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Terminating Cover Crops Before They Become Weeds

Spring grass cover crop Figure 1: Photo credit of University of Nebraska - Lincoln
  • A burndown program will be crucial to successful termination of cover crops not killed by winter temperatures.
  • The species of cover crops present determines what herbicide and rate to use.
  • Profit is maximized when cover crops are properly terminated.

Cover crops provide solid agronomic benefit regarding soil erosion, nutrient scavenging, winter annual weed suppression, soil health, and residue management. However, they can become weeds if not terminated correctly. Penalties for improper termination include additional herbicide expense, delayed planting, and establishment of a “weedy” seed bank that must be managed in the future. This article discusses options for setting up for successful cover crop termination for broadleaf and grass cover crops.

Broadleaf Cover Crops

Early March, just as temperatures begin increasing, is an ideal time to scout and assess the winter survival of cover crops. While most broadleaf cover crops winterkill, this does not occur for some species, such as crimson clover, vetch and Austrian winter peas.

The key to successful termination of surviving broadleaf cover crops is to avoid relying solely on glyphosate as your chemical means of control. The addition of a growth regulator such as 2,4-D or dicamba may provide more consistent control and another effective site of action for resistance management.

Grass Cover Crops

Grasses fall into two major groups—annual ryegrass and anything but annual ryegrass. While glyphosate is probably the most widely used herbicide to manage all these grass species, herbicide rate and timing may vary depending on the grass species present.

Annual ryegrass can become difficult to terminate if it becomes too large. The standard recommendation for annual ryegrass is to apply glyphosate at 1.25 lb ae/acre (Roundup PowerMAX® 3 at 33 fl oz/A) with the appropriate adjuvants to plants less than 6” in height, and actively growing.

Research suggests that the ambient air temperature must be above 45°F for 3 consecutive days prior to the application for more consistent control. If annual ryegrass is taller than 6”, increasing the rate of glyphosate is necessary to get control. In terms of growth stage, it is best to control annual ryegrass prior to jointing. 

We have a little more flexibility with other grass species, including cereal rye. The standard glyphosate recommendation is to apply at a rate of 1.13 lb ae/acre (Roundup PowerMAX® 3 at 30 fl oz/A) with the appropriate adjuvants to stands less than 18” tall.

Again, if the cover crop is taller, the rate of glyphosate may need to be increased for optimum control. The addition of saflufenacil (Sharpen®) to a tank mix with glyphosate can help improve control of grass cover crops. Refrain from using photosystem II (PSII) inhibitors (atrazine or metribuzin) with glyphosate due to the antagonistic effects resulting in poor control.

Regardless of grass cover crop species, there are additional herbicides that can be used in addition to or in place of glyphosate. For example, the use of paraquat-containing products will provide control of grass cover crops.

One thing to keep in mind is that either COC or MSO must be tank mixed with PSII inhibiting herbicides (e.g. atrazine for corn or metribuzin for soybeans). If these adjuvants are left out, you may not obtain adequate activity.

When using paraquat, nozzle selection is important to provide adequate control of the species. Repeated applications may also be necessary the larger the cover crop gets.

Are there other ways we can terminate cover crops aside from using chemicals? Absolutely! Cultural methods such as mowing, tillage, and crimping can all aid in cover crop termination. Mowing is only successful if the cover crop is cut at the right timing, when the growing point is high enough that the mower can cut it off.

This can lead to more above ground biomass that may be harder to plant into; however, the excess residue may be beneficial for weed suppression. Tillage may be effective if the cover crop is small, but keep in mind that tillage may defeat the purpose of what a cover crop was intended for in the first place (i.e. erosion control, nutrient cycling, building organic matter, etc.).

Lastly, crimping allows the above ground biomass to be used as a thick weed barrier. These methods all come with benefits, but also with have downfalls, such as more trips needed across the field, additional equipment, and a potential harbor for certain insect and animal pests and plant pathogens.

The key to successfully terminate cover crops is to manage them with the right herbicide while they are still small. Complete termination is the key to beginning a successful cash crop.

References

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