Cover Crops for Weed and Nutrient Management

Cover cropping is a sustainable conservation practice that can enhance the health of agroecosystems. Cover crops have many benefits, including weed suppression and assisting with nutrient cycling. This article will help producers understand how cover crops assist in weed suppression and nutrient management. Specific strategies for weed and nutrient management utilizing cover crops are also discussed.

Introduction

Conventional agricultural management practices like tillage, monoculture cropping, pesticides, and chemical applications negatively impact the environment by increasing soil degradation and greenhouse gas emissions and decreasing biodiversity.¹ Cover cropping is a management practice that is gaining interest in production agriculture as a sustainable conservation practice that can support and enhance agroecosystem health. In agriculture, cover crops are planted in a cropping system primarily as non-cash crops. Advantages of cover cropping are protecting the soil from erosion and evaporation, improved weed and insect pest suppression, improved soil physical properties, and nutrient recycling.^2,3,4 Historically, cover crops were used as green manure to correct soil degradation caused by erosion and runoff.⁵ Broadleaf cover crops tend to prevent water run-off from fields and reduce the intensity of rainfall hitting on the ground. This allows water to infiltrate the soil rather than causing sheet and rill erosion that removes nutrients from the soil surface.⁶

Figure 1. Cereal rye, hairy vetch, and daikon radish cover crop mixture planted in a cotton field at the Edisto Research and Education Center in Blackville, South Carolina. Image credit: Mandeep Singh, Clemson University.

Many different plants are used as cover crops (table 1). Legumes and grasses are used widely as cover crops in production agriculture, with brassica species increasing in use. A diverse mixture of cover crop species in a production system is the best approach. For example, hairy vetch fix nitrogen (N) may be available to a subsequent cash crop, and cereal rye is an excellent plant for adding organic carbon (C) to the soil in addition to other benefits such as erosion prevention (figure 1). When these two are planted in a mixture, the grower has the potential to add both organic C and N to the soil. For example, soybean yields in Tennessee were higher with a multispecies cover crop compared to a single or double species cover crop, after three years of incorporation into the cropping rotation. In addition, higher soil water content and inorganic N content were documented from fields where the multispecies cover crop was grown.² The benefits from cover crops vary with species and region, so it is important to select cover crops that are suitable for the cropping system and desired outcome. In a sustainable agricultural system, establishing best management practices suitable for the farm is critical. Matching cover crop species to be compatible with the main crop for a desired outcome can help also reduce energy, labor, and machinery costs.

Weed Management Using Cover Crops

Weeds compete with crops for light, nutrients, and water. Previous research has shown that weeds are highly competitive in corn, soybean, and cotton.⁷ The inclusion of cover crops can have a significant effect on weed seed survival in row crop systems.^8,4 Grass species, such as cereal rye, are highly effective in producing biomass that can suppress weeds and mitigate soil compaction.³ In other studies, high levels of weed control were achieved using cover crops compared to other conventional systems.⁴ There are three main ways cover crops reduce weed pressure.

1. Cover Crop Residue: Cereal rye (Secale cereal L.) residue has been shown to have an allelopathic effect on weeds (inhibited germination and reduced shoot dry weights) for up to 6 weeks.^9,10,11 Cover crop residues significantly reduce weed biomass in cotton and peanut fields and required substantially less herbicide inputs compared to conventional (no-cover) production systems.^3,12 Palmer amaranth populations are also reduced when cover crops are utilized as part of an integrated pest management approach.¹³ In the cover crop + herbicide treated plots, the presence of Palmer amaranth (Amaranthus palmeri S. Wats.) and annual grasses were reduced 100% and 93%, respectively (figure 2).
2. Live Cover Crop Canopy: A vigorous and fast-growing cover crop suppresses weed germination by strongly competing with weeds for space, light, moisture, and nutrients and can reduce weed growth by 80%-100%.^14,15,16,17
3. Root Exudates and Compounds Released from Decomposition: Some cover crops can exude substance through their roots into the soil directly around the plant’s roots. The organic compounds can support different microflora, such as fungi and bacteria, assist in developing a well-structured soil, and prohibit certain weed germination.^18,19 Cover crops with an extensive root system enhance the beneficial association with soil microbes that are harmful to certain weeds. For example, pigweeds (Amaranthus spp.), lambquarters (Chenopodium album L.), and weeds in the buckwheat family (Polygonum spp.) show reduced growth if their roots are infected by mycorrhizal fungi.²⁰ Considering the main crop-weed-soil-microbe nexus is a relatively new strategy in integrated organic weed management.

