WISCONSIN DELLS — When UW-Madison Extension cropping systems weed scientist Rodrigo Werle asked a room full of farmers if they were concerned or dealing with waterhemp and/or giant ragweed in their corn and soybean fields, nearly every farmer raised their hand. And Werle wasn’t at all surprised.
“These are the two major weeds driving the way we manage crops in terms of weed control,” he said during his session at the Corn/Soy Expo in late January.
Waterhemp is the No. 1 weed found across the Midwest, with male and female plants each capable of producing more than a million seeds per plant. It can also be challenging to manage as the plant can adapt resistance over time, forcing farmers to use herbicides wisely.
In 2015, 15 counties in Wisconsin had confirmed glyphosate-resistant strains of waterhemp; in 2018, 28 counties had confirmed glyphosate resistance, with 10 counties also reporting confirmed protoporphyrinogen oxidase resistance.
Also just this past year, a six-way resistance was confirmed in Maryland.
“This thing evolves — and it evolves fast,” Werle said.
Because of this, Werle recently asked farmers to send waterhemp seeds to the university. While he expected to receive 30 samples, he was pleased to receive 90 samples from across the state, with plans to study resistance for glyphosate and PPO over the next few months.
“Hopefully, a year from now we’ll have some interesting information to present to you all,” he said.
Until then, he offered farmers a few tips on opportunities each season that they can take advantage of to lower the presence of waterhemp in their fields. In the winter, tillage can help, followed by an application of a pre-emergence herbicide in the spring. In the early summer, a post-emergence herbicide can help and in late summer, enhanced crop competition can keep waterhemp down. Then in the fall, there is potential for seeds to be destroyed with equipment — and if worse comes to worst, Werle suggested farmers take an hour and walk their fields, pulling weeds by hand and then burning the plants off-site.
Research has also suggested that cover crops, particularly rye, can delay the emergence of weeds, which could be another management practice farmers could incorporate to combat waterhemp in their fields.
Werle recommended farmers use a layered residual herbicide program, with a focus on pre-emergence sprays, a conclusion he came to after completing his own research on herbicide spray programs for waterhemp at UW-Madison. Pre-emergence herbicides can reduce the number of weeds that come up and make it less likely for those that survive to grow very large as the crop canopy will discourage that. Management past pre-emergence will present a challenge, he added.
For giant ragweed, Sarah Striegel’s graduate research suggested that a combination of pre- and post-emergence herbicides can prove the best management practice for this very competitive plant.
Giant ragweed is indigenous to Wisconsin, and has been classified as a noxious weed in three states. It’s one of the first weeds to emerge in the spring as it requires overwintering, which makes early control management important.
Striegel offered the following tips for early control management: start with a weed-free seedbed in fields by using tillage or burndown; use a pre-plant or pre-emergence control product; and apply a post-emergence product in two passes. Her research suggested that one pre-emergence pass won’t be enough, she said.
Unlike waterhemp, there are significantly fewer confirmed instances in Wisconsin related to resistance. There have only been two instances recorded in Wisconsin, while a few others have been confirmed in the Midwest.
The weed’s seed bank persistence is relatively short-lived as well, with seeds surviving two and four years, which allows farmers, with good management, to get those numbers down over time.
She recommended farmers know their emergence window each spring, start with as clean of fields as they can and use an effective pre-emergence program followed by an effective post-emergence program. Farmers can also rotate traits and herbicides and keep a close eye on the weeds so they do not go to seed.