The War Against Weeds Evolves in 2018

According to the latest Stratus Ag Research survey, 73% of nearly 4,000 growers polled across the United States reported having glyphosate resistance on their farm in 2017. That figure represents 120 million acres, and it’s a 15% jump in just one year – even after more than a decade of growing awareness, and the industry doing basically everything short of discovering a new mode of action itself, to curb the problem of weed resistance.

The numbers, unfortunately, surprise no one.


“When you get to a certain level with these pigweed species, they are so much different than any other weed with the explosion they can have. That’s because they are able to transfer these traits by pollen,” says Dr. Kevin Bradley, Professor of Plant Sciences with the University of Missouri. To illustrate, he shares how the world’s first glyphosate-resistant waterhemp was found in his home state in 2004. “By 2006 or 2007 more than half of our counties had glyphosate-resistant waterhemp.”

The breakdown of the Stratus Ag survey shows the top three weeds in 2017 as marestail (49 million acres), waterhemp (47 million acres), and Palmer amaranth (43 million acres).

“Everybody talks about Palmer, but it was actually number three,” says Arlene Cotie, Senior Product Development Manager with Bayer CropScience. Cotie, who also serves as U.S. Chair of the Herbicide Resistance Action Committee (HRAC), points out the most significant changes from 2016 to 2017 were expanded acreage of resistant Palmer amaranth in Nebraska, waterhemp in Minnesota and Nebraska, and marestail in Nebraska, North Dakota, Illinois, and Iowa.

Cotie’s work has involved LibertyLink for more than 25 years, and she says she has “never been more excited” about the system’s growth opportunity, which is expected to be around 25% in 2018.

Kent Bennis, Market Development Specialist with Dow AgroSciences, is relaying tips to growers for 2018 to ensure that the weed situation on his or her farm remains under control.

“We’re seeing several trends, as part of a sound herbicide program, that are helping farmers improve ROI and counter weed pressure,” Bennis says. To overcome challenging weeds in corn and soybeans, here are the top weed-fighting strategies to keep in mind for the 2018 season:

1. Forget skimping on herbicide passes
The timing of herbicide passes is shifting, Bennis says. There are instances when farmers prefer to apply a residual herbicide shortly after planting rather than before. Herbicide timing largely depends on the weed spectrum and density. With increasing herbicide resistance, a two-pass program is the best way to keep weeds small throughout the season.

Commodity prices may tempt farmers to cut down on residual treatments, but this approach allows weed escapes and can damage yield and profit potential as early as V2 in corn.

“With the terrific yield potential we’ve seen the last couple of years with corn hybrids and soybean varieties, I just don’t see why you would want to jeopardize that by putting yourself in a rescue situation. If you use a strong residual on the front end you can prevent that from becoming an issue,” Bennis says.

“I’ve said it for a long time: the best way to have clean soybeans is to have spotless or very clean corn the year before. When looking at a two-year rotation, make sure you’re truly changing modes of action. When I visit with growers, I make sure that they’ve got six, seven, or even eight modes of action in the two-year rotation herbicide plan.”

2. Be on the watch for marestail
Warm conditions last February gave a major boost to marestail, and it never let up – the weed proliferated reduced-till and no-till fields in the Corn Belt through 2017. Marestail has adapted all too well to our cultural and chemical practices, becoming tougher to control because not only does it overwinter, it’s germinating later than it has in the past. “If you don’t have a strong residual product in there to control the ones that are going to emerge in April or May, then you’re going to have a problem,” Bennis cautions.

A new herbicide this year from Dow AgroSciences, Elevore, will be the only product on the market that has an 8-inch label for marestail. Elevore, which contains the new Group 4 growth regulator Arylex active, can be applied up to 14 days before planting in soybeans and corn. It works well in cool conditions as well and can be tank-mixed with other products such as 2,4-D and glyphosate to provide effective burndown and residual control for emerging annual weeds throughout the spring and early summer.

In Ohio, marestail remains the top weed, but it is waterhemp that is presenting the biggest learning curve in the eastern half of the Corn Belt, where the weed is developing multiple resistances and spreading faster than Palmer, says Dr. Mark Loux, Professor and Weed Scientist with The Ohio State University.

“Waterhemp and Palmer have changed the formula, because they produce so much seed and develop resistance so rapidly. Guys in Illinois who have been dealing with it longer than us (in Ohio), say, ‘You won’t win against weeds with herbicides alone.’ We’re trying to look into things like cover crops, and more sanitation at the end of the season to manage the seed bank,” Loux says.

Giant ragweed is also problematic, and he is expecting more Xtend beans and postemergent use of dicamba this year in the state, “partly because dicamba annihilates giant ragweed.”

Weed scientists including Loux point to the potential introduction of Dow AgroSciences’ Enlist soybeans in 2019, pending Chinese approval, and the advantage that system offers via 2,4-D and glufosinate tolerance to take some of the pressure off dicamba.

