dicamba herbicide damage to soybeans

Several thousand acres of Mid-South soybeans have been affected by either drift, volatility, temperature inversions or tank contamination from dicamba herbicide applications.

Pigposium III slated for Feb. 28 in Forrest City

Where can you find the latest on pigweed control?

The first Pigposium was held in 2010, after one of the worst pigweed years growers have experienced.

“Glyphosate resistance had become an issue for everyone it seemed and growers were desperate for solutions,” says Tom Barber, University of Arkansas weed scientist. “That meeting drew around 800 attendees and as far as I know still holds the unofficial record for the most highly attended meeting conducted by the University of Arkansas Cooperative Extension Service, at least in recent years.”

The second Pigposium was a hands-on field tour in 2014 at the Northeast Research and Extension Center in Keiser, Ark.

“Among other things, that meeting focused on the importance of crop and technology rotation to preserve the remaining tools for pigweed control. The importance of residuals and utilizing multiple effective herbicide modes of action was highlighted in all crops as well as timing and rates of Liberty applications. We also talked about new technologies and alternative methods for pigweed control such as ‘zero tolerance’, cereal rye cover crops and narrow-windrow burning.

For Pigposium III, set for Feb. 28 in Forrest City at the East Arkansas Community College Fine Arts Center, there are several main thrusts.

“What can we do for pigweed control besides the use of herbicides? What will the pigweed situation look like in five years? I’m sure windrow burning and the Integrated Harrington Seed Destructor will be covered.”

Another obvious focus this time around is the dicamba-tolerant crops and the proper use of dicamba.

PPO resistance

“Honestly, we weren’t planning to do this symposium at this time,” says Barber. “But we figured it was needed after the widespread PPO resistance producers experienced last summer. There are now pigweed populations in the state resistant to four modes of action: ALS chemistry such as Classic and Pursuit; DNA resistance, products like Prowl and Treflan; Roundup; and, now, PPO herbicides such as Valor and Flexstar. All of the PPO-resistant populations in Arkansas are likely multi-resistant.”

At the meeting, Barber will speak on expectations for PPO-resistant pigweed, pre-emergence and post-emergence herbicides.

“We are lucky to have a ton of data because three farmers were willing to work with us last summer. So, we studied three locations of PPO-resistant pigweeds and looked at a number of pre-emerge and post-emergence herbicide programs for control.”

At the end of the day, the researchers found one pre-emerge product isn’t enough. “We need to use a combination of herbicides. For years PPO herbicides such as Valor and Reflex have really saved our butts in regards to holding glyphosate-resistant pigweed at bay.

“There are a lot of pre-emerge herbicide combinations containing a PPO herbicide such as Valor. The best control we got from any of the locations with Valor alone this year was around 80 percent and the worst was lower than 50 percent. Unfortunately, Valor isn’t bringing a whole lot to the table anymore when it comes to PPO-resistant pigweed control.

Eye-opener

“The biggest eye-opener for us was that these PPO herbicides won’t give effective control pre-emerge. That was a surprise because in the Midwest, where there is PPO-resistant waterhemp, they were still getting decent residual control from Valor. That was true even if the post-emergence control of a PPO herbicide like Flexstar failed.”

Going into a new season, “growers need to have a good game plan developed to control PPO-resistant pigweed that includes metribuzin plus a group 15 herbicide such as Zidua or Dual Magnum.

“Last year, we found metribuzin and Zidua, or a product like Boundary (metribuzin + Dual Magnum) are combinations that worked well. Anywhere we added Zidua or Dual into the mix, we saw increased pigweed control.”

One thing to consider if you’re going to use metribuzin: double-check the list of sensitive soybean varieties at the University of Arkansas website (UAEX.edu).

Dicamba issues

What about dicamba-tolerant crops and educating growers about label requirements for new dicamba formulations (XtendiMax VaporGrip and Engenia)?

“Ples Bradley, (University of Arkansas) pesticide safety specialist, will be talking about the labels. And he’s got quite a task in front of him.

“Seriously, I’ve been reading herbicide labels for a long time. These are the most confusing labels for any herbicides I’ve ever tried to decipher.

“I’ve been trying to explain to cotton growers how to use Engenia in their Xtend crops. They can’t use XtendiMax if the new Arkansas regulations go through. But it’s very hard to give them precise answers because the more you dive into that label, the more difficult it seems the product will be to use.”

In cotton, the system “seems to work better because growers can use Liberty and Liberty has been really effective for managing pigweed if timings are correct.

“In Xtend soybeans, if I have PPO-resistant Palmer amaranth, even if I use a broad-spectrum residual herbicide combination, sooner or later, there will be some escapes. When I get those escapes there’s nothing else to use but dicamba. Flexstar won’t control them, Roundup won’t control them and the Xtend soybeans aren’t tolerant to Liberty.

“That means I’m down to Engenia. Engenia can do a good job as long as the pigweeds are caught small. But in our PPO-resistant research sites, we still had some pigweeds left after spraying two applications of Engenia on 2-inch weeds.  I also had some pigweeds that were larger than 4 inches when they were sprayed.”

Buffers

Another major concern is field configuration, how producers will be able to spray a field with all the buffer requirements.

“Well, if I can’t spray Flexstar or Roundup, what can I do on those unsprayed acres? At that point, I’m down to hiring a chopping crew or letting the pigweeds go to seed in the buffer area. Neither of those are good options.

“If the soybean price leaped back up to $14, growers wouldn’t mind spending money on a chopping crew so much. But those prices aren’t going to make that leap. It’s going to make tight budgets even tighter if we have to resort to hand-weeding.”

TAGS: Weeds
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