Figure 2. Weed suppression in cotton following (a) a cereal rye cover crop (note the cereal rye residue in between cotton rows), and (b) no cover crop. Image Credit: Michael W. Marshall, Clemson University.

Strategies for Successful Weed Control Using Cover Crops

1. Weed Scouting and Identification: Identify the weed species present in the fields to gauge the effectiveness of a cover crop on those species. Typically, annual weeds (i.e., Palmer amaranth, large crabgrass [Digitaria sanguinalis (L.) Scop.], and goosegrass [Eleusine indica (L.) Gaertn.]) are more easily suppressed compared to those with a perennial lifecycle (i.e., yellow nutsedge (Cyperus esculentus L.), dewberry (Rubus trivalis Michx.), Johnsongrass [Sorghum halepense (L.) Pers.] ). Consult the Southern Cover Crop Resource Guide’s Cover Crop Functions and Rankings for identifying which cover crops work best for weed management in your area (https://southerncovercrops.org/cover-crop-resource-guide/).
2. Optimum Cover Crop Planting Time: Early planting of cover crops will reduce the overall emergence of weeds. Planting the cover crop early enough in the fall allows enough time for sufficient biomass production, which will provide soil coverage and suppress weeds. Be sure to plant summer cover crops before the emergence of weed seedlings. Consult the Southern Cover Crop Resource Guide’s Cover Crop Planting Dates chart to identify when cover crops can be planted in your area.
3. Select Cover Crop Species that Produce High Biomass: Certain cover crop species are known to produce significantly higher biomass. Cereals such as barley, oats, or cereal rye are good choices. These cover crops produce a high level of plant biomass and cover soil surface and reduce sunlight penetration, which is critical for the germination of small-seeded weed seeds. Consult the Functions and Rankings for identifying which cover crops produce the greatest amount of biomass in your area in the Southern Cover Crops Council’s Cover Crops Resource Guide.
4. Use a Diverse Cover Crop Mixture: Cover crops can be planted in mixtures or as a single species. For example, planting legume cover crops with cereal rye cover crop will add N to the soil and improve the cereal rye growth and canopy production.
5. Choosing Cover Crops Based on the Season and Your Goals: Choose a cover crop that is the best candidate for a given season. For example, planting sorghum or millet (warm-season cover crop species) during the early summer can help to minimize the emergence of fall weeds. Cover crops can also be used for grazing during this time, so the choice of the cover crop species also depends on your goals. Consult the Southern Cover Crop Resource Guide for identifying summer and winter cover crops that are successful in your area, and the Guide’s Cover Crop Functions and Rankings to identify the benefits of each cover crop.
6. Cover Crop Termination and Allelopathy: After terminating cover crops (e.g., cereal rye or brassicas), the allelopathic chemicals that are released from the decomposing residue can act like a preemergence herbicide, keeping weed seed from germinating. The allelochemicals produced by cover crops can have the most significant impact on weed seed germination. For example, Brassica spp. like rapeseed, mustards, and radishes produce several chemical compounds called glucosinolates. During the decomposition process, these chemical compounds break down into powerful allelochemical substances called isothiocyanates, which can reduce weed germination.²¹
7. Seed Rate and Method of Planting (table 1): Cover crops are usually planted by two methods, broadcast and drill. The drill is typically used for a narrow row (~7.5 in) planting of crops, such as small grains. Cover crops planted with a drill will result in a more uniform stand compared to the broadcast method.²² This planting method ensures better seed to soil contact and also uses much less seed per acre. Broadcasting cover crops over crop residues can often lead to low germination and uneven growth. Increasing the seeding rate can help compensate for an anticipated uneven or poor cover crop stand. Increasing the seeding rate is critical when planting late.²³ Aerial broadcasting of cover crop seeds offers the ability to cover more acres in less time than with ground equipment. When crop harvest will be late, aerial broadcasting of cover crop seeds will help in planting in a timely manner. However, if the farmer has access to a drill, cover crops planted with a drill will result in a more uniform stand compared to the broadcast method.²² Seeding (and subsequent cover crop germination and growth) can be conducted before harvesting of some main crops (e.g., cotton, soybean, corn). This will ensure a good cover crop stand before the onset of cold temperatures or moisture stress conditions.
8. Soil Conditions: Prior to planting, the soil should have adequate soil moisture (based on visual examination of the soil profile with a shovel). A soil surface that is covered with a layer of residue or loose and rough surface works better for aerial broadcasting and seed germination.
9. Monitoring the Growth and Performance of the Cover Crop: The cover crop may not eliminate all types of weeds or replace the need for herbicides. Cover crops do reduce weed populations to a number, which makes subsequent scouting and control with herbicides much easier and economical (figure 3). By monitoring your fields over time, you can evaluate the performance of the cover crops as a tool in the field’s integrated weed management program.

