How might crop rotation be used to manage the burgeoning glyphosate-resistant pigweed problem in the Mid-South?

“In Mississippi, there are currently 11 counties that we know have resistant pigweeds,” said Jason Bond at the recent Pigposium forum held in Forrest City, Ark. Bond, an associate professor of weed science at Mississippi State University, said the “number is sure to go up as we continue the screening programs this fall and winter. Ten of the 11 counties are located in the Delta area of the state — we consider that the northwest (quarter) of the state.”

All of the counties that border the Mississippi River are infested with glyphosate-resistant pigweed.

 “I think we have two real opportunities for managing pigweeds in the Delta. In Mississippi, we have really good yield potential with rice, as does Arkansas. But nearly all the counties on the Mississippi side of the Delta have excellent yield potential with corn.

“So I want to focus on those crops — corn and rice — and how to manage populations of pigweeds in those crops.” 

Checklist

To manage a resistant weed population, “first, you must break a continuous cycle of crop-weed association. A lot of times, an environment favorable for a particular crop is also favorable for a certain weed.”

Bond referenced barnyardgrass and rice — “very similar plant types, both favor aquatic environments. Therefore, barnyardgrass proliferates in a rice crop.

“Personally, I think Palmer pigweed and cotton are closely associated. They thrive in similar environments. They tend to have similar soil temperature requirements for germination. Pigweeds seem to begin germinating about the time we plant cotton in the spring.”

Second, growers should “rotate crops with contrasting growth and cultural requirements. For instance, rotate a row-crop with a drill-seeded crop. … Drill-seeding soybeans (can be) a way to supplement your Palmer pigweed control program.”

Narrower rows canopy quicker, shade out the ground and prevent additional pigweeds from germinating after canopy closure.

“Third — and what I’ll spend most of my time talking about — is rotating weed management tools. Obviously, the big component of this is rotating herbicide chemistries. But you can also manipulate cultural tools such as flooding or planting dates.”

Many weed specialists advocate rotating herbicide chemistries or modes of action. However, Bond cautioned that “just because you’re rotating a crop doesn’t automatically mean you’re rotating herbicide chemistries. For instance, you have access to ALS inhibitors in all four major Delta crops. The same is true for the PPOase inhibitors. So, you must be conscious not only of the crop you’re growing but also the herbicide chemistry being used.”

Bond said the two to focus on are the synthetic auxins (2,4-D, dicamba, Grandstand in rice) and the HPPD inhibitors (currently used solely in corn, including Callisto and Laudis). “You’ll notice we don’t have herbicides in either of these families available for in-crop use in cotton or soybeans.”

At least on the Mississippi side of the Delta, “corn is an excellent rotation crop for either soybeans or cotton. We have chemistry to control pigweeds in corn — particularly in the HPPDs and synthetic auxins.

“And you can use planting date for corn to avoid pigweeds. Start planting early, before the pigweeds begin emerging. But the big thing with corn is avoiding seed production after harvest. That’s a major aspect of managing pigweed populations in corn.”

A trial done last summer looked at single herbicide applications on V-4 corn. There were no tank-mixes with glyphosate or atrazine.

“The HPPDs, Laudis and Callisto, were right near the top providing 85 to 90 percent (control). The next group, synthetic auxins, included Yukon and Status, both pre-mixes that contain dicamba.

“Near the bottom was Cadet, a PPOase inhibitor, applied to a bit larger weed. Cadet is a contact herbicide and coverage wasn’t quite good enough to gain adequate control.”

Resolve Q is an ALS inhibitor. “Fifty to 60 percent of the pigweeds that have gone through the screening programs at Stoneville are ALS-resistant. And I’d say 90 percent of the pigweeds in my fields at the research station in Stoneville are ALS-resistant. So, Resolve isn’t really a good option there.”

One unknown that could impact pigweed control: the future of atrazine. 

“If we lose atrazine, we’ll need to get a group together next year to talk about managing pigweeds in corn. Without glyphosate or atrazine, pigweed management in corn becomes complicated very quickly.”

Another study considered three corn planting dates: March 15, April 5, and April 21. “For my purposes, those roughly simulated early-, mid- and late-planting dates. They probably should have been about 10 days earlier to be ‘real world.’ But that’s what the weather allowed us to do in 2010.”

By planting early, “we were able to delay the herbicide application to V-4. Basically, that’s as late as we could go with the growth stage restriction for atrazine. And that was the best corn we cut — over 200 bushels.

“When we delayed planting to the mid-date, the best yield was produced with a pre-emergence application of a residual herbicide.”

Bond and colleagues “introduced the residual herbicide into the system when it was needed most: when the pigweeds were emerging. You can supplement a control program by using a combination of the residual herbicide and planting date.”

