Managing no-till systems with cover crops to lower herbicide use
by Karl H. Kazaks
With the price of glyphosate and other herbicides so much higher than in recent years – and in more limited supply – finding ways to reduce herbicide inputs in no-till systems is on the minds of many in the ag industry.
Recently, NRCS hosted a webinar on managing cover crops to lower herbicide use. The feature presenters were Dr. John Wallace, assistant professor of weed science and Extension specialist at Penn State, and Dr. Mark VanGessel, professor of weed science and crop management and Extension specialist at the University of Delaware.
Though there are a variety of factors which determine whether usage of cover crops will lead to a reduced population of weed species (and thus a lower herbicide requirement), “optimal management of cover crops can lead to lower herbicide use,” said Wallace.
Two primary management goals in cover crop usage in no-till systems, Wallace said, are the optimization of biomass production and the rapid establishment of a cover crop stand.
He suggested a few tactics to attain those goals. First, understand growing season windows and look for opportunities to lengthen those windows. If you have an early seeding window, use it. Also, at the tail end of the cover crop season, delayed termination may also help in controlling weed populations.
Seeding method is another tactic Wallace suggested producers consider in evaluating their cover crop management. “Drilling will typically get a more consistent stand than broadcasting,” he said. The termination method may also play a role – is it more effective to leave the crop standing dead or use a roll crimper to create a mulch layer?
This photo shows the effectiveness of cover crops in managing weed populations. The left side of this corn silage field was interseeded with annual ryegrass, so there was a crop growing in early fall when marestail starts growing. The right side of the field did not have a cover crop and the following spring experienced strong marestail pressure. Photo courtesy of Corey Dillon
Species selection is another tool in the producer toolkit. “Fall-seeded cereal rye has proven hard to beat when it comes to weed suppression,” Wallace said. “It can be established late, is winter hardy and has a high biomass potential.”
When planting cereal rye, Wallace suggested, “Follow the combine with the drill. Get as many fall growing degree days as possible.”
What’s more, delaying the termination of cereal rye can be a real benefit. A delay of 10 to 14 days “can double or more than double total biomass of cereal rye,” Wallace said.
Knowing what weeds need to be suppressed will help you tailor your cover crop management. The size of the weed seed is an important factor. Small-seeded weed species are typically more prolific seed producers. But, because they are smaller, they have a harder time getting established in a cover crop mulch. Species which are less suppressed by cover crops include large-seeded annuals and perennials, especially taprooted perennials, Wallace said.
Many of the herbicide-resistant weed species, like horseweed, Palmer amaranth and common waterhemp, are small-seeded annuals. Because they’re small-seeded, producers have the potential to use cover crops to manage those weeds.
VanGessel elaborated on best practices in the termination of cover crops. “The optimal time to terminate is the early reproductive stage,” he said. “Too late, and you will have seeds. Too early, and you’ll get regrowth. For cereal rye, that window tends to be fairly narrow.”
Multi-species cover crop mixes are a challenge because they have differing maturities – you might be early to terminate one but late to terminate another. What’s more, while a mixed grass/legume cover crop will provide higher nitrogen availability in soils, there is shorter residue persistence as compared to grass monoculture cover crops, so that’s a factor to consider in your management.
Roller crimpers are preferred to mowers, VanGessel said. “We have in our experiments limited success with mowing. A shredded cover crop doesn’t provide mulch for weed suppression, and you can also get regrowth.”
When terminating with herbicides, which chemical you use depends on what cover crop you grew. When terminating later in spring – closer to cash crop planting – “oftentimes you have better environmental conditions for spraying and a successful kill,” VanGessel said.
When you delay cover crop termination, you tend to get more cover crop residue. That can be an issue when planting, VanGessel warned. Make sure your planter is adjusted for the cover crop residue. “Does it have enough weight?” VanGessel asked. “Enough hydraulic downforce to cut through cover and get seed placed properly in the soil?”
It’s easier to plant into “dead or green cover than dying cover,” VanGessel said. That’s because a dying crop doesn’t have the stiffness of a green or dead cover crop. If planting into a growing cover crop, make sure your discs are good and sharp.
Delayed termination of cover crops can have effects in the growing season. For example, soybean seedlings grow taller when planted into a field of cover crops which were terminated later. Also, there could be an effect on soil moisture. With delayed termination, there could be lower soil moisture at planting (as the cover crops are holding a good bit of moisture). But the soil moisture could be higher later in the season due to cover crop mulch.
Cover crops can help your growing season herbicide practices, VanGessel said. Cover crops tend to slow weed emergence and also weed growth rate, which means you’ll have a wider window of opportunity to apply post-emergent herbicides.
If row cleaning is in your practices, it may not lead to lower herbicide usage. That’s because there is a connection between row cleaning and in-season weed pressure.
The bottom line, VanGessel said, is that there are a lot of factors to consider when you incorporate cover crops into your no-till growing system. As he put it, “using cover crops requires more planning.”
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