New herbicide modes of action are few and far between, with almost 20 years since the most recent new herbicide and many more years before upcoming herbicides will receive approvals. During the 2019 Soybean School in Parsons, Kansas State University weed expert Dallas Peterson shared his most influential old-school management techniques to help producers through the new technology drought.
“We need to get out of this mind-set that we just wait until we see the weeds and then go out there and spray them and control them,” Peterson said. “We all know that weed management has become much more difficult, especially in soybeans, than it was 10 years ago.”
The ease, effectiveness and low cost of Roundup Ready technology eliminated the need for strategy in weed management and the result was ultimately herbicide misuse and weed resistance, Peterson said.
“Waterhemp was one of our first species in Kansas to develop a resistance to glyphosate,” Peterson said. “Prior to that point in time, it seemed like we had it made in the shade with Roundup and Roundup Ready technology but once that resistance happened we could no longer continue managing things the way we were.”
While weed control products often come in pre-emergence and post-emergence varieties, Peterson urged producers to think of weed control throughout their entire cropping system rather than one growth stage at a time.
“Some of the integrated weed management approach would include cultural practices, the use of sequential herbicide programs, incorporating residual herbicides, and utilizing multiple effective herbicide sites of action,” Peterson said. “Timing is everything because no other herbicides will work as effectively on big weeds post-emergence as glyphosate did.”
For producers focused on efficiency, controlling weeds can be much more cost-effective than not controlling weeds, Peterson said. In an integrated weed management program, crop rotation could play a vital role in supporting and extending the life of current technologies.
“I think crop rotation is the most powerful tool we have to help manage our weeds through time simply because we have different growing seasons and can use different herbicides through those different cropping systems,” Peterson said. “The more diverse and complicated your crop rotation is, the easier it is going to be to manage those weeds through time.”
Along with crop rotation and cover-cropping practices, Peterson said seed bank management should rank highly among weed control tactics.
“Another thing we lost sight of was seed bank management,” Peterson said. “The old timers appreciated seed bank management because they did not have the silver bullet to go out there and control those weeds.”
Peterson said seed planting date makes a difference on when weeds come up relative to the crop and when combined with thoughtful row spacing and plant population, it can be a useful management tool.
“You’re going to get good weed control up front whether you are in wide rows or narrow rows,” Peterson said. “When it gets close to canopy, of course, a narrow row will canopy quicker than a wide row and that’s when row spacing really comes into play for weed control.”
Creating a dark environment for weeds is one of the key benefits of narrow rows, as well as cover cropping systems. While using cover crops for weed suppression is highly variable — depending on the weed species producers are seeking to control and the type of cover crop they are using — against some weed varieties, a dark environment can be a valuable tool.
“Pigweed actually requires a flash of light on that seed to stimulate germination,” Peterson said. “If you go out and cultivate in the middle of the night versus in the middle of the day, you’ll have more pigweed come up after the daytime cultivation versus the nighttime cultivation.”
In the era of herbicide resistance, proactive scouting becomes an essential weed management tool. With most post-emergence herbicides losing effectiveness after a weed is 4 inches tall, catching and eliminating weeds early is essential.
“Timing is obviously very critical from a standpoint of herbicide performance,” Peterson said. “It’s not true just for our post treatments but it is also true for our residual herbicides and we have to rely more on our residual herbicides.”
For post-emergence treatments, Peterson recommended treating within 21 days after planting for the best control of burgeoning weeds.
“With our post-emergence burndown-type treatments we know that those work better on smaller weeds than bigger weeds,” Peterson said. “That’s because not only are they more susceptible but we also get better coverage if those weeds are small.”
For Peterson, sound weed management is a holistic approach that considers many factors throughout a cropping season to give the best chance of producing an abundant and efficient crop.
“Anything we can do to help that crop get a competitive advantage on the weeds is going to work to our benefit,” Peterson said. “Having the pH where it needs to be, good soil nutrients, planting at the proper time, getting good seed placement for quick establishment — all of that will help us in terms of suppressing those weeds.”