“As far as utility Enlist is probably really similar to Xtend, but the inherent weakness is that the 2,4-D trait doesn’t solve issues with marestail in the spring. If you don’t do treatment in fall, it’s not good enough, whereas a dicamba preplant application is better on marestail,” Loux explains.

New from Dow AgroSciences this year is Enlist One, a straight-goods 2,4-D choline product featuring Colex-D technology, with the added bonus of the ability to tank-mix with glufosinate to help control Palmer amaranth, marestail, giant ragweed, and other weeds.

3. Control pigweeds early before they progress across fields
Waterhemp and Palmer amaranth continue to be driver weeds in corn and soybeans. Waterhemp is slowly taking over more acres each year, Bennis says, and if a farmer is in an area where it was just starting to get bad last year, it’s likely getting worse.

Identifying waterhemp can be difficult because it looks like Palmer amaranth and other pigweed species in early growth stages. The petiole of Palmer amaranth is always longer than the leaf, which is one way to distinguish Palmer amaranth from waterhemp. The first true leaves of waterhemp are generally longer and more lance-shaped than other pigweeds.

In soybeans, farmers can also improve control of tough weeds, including the pigweed species, by planting narrow rows of seven to 15 inches, Bennis says. This allows the crop to shade the row much quicker, which reduces weed germination and emergence.

John Appel, Herbicide Commercial Product Lead with Syngenta, says, “With waterhemp already resistant to six different sites of action – if you can believe that – it’s going to be important to control them when they’re very small to give growers their best chance.”

“In continuous crop systems with early and late postemergence applications with one site of action, resistance can lead to weed control failure in as little as two years. By comparison, the addition of a preemergence herbicide with two effective sites of action can delay resistance for 18 to 20 years,” Appel says.

EPA has cleared a slate of Syngenta herbicide tank-mix options for inclusion with both XtendiMax and Engenia for use in their respective soybean and cotton systems in 2018. The options include Boundary 6.5EC, BroadAxe XC, Caparol 4L, Dual Magnum, Flexstar (for XtendiMax only), Prefix, Reflex, and Sequence. Syngenta expects to add several more in the coming weeks. The products can be used once listed on the Engenia and XtendiMax tank-mix websites.

Also coming online for 2018 is AMVAC’s ImpactZ broad-spectrum herbicide for corn growers. It contains both Impact and atrazine for control of grass and broadleaf weeds in corn from weed emergence until corn reaches 12 inches high as a sequential, early post or total postemergence program.

From UPI in 2018 is TRIPZIN ZC, a patented premix that combines metribuzin and pendimethalin for preemergent control of a wide spectrum of broadleaf and annual grass weeds, including Palmer pigweed and other pigweed species, ragweed species, lambsquarters and velvetleaf. Crops on the label include soybeans, alfalfa, field corn, garbanzo beans, lentils, peas, potatoes and sugarcane.

4. PPO resistance heads South
If only glyphosate resistance was the only worry. Weeds developing multiple cross-resistance including to PPO inhibitors is the issue that more and more growers are talking about, from Illinois to Missouri, says Joe Sandbrink, Technical Specialist with West Central Distribution. Arkansas and Tennessee have also been affected.

The University of Missouri’s Bradley says: “The real eye-opener for the southern U.S. is that Palmer much more recently started having PPO resistance, just in the past year or two. This is new for people, because they could always use their PPO herbicides to control glyphosate-resistant Palmer. The fact that they’re now having multiple resistance is definitely scary for them.”

Bradley says that while Liberty and Xtend will still control multiple resistant populations of Palmer and waterhemp, “I always try to make sure people realize that the more important part is using residual herbicides,” such as metolachlor and pyroxasulfone.

While Group 14 PPO herbicides won’t control resistant species postemergently, they still provide residual control – for now.

“We are moving the needle, and it is a concern that someday we might have true, full-fledged, soil residual resistance to these herbicides, but right now we don’t have very many options other than Group 14 and 15s to manage PPO-resistant weeds,” Bradley says.

FMC is providing growers with such a tool with its launch this season of Authority Supreme herbicide, a premix of sulfentrazone (Group 14 PPO) and pyroxasulfone (Group 15), for use in soybeans, sunflowers, and dry field peas in both Canada and the U.S. It’s effective against ALS, triazine, HPPD, glyphosate, and PPO-resistant weed biotypes, including waterhemp, Palmer pigweed, lambsquarters, kochia, morningglories, and others.

New from HELM, ARGOS ULTRA herbicide is a co-formulation of metolachlor (Group 15) and mesotrione (Group 27). Approved on field corn, sweet corn, yellow popcorn, and grain sorghum, Argos Ultra combats difficult grasses and small-seeded broadleaf weeds. It also controls weed biotypes resistant to glyphosate, ALS-inhibiting, PPO, and triazine herbicides.