Effective weed management is a significant challenge for farmers as herbicide resistance increases.
Kurtis Pilkington, a customer solutions agronomist for Bayer CropScience, said that against herbicide-resistant weeds, particularly waterhemp, keeping fields weed-free for the entire growing season is critical.
“A planned two-pass herbicide program enables a grower to start clean with a pre-emergence residual herbicide and stay clean throughout the season with a post-emergence application to tackle weed escapes and late-emerging weeds,” Pilkington said.
Why it Matters: Experts increasingly recommend a two-pass herbicide program, not only to address resistance but also to account for weather variability that can affect a pre-emergent herbicide’s activation.
Using multiple and different herbicide modes of action between application timings also helps to control resistant weeds and manage against further resistance.
A two-pass approach is key for weed seedbank management, ensuring that any weed escapes or late emerging weeds don’t go to seed increasing weed problems in future years, Pilkington said.
This is a key component of herbicide resistance management: making sure any weeds that may have survived the initial herbicide application don’t set seed.
“Keeping fields weed-free with a second herbicide application can assist with harvest efficiency, eliminating green weed stems at harvest and ensuring that harvest equipment doesn’t spread weed seed,” he said.
Crops have a critical weed-free period (emergence until third trifoliate for soybeans; emergence until six-leaf stage for corn) and weed competition during those periods can hurt yields.
Using a pre-emergent residual herbicide, Pilkington said, growers can protect against yield loss at those times — an approach he said also helps prepare for the unexpected.
“Pre-emergent herbicides can underperform in situations of less-than-ideal weather conditions, particularly dry weather after application. A planned two-pass program can help counter an unanticipated situation where a pre-emergence application fails,” he said.
Costs and benefits
While a two-pass approach to weed management can be more costly than a one-pass program, the agronomic benefits add value that may not be captured strictly in yields.
Dr. Peter Sikkema’s lab group at the University of Guelph ran research into the yield and profitability advantages to a two-pass versus a one-pass program in glyphosate-resistant corn, and found a two-pass program of a residual pre-emergence herbicide, followed by a late post-emergence herbicide application, provided the most effective and profitable weed management.
Outside of Ontario, a review of corn herbicide trials at the University of Missouri found that of 61 trials, a two-pass program that included a pre-emerge herbicide followed by a post-emerge herbicide provided the highest corn yields 67 per cent of the time.
Pilkington said a two-pass herbicide system can be beneficial with a general weed spectrum, but is particularly effective when dealing with weeds that emerge throughout the growing season.
“A problematic example in Ontario is waterhemp, which begins to emerge in May and can continue to germinate into the fall. A planned two-pass approach can help combat later-emerging plants,” he said.
Pilkington recommended starting clean by targeting the first application for pre-plant through to early post-emergence (depending on product) and taking advantage of residual products to provide a long-lasting protection.
A second pass as an in-crop application toward the later end of the safe application window would go to tackle any escapes or later-emerged weeds, he said.
“An example of this approach in Xtendflex traited soybeans would be to apply Roundup Xtend 2 with VapourGrip Technology pre- or early post-emergence to provide burndown of present weeds along with short-term residual control. Then follow up in-crop at first flower with Roundup Transorb or Liberty to control later flushes of weeds.”
Before beginning a two-pass management protocol, Pilkington suggested farmers scout fields to understand the specific weed pressures.
Stay flexible, he added: if the first herbicide application achieves effective control, a second pass may not always be needed.
Also, he said, always double-check labels to confirm application timings for crop safety and effective weed control — also to confirm tank-mix compatibility, and if the chosen products need adjuvants or surfactants.
More reading
Soltani, N., Nurse, R.E., Gillar, C.L., and Sikkema, P.H. 2013. Weed control, environmental impact and profitability of two-pass weed management strategies in glyphosate-resistant corn. The Open Plant Science Journal. 7:31-38. https://benthamopen.com/contents/pdf/TOPSJ/TOPSJ-7-31.pdf.
Bradley, K. 2014. Two-pass corn herbicide programs are almost always the best. University of Missouri Extension. https://ipm.missouri.edu/IPCM/2014/3/Two-pass-Corn-Herbicide-Programs-are-Almost-Always-the-Best/.
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