Cover Up

Cover crops as a weed management tool

Published in the April 2014 Issue Published online: Apr 20, 2014
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Cover crops are crops grown between harvest and planting of cash crops, usually not for harvest, but for the production of biomass and the various agroecological benefits this additional biomass can provide. While cover crops are most often thought of as a means of preventing erosion and improving soil health, they can also be applied as an effective weed management tool.

Cover crops can suppress weeds in four primary ways:

•By diversifying and filling gaps in a crop rotation;

•By competing with weeds for light, nutrients and moisture;

•By releasing chemicals that inhibit the germination and growth of weeds (allelopathy);

•By attracting beneficial organisms that feed on weeds and weed seed.

The following will expound a bit on how each of these suppressive mechanisms function and can be successfully applied.

Diversifying and Filling Rotation Gaps

Diverse crop rotations suppress weeds by forcing them to grow in different crops with varying life cycles, growth habits, spatial arrangements and management requirements. Rotating dissimilar crop species also allows the application of a wide range of control measures and exposes weeds to more natural mortality factors. For example, adding a year or two of a perennial legume crop like alfalfa between potato crops can reduce the germination of annual weeds normally triggered by soil disturbance and also permits regular mowing that can suppress problem perennial weeds like Canada thistle.

It is also important to remember that many weeds are pioneer species which tend to establish quickly and thrive in disturbed habitats. While mechanical control methods like cultivation can knock back weeds for a period of time, disturbing the soil essentially resets the successional clock, bringing new weed seed to the soil surface and initiating more weed germination and growth. It is certainly possible to till often enough to keep weeds down, but tillage degrades soil health, and equipment is costly to operate.

As an alternative, cover crops can be grown in short windows before and after cash crops to fill gaps in a rotation, minimizing exposure of bare soil and limiting the ability of most weeds to become established. For example, a 1995 study by Rick Boydston and Ann Hang found that fall-planted rapeseed reduced weed biomass in the following year’s potato crop by an average of 73 percent compared to potatoes after fallow.

Weed Competition

A vigorously growing cover crop, whether alone or interplanted with a cash crop, can prevent weeds from germinating and/or growing to the point that they produce seed or compromise cash crop yields. However, successfully applying this strategy requires knowledge of weed ecology, careful selection of cover crop species, and favorable conditions for cover crop establishment and growth.

Unfortunately, the competitive impact of cover crops is not restricted to weed species. Research has shown that cover crops capable of out-competing weeds will likely also suppress an interplanted cash crop. In the case of living cover crops, cash crop suppression is most often the result of competition for water. Cover crop residues can also negatively impact the establishment of a cash crop. This is usually related to physical interference with seed placement in the soil, prevention of soil warming, tie-up of nitrogen, or the release of allelopathic compounds.

Releasing Chemicals to Inhibit Weed Growth

Some cover crops produce compounds that inhibit the germination and growth of other plants, including weeds and crops. This property is known as allelopathy. Known allelopathic cover crop species include cereal rye, hairy vetch, crimson clover, buckwheat, alfalfa, barley, rapeseed, oat and sorghum. Allelopathic compounds are present in the roots and above-ground portions of these plants in varying concentrations, and can act against weeds while the cover crop is alive and growing, or when cover crop biomass is incorporated into the soil. In some cases, the phytotoxicity of allelopathic compounds is increased when they are modified by soil microbes in the process of decomposition. Small seeded weed and crop species seem to be most affected by allelopathic compounds due to their greater surface-to-volume ratio and orientation nearer the soil surface where allelopathic crop residues are usually concentrated.

Unfortunately, allelopathic effects can be difficult to isolate and apply in the field. This is because the suppressive effect created by these cover species is a combination of allelopathy and the sort of run-of-the-mill plant competition described above. Also, the production, activity and decomposition of allelopathic compounds can be highly variable based on environmental factors like soil fertility, moisture, pH and temperature. In general, the allelopathic effects of cover crop residues are quite short-lived and hard to predict.

Attracting Beneficial Organisms

The final mechanism through which cover crops can suppress weeds is predation. Weeds are an important source of food for many organisms. While most weed predation is focused on seeds, various organisms prey upon weeds at every stage in their life cycle. Common weed predators include birds, insects, rodents, earthworms and microbes.

Supporting populations of control agents that already exist in a field is known as conservation biological control. Conservation biocontrol of weed predators can function in a few different ways. Planting and/or maintaining between-field cover in the form of herbaceous strips, fence rows, and hedge rows provides habitat for weed predators year-round and increases their abundance. Including cover crops within a field provides protection for weed predators, encouraging them to stay in the field longer and eat more weed seed. There is also strong evidence that reducing tillage to leave weed seeds on the soil surface contributes to increased seed predation.

What It Means

Cover crops can enhance many aspects of your potato production system, including weed management. However, there are lots of cover crop options, and each farm is different. Successful use of cover crops for weed management depends on strategic implementation.

Several things to consider before planting a cover crop include:

• Is a particular species/variety likely to fit your objective of weed suppression via a particular mechanism?

• Is the seed readily available at an acceptable cost?

• Is the cover crop suited to your field conditions, soil, cropping system, traffic patterns, etc?

• Is there a high likelihood of good establishment given your equipment and labor availability, timing, etc.?

• Do you have a viable strategy for suppression and residue management?

• Is the cover crop likely to cause any additional problems due to weediness, insects, etc.?

If you can satisfactorily answer all of these questions, the cover crop under consideration might be right for you and contribute to your success with cultural weed management.