Coping with herbicide-resistant weeds, after decades of the easy, sure control afforded by glyphosate (Roundup), will require a multi-pronged approach, Mississippi growers were told at the 2011 Delta Ag Expo.

As glyphosate-resistant pigweed (Palmer amaranth) spreads in the Mid-South, and resistant Italian ryegrass continues to be documented, producers will need to include alternate chemistries — many of which are decades-old active ingredients — in their weed control programs, a panel of specialists noted.

And after wide-scale adoption of no-till and reduced till practices, in many cases, tillage may also be a necessary component for combating resistant weeds.

“There are many farmers who can tell horror stories of the problems we had with insect resistance to pesticides in the 1970s and 1980s,” said Robert Martin, Extension director for Issaquena County, who chaired the forum. “Weed resistance has the potential to be even worse.”

Panel participants were Tom Eubank, assistant research/Extension professor at the Delta Research and Extension Center, Stoneville, Miss.; Darrin Dodds, assistant Extension professor of plant and soil sciences at Mississippi State University; Daniel Stephenson, assistant research/Extension professor of weed science at Louisiana State University; Jason Bond, associate research professor of weed science at the Delta Research and Extension Center; and Erick Larson, associate Extension/research professor of plant and soil sciences at Mississippi State University.

Excerpts from their remarks follow:

Eubank: “We’ve had instances of herbicide resistance since the 1970s. We’ve had a long run with Roundup: 30-plus years before getting resistance. But we’ve used this herbicide continuously on millions of acres, in many cases with no-till, and with fewer alternate chemistries. We need to break that cycle.

“This is an issue that’s not going away. These resistant weeds are going to continue to spread. We’ve got to be proactive and timely in using alternate chemistries in conjunction with glyphosate.

Best control obtained when weeds are small

“And it’s critical that these alternative chemistries, such as Ignite and Flexstar, be applied to pigweeds when they’re 2 inches to 3 inches high, no more than 4 inches. The days are gone when you’re going to get control of large pigweeds with any herbicide.

 “There is no residual herbicide you can put out in the fall or late winter that will carry over to provide control the following season. It’s important that you start with a clean field in the spring — but you may still have to come back later with other materials to control escapes. Metribuzin is still a good burndown material that offers some residual control.”

Dodds: “The move by producers to Roundup Ready is like a gerbil on a revolving wheel — it’s hard to get off. Once growers got on the wheel, they felt they couldn’t afford the cost and/or didn’t need tillage and other measures because of the technology fees they were having to pay.

“We’ve never quit making recommendations for tillage, but production economics and farm size have led many growers away from it. In Mississippi, tillage is still a good option as part of a pigweed and ryegrass control program. It’s not a solution in every situation, but we need to keep it in mind as an option.

“We have a limited number of modes of action in herbicides and we need to do all we can to effectively use them and preserve them. In many cases, we’re recycling herbicides we’ve had for 50 years or more. New chemistries don’t come along very often — I can count on one hand those I’ve seen in 10 or 12 years.”

Bond: “Unfortunately, we’re too often in a reactive mode rather than proactive. Crop rotation, use of alternate chemistries, and tillage are long term, proactive measures in dealing with weed resistance.”

'A definite change in the way we farm'

Larson: “This is definitely going to change the way we farm. Crop rotation is one practice that can play a significant role in dealing with resistant weeds.

“A rotation including corn serves as the most effective tool to combat resistant Palmer amaranth and marestail. However, ryegrass is a significant issue for corn production.

“We have documented glyphosate-resistant ryegrass in 12 Delta counties and ALS-resistant ryegrass in 14 counties. This is a very serious issue in corn, and control requires some very aggressive, proactive methods. Ryegrass can be a significant competitor for corn and it needs to be controlled before you ever put the planter in the field. One ryegrass plant per 7 square feet can result in corn yield loss of as much as 27 percent, or 65 bushels per acre, as documented in a Mississippi State University corn verification trial last year.

“This is why we’re recommending a more aggressive strategy with burndown with products other than glyphosate and 2,4-D.”

[A field day highlighting management programs for glyphosate-resistant Italian ryegrass will be held March 10 at the Capps Center on the campus of the Delta Research and Extension Center at Stoneville. Registration will begin at 7:30 a.m. Plans are to have an actual field tour of research plots, but the program will be moved indoors in the event of inclement weather.]

Dodds: “Glyphosate-resistant pigweed is a major threat to cotton. I saw more in-season tillage and chopping crews last year than in the previous six years combined.  We can manage this problem, but growers are going to have to be willing to adapt and to adjust their practices.”

Stephenson: “Georgia has good data that turning under pigweed seeds can help control the weed, but if you have to grow a crop on a bed, it can bring the seed right back up.

