Monitor Corn Crop Environment to Improve Residual Weed Control
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When Mother Nature or irrigation provides ½ to 1 inch of timely rainfall within 10 to 14 days to activate preplant or preemergence herbicides, corn wins against weeds. But what environments allow weeds to gain the upper hand, and why?
Like most years, the spring of 2023 delivered everything from cool and wet environments causing issues across the upper Midwest to more severe drought areas lingering from Nebraska into the southern plains and Missouri.
“Dryland crops in Nebraska currently suffer from lack of moisture that is not activating preemergence residual herbicides,” says Amit Jhala, University of Nebraska Extension weed management specialist. “In southern Minnesota, you have the opposite extreme of flooded fields that leach herbicides out of the top 1 to 1.5 inches of topsoil where weeds germinate.”
Scout Fields
In both cases, farmers likely await a more moderate environment to apply an early postemergence herbicide application. Jhala recommends crop scouting to examine residual herbicide performance to determine what weeds are breaking through.
Dry conditions cause positively-charged herbicide molecules to attach to negatively-charged soil particles (called adsorption). That’s why rainfall is needed to move herbicides from the soil surface into the seedling zone so the weeds can absorb them. In addition, water-stressed weeds are tougher to control as plant uptake of foliar postemergence herbicides is slow.
In wet conditions, negatively charged herbicides won’t bind to negative soil particles, causing them to be more water-soluble and leach out of the 1- to 1.5-inch weed seed germination zone.
Other environmental factors that can impact residual herbicide effectiveness are soil organic matter, texture and pH. For example, surface-applied herbicides can be subject to photodegradation thanks to sunlight on drier sandy soils that are lower in organic matter.
Check Product Labels
“Soil pH is relatively less important than organic matter since organic matter and soil texture determines the application rate used for some residual herbicides,” Jhala says. “That’s why it’s always important to refer to the product labels of each component in a premix or tank-mix.”
Some herbicides can last longer or degrade more rapidly when soil pH is outside the neutral 6.0 to 7.0. For example, the triazine (Group 5) and sulfonylurea (Group 2) chemical families persist longer when pH is greater than 7.0, allowing for more plant uptake. Conversely, imidazolinone (Group 2) herbicides gain persistence and greater uptake when pH is less than 6.0.
In addition, residual herbicides can also break down due to soil microbes that feed on herbicides, reducing residual control. High organic matter, lower pH, warmer soil temperature and excessive moisture cause microbial degradation.
Corn crop safety is a lesser concern with corn residual herbicides that can be applied early postemergence compared to soybean residual products like sulfentrazone, chlorimuron or flumioxazin that must be applied within three days of planting. And even though herbicide-resistant weeds may not be deemed an ‘environment,’ it is vital to grasp the weed spectrum strengths and weaknesses of corn residual herbicides while alternating their use.
Postemergence Environment
Crop safety in corn plays a role when shifting to postemergence weed control, and soil attributes take a back seat. “Broadcast corn herbicides have different corn height maximums to reduce crop injury, so check out these label restrictions,” Jhala says.
Wind becomes a significant factor in postemergence herbicide applications to achieve weed coverage and not drift off target. “Growers should not apply herbicides when wind speed exceeds 12 mph. For dicamba-based herbicides, wind speed should be between 3 and 10 mph,” Jhala adds.
Be sure to check weather forecasts when planning a postemergence spray due to the rainfast intervals needed to get the product into the weed. For example, glyphosate-based herbicides require one hour of no rain after application, and glufosinate-based herbicides are rainfast in four hours.
Watch Temperature Inversions
The other critical environmental factor to monitor is air temperature inversions. This summer occurrence happens when cooler air is near the ground with warmer air above. It often begins around two to three hours before sunset and dissipates 30 minutes to two hours after sunrise. This event is often associated with calm mornings and evenings (0 to 3 mph winds).
Inversions cause fine spray droplets of low-volatility formulations like dicamba to stay suspended and move off-target to harm sensitive plants.
Be sure to add the label-required/suggested adjuvants to the tank-mix to improve weed control efficacy. “One caution is that these adjuvants cannot overcome temperature inversions or high wind speeds. Their task is to help the herbicide enter the weed leaves’ cuticles to reach the plant’s target site,” Jhala says.
Finally, keep good records of these environments each year by field. Knowing what worked, or didn’t work, provides valuable knowledge when similar future issues arise.
Content provided by DTN/Progressive Farmer.
Additional resources:
2023 Guide to Nebraska Weed Management
2023 North Dakota Weed Control Guide
2023 Weed Control Guide – Ohio, Indiana, Illinois and Missouri
2022 Herbicide Guide for Iowa Corn and Soybean Production
Herbicide Classification Guide
Take Action Herbicide-Resistance Management
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