Since many postemergence herbicides have already been applied, now is a good time to begin evaluating the effectiveness of our weed control measures in order to plan better for future years. Evaluation following postemergence applications should include scouting of fields through the end of the growing season, notation of problem weed species and their locations, and careful consideration of opportunities to diversify weed management in soybean fields for future years.
Strong herbicide programs
While herbicide programs are an easy fix for most farmers, there is probably room for improvement in most herbicide programs to better manage problem weeds in both corn and soybean. A recent study by Iowa State University weed scientist Prashant Jha evaluated the effect of the prior year’s level of weed control, soybean row spacing and inclusion of a cereal rye cover crop on control of waterhemp in soybean.
The study showed that the number of weed escapes in corn played an enormous role in the success of controlling waterhemp in soybean. Waterhemp emergence was reduced by 75% in plots, with complete control in corn, compared to plots with marginal control the previous year. These results clearly illustrate the importance of looking at weed management as a long-term effort — how well you manage weeds this year directly affects your success in the future.
If you have struggled to manage waterhemp in the past, it is critical to determine why your current programs are failing. An effective preemergence program should provide at least three to four weeks of weed control. If your program isn’t providing that, consider these factors: product choice, rate, potential for herbicide resistance, and weed populations. If the product is correct for the weeds and soil type, then carefully evaluate the rate of that product. If the rate is sufficient to provide residual control, then consider the weed population you’ve got in the field.
Two things to consider are whether the weed population is resistant to the herbicide, or whether your population density is so high that it is overwhelming the herbicide. Use of multiple effective sites of action (herbicide groups) in the preemergence program and a layered soil residual program is one important strategy to reduce waterhemp populations, and, ultimately reduce the number of seed that can germinate next year.
If your problem seems to be more due to waterhemp surviving the postemergence program, factors to consider include: product choice, rate, size of waterhemp at time of application, additives, spray coverage, weather conditions during application, and potential for resistance. Without determining why the current program isn’t working, you will be shooting in the dark to try and fix it.
One thing we’ve learned over the past few decades is that a strong herbicide program alone is not enough to combat waterhemp. A near total reliance on herbicides has led to a quickening appearance of multiple-resistant waterhemp across the state. Now is a good time to consider what other integrated weed management tactics might work on your farm in future years.
Growing soybeans in rows narrower than 30 inches, such as 15-inch rows or drilled rows, means closing the soybean canopy earlier in the growing season. Quicker canopy development reduces the number of late-emerging weeds able to survive in the shade of the soybean. Weeds that do survive will produce less seed. Since waterhemp is a later-emerging weed, this management tactic is a useful option to pair with a strong herbicide program to reduce the competitiveness of waterhemp with soybeans, and ultimately reduce its seed drop for future years. Jha’s research found that 15-inch soybean rows reduced waterhemp emergence by 15% and waterhemp growth by 40% compared to 30-inch rows.
Cover crops as weed suppression
A cereal rye cover crop can provide many benefits, but it requires specific management when used for weed suppression. If a priority of using a cover crop is to gain weed suppression benefits, it must be managed to produce sufficient biomass to smother weeds. Farmers should prioritize timely seeding, getting a uniform stand, and delaying termination until the cover crop develops enough aboveground growth to control weeds.
Jha’s study conducted in 2020 showed that soybean planted into a stand of cereal rye with approximately 4,600 pounds per acre biomass (May 22 termination date) had 30% fewer waterhemp plants. Growth of waterhemp that emerged in the cover crop was reduced by 75%, compared to waterhemp with no cover crop. Farmers who have historically grown no-till soybeans may find this is an easy change in their system and can see real benefits from this management change.
While these are just two possible strategies – narrow rows and cover crops – to help protect the value of herbicides, there is more innovation happening that will help us diversify weed management in future years. Even with more innovation coming in the future, NOW is the time to evaluate what changes you can make on your farm to better manage waterhemp and other problem weeds. To learn more about Jha’s research, read this recent Integrated Crop Management News article about using cover crops and narrow-row soybeans to manage herbicide-resistant waterhemp.
Anderson is a field agronomist with Iowa State University Extension.