A show-me weed showcase
Weather good for crops…and weeds
In many areas, this growing season featured ample -- if too much -- moisture coupled with summertime heat. Moisture and heat units are a perfect prescription for crop growth. Unfortunately, these conditions are also perfect for weeds, too. Earlier this summer, University of Missouri (MU) weed scientists talked about weed management at an MU Bradford Research Facility field day near Columbia, Missouri.
Missouri has had its share of glyphosate resistant weeds. A 2008 to 2009 survey revealed 12 out of 27 giant ragweed populations in 8 Missouri counties resisted glyphosate, says Kevin Bradley, University of Missouri Extension weeds specialist.
Waterhemp is worse
If Missouri legislators ever designate a state weed, waterhemp would probably win. It’s everywhere. Particularly disconcerting are the glyphosate-resistant biotypes that are multiplying across the state.
“It is a widespread phenomena,” says Bradley. Eighty-four out of 144 fields in 38 Missouri counties -- or 58% -- infested with waterhemp from 2008 to 2009 had glyphosate-resistant waterhemp.
Multiple herbicide resistance takes resistance problems to a whole new level. There are biotypes of giant ragweed (left) and waterhemp (right) in Missouri that resist both glyphosate and ALS inhibitor herbicides (such as Pursuit and Scepter). “If a soybean is cross resistant to both ALS inhibitors and glyphosate, you can come in with Cobra, a PPO inhibitor,” says Bradley. Another solution is a mix of a treatment of glyphosate and Flexstar, another PPO inhibitor, he says.
Timing is critical, though, as MU graduate student Eric Riley shows. One effective giant ragweed treatment that effectively can control both glyphosate-resistant and ALS inhibitor-resistant weeds is including 1.3 pints of Flexstar, a PPO inhibitor, with glyphosate. This can nix weeds that resist both glyphosate and ALS inhibitor herbicides. In MU tests, a Roundup PowerMax and Flexstar mix controlled 82% of the 4-inch giant ragweed. In contrast, a 22-ounce rate of Roundup PowerMax controlled just 20% of 4-inch high giant ragweed. Meanwhile, just 1.3 pints of Flexstar per acre controlled 70% of giant ragweed. It’s important to spray weeds early. At an 8-inch height, the Roundup PowerMax and Flexstar combination controlled just 36% of giant ragweed.
The Kixor stable of products from BASF debuted this year. It contains several brand names. For example, Integrity is designed for preemergence treatments on corn, while Sharpen is designed for broadleaf burndown on crops including corn and soybeans. Bradley notes an Integrity and atrazine mix works well as a preemergence set-up treatment for a later postemergence option of a non-selective herbicide on a herbicide-tolerant crop. Meanwhile, Sharpen works well as a burndown treatment. “The neat thing about Sharpen as a burndown is it is can be a replacement for 2,4-D,” says Bradley. “Unlike 2,4-D, there is no seven-day waiting period prior to planting.”
All pre, all post, or a pre-post herbicide split? That’s a herbicide application question corn farmers face each year. Brett Craigmyle, an MU graduate student, and Kevin Bradley showed compiled 2000-2009 trials on these options. The preemergence/postemergence split topped yields 37 times, compared to 16 for an all postemergence strategy and 2 for a one-pass preemergence strategy. “It has the best chance of success in Missouri in most years,” says Bradley.
Postemergence residual herbicide mixes
Still, there’s much interest regarding postemergence tank mixes for LibertyLink and Roundup Ready systems laced with residual herbicides.
“Some years, they work pretty well,” says Bradley. “This year may have been one of those years. It looks like waiting to apply at postemergence paid due to late flushes.” When yields are tallied, though, the pre/post split may again win. That’s because the preemergence residual component zaps early weeds that curtail yields early on.
Adjuvants help herbicides, don’t replace them
Field days are good days for fleshing out those “urban myths” that abound in agriculture. One is that adjuvants can help kill glyphosate-resistant weeds. They can, but only when another herbicide is added to the mix that kills weeds that glyphosate can’t. By themselves, adjuvants can enhance performance of herbicides, but they can’t kill weeds by themselves.
“You can’t put it (an adjuvant) on a glyphosate-resistant weed alone and kill it,” says Bradley. “You need another herbicide in the mix.”
Volunteer corn losses quickly add up
Volunteer corn can quickly tally up due to harvest losses, says MU grad student Tye Shauck. Even an amount as low as a 1 bushel per acre loss behind your combine leaves 80,000 seeds to germinate in next year’s crop. Volunteer corn growth, too, is more spotty than corn planted by a corn planter, as it is bunchy.
Controlling volunteer corn
Control of volunteer corn in soybeans is relatively simple. “Use selective herbicide in soybeans,” says Shauck. “Controlling volunteer corn in corn is tougher, though. For example, how do you get rid of Roundup Ready volunteer corn in a newly planted Roundup Ready field? The most obvious option is to rotate with LibertyLink or Clearfield corn, Shauck says.