“We’ve had only one confirmed case of glyphosate-resistant pigweed in Louisiana, but we have glyphosate-resistant ryegrass in several areas. We also are concerned about resistant water hemp, and we have confirmed resistant johnsongrass, which took massive amounts of herbicide to kill.

Some grower attitudes 'a recipe for disaster'

“Unfortunately, we have some growers who take the attitude, ‘I’m going to keep spraying with Roundup until it doesn’t work any more.’ That’s a recipe for disaster.

“Even if you don’t have pigweed, but your neighbor does, it’s likely going to spread to your fields. The seed will stick to equipment, or to you, like white on rice. A lot of spread is by equipment movement. If you use a custom harvester, make sure they clean their equipment thoroughly before coming to your fields.”

Dodds: “There was a grower in the Mississippi hills who bought a sprayer in Arkansas that has a tremendous amount of pigweed seeds in many crevices of the machine. Had he not cleaned it before putting it in his fields, he’d have spread those seeds everywhere. Be extremely careful about buying equipment in areas where there is resistant pigweed — it could be a very costly purchase.”

Bond: “Equipment movement is a major way ryegrass is spread. Landplanes do an unbelievably effective job of spreading the seed. It is also easily spread by combines.

 “Ryegrass doesn’t spread as fast as pigweed and it’s usually worse in field margins, or along roads, so you have an opportunity to control it in those areas and limit it spreading to your fields.

“We had many calls last year about fields that were infested with pigweed where they’d never had it before. But it’s mind-boggling how big these plants can grow, how much seed a large pigweed plant can produce, and how easily those seeds can spread.

“When I was in graduate school, we grew a pigweed that was 12 feet tall and weighed 45 pounds. In Arkansas, researchers counted 1.7 million seeds on a single plant. Even if you get 96 percent or 97 percent control, with that kind of seed numbers you can get behind the math curve pretty quickly.

Only a matter of time for resistant weeds

“Currently, we have 10 counties in Mississippi, bordering the river, with glyphosate-resistant pigweed. If you don’t have them, it’s only a matter of time until you will. In 2010, we had some resistant pigweeds in areas near the coast, a long way from the Delta. Most of the coastal counties of Alabama have resistant pigweed, and it’s probably only a matter of time until it spreads to the Mississippi hills.”

“Residual herbicides now represent a key line of defense against resistant pigweed. How long they will be effective depends on how carefully we manage them.

“Resistance is a moving target and can vary from one year to the next. In some cases, control is textbook; in others, it depends on weather and other factors.

“Remember: residuals need water for activation. If you don’t get rain, they won’t work. If you look at historical weather data, there usually will be rainfall to activate residual herbicides applied in April. After that, it becomes chancy.

“If you have the ability to irrigate, you can facilitate activation. Overhead irrigation trumps furrow irrigation for herbicide activation.”

Dodds: “With residual herbicides, be aware of label restrictions regarding the number of days from application to planting. Counting starts on the date you get the specified amount of rainfall — not the day the material was applied.”

Eubank: “Treflan, which goes back many years, is still in our recommendations. But remember, it’s a single mode of action — you can’t use just that one material — and if you don’t incorporate it properly, it’s not going to work. “Nothing is going to give you 100 percent control of pigweed, just because of the sheer numbers of seed produced.

Some crop injury may be necessary

“Sencor (mostly generic versions now) is also a good option we haven’t used much — not only for pigweed, but for horseweed and ryegrass. . There currently is no pigweed resistance to this material in the Mid-South. Boundary is a combination of Sencor/Dual and is a jam-up combination for these weeds.

“In many situations, some amount of crop injury will be something you will have to live with in order to get control of pigweed.”

Bond: “You can expect some injury from a residual herbicide. Some of these materials have been around forever — in the grand scheme of things, they’re antiques, but they work the same as they did decades ago, and you’re still going to get injury. You’ve got to seek a balance between some herbicide injury loss and major loss from resistant weed competition.”

Dodds: For the last 15 years, we’ve become accustomed to seeing a robustly-growing cotton crop with little or no injury; however, cotton can sustain some herbicide injury and still produce well.”

Eubank: “As we shift more to contact herbicides, it’s very important to have adequate coverage. You can’t apply these herbicides with AI nozzles, low water volume, and a high ground speed and get the coverage you need.

“If you’re going across the field at 20 mph, a lot of your smaller spray particles are being caught up in the vortex of wind and dust behind your sprayer and coverage isn’t as effective as it should be. If you miss control the first time around, there’s no coming back for a do-over.

“You need an orifice size that will put out the necessary volume of water for thorough coverage. I like 10 gallons minimum, preferably 15 and no more than 10 mph.”