Waterhemp woes

By Dusty Sonnenberg, CCA, Ohio Field Leader: a project of the Ohio Soybean Council and soybean checkoff.

Waterhemp is a weed that some Ohio farmers have not experienced on their farms. Other farmers in Ohio have joined the ranks of those across the country who know it all too well, and wish they did not. Waterhemp is a weed that Ohio State University Extension personnel have been warning farmers around the state about at numerous agronomy meetings. The impact of waterhemp on soybean yields is very real. “If left untreated, it will compete with soybeans all season long, and can reduce yield by 44%,” said Jeff Stachler, OSU Extension Educator in Auglaize County, and Weed Specialist. 

Waterhemp is an annual weed with enormous genetic diversity. It begins emerging in early May and continues to emerge until late July. Waterhemp is a prolific seed producer.

“Most plants will produce at least 100,000 seeds per plant. Some plants can produce over 250,000-500,000 seeds per plant. It is possible that a single plant could produce over 1,000,000 seeds. That is why a field with a single weed escape can become a field that is totally infested in just a few short years,” Stachler said. “Waterhemp is a weed that grows quickly, however the seeds do not have great longevity. Approximately 95% of the seeds are gone in 4 years.”

On farms where waterhemp has just been discovered, getting control of the situation early is a key.

“This was a weed I had heard about at some meetings, and knew we did not want it on our farm. We first found it in some CRP ground down along the river when we were mowing. I think some seed must have gotten on the chopper, and when we brought it back to the farm and pressure washed it off, the water and the seeds ran into the field by the shop. Now we have waterhemp coming in that field,” said Jeff Duling who recently discovered it on his Putnam County farm. “I am also finding in on farms in some of the lower swales where I did not have the seeding rates and plant populations I wanted in my soybeans, and did not get the canopy early enough. I am now converting my air seeder to be able to variable rate population across the field, so I can plant higher populations where there is more opportunity to have the waterhemp in the future.”

A solid herbicide program is vital to overall weed management, and especially the control of difficult to control weeds.

“We have a pretty solid herbicide program we follow that involves both pre- and post- applications, but I cut the post- program back in the corn this year and reduced the rates because I wanted to inter-seed cover crops,” said Duling. “I had hoped that the cover crops would establish and crowd the later emerging weeds. That proved to be a mistake. Now I have late emerging waterhemp coming, and no good way to control it. I have been walking fields, and pulling it where I find it, but when you pull one, you typically find two to three more that are still coming.”

Late emerging waterhemp can be an issue on farms attempting to establish cover crops in July, after the wheat is harvested.

“We harvested the wheat then worked it lightly to incorporate a cover crop mix that contains rye, tillage radish, buckwheat, and rape,” Duling said. “Now in the swales where water would run across the field, I have late emerging waterhemp. I cannot spray it or I will kill the cover crop mix that is up, and mowing it just causes it to branch off lower and keep growing. Every one of those new branches is producing flowers and seeds. It’s a mess.”

One reason some farmers have implemented using cover crops is to aid in weed control.

Mark Loux OSU Soybean Researcher Field Leader
Dr. Mark Loux, OSU Extension State Weed Specialist

“A good cover crop stand will reduce weed pressure in the spring,” said Mark Loux, OSU Extension State Weed Specialist. “Cereal rye has allelopathic properties (release biochemicals into the soil) that inhibit the germination of other plants, especially some of the small seeded weeds like waterhemp and marestail. Waterhemp that would typically germinate early (in May) would be impacted by the allelopathy. One study showed that rye gave upwards of 40% to 50% suppression early in the season. Grass cover crops, or mixes that are primarily grasses still allow the farmer to go back with a broadleaf herbicide to try to control some of the late emerging weed pressure. It comes down to planning and management. A farmer needs to create both a cover crop plan and weed control plan that can work together at the same time.”

Another benefit of establishing cover crops is to take some pressure off the herbicide program.

“It is still advisable to have a herbicide program that involves both a pre- and post- herbicide application, but if the weed pressure is lower because of a cover crop, that takes some pressure off the herbicides. The goal of the pre- is to prevent emergence of weeds for a month or so after planting, and the post- program should contain a foliar herbicide to control existing waterhemp.  In some cases, there is a benefit to including residual in the post- for control the even later emerging weeds. Cutting either of these can have negative impacts on the effectiveness of the weed control. A good rye cover crop stand in the fall will help reduce marestail and other winter annual weeds,” Loux said. “Sometimes a farmer’s goal with cover crops is to be able to reduce herbicide use. We really are emphasizing the importance of late season weed scouting. Weeds are adapting, and we are seeing an increase in the herbicide resistance curve, and also an increase in the weed seed bank. Farmers need to be in a preventative mode when it comes to waterhemp and not let it get established.”

When designing a herbicide plan, there are several products available if a farmer is concerned with waterhemp.

“Preemerge options would include the Group 14 herbicides, which are PPO inhibitors such as flumioxazin (Valor etc) and sulfentrazone (Authority etc). The Group 5 PSII inhibitors such as metribuzin, as well as the Group 15 mitosis inhibitors such as metolachlor (Dual, etc.), Outlook, Zidua, Warrant also have activity. The Group 3, Mitotic inhibitors such as pendimethalin and Treflan, are also options,” Stachler said. “Most effective postemergence options in soybean are dicamba (Xtend), 2,4-D (Enlist), and glufosinate (LibertyLink, LLGT27, Enlist, ExtendiFlex).  A combination of two of these can be most effective. Most waterhemp populations in Ohio are resistant to glyphosate and many are resistant to site 14 herbicides (Flexstar, Cobra, etc), and growers would need to confirm lack of resistance to these prior to depending upon them.

“There are a few limitations to some of the post-emerge products. Metolachlor and Zidua need to be applied before the soybeans reach the third trifoliate according to the product label and Outlook needs to be applied prior to the fifth trifoliate stage in soybeans. Warrant can be applied up to R2. Farmers need to keep in mind that each of these products has a maximum total amount that can be applied per acre in a season, so if the product is used in multiple applications, that needs to be factored in.”

Farmers are also looking at the new traited soybean packages to give more options for weed control.

“I have grown the Pioneer Plenish high oleic soybeans — which are Round-up beans — the last several years and really like the premium. Unfortunately, having weeds that are glyphosate tolerant becoming more of an issue. I am looking at growing a different platform, such as the Enlist beans in the future,” Duling said. “I need to grow something that gives me more options for weed control later in the season.”

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