Figure 3. (a) Bare soil without cover crop and (b) with a rye and vetch cover crop mixture. Image Credit: Mandeep Singh, Clemson University.

Cover Crops for Nutrient Management

Catch Crops for Nutrient Retention

Cover cropping has been shown to influence nutrient cycling.²⁴ Soil macronutrients like N, phosphorus (P), and potassium (K) are likely to be lost from the soil through the leaching process.²⁵ Cover crops can absorb excess nutrients left over from the previous cash crop, preventing nutrients from being lost deeper in the soil profile with percolating water.²⁶ When fertilizers and manure are applied to the soil, cover crops can absorb the nutrients and prevent them from leaching. Growing cover crops can take up excess nutrients from the soil (cover crops that do this are commonly termed a catch crop), and release the nutrients back to the soil upon decomposition.^2,9,24 For example, deep-rooted cover crops have the potential to extend their root networks into the soil profile and scavenging residual nutrients that moved beyond shallow-rooted cash crops. Deep-rooted catch crops such as annual ryegrasses, alfalfa, red clover, and sweet clover grow roots deeper into the soil profile and bring the soil nutrients to the upper layers. Supply nutrients to the following crops to efficiently produce good yield while minimizing environmental degradation.^24,27 Cover crops with fast-growing ability have the potential to produce huge biomass in a short period and improve soil organic matter, thus increasing nutrient availability and stimulating soil biological activity.^2,28 Deep-rooted catch crops can also break subsoil compaction through their root structure, and in addition to leaving organic matter, they also leave channels in the soil that help the following cash crop to grow roots deeper down the profile. The selection of cover crops is based on their potential positive effects on both soil nutrients and the environment. Cover crops can be used to replace or reduce the use of off-site sourced fertilizers (such as inorganic fertilizers and organic fertilizers such as poultry litter and swine sludge).

