.

As far north as southern Virginia, long-time successful farmers, like Dean Stallings and Robby Taylor say herbicide resistance is the major topic of discussion. The big question is what to do about it. Already, they are looking at common lambsquarters in their area that is highly suspected of being resistant to glyphosate.

In South Carolina, Clemson Extension Weed Specialist Chris Main says farmers are in a situation where they have to have a solution, but it has to be an economic solution to allow them to stay in the cotton business. Growers simply can't pay $80-90 per acre for weed control, when they have been living in a world since 1997 in which Roundup Ready varieties reduced cost to $20-25 per acre.

Main, a veteran of glyphosate resistance battles with horseweed, while he was at the University of Tennessee, contends the battle is winnable, but that it will require that management decisions at the grower level be made long before a cotton or soybean crop is planted.

South Carolina has seen a meteoric rise in peanut production in the state. Peanuts are an ideal rotation with cotton. Peanuts break up nematode cycles, have relatively few disease problems, and fit in well time-wise with other crops. Cadre is a widely used herbicide for weed control in peanuts, as is Staple for weed control in cotton. Unfortunately both are ALS-inhibitors.

More unfortunately, there is widespread documentation of ALS resistant pigweed.

The worse case scenario imaginable for farmers with large acreages of peanuts and cotton is to have cross resistance to both glyphosate and ALS inhibitors. Though not quite ready to roll out the scientific evidence, weed scientists in the Southeast privately contend just such a cross resistance is more likely than not.

“In South Carolina, we have a number of growers who are in their second set of land growing peanuts,” Main says. Now, they are in more marginal land, spraying 2-3 inch pigweed with Cadre, and it doesn't kill the weeds. The next option for control is Blazer, Cobra, or similar post-emergence herbicides, but there is only one reliable herbicide to control pigweed when these weeds get 6-10 inches tall — glyphosate. If those weeds are resistant to glyphosate, the grower is in big trouble, Main adds.

Main explains that back in June he did a number of tests in which he measured growth of weeds. Palmer pigweed averaged two inches growth per day. If a grower sprays any herbicide on a four-inch pigweed and doesn't kill it, for whatever reason, and he can't get back in the field for three or four days, he is then trying to kill a 10-12-inch tall pigweed, and the only reliable chemical option is glyphosate, he stresses.

The Clemson scientist says any farmer, who, in the summer of 2006, has a history of pigweed in his fields, depended largely on glyphosate and ALS systems for controlling these weeds, has seen patches of pigweed he knows has been sprayed, but these weeds don't die, needs to be planning a 2007 herbicide resistance management strategy long before he is ready to plant next spring.

For the 2007 season, Main says growers planting cotton in strip-tillage systems should look at Valor. It has to be timely, applied 14 days prior to planting. If weather conditions are right, it will help with burndown and will provide 4-6 weeks Palmer pigweed control, again depending on soil moisture.

If using conventional-tillage, a pint to a pint and a half of Reflex is a good option, he says. The lower rate will provide 4-5 weeks of residual weed control and the higher rate 6-7 weeks, again depending on weather conditions.

With either Valor or Reflex, farmers should consider a tank-mix with Prowl, or generic pendamethalin. Though providing only 70-75 percent control of pigweed, even under ideal weather conditions, this family of herbicides does provide excellent grass control.

A big key to managing herbicide weed resistance is for the farmer to have, or to develop accurate records of what herbicide has been sprayed in each field. He should know which materials are in the same family of chemistry and the mode of action of these materials.

By rotating use of chemicals from crop to crop, growers should reduce resistance to any one chemical family and be able to develop chemical systems targeting a specific resistant weed variety to a specific family of herbicides.

Palmer amaranth, or Palmer pigweed, as it is commonly called, is an obligate out-crosser, meaning pollen has to transfer from male to female plants to make seed. Unless resistance is showing up only in female plants, resistance has to be transferred by pollen.

In test fields in Georgia, nearly 90 percent of pigweed plants are resistant, thus the population cannot be all female.

A common misconception is that resistance is not there one year and it is an unmanageable problem the next year. It doesn't work that way. More likely, a single plant develops an alternative site for herbicides to bond, and develops resistance to the herbicide. That plant produces 400,000 to 500,000 seed, which produces a few more resistant plants, usually in a small clump around the original resistant plant.

There are thousands of reasons for pigweed escapes, none of them related to resistance. So, in many cases 25 percent or more of the field is resistant before a farmer recognizes the problem. In a 100 acre field with 25 percent resistant pigweed plants, the seed production is phenomenal — the long-range result: pigweed will take over the field in 4-5 years after resistance begins, unless the farmer makes dramatic changes in his weed management program.

The problem is compounded by applying higher and higher rates of glyphosate, or ALS, or whatever herbicide to which the weed is resistant, because killing off susceptible plants provides a better environment for resistant weeds to survive.

Dependence on any one herbicide as an alternative is likely to result in resistance to that material — again because of pigweed's ability to produce so many seeds.

If a grower has glyphosate resistant weeds, the option is to find a management system that will kill the resistant weeds. Glyphosate still kills over 99 percent of the weeds and grasses in a cotton field, so abandoning this herbicide doesn't make much sense.

“In a Palmer pigweed resistant world, the farmer has to be on top of the problem from day one. He cannot wait to get feedback from a cotton scout, who typically doesn't see the crop until mid-June. By that time they have sprayed twice with glyphosate and resistant weeds are too large to manage, and they can't use glyphosate,” Main stresses.

“We may have to follow the rule of planting and harvesting — don't plant more in a day than you can harvest in a day. In a Palmer pigweed resistant world, we may have to go back to not planting more cotton in a day than you can spray with a residual herbicide in a day,” Main contends.

Though years of research prior to 1997 demonstrated the low reliability of yellow herbicides to control pigweed without adequate moisture, Main contends these pre-emergence materials play a vital role in the battle against glyphosate resistant pigweed.

A herbicide, like Prowl, may give 70 percent control, and in combination with Valor or Reflex up to 90 percent control. If a grower can get 90 percent control of pigweed, without using a drop of glyphosate, up until he puts a lay-by rig in the field, he is way ahead of the game, Main explains.

With low populations of pigweed, a grower can do a good job of post directing materials like Sequence, then coming over-the-top with a Staple-Roundup combination, if he doesn't use other ALS-inhibitors for other crops, then he can come back with pigweed specific materials like Direx, Caparol, or Suprend, he says.

For cotton and soybean growers, glyphosate resistance is here. So is ALS resistance for cotton and peanut growers. The more heavily the dependence on these herbicides, regardless of the crop, for weed control, the more likely resistance will occur.

Denying the problem exists is a blueprint for failure, while managing it ahead of time is the best chance for success.

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