Old Weeds, New Worries

Say “cocklebur” or “velvetleaf,” and Zack Rendel is suddenly 8 years old again, sitting in a John Deere 4020, pulling a six-row cultivator.

It was the mid-1990s, and weeds such as these two dominated the Oklahoma farm boy’s summers.

“We were cultivating the same areas two or three times just to try to rip those things out and get rid of them,” Rendel recalls.

Enter Roundup Ready. Rendel’s family operation, in northeast Oklahoma, transitioned primarily to no-till. The cultivators were sidelined. Cocklebur and velvetleaf retreated.

But, the weeds were playing the long game.

“We don’t think it’s chemical resistance, but we do think we’re seeing a shift in emergence patterns,” Michigan State University weed scientist Christy Sprague says. “Cocklebur and velvetleaf are coming up later in the season after a lot of postemergence herbicides have worn out.”

Environment may be playing a role, she adds.

“Our weather environment is so unpredictable, and in certain years, these weeds seem to be more prevalent than others,” Sprague says. “When they come up seems to be triggered by certain combinations of temperature and moisture.”

Rendel first noticed the shift a few years ago, when fields started to need cleanup sprays later in the season, beyond June. Cockleburs have become particularly difficult to manage in some sorghum fields.

A new generation of farmers may need to become reacquainted with these weeds, which plagued their grandparents and are staging a comeback, Sprague says.

STICKY HITCHHIKER. Surely, every child and animal in the countryside has been on the losing end of an encounter with cocklebur at one time or another. The oval-shaped burs, which resemble tiny hedgehogs, have hooked prickles that behave like a Velcro fastener.

“The burs are a big deal because they let the weed seed transport easily,” Sprague notes. “They grab onto things, like animal furs, and are very easily transported by animals.”

The weed traditionally emerged shortly after planting, in late May or early June, and posed a problem in row crops, particularly soybean fields. Because the seeds were safely cocooned in the bur, soil-applied residual herbicides weren’t very effective on them.

“The trick was trying to get rid of all the burs before they went to seed, so you wouldn’t fight the seed for years on end,” Rendel recalls. “Then came this glorious thing--Roundup Ready beans.”

“With a 60-foot boom, we could cover half of an 80-acre field in an hour--a field that would have taken a full day to cultivate,” he says. “You’d put Roundup and AMS in the tank, and come back a week later, and every weed was dead.”

Glyphosate--and other older chemistries--are still effective against cocklebur, Sprague notes. But, now the weed is waiting until late June to sprout after herbicides have come and gone.

“It’s tough because not a lot of postemergence herbicide options that would work have enough residual to help with it,” Sprague explains. “Growers just need to be aware that they might need an additional application to get it under control.”

Those later applications can be hard to squeeze in before growth-stage restrictions on the label kick in, Rendel notes.

“I’ve seen cocklebur be 4 inches tall and still put seed on,” he says. “It could potentially be a big problem. The later those come up, the more seed they leave out.”

A WEED BY MANY NAMES. Velvetleaf, buttonweed, butterprint and wild cotton are just a few of the names for this troublesome summer annual.

Unlike cocklebur, velvetleaf was never completely controlled by glyphosate, Sprague notes.

“With this particular leaf surface, glyphosate absorption can be challenging,” she says. “There’s been a lot of work done with adjuvants, and we still recommend including a nitrogen source like AMS to the tank.”

Some older chemistries are still effective on the weed, especially HPPD inhibitors in corn, Sprague explains. Postemergence options in soybean are trickier, but older “burners” like Resource and Cadet still work. “They’ll cause some leaf burning in soybeans,” Sprague cautions.

However, like cocklebur, velvetleaf appears to be emerging later in the season, when these products don’t have enough residual to affect it.

WHAT LIES AHEAD. The return to a pre-Roundup Ready-era weed-control system has been a slow but steady march of necessity on Rendel’s operation. They make time for aggressive preemergence applications and run cleanup passes with older chemistries such as Cobra, Classic and, sometimes, Select. They have even reluctantly added some tillage back in, as well as some conventional, non-GMO soybeans.

New weed-control systems are making their way into the marketplace as well: Xtend dicamba-tolerant soybeans and cotton are headed into their third year of commercialization, and Enlist 2,4-D-tolerant corn is
also commercialized, with Enlist soybeans waiting in the wings for final import approvals.

Next year, LibertyLink GT27 soybeans will also join the landscape. They tolerate glufosinate, glyphosate and a formulation of isoxaflutole called ALITE 27, which, at press time, still needed EPA approvals.

Both dicamba and 2,4-D are ranked “fair” on velvetleaf and “good” on cocklebur, Sprague says. She pegs Liberty as “good” on velvetleaf and “excellent” for cocklebur, as long as the weeds are sprayed at 2 to 3 inches tall. Isoxaflutole would also be very effective for preemergence use on these weeds, once it is fully registered for use in soybeans, Sprague says.

However, none of these options can fully combat old weeds playing a new waiting game, she adds.

“Again, it’s just a matter of timing,” she says. “The bigger issue is being able to control these weeds when they’re actually there.”

[PF_1218]

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