Role of Legumes in Nitrogen Management

Leguminous cover crops fix atmospheric N by forming a symbiotic association with soil bacteria and convert them into a plant-available form. For example, hairy vetch can fix 38 to 170 lbs per acre of N, and red and white clover can fix 75 to 200 lbs per acre of N.²⁰ This fixed N can be available for plants after microbial decomposition of cover crop residues. The quantity of N fixation depends on many factors such as species of cover crop, biomass of cover crop, termination timing, moisture conditions, and the amount of plant-available soil N.²⁹ By limiting the nutrient losses, cover crops also potentially increase the fertilizer and nutrient use efficiencies by recycling residual nutrients for the subsequent cash crop.²⁴ Termination timing of the cover crop should be synchronized with the subsequent cash crop needs in order to increase nutrient use efficiency and lessen the dependency of off-site fertilizers. Depending on the type of cover, crop residue nutrients will be released at different rates upon decomposition. Generally, crop residues with high C:N ratios (>35:1) will decompose and release nutrients slowly into the soil. However, cover crops like legumes and legume mixtures with a C:N ration of about 20:1 decompose faster and deliver nutrients quickly when compared to cereals and grasses. Consult the Land-Grant Press publication “Management Practices Affecting the Nutrient Provision Capacity of Cover Crops” for more information.

Cover Crops Impact on Soil Macrofauna and Microorganisms

Figure 4. Hairy vetch, cereal rye, blue lupins, crimson clover, and daikon radish cover crop mix grown in a cotton field at Edisto Research and Education Center, Blackville, South Carolina. Image credit: Mandeep Singh, Clemson University.

Growing cover crops will help increase the activity of earthworms.²⁸ These earthworms create more micropores in the soil, which allow water and nutrients to enter the soil profile more easily. Earthworms are a significant component of soil fauna communities. Their activity is beneficial because they can enhance soil nutrient cycling through the incorporation of crop residues rapidly into mineral soils. Earthworms also concentrate large quantities of soil nutrients such as N, P, K, and calcium that are easily assimilable by plants in fresh cast depositions.³⁰ In highly leached soils, earthworm activity is beneficial because of the rapid incorporation of crop residues into the soils. In addition to this, water excretion in the earthworm gut associated with mucus (coelomic fluid) production is known to enhance the microbial activity.³⁰ Growing a multispecies cover crop (figure 4) also increases soil biodiversity. Increased soil biodiversity, microbial biomass C, and earthworm’s activity can improve soil aggregation and store C within stable micro-aggregates (very small soil clusters). Earthworms casts are enriched with organic C and N and appear in all particle-size fractions. Earthworms increase the incorporation of cover crops derived C into microaggregates formed within macroaggregates. This clearly indicates the direct involvement of earthworms in protecting soil C in microaggregates within large macroaggregates and stabilize soil C for the long term. The increased storage of C and N into soil aggregates indicates the potential of earthworms in soil organic matter stabilization and accumulation in the agricultural system. Using cover crops increases the earthworm population by increasing their food supply (crop residue) and by giving longer periods to feed and reproduce. Leguminous cover crops have the ability to obtain a large portion of N from atmospheric N fixation and also can contribute large quantities of organic materials rich in N. Research suggests that leguminous cover crops are particularly beneficial in supporting a large population of earthworms and influence soil nutrient cycling.^30,31

Cover Crop Recommendations for South Carolina

Table 1. Common cover crop mixtures recommended for South Carolina. Regional specific information, planting and termination guidelines, and other cover crop relative information can be found at the Southern Cover Crops Council’s Cover Crop Resource Guide website.

Sources: NRCS South Carolina, Midwest Cover Crop Field Guide 1/2012, and Managing Cover Crops Profitability 3^rd Edition.
Note: *Seeding rates are for drilled plantings. For broadcast seeding, consider increasing rates by 75%; mixes are suggestions ONLY; consideration should be made based on the instrument available, the purpose of cover crop, cropping system, time of year, and weather conditions.

Key Points

1. Cover crops are primarily planted for covering the soil to get multiple soil, plant, and environmental benefits rather than for being harvested.
2. Integrating cover crops into a rotation can decrease the incidence of in-field weed populations. This is critical due to the increased prevalence of herbicide-resistant weeds in recent years.
3. The inclusion of cover crops in the cropping system has the potential to increase soil physicochemical properties and microbial communities. Cover crop biomass protects the soil surface from being eroded during a typical fallow period, can catch excess nutrients, and release those nutrients for the next crop in the cropping system upon biomass degradation.

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