This year, Mississippi’s harvest was very early. “Particularly in corn — and also in early-planted soybeans — it wasn’t uncommon to see combines running in the last week of July. For sure, most years, we get ramped up with harvest the first couple of weeks in August.”

Depending on the weather between early August and mid-November, “you can have several months of good growing conditions. Pigweed emerging after harvest can grow to maturity and produce seed that will cause problems in following years.

“Every effort you make to grow corn with the intent of managing resistant pigweeds can go right out the window after harvest while you’ve moved on to (harvesting other crops) in the fall. Once you harvest, don’t forget the fields need additional attention.”

Bond tried to address that this year, although the effort began late. Bond and Daniel Stephenson, with the LSU AgCenter in Alexandria, La., evaluated “some different programs after harvest: two non-selective applications spaced about three weeks apart with the first two weeks after harvest; flail mow and, about three weeks later, come back with a non-selective herbicide; or, disk the field a couple of weeks after harvest and then come back with a spray three weeks later.

“It didn’t rain at my house from the first of August until (early November). So, in 2010, it didn’t really matter what program we used. Because it was so dry in the fall, we were able to reduce pigweed seed production with all three programs.”

But in 2009, “we got most of our corn out and the rains set in during September. That didn’t allow us (any control measures) for five or six weeks and led to some real problems with pigweed seed production after harvest.”

Rice

In Mississippi, 90-plus percent of rice is grown in rotation with soybeans.

“We have some real opportunities in rice to manage pigweeds. My favorite herbicide combination is propanil plus Grandstand. But you also have the flood — and you can’t beat a flood for managing Palmer pigweed” as it is native to an arid climate.

However, Bond cautioned about one big drawback for managing pigweed populations in rice: preventing seed production on levees. Pigweeds may not survive a flood but they can thrive on a levee throughout the summer.

How can Palmer amaranth control work with a general rice weed control program targeting grass and broadleaf weeds?

“SuperWham plus Grandstand applied at a mid-post timing (three- to four-leaf rice) provided 98 percent control.”

Another option is Broadhead, a pre-mix of quinclorac plus Aim. “Broadhead, in the limited experience I’ve had with it, has done a very good job on very small pigweeds. It has done a poor job on larger pigweeds because one of its components — the one doing the major effort to control pigweeds — is Aim. Aim is a contact herbicide that needs excellent coverage and, obviously, works better on smaller weeds.”

Because of the ALS-resistant pigweeds in fields around Stoneville, “we only get around 65 percent control with Aim plus Permit. But if we can injure those plants and then get water on them within 10 days or two weeks, it seems that even though they don’t die from the herbicide application they’ll melt down and die quicker if they were at least injured.”

Bonds said it’s too common for growers to “wait for the pre-flood timing to treat most broadleaf weeds in rice. At that point, the weeds are just too big. Even though the flood will control the pigweed you have endured four or five weeks of competition. If the pigweed population is thick enough, you can incur some real yield losses before the field is flooded.”

Levees

The biggest problem spot with rice and pigweeds is the levees.

“If you leave a pigweed on a levee and it makes seed, it’s just like allowing Palmer pigweed to go to seed after corn harvest. (Almost) every effort you’ve made to manage that population will go out the window. Because once you’ve swiped that levee down (in preparation) for next year’s soybeans, you’ll do a good job of evenly spreading those pigweed seeds from the levee across the field.”

Data from work done by Jason Norsworthy, weed scientist with the University of Arkansas Division of Agriculture, showed that “no herbicide or combination does an excellent job of controlling pigweed on levees. Plants are probably 18, 24, or even 30 inches tall at time of application because we normally spray levees later in the season.

“Propanil and 2,4-D was the best treatment they had in (the test). In my opinion, the major goal of pigweed control on levees is preventing seed production. That’s particularly true if you haven’t seeded the levees to supplement your per-acre grain yield. Do what you must to keep it from making seed.”

There are other benefits of crop rotation including breaking disease, insect and nematode cycles and improving soil conditions. But from a weed control standpoint, “if the appropriate herbicides are chosen and applied correctly, then rotation can be a big tool we can use to manage pigweed populations.

“Again, rotate herbicide chemistries and exploit cultural tactics available in (each) rotational crop. If you’re going to grow a rotational crop with an eye towards managing resistant pigweeds, then you must target 100 percent control.”

Italian ryegrass

Farmers and consultants, mark your calendars. Bond announced an event at the Delta Research and Extension Center in Stoneville on March 10.

“We’ll have a field day revolving totally around glyphosate-resistant Italian ryegrass. We have a bad pigweed problem in Mississippi. But we also have a severe problem with resistant Italian ryegrass. We hope to have some of the same folks involved” that worked the Pigposium.