Utilizing Cover Crops For Weed Control: Be Sure To Consider All Aspects

Sep 26, 2016

By DeAnn Presley

Soil Management Specialist

Cover crops may be able to suppress weeds in some cases, but this is just one of many considerations that producers should take into account when selecting cover crops. The following are some questions to guide you when considering cover crops for your field, with weed management as a goal.

What are the potential benefits and costs of cover crops?

Cover crops provide a range of possible benefits:

Reduce fertilizer costs by scavenging nutrients or adding N to soil through fixation
Reduce or prevent soil erosion
Reduce soil compaction
Conserve moisture by reducing evapotranspiration
Use up excess soil moisture when soils are very wet
Protect water quality by reducing phosphorus runoff
Provide weed management benefits
Adding diversity to soil biosphere

Be aware of the costs associated with cover crops:

Seed cost and equipment to plant
Requires one or more additional passes through the field for planting, terminating
Unwanted use of soil moisture when soils are dry
Can becomes a volunteer weed
May be difficult to control volunteer wheat or other pest problems
Timing of termination can be inconvenient and ability to terminate (mowing, tilling, rolling, spraying, etc) can be a challenge in some cases

You’ll need to match the choice of a cover crop with your specific goal(s).

How will you plant it, and when?

Consider the crop rotation that you have planned and determine the best time to seed and establish the cover crop. For the greatest weed management benefit, know when the key weed species you are targeting germinate and emerge. Establish the cover crop prior to that key point in the lifecycle of the weed for most impact.

Classification of weeds based on emergence timing:

Sept – Nov (winter annuals) – marestail, mustard species, cheat, downy brome

Very early spring (April) - kochia

Mid-spring (May) – common sunflower, giant ragweed, common lambsquarters

May – June (summer annuals) – Palmer amaranth, waterhemp, velvetleaf, foxtails, large crabgrass, barnyardgrass, shattercane

How can cover crops help control weeds?

Ways that cover crops can provide weed management benefits:

Living cover crops or a layer of residues will reduce sunlight reaching the soil surface; smother and outcompete weeds for light, water, and nutrients
Alter the moisture and temperature environment in the soil surface layer during weed seed germination and emergence
Some cover crops release chemicals from roots or decaying residue, inhibiting weed seed germination
Improve overall soil health and benefit crop growth and vigor to compete effectively against weeds

Figure 1. A cover crop mixture of oats and spring peas. All photos by Anita Dille, K-State Research and Extension.

Figure 2. Palmer amaranth in field without cover crop.

Figure 3. A cover crop mixture of oats and spring peas has suppressed Palmer amaranth growth.

All three photos above were taken May 12, 2015 before termination and spraying with either glyphosate, or glyphosate with a residual herbicide. Many of these Palmer amaranth are glyphosate-resistant, so some would survive the glyphosate-only termination method, but there would be a better chance of control with the glyphosate-plus-residual treatment. Also, the cover crop residue remaining on surface and the subsequent soybean crop no-till planted into that residue provided further suppression of the weeds. The Palmer amaranth was much larger by in the field without a cover crop at the time of these photos, and Palmer amaranth is more difficult to control when it gets bigger. In the field with the cover crop, there were fewer Palmer amaranth plants and the plants were smaller, easier to control, and smothered by the cover crop after terminating.

What will precede and what will follow the cover crop in your rotation?

Some cover crops tie up nitrogen, so it is important to consider the carbon-to-nitrogen ratio of the cover crops being considered. This will influence the rate of residue breakdown and future release of nutrients in the subsequent crop.

Some pre-emergence herbicides applied in the spring before corn or soybean, can persist into the fall, impacting the establishment of some cover crop species, causing some injury, and sometimes loss. The length of residual activity will be influenced by herbicide chemical properties (half-life) and environmental conditions, such as rainfall, temperature, soil pH, soil type, and soil organic matter. Conduct a soil bioassay if uncertain about soil-applied herbicides and your cover crop of choice.

Which cover crop will you plant?

Depending on your geographic location, many options are available. Resources:

Midwest Cover Crops Councils’ Cover Crop Decision Tool; data available for Kansas
Managing Cover Crops Profitably, 3rd Edition, SARE publication
Integrating Cover Crops in Soybean Rotations, Challenges and Recommendations for the North Central Region. Published by Midwest Cover Crops Council.

There are many questions about whether a single species or a mixture is most beneficial for weed suppression. The key aspect is trying to achieve enough biomass by the cover crop to minimize weed growth.

How will you terminate your cover crop?

Consider how to terminate the cover crop along with what will need to be done to control any weed species present. A residual herbicide may need to be included with the burndown application in some cases. Some cover crops will die out over the winter and leave residue on the soil surface (e.g., mustards, peas, spring cereals). Others may require some active methods to terminate, with proper timing being important. For example, to terminate overwintering cereal rye or wheat, apply glyphosate and a residual herbicide at 9 to 12” cover crop height, or use a roller/crimper at the soft dough stage. For perennial clover, treat with herbicide tank mixes (2,4-D, glyphosate, and a residual herbicide) 2 to 4 weeks before planting the following cash crop. A roller/crimper is not effective on clovers.

The standard recommendation is to spray / terminate the cover crops at least 2 weeks before planting corn or soybean crops in eastern Kansas. Check with crop insurance providers, USDA-FSA, or NRCS offices for local rules on termination timing, particularly in the western half of Kansas.

Source: ksu.edu