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.”