Cover Crop Termination and Preparation for Planting

Cover cropping is becoming a popular agricultural topic, primarily driven by the goal of improving soil health and water quality, but the concept has been around for years. Cover crops produce vegetation which can help protect the soil from erosion, improve soil organic matter and provide other benefits during the winter when traditional summer crops are not alive. However, this additional vegetation presents various challenges for cropping systems which can limit return on investment, reduce yield, and discourage adoption. Abundant vegetation produced by cover crops can physically impede mechanical operations, such as planting, and alter the cultural dynamics affecting cropping systems and their management. For instance, cover crop vegetation can introduce issues with crop production by immobilizing nitrogen in the soil, and promoting various insect, disease and weed issues which threaten subsequent crops and increase management expenses. Cover crops also challenge planting efficiency and restrict opportunities. Winter cover crops naturally grow well past optimal spring planting times for primary crops. This introduces a huge conundrum because crops are typically far more productive and less vulnerable to stress and risks when they are planted early in the spring. Early planting is favorable because it allows crops to develop and mature when they are less subjected to summer environmental extremes, including heat and drought, and rising pest issues. Furthermore, ample cover crop vegetation will shade the soil, restricting absorption of solar radiation, keeping soils cooler and wetter. This can delay spring crop planting and hamper seedling establishment, particularly in our moist climate. Thus, growers must find a practical management solution between these plant systems, which will require compromise.

Cover crops often interfere with subsequent row crops by hampering planting, seedling establishment and uniformity of growth. This limits success and future adoption.

Winter cover crops naturally grow well past optimal spring planting times for primary crops, so terminating them prior to crop planting could potentially address many issues causing interference. However, the extent of potential benefits associated with cover cropping, such as biomass contribution and nitrogen fixation, will likely be meager if they are terminated too soon. Conversely, profuse cover crop growth amplifies many problems documented to increase failure.

We have been conducting research evaluating methods to incorporate cover crops into successful corn production systems. Cropping systems are very reliant upon early planting to optimize productivity by enabling reproductive development during more favorable weather. Abundant spring rainfall common in the Southeast region of the United States presents considerable difficulty because wet soil restricts planting opportunity and increases seedling mortality and stunts growth, which will reduce crop productivity. Cover crop vegetation can intensify these issues. Therefore, integrating these cropping systems presents a considerable challenge.

Delaying termination of cover crops until planting, as shown on the right, can impede subsequent crop growth and productivity. The cover crop on the left was sprayed with herbicide 6 weeks prior to planting. Corn was planted the same day.

Using herbicides to terminate cover crops prior to planting is a crucial management factor affecting the successful integration of cover crops with corn. Terminating cover crops with herbicides is much more dependable and practical compared to mechanical methods, such as rolling crimping, mowing or tillage. This is because abundant spring soil moisture inherent in the Midsouth restricts opportunities to execute mechanical methods to terminate cover crops. Cover crop vegetation and decaying plant residue can hinder planter efficiency and stand establishment and reduce grain yield if cover crop termination timing is delayed close to planting, or when “planting green” into live cover crops. For example, a standard cover crop mixture consisting of cereal rye and berseem clover reduced corn grain yield 8 to 15% (14 to 23 bushels/acre) when terminated 2 or 0 weeks preplant, compared with no cover crop in a traditional stale seedbed system. Crop productivity was reduced by cover crop vegetation reducing solar radiation absorption and limiting soil temperature, which stunted seedling growth and subsequent crop development. However, a cover crop system produced similar corn yield as no cover crop when terminated 4 or 6 weeks preplant. Furthermore, this research showed that interference, or corn yield reduction was greater and occurred earlier preplant for cereal rye, compared to berseem clover. This indicates more troublesome problems associated with cereals are likely related to their vegetation having high carbon content, which is slow to decompose, compared to legumes. These results suggest that terminating cover crops near when cereals begin stem elongation should reduce potential interference with row crops. In fact, timing cover crop termination relative to stem elongation growth stage of cereals may be more practical than calendar date, due to varying environmental conditions and intended crop. This normally occurs in early to mid-March, depending upon latitude and seasonal temperatures in Mississippi.

Herbicide timing dramatically affects the soil seedbed environment in cover crop systems. Early timing as shown on the right allowed sunlight to warm the soil and promote subsequent crop growth and higher yield.

Another method to limit interference is to manipulate distribution of cover crops in the row where seeds will be planted. Our research evaluated numerous seeding methods limiting cover crop presence in the intended planting zone or row. This included mechanical tillage treatments which concentrated cover crop seed in the furrow, rather than the row, as well as strip tillage conducted after cover crop establishment, which physically removed cover crop plants within the row. Our research showed cover crop plant distribution manipulated by various methods did not affect crop productivity when cover crops were terminated with herbicides several weeks prior to planting. In other words, cover crop termination was a more effective method of reducing cover crop interference, compared to manipulating plant distribution.

Timing cover crop termination near or preceding stem elongation of cereals is key to limit interference. This method may be more practical than relying on calendar dates.

The best method to address the primary issues associated with growing cover crops is to terminate them with herbicide prior to planting the primary crop. This interlude reduces opportunity for problems to carry over to the subsequent crop, as verified with the “green bridge” phenomenon. Problems are also closely correlated to the amount of vegetation generated. Therefore, early cover crop termination will reduce many complications, including vegetation obstructing seed placement, furrow soil coverage and seed water imbibition, which hamper crop stand establishment. Additionally, row crop planters are fundamentally designed to function in bare soil. Thus, planter modifications are usually needed to help displace troublesome cover crop vegetation. There are a host of attachments which can supplement, or replace planter parts, including coulters, row cleaners and closing systems designed to improve performance in cover crops or any high residue situation. The purpose of these products is to cut and remove vegetation from the row, so the planter will perform more efficiently, and maximize seedling establishment. Accordingly, effective row cleaners and closing systems rely on sharpened disc blades with serrated, angled teeth to cut vegetation, remove it from the planting zone and close the seed furrow to ensure seed to soil contact. Conversely, popular row cleaners relying on spikes or fingers not designed to cut or remove vegetation and roots anchored in the soil, may struggle when used in cover crops.