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Plants out of place


They persist and resist, and every growing season presents new hurdles. The most successful survive and reproduce. They outcompete crops for resources such as water, sunlight and nutrients. They pose ever-increasing challenges for producers.

“A weed is defined as a plant out of place,” said Dr. Reid Smeda, professor of weed sciences at the University of Missouri in Columbia. “Every weed has a unique niche and how it’s going to be successful in the soil.”

Many farmers have long relied on the quick-fix control provided by post-emergent herbicides such as glyphosate, glufosinate and 2,4-D, but with resistance on the rise and limited new technologies on the horizon, management strategies have to change. The key is how.

“Weeds will only respond to our practices,” Smeda said. “If we do the same thing over and over again, no matter what it is, the weeds will adjust, the species will shift. Then we’re back to a prob­lem again.”

Overlapping residuals combined with a post-emergent herbicide is the best bet for sustaining these chemistries in the long run, said Dr. Jason Weirich, MFA vice president of sales and agronomy. It’s a practice he’s been preaching for the greater part of the last decade.

“It’s a double-edged sword when we go to the farm gate,” Weirich said. “When we make the rec­ommendation for overlapping residuals and to use multiple modes of action, it’s not about a sale. It’s about sound science and stewardship. There’s nothing new coming down the pipeline. Ultimately, it’s the producer’s choice, but we want to make sure that our farmers have options as we go into the next growing season and five years down the road.”

Before Bill Fry of Burlington, Kan., began working with his local AGChoice agronomists in 2016, he could see the writing on the wall. His current weed management plan wasn’t working optimally for his operation, and he was spending too much money on expensive chemicals to control what he could.

“We put a burndown on in the spring, but we had to come back at least twice with a post appli­cation to keep the weeds even halfway under control,” Fry said. “Even then we probably only had about 80% control in our fields.”

For the last 30 years, he has been no-till farming ground near the Neosho River. There, he grows corn and soybeans in addition to managing 250 head of cattle.

“Bill really wants to do everything he can to keep his fields clean,” said Colin Kraft, MFA preci­sion agronomy specialist, who works with Fry to develop recommendations for the farm and scouts roughly 1,000 acres of the farmer’s row-crop fields. “He wants to be progressive and do everything right for his operation.”

When Fry began working with MFA’s precision team, together they came up with a new plan. In the fall, Fry applies a burndown application of glyphosate and 2,4-D in the fields he plans to plant into corn the next spring. In the spring, he uses a pre-emergent herbicide on those same fields roughly two weeks before planting along with an overlapping residual. Fry is seeing a difference.

“Our crop scouts come around about once a week, and we’re ahead of the weeds all the time,” Fry said. “It saves us a lot of money and chemical because we don’t have to spend as much trying to get rid of a weed a foot tall. I would say our fields are 95-98% weed free now.”


The type of vigilant weed control found on Fry’s farm shows that herbicide programs still work if used properly and judi­ciously, Weirich said.

“A combination of some of these products will pick up controls,” he said. “The main takeaway is we need to be making sure we are taking the best steps we can to steward the products that we do have.”

If there is a break in treatment, weeds have the potential to take root and grow, and there’s a greater risk of developing herbicide resistance. That’s why overlapping residuals are needed to suppress those weeds, Smeda explained.

“A farmer applies a herbicide in the spring at a certain rate,” he said. “Once applied, herbicides degrade over time to lower concentrations, and this is aided by warm tem­peratures and soil moisture. When the concentration of that herbicide drops below a specific threshold, weeds will germinate.”

The threshold for each weed species is different, Smeda continued. Grasses may germinate at a lower threshold than waterhemp, and waterhemp may germinate at a lower threshold than morning glory.

“At different times through the season, you’ll start to notice weeds break through,” he said. “That’s a reflection of the degradation of the chemical in the soil through natural means. Natural chemical degradation can be facilitated by microbes including bacteria and fungi. Some herbicides on the soil surface can also be broken down by photodegrada­tion, which is the result of exposure to UV light.”

As time passes, less chemical exists in the soil.

“At the point where weeds start breaking through, a post-emergence application is needed to kill emerging weeds. Along with that, additional residual herbicide is added to boost the level of herbicide present,” Smeda said. “By doing that, you’re extending the suppression of sensitive species for a longer period of time and reducing the poten­tial that you have to put another herbicide on.”


A good herbicide program requires more than just residuals, Weirich added. A multi-pronged approach is needed. Each chemistry has different modes of action for control of certain weed species. By varying modes of action in accordance with a proper herbicide recommendation, the probability of resistance is exponentially decreased.

And preventing resistance is an ongoing battle in the world of weed management. In Missouri, 15 weed species have confirmed herbicide resistance. Of those 15, water­hemp is considered to be the most problematic, followed by horseweed, Palmer amaranth and giant ragweed. Now there are suspicions about Johnsongrass.

