Research HighlightsHow Residual Herbicides Handle Heavy Rain
In this article, you’ll find details on:
- Intense rainfall events are increasing, especially in the Northeastern U.S.
- Soil residual herbicides, valuable tools to manage against weed resistance, rely on rainfall for activation. However, heavy rains can reduce the effectiveness of more soluble active ingredients.
- Cover crops help suppress weeds, but they also can intercept residual herbicides, which need to get into the soil to work well
By Laura Temple
When it rains, it pours — more often than it used to.
Weather data shows that extreme precipitation events are increasing, says Carolyn Lowry, assistant professor of weed ecology and management at Penn State.
“The Northeast has seen the greatest increase in recent years compared to other parts of the country,” she explains. “Our region has experienced a 40% increase in 2-inch rain events.”
As a weed ecologist, Lowry explores how changes like this affect weed communities and weed control, especially early in the growing season. She notes that more extreme spring rains could increase populations of problem weeds, due to reduced weed management efficacy.
“Many farmers rely on soil-applied pre-emergence herbicides because of resistance to other herbicides,” she says. “To be effective, these herbicides need to persist in the soil. But do heavy rains impact efficacy?”
With support from the Pennsylvania Soybean Board, she is exploring how heavy rainfall influences residual soybean herbicides that rely on rainfall for activation and how these factors interact with cover crops.
“We constantly promote integrated weed management to help buffer against weather extremes, but it is hard to test that in the field,” she says. “My lab specializes in doing in-field climate manipulations to study how agronomic management responds to weather variation.”
In the plots for these trials, her team planted soybeans along with weed seeds like smooth pigweed and giant foxtail to ensure a noticeable level of weed pressure.
Simulating Intense Rains
Lowry and her team compared the performance of several pre-emergence herbicides for several weeks after planting under three different scenarios.
- All treatments, including the controls, received ambient rainfall for the season.
- One set of treatments received 5 inches of simulated rain in one day.
- Another set of treatments looked at frequent events, receiving two simulated rains of 2.5 inches in one week.
In the field, one-meter-square rainfall simulators mimic intense rains.
“The small area allows us to control water applied, and allows us to observe results,” she says. “However, it is still challenging to extrapolate what we see to the field level.”
Treatments included pre-emergence herbicides from Group 14, PPO inhibitors, and Group 15, very long-chain fatty acid synthesis inhibitors. All the Group 14 options, including sulfentrazone in Authority and flumioxazin in Valor, proved very effective regardless of rainfall conditions.
“We did see differences in weed control efficacy from Group 15 herbicides based on water solubility,” Lowry reports. “More soluble active ingredients appear to wash away more easily under intense rainfall. However, we only have one year of data. We need at least one more season of observation before making recommendations, but that may become a characteristic for farmers to consider.”
Her Group 15 treatments included highly soluble dimethenamid-P in Outlook, S-metolachlor in Dual II, which has medium solubility, and the less soluble pyroxasulfone in Zidua.
Evaluating Pros and Cons of Cover Crop Mulch
Lowry and her team are also testing how residual herbicides and heavy rains interact with cover crop surface residues to impact weed control.
“We know cover crop residue provides backup weed suppression,” she says. “It is unclear how wet soils combined with extreme rain events will affect residual herbicide efficacy.”
All the treatments described previously were repeated in plots that had a cereal rye cover crop terminated two weeks before planting, as well as on bare ground.
“Other research shows that the residue intercepts a residual herbicide, keeping it from hitting the soil,” Lowry continues. “In the case of heavy rainfall, will that increase or decrease weed control?”
The first year of data does suggest that cover crops interact with residual herbicides and intense rain.
“Overall, we saw the greatest reduction in weed control when extreme rainfall was simulated on the more soluble herbicide,” she says. “The effect was exacerbated by cover crop surface residues. However, weed control remained consistently effective with the less soluble herbicides under extreme rainfall, even with cover crop present.”
She would like at least one more year of data to see if that’s a real trend or not.
Lowry believes that as more farmers use cover crops, this research will help them understand how to take advantage of the benefits of the practice while minimizing potential drawbacks.
“We know it’s important to preserve efficacy of the herbicide tools we have,” she adds. “Understanding how they work with integrated practices and changing weather patterns will help.”
Additional Resources
- Weed control research – SRIN page
- Cover Crops Do’s & Don’t’s – Crop Protection Network
- Cover Crops and Residual Herbicides Team Up Against Palmer Amaranth – GROW article
- New Research on How to Apply Residual Herbicides When Planting Green – GROW article
Published: Sep 24, 2024
The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.
