Already a chief torment of Mid-South producers, last year pigweed was discovered to have added PPO products to the list of chemistries it no longer fears. The development has only intensified the need for growers to carefully scrutinize herbicides and understand how best to attack the weed.

In early June, following a torrent of calls from producers with problem pigweed fields, Tom Barber visited with Delta Farm Press. Barber, Arkansas Extension weed specialist, spoke on the need for grower education, where to find solid product information and treatment regimes that remain effective. Among his comments:

On using Arkansas’ 2016 MP44 guide…

(note: for this section, Barber refers to MP44 Page 42 for soybean herbicides)

“On the left side of the page are weed response ratings for various herbicides in soybeans, although we have similar tables for all crops. Look at the very first column and that includes the herbicide’s ‘family.’ That relates to the group, or mode of action, for each herbicide listed in the table.

“So, if you have a product like Trivence, you’ll see it has three modes of action as a pre-mix: ALS (Groups 2, 5 and 14 which represent acetolactate synthase), photosynthetic inhibitors and PPO (protoporphyrinogen oxidase) inhibitors.

“The main reason for that to be included in the weed control table is so growers and consultants will know which herbicide modes of action are included in the single or pre-mix product they are spraying. Knowing which modes of action included in a specific product is very important moving forward in the battle with Palmer pigweed. 

“We have numerous pigweed populations in the state that are resistant to four herbicide modes of action including ALS, DNA, EPSPS (glyphosate) and PPO (Valor or Flexstar), or groups 2, 3, 9 and 14 respectively. This means that our alternatives or options for control moving forward are very limited.

“For pre-emergence applications basically we are down to two herbicide groups (5 and 15) which include metribuzin and the chloroacetamides such as Dual Magnum, Warrant or Zidua. From a post-emergence standpoint the only herbicide labeled in cotton and soybean that has activity on PPO-resistant pigweed postemergence is Liberty.”

More on modes of action…

“Pigweed populations resistant to PPO herbicides are much more widespread than what we thought, especially in northeast Arkansas (North of I-40). Moving into an era where a producer has pigweed resistance to PPOs, ALS, and Roundup -- which is the majority of pigweeds where there’s PPO resistance -- removes three separate modes of action from treatment considerations.

“Given this current situation with resistance, growers, consultants and other on-farm decision makers need to understand what a mode of action is and what products fall into each herbicide family.

“It’s not good enough to switch product names and say you’ve switched modes of action. You can’t say ‘(X) product has two modes of action’ and leave it there. True, (X) product may have two modes of action, but only one of those may still work on PPO-resistant pigweed. Unfortunately, I see and hear too much of that.”

Rotations, education

On understanding true rotation of chemistries…

“In other words, if you only use Dual Magnum pre and Warrant or Zidua post in beans from a residual standpoint, you are not rotating chemistries in regards to resistance management.

“Dual, Warrant and Zidua, if you look at the MP44 list, have the same group number: 15. While one is metolochlor, one is acetochlor and the other is pyroxasulfone, they’re in the same herbicide family/mode of action. They basically control weeds in the same way so if pigweeds ever become resistant to Dual they’ll also be resistant to Warrant.”

On the great need for growers to be educated…

“It’s very important growers know the MP44 contains this information and is available and how to use it. That’s especially true as we move further into management of PPO resistance.

“Moving forward this year as we’re still planting some beans, and especially for next year, we must focus on using products that contain modes of action that will work effectively on these pigweed populations. If it is PPO-resistant, any product that’s a Group 14 won’t have the effectiveness needed.

“If you’re putting out a generic product with an odd name and don’t know what all is in it, find out! We can’t be working blind anymore.”

On pigweed plot work…

“This is serious business. From a pre-emerge standpoint in our plots in Marion, where we used to get 100 percent control with Valor for three to four weeks, we’re now seeing 70 to 80 percent control. And most of the ones we miss are emerging with the beans so there’s an immediate mess on our hands. There are several products out there where Valor is the main component for pigweed control and these will not be as effective in the future.

“So, if I’ve planted a Roundup Ready bean, all that’s left in my arsenal is Flexstar or products that contain Flexstar like Prefix or Warrant Ultra. In our locations with PPO-resistant pigweed, Flexstar at the highest rate doesn’t check them up. There is no alternative herbicide available to control pigweed once up in Roundup Ready beans. The plots in Marion are ample evidence of that.

“The last two weeks, I’ve gotten call after call of that exact scenario playing out in fields. Our options are to get a big chopping crew to go through the field chopping or pulling by hand or disk the field up. In almost every case, the grower decided to disk the field and replant with Liberty Link varieties.

A good idea

A good idea…

“As a practical matter, a good idea that started in Australia is to put the herbicide group numbers on the jug labels. For instance, a jug of Dual Magnum will have a big ‘15’ printed somewhere on that label.

“Back here, with where we’re at with pigweed that sure sounds like a good idea to keep our herbicide programs straight. That will help growers know what they’re dealing with, what they’re putting out. Unfortunately, right now the information on the labels isn’t always clear.

“I get calls on a ton of generic herbicides that I’ve never heard of. Once they provide the chemical name it’s ‘oh, yeah, that will do this and that.’ But if you don’t know that going in, you’re at a serious disadvantage.”

Moving forward…

“Looking at our Marion plots -- and we have folks touring them weekly -- we’re looking at 70 to 80 percent control of pigweeds with Valor pre. Move down the road to plots in Marianna where there’s no PPO resistance, we’re getting 100 percent control.

“It seems crazy to have that gap from a pre-emerge standpoint. We thought we’d get much better control. And that’s a disaster. Only 80 percent control with Palmer amaranth means you’re likely disking the field up.

“This year, our main recommendation, has been a product like Boundary. That has metribuzin (TriCor) and Dual Magnum in it, two effective modes of action on all pigweed, including PPO-resistant. All herbicide products/premixes that contain a Group 5 (metribuzin) and 15 (Dual, Warrant, Zidua) herbicide are doing a good job controlling pigweed in our plots at Marion.

“Fierce, a premix of Valor and Zidua is still looking good on PPO-resistant pigweed in our research; however, the Zidua component is doing the heavy lifting in regards to pigweed management.

“Moving forward we need to shift our focus to products that contain more than one EFFECTIVE mode of action on pigweed. In fields where numbers are high it takes a program with two effective modes of action up front to keep pigweeds at a manageable level for the rest of the season.  

“One thing is for sure, it’s getting tough for our growers. We live in a world of pre-mixes and it’s crazier than I’ve ever seen it. If I had a list of pre-mixes for soybeans it would fill a binder to carry around; there are that many. PPO-resistant pigweed is a game changer for soybean and cotton producers and it is crucial that we know exactly what is in each product before it goes into the tank.”