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What can we learn from the loss of the world’s greatest herbicide?

For me, the world’s greatest herbicide was – and I say that in the past tense, was – glyphosate. It’s unfortunate but in my geography it is a herbicide of the past on many driver weeds. For me Palmer amaranth is a driver weed. For you that may be kochia. That may be wild oat. That could be green foxtail.

July 4, 2016  By Jason Norsworthy University of Arkansas

Palmer amaranth three years after the introduction of one patch of seeds.

Simplicity has a cost. We as humans like to move forward in the simplest manner. Farming, we like to look at simplicity. I want everyone to understand today that rotating Roundup Ready soybean with Roundup Ready corn and following that with Roundup Ready cotton, which is what we did, is absolutely no rotation at all. 

Glyphosate resistance has caused an increase of about $40 to $50 per acre in our weed control systems. Those herbicide costs are directly driven as a result of the resistance issues that we’re dealing with: Reduced harvest efficiency. Complete crop loss. Loss of the family farm. It’s compromised conservation tillage for us.

Glyphosate has become a grass herbicide or an adjuvant in some fields. We have glyphosate-resistant giant ragweed, glyphosate-resistant Palmer amaranth, glyphosate-resistant Johnsongrass, glyphosate-resistant goosegrass.


Here’s how we got into the situation that we’re in. In the U.S., corn acreage is 89 million acres, and 89 per cent of this is Roundup Ready. We have 85 million acres of soybean and 94 per cent is herbicide-resistant; and of that, 89 per cent of these acres would be Roundup Ready.

We have soybean fields with glyphosate-resistant kochia. Our kochia is also ALS resistant. I’m told dicamba-resistant kochia is spreading like crazy across the northern U.S. I understand there are a few populations of dicamba-resistant kochia in Canada today, but you’re going to see increased populations of dicamba-resistant kochia move across Canada rather quickly.

Glyphosate-resistant Palmer amaranth is a national issue in the U.S. I’m also convinced that it will soon be a Canadian issue. In 2005, glyphosate-resistant Palmer amaranth was found in the southeastern U.S. This is a weed that thrives in hot, dry conditions. Today we have 30 states in the U.S. that have glyphosate-resistant Palmer amaranth, and I am convinced southern Ontario will soon have glyphosate-resistant Palmer amaranth.

Glyphosate-resistant Palmer amaranth is spreading because almost all of our acreage was planted to Roundup Ready technology and sprayed with one herbicide, glyphosate. We’ve got to bring more diversity into these production systems.

The resistance treadmill. There’s nothing novel about this. Back in the mid to late ’80s we adopted ALS herbicides throughout the U.S. but weeds developed resistance. In the mid ’90s glyphosate came along. Glyphosate became the solution to ALS resistance.

Once we had glyphosate resistance we took the PPO chemistry and applied it pre-emergence. We also applied it post-emergence and that allowed us to have continued planting of Roundup Ready crops. In 2015, we found PPO-resistant Palmer amaranth. We now have found it in five states in the southern U.S. It appears to be spreading rather quickly. Glyphosate resistance is the reason we have PPO resistance today. Stacked resistance or multiple resistance – it’s a game-changer.

We have found that ALS resistance (Group 2), DNA resistance (Group 3), glyphosate resistance (Group 9) and PPO resistance (Group 14) stacked in Palmer amaranth. I challenge anyone to come up with an effective herbicide program that I could place in that field and successfully grow a Roundup Ready or conventional soybean crop – because it’s not possible.

How do we move forward? I have 12 BMPs or best measurement practices but will only talk about six ways of moving forward today.

  1. Know the enemy. If you take a look at the weed science literature, generally 25 to 30 weed species is what you would find in a field but you have five driver weeds for which you’re going to develop management programs. When does it emerge? How long does it emerge? If I’m going to use tillage or use a herbicide, I need to understand when that weed is going to emerge and can be controlled. Growth rate is very important. Palmer amaranth in my geography takes five or six days to get about four inches tall. From that point on, it grows two to three inches per day. When does it flower? How long is it going to flower? That’s important if we’re looking at preventing seed production. How is it dispersed? How many seed are produced? How persistent is the weed in the soil seedbank? Ultimately, what we’re getting after is the weakness of the weed.
  2. Apply labelled use rates. The labels tell you what rates to use at what stage of the weed. Research on dicamba and glyphosate found that reduced rates help select for herbicide resistance. These technologies will all be short-lived if we do not effectively use them in combination with other herbicides and apply them at the proper timing. Proper timing and proper rate – that’s partly the key to being successful.
  3. Multiple effective modes of action. We have to understand which actives are actually effective. Pre-mixes are often promoted as a resistance management strategy. Most pre-mixes have a half X rate of one herbicide and a half X rate of another herbicide.  Each active ingredient is not a standalone, effective, product at a half X rate. Another example is Enlist Duo, a combination of 2,4-D and glyphosate that is going to be labelled in soybean. If you have a glyphosate-resistant broadleaf, glyphosate is not an effective mode of action. The 2,4-D is providing an effective mode of action on the broadleaf, but what about the grass weeds out there? What’s controlling the grass weeds? Glyphosate. There may be two modes of action but there are not two effective modes of action on both broadleaf and grass weeds. There are very few options out there for multiple effective modes of action in a wheat-canola rotation.
  4. Prevent weed seed production for resistant prone weeds; economic thresholds do not apply. About seven years ago we did some research where we introduced 20,000 Palmer amaranth seeds into a little circle into a crop field to mimic one plant setting seed. Palmer amaranth produces half a million seeds per plant. We sprayed it three consecutive years with glyphosate. The first year it looked okay. The next year you could see where the combine or the cotton picker actually moved the seed. Palmer amaranth was so bad in year three that the crop had to be abandoned.
  5. Exploit biological weaknesses using cultural and mechanical management. We don’t talk enough about row spacing, seeding rate or orientation of rows. If we look at weed emergence, at 90 per cent light interception or canopy formation, weed emergence ceases. The point is you’ve got to establish a dense canopy to be ahead of the game when it comes to the fight on resistance. We must have a strong non-chemical and chemical strategy. If weed seeds are going into the combine they are ultimately going out the back of that combine and we’re spreading weed resistance. Harvest weed seed management can help.
  6. Diversify your weed control programs and focus on managing the soil seedbank. Steve Powles with the University of Western Australia said if it works, do something different next year. If you have a program that works, you’ve got to diversify that program. You’re not going to continue to be able to use that program and expect to be successful long term.


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