By Steve Watson and Danielle Comstock

Cover crops may be able to suppress weeds in some cases, but that’s just one consideration producers should take into account when selecting cover crops for their farm fields, according to Kansas State University agronomists DeAnn Presley and Anita Dille.

Presley, a soil management specialist and Dille, a weed scientist, both with K-State Research and Extension, provided guidelines to use when considering cover crops for your field, with weed management as a primary goal.

“To plan for the greatest weed management benefit with cover crops, you should start by knowing when your key weed species of concern germinate and emerge,” Dille said. “Your goal should be to establish the cover crop prior to that key point in the lifecycle of the weed for most impact.”

Weeds that emerge between September and November, or winter annuals, are marestail, mustard species, cheat, and downy brome. An early spring weed is kochia, which emerges in April. Common sunflower, giant ragweed, and common lambsquarters are mid-spring weeds that emerge in early May. Weeds that emerge between May and June, or summer annuals, are Palmer amaranth, waterhemp, velvetleaf, foxtails, large crabgrass, barnyardgrass, and shattercane, Dille said.

Cover crops provide weed management benefits in a number of ways, she said.

“Living cover crops or a layer of cover crop residues will reduce sunlight reaching the soil surface. This will serve to smother and out-compete weeds for light, water, and nutrients,” Dille said.

In addition, cover crops have the effect of altering the moisture and temperature environment in the soil surface layer during weed seed germination and emergence. Some cover crop species also release chemicals from roots or decaying residue, which can inhibit weed seed germination.

If weeds or volunteer wheat do get established in cover crops, however, they become difficult if not impossible to control as long as the cover crops are alive, Dille said.

There are other factors to consider, including benefits and potential drawbacks, when deciding whether to plant cover crops, the specialists said.

Cover crops provide a range of possible benefits besides weed management, Presley said. They can often build soil organic matter which help reduce soil compaction and can prevent soil erosion. Plus, some cover crops can save fertilizer costs by scavenging nutrients or adding nitrogen to the soil through fixation.

They also conserve moisture by reducing evapotranspiration while using up excess  moisture when soils are very wet, she said. Cover crops protect water quality by reducing phosphorus runoff and they add diversity to the soil biosphere, she added.

There can be drawbacks, however, starting with costs.

“There are costs involved in the seed and equipment use for planting cover crops,” Presley said. “Planting and terminating cover crops may require one or more additional passes through the field, and those costs have to be considered.”

Cover crops can lead to unwanted use of soil moisture when soils are dry, which can cause problems for the following cash crop, Presley said.

When it comes to terminating the cover crop, the timing can be inconvenient and the ability to terminate can be a challenge in some cases, she added.

It’s important to consider ahead of time how to terminate the cover crop along with how any weed species present will be controlled, the specialists said.

“A residual herbicide may need to be included with the burndown application in some cases when terminating the cover crop chemically,” Dille said.

The standard recommendation is to spray/terminate the cover crops at least two weeks before planting corn or soybean crops in eastern Kansas. Check with crop insurance providers, U.S. Department of Agriculture-Farm Service Agency, or Natural Resources Conservation Service offices for local rules on termination timing, particularly in the western half of Kansas, Presley added.

Some cover crops will die out naturally from freezing and not require active termination.