Cover Crop Termination and Preparation for Corn Planting

Cover cropping is a very popular agricultural topic primarily driven by the goal of improving soil health and water quality. 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 grown. However, this additional vegetation presents many different challenges for cropping systems which limit return on investment and adoption. Abundant vegetation produced by cover crops can physically impede mechanical operations, such as planting, but may also potentially 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 limit productivity. Cover crops also challenge planting efficiency and may restrict opportunities. Winter cover crops naturally grow well past optimal spring planting times for primary crops. This introduces a huge conundrum because crops are usually far more productive and less vulnerable to environmental risk when they are planted early in the spring since they are generally less exposed to summer environmental extremes, such as heat and drought, and cumulative pest issues. Furthermore, cover crop vegetation will shade the soil, restricting absorption of solar radiation, keeping soils cooler and wetter. This compounds issues with crop planting and seedling establishment, particularly in moister climates. Thus, growers must find a practical management solution between these plant systems, which will require compromise.

Cover crops often interfere with crop emergence and growth, which greatly restricts the outcome 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 the 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 greatly increases many problems documented to increase failure.

We have conducted research evaluating methods to incorporate cover crops into successful corn production systems. Corn is the first crop suitable for planting in the spring which stresses the timing conflict with cover crops.  Furthermore, Corn is also extremely sensitive to emergence failure and plant growth disparity, compared to other primary crops, including soybean and cotton. Cover crop vegetation can aggravate these issues. Therefore, integration of these cropping systems presents considerable challenge.

Cover crop termination timing relative to corn planting date dramatically affected corn productivity. This research is supported by the Mississippi Corn Promotion Board and the Mississippi Corn Checkoff.

Using herbicides to terminate cover crops prior to planting is a crucial management factor affecting the successful integration of cover crops with corn. Cover crop vegetation and decaying plant residue not only hindered planter efficiency and stand establishment, but also stunted corn growth and reduced grain yield if cover crop termination timing was delayed close to planting. This was caused by vegetation reducing absorption of solar radiation and limiting soil temperature, which substantially retarded corn seedling growth and resulting development. Herbicide termination treatments were evaluated at two week intervals from six weeks preplant until corn planting. A standard cover crop mixture consisting of cereal rye and berseem clover reduced corn grain yield 8 to 15% (14 to 23 bu/a) when terminated 2 or 0 wk preplant, compared with no cover crop in a traditional stale seedbed system. The cover crop produced similar corn yield as no cover crop when terminated 4 or 6 weeks preplant. Accordingly, we recommend cover crops be terminated at least four weeks prior to planting corn. Furthermore, this research showed corn yield reduction was greater and occurred further preplant for cereal rye, compared to berseem clover. This shows more interference was associated with cereal rye and is likely related to cereal vegetation having high carbon content which is slow to decompose and thus produce complications, compared to legumes.

Another method to limit interference is to manipulate distribution of cover crops relative to the row where corn is planted. Our research evaluated numerous seeding methods which limited cover crop establishment in the intended planting zone or row. This included mechanical tillage treatments which concentrated 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 results showed cover crop distribution managed by various methods did not affect corn productivity when cover crops were terminated with herbicides several weeks prior to planting.

The best method to address most of the primary issues associated with growing cover crops is to terminate them prior to planting the primary crop. More time reduces opportunity for problems to be transferred to the subsequent crop, as documented as a “green bridge” phenomenon. Problems are also strongly related to the amount of vegetation generated. Regardless, cover crops will produce vegetation or residue which can physically obstruct seed placement, soil coverage and water imbibition which hamper stand development. Row crop planters are ultimately designed to function in bare soil. Thus, substantial planter modifications are usually needed to help removing either live or dead cover crop plants anchored by their root system, which presents a significant challenge. There are a host of attachments which can be added to or replace parts on a planter including coulters, row cleaners and closing systems designed to improve planter performance in cover crops or any high residue situation. The underlining principle of these products is the ability to efficiently cut and remove vegetation from a strip where seed can be planted. Accordingly, row cleaners and closing systems generally rely on sharpened wheels with deeply serrated angled teeth to cut vegetation, remove it from the planting zone, and close the seed furrow to obtain optimal seed to soil contact. Popular row cleaners relying on spikes or fingers are not intended to cut or remove vegetation anchored in the soil and thus, will likely struggle when used in cover crops.