Working with Waterhemp
By Jason Jenkins, Mill Creek Communications
Tucked safely inside greenhouses on the University of Missouri campus in Columbia, thousands of waterhemp seedlings grow in black plastic flats, roughly 2 feet long and 1 foot wide, filled with soil to a depth of about 2 inches.
Like a gardener expertly tending to rare orchids, MU graduate student Travis Winans sowed these seeds about six weeks ago. Every day since, he’s checked on them. He’s fertilized them, watered them three times a day and ensured they had abundant light. There’s a reason behind this constant and consistent care, Winans explains.
“We want to eliminate any reason for mortality,” he says, “except for the herbicide we spray them with, that is.”
Winans is working under the guidance of MU Extension weed scientist Kevin Bradley to identify populations of herbicide-resistant waterhemp, widely considered the weed of greatest concern for soybean producers in the Show-Me State. He is treating suspected populations with two of today’s most popular “over-the-top” herbicides — glufosinate and dicamba, the herbicides employed in the LibertyLink and Xtend soybean systems, respectively.
“We’re seeing some failures at the field level, and we’re worried about resistance,” Bradley says. “We’re worried about losing Liberty because we use Liberty a lot.”
The work is just one component of a collaboration with weed scientists
at land-grant universities in Illinois, Indiana, Ohio and Tennessee where other weed species — such as Palmer amaranth, marestail and giant ragweed — also are being evaluated for their ability to survive applications of today’s post-emergence herbicides. The work is primarily supported by checkoff funds directed by the United Soybean Board.
According to the International Survey of Herbicide Resistant Weeds, there were 514 unique cases of herbicide-resistant weeds worldwide as of July 2020. Resistance was reported in 262 different species and in 93 crops across 70 countries. Of the 26 known herbicide sites of action, weeds have evolved resistance to 23 of them and to 167 different herbicides. Winans’ work could add to the list.
“We’re working with 137 different waterhemp populations from five states,” says Winans, who received his bachelor’s degree in crop sciences from the University of Illinois. “They’re all suspected to be resistant populations. In some cases, we’re growing it out to see if it’s truly resistant or if there wasn’t control because of an application issue.”
Herbicide-resistant waterhemp is already problematic for soybean producers. The small-seeded weed is one of the toughest to control. A single plant can produce a quarter-million seeds, which can remain viable for up to four years in the soil. Multiple flushes of waterhemp can emerge throughout a growing season. When the plants appear, they can grow quickly — more than an inch per day.
“We planted our flats during the last week of May, and by the first week in July, we had some plants that were 12 to 16 inches tall growing in just 2 inches of soil,” Winans says. “Waterhemp has really good developmental plasticity, which means it’s able to adapt to a changing environment. It can grow with hardly any water or in just a little soil. It’s able to outcompete other plants for water and nutrients and survive, producing seed to keep the population going. It has different resistance mechanisms and can metabolize herbicides, which makes it tough to control. That’s why the size of the plants and the timing of application are so important.”
Bradley says overall, season-long competition with waterhemp can reduce soybean yields by 44 percent. In the past decade and a half, almost every waterhemp population has developed two-, three- or even four-way herbicide resistance.
In 2018, a population in Randolph County was identified that couldn’t be controlled with six different herbicides, including 2,4-D, atrazine, chlorimuron, fomesafen, glyphosate and mesotrione. Should waterhemp develop resistance to Group 4 dicamba or Group 10 glufosinate, it would severely limit producers’ ability to acheive chemical weed control.
For his weed survey, Winans recreates a burn-down herbicide application scenario. Each of the suspected resistant waterhemp populations is grown in its own plastic flat. Once they reach a height of 4 inches, they are sprayed. Winans is repeating the survey four times at two rates.
“We’ll do four reps at a full 1x rate and four reps at a 1/2x rate,” he explains. “Fourteen days after we spray, I take a rating, looking at percent control of the flat. We also do ratings at 21 and 28 days. At 21 days, we do percent survival. I count how many plants were in a given flat and then how many are going to survive. We’ll do it for dicamba and then we’ll do all over again for glufosinate.”
While some plants may curl up and die after application, others will curl up
but remain green and grow out of the herbicide injury. Plants that do survive will be allowed to grow to maturity. Then, Winans will collect the seed and repeat the entire process. Should those plants survive, they are considered truly herbicide resistant.
After having his project delayed a few months due to COVID-19, Winans hopes to complete the trials within the next year. The graduate student wants to give producers better knowledge about what’s growing in their fields. He says watching the waterhemp in the greenhouse has given him better appreciation for the need for weed management strategies that go beyond a single herbicide.
“Even though the different populations were all planted at the same time and given the same treatment, they didn’t all germinate at the same time, even in the same flat,” Winans says. “So, that shows you why it really does pay to put down a pre-emergent herbicide at planting to hold down that first flush of waterhemp until you can come back with your post application.”
The project also has reemphasized the need to rotate modes of action whenever possible — and to combine chemical control with mechanical control and other techniques, such as tillage, weed seed management and planting cover crops to suppress weeds.
In addition to his work in the greenhouse, Winans has another component to his graduate project — helping to study the effectiveness of a weed seed destruction implement called the Seed Terminator. The device is attached to the rear of a combine and uses multistage hammer mills to pulverize weed seeds present in the crop residue as it passes out of the rear of the machine.
Initial results indicate the Seed Terminator could be a powerful tool against herbicide-resistant weeds. When tested in four fields in 2019, more than 98 percent of the waterhemp seed was destroyed or damaged to the point that germination was unlikely. The research is funded in part by the Missouri Soybean Merchandising Council.
“Honestly, I never thought I would ever be planting waterhemp,” says Winans, who grew up in central Illinois and comes from a farming family. “But that’s just what I did. I think this project is pretty needed to give us better knowledge of weeds, the mechanisms of resistance and finding new solutions that go beyond herbicides.”
EDITOR’S NOTE: This is the fourth article in a yearlong series examining the past, present and future of weed control in the production of soybeans in the Show-Me State. This issue, we take a look at the science behind determining herbicide resistance in troublesome weeds.
Find the entire August issue here.