The first documented case of glyphosate-resistant John­songrass was collected from a soybean field near West Mem­phis, Ark., in 2008. Recently, Smeda collected some rhizome samples from western Missouri to study.

He and some of his graduate students are now conducting Johnsongrass trials in MU greenhouses. “We want to verify if there is resistance and determine the mechanism. Johnsongrass is difficult to manage in corn, and the potential loss of glypho­sate as a management tool could be problematic.”

If Johnsongrass and other significant grasses such as giant foxtail and fall panicum become resistant to glyphosate, Smeda said there would be large holes in the Roundup Ready technology.

“Roundup Ready’s real effec­tiveness is on a lot of the grasses and secondary species,” Smeda said. “But we’re already having to put other chemicals in. That’s why Xtend and Enlist soybeans were developed. It’s going to really require us to think about new strategies of how we’re going to manage this in a corn system as well as soybeans.”

Last year, University of Illinois weed scientists also document­ed waterhemp resistance to the Group 15 class of herbicides— long-chain fatty acid inhibitors, which include products such as Dual, Warrant, Outlook, Degree, Harness and others.

Sometimes called “the super­weed,” waterhemp has demonstrated resistance to six herbicidal modes of action: atrazine, 2,4-D, chlorimuron, fomesafen, glyphosate and mesotrione.

“That doesn’t leave us much, does it?” Weirich said. “Group 15 herbicides are commonly used as part of our residual pro­grams and not only heavily used in soybeans but also in corn. Losing that effectiveness on waterhemp on some populations we know is possible, and it’s something we need to be aware of because currently we don’t have any new modes of action from a residual standpoint.”

If herbicides continue to lose their effectiveness, me­chanical measures like the chopping crews and culti­vators used in some southern states are still an option. However, these control methods are expensive and unreasonable for the size of most farming operations in MFA’s territory.

“Missouri is predominantly a no-till state,” Weirich said. “I’m not sure that we’re ready to go back to that kind of system here from the standpoint of equipment, stewardship and the farming practices we have today.”


Implementing cultural controls such as cover crops could also help farmers fight weeds. Cereal rye, specifically, has allelopathic properties, meaning the plant itself produces biological chem­icals that affect the growth and survival of other plants.

“Rye is one of the most allelo­pathic of the cereal cover crops,” Smeda said. “Compounds re­leased from the roots of rye have unique herbicidal-type activity. When planted in the fall, it will keep out winter annual weeds. Once the crop is terminated, there are two to six weeks of soil residual activity from those allelopathic compounds.”

In addition, Smeda said 50% of the weeds in Missouri and surrounding areas are light sen­sitive. Because of this, cereal rye can have a suppressive effect.

“Cereal rye forms a sort of mat,” Weirich said. “It basically intercepts the light, causing a smothering effect on the soil. Is it 100% effective? No, it’s not, but it does take out a few of those populations that might be at risk for herbicide resistance and provide a different control method.”

Even before working with MFA and AGChoice, Bill Fry was planting cereal rye as a cover crop for his cattle to graze through the winter. In the fall, when the corn is starting to dry down, he has the seed flown on with a plane. The next spring, he removes the cattle and lets the rye continue to grow. He terminates the cover crop prior to planting soybeans in the residue.

For Fry, the cover crop offers many benefits.

“If you have a good stand, the rye not only fills in the tracks from the cattle, but also helps keep weeds out,” he said. “The cows prefer it to hay. Plus, it provides mulch for the plants.”

Fry stressed that farmers interested in growing rye as a cover crop should do their research.

“Cereal rye is easy to terminate; rye grass is not,” he said.


New traits in crop genetics also offer promise for combating weeds, but stewardship of these technologies is a concern.

Last year, Corteva Agriscience released the Enlist E3 trait of soybeans. Enlist soybeans are tolerant to three classes of herbicides: 2,4-D, glyphosate and glufosinate.

“It’s really going to be the first year where we’re going to have a large number of acres of Enlist,” Weirich said. “Pre­viously, when we had a new trait come out, we saw a step back in adoption of residual herbicides, and growers relied more on post-emergent applications only. Due to current commodity prices and the farm-gate value, some people think we will see that again. If we do, we will see weed resistance.”

Smeda also worries about future management practices surrounding these new traits.

“GMO crops have been a great advantage for farmers,” he said. “But it made us a little management lax and too long dependent on those tools that were successful. Now we’re trying to get back to other tools like residuals combined with post applications that were also successful in the past but abandoned because of the ease of these other programs.”

After the drought of 2018 and flooding in 2019, proper weed management is particularly important this year, Weirich stressed.

“Due to the environmental factors of the last two seasons, I do expect to see increased weed presence in our fields,” he said. “In areas of flooding, we could also see weeds we haven’t seen in a while from soil deposits. We just can’t forget what we’ve learned over the past 20 years. This is definitely not the year to cut back on herbicides, especially residuals.”

For more information on how MFA can help with weed management, contact your local MFA or AGChoice agronomist.

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