Cover crops are plants grown primarily to protect and enrich the soil rather than for direct harvest, serving purposes like erosion control, weed suppression, and nutrient management. Spring marks the decisive moment when these beneficial plants must be managed to prepare the ground for the subsequent cash crop. The transition to a field ready for planting requires careful decisions that directly influence soil health, moisture levels, and the eventual success of the main crop. Successfully navigating this period depends on timely choices about how and when to terminate the cover crop, balancing cover crop benefits against minimizing risk to the incoming crop. The specific strategies employed will vary significantly based on the cover crop species, local climate, and the farmer’s long-term soil health goals.
Timing the Termination
The decision of when to terminate a cover crop is a balance between gaining maximum soil health benefits and ensuring a successful planting season for the subsequent crop. A primary consideration is the cover crop’s growth stage, as termination is most effective when the plant is transitioning into its reproductive phase. For grasses like cereal rye, this stage is typically at flowering, or anthesis, because the plant has invested most of its energy into above-ground biomass, making it vulnerable to termination methods like roller-crimping.
Delaying termination allows the cover crop to produce more biomass, which provides greater weed suppression and soil organic matter, but this delay comes with increased risk. The standing cover crop actively uses soil moisture, which can deplete water reserves needed for the cash crop, especially in dry spring conditions. Therefore, if soil moisture is a concern, it is recommended to terminate the cover crop at least two weeks before the cash crop’s planting date to conserve water.
Environmental factors like soil temperature also influence timing, particularly for cash crops like corn that require warmer soil for germination. The residue left by a terminated cover crop can keep the soil cooler, potentially delaying planting, so termination may need to occur earlier in the spring to allow the soil to warm up. Generally, growers aim for a “kill period” of 10 to 14 days between termination and planting to ensure the cover crop is completely dead and poses no competition to the emerging cash crop seedlings.
Choosing the Termination Method
The how of cover crop termination falls into three main categories: chemical, mechanical, and natural, each with distinct trade-offs for labor, cost, and soil disturbance. Chemical termination, typically involving non-selective herbicides, is a common choice due to its speed and effectiveness across different growth stages. Herbicides are particularly useful when a large area needs to be treated quickly, and this method leaves the plant residue intact on the soil surface, which is ideal for no-till systems.
Mechanical termination includes methods like mowing, tillage, and roller-crimping. Tillage is highly effective at killing the cover crop by uprooting it, but it negates many of the soil health benefits by disturbing the soil structure and increasing the risk of erosion. Roller-crimping is a specialized no-till method that physically breaks the plant stems, requiring precise timing when the cover crop is in a mature reproductive state to ensure a complete kill.
Mowing can terminate some species when timed correctly, but it is energy-intensive and may lead to regrowth if they are not at the proper growth stage. Some cover crop species are naturally terminated by winter conditions, known as winter-kill, which eliminates the need for spring intervention. However, even with winter-killed plants, spring management is still required for the remaining residue and any plants that may have survived a milder winter.
Handling the Remaining Biomass
Once terminated, the dead cover crop material, known as biomass or residue, becomes a surface mulch that requires management for the subsequent planting. The two primary approaches are leaving the residue on the surface or incorporating it into the soil. Leaving the biomass on the surface, a practice common in no-till systems, maximizes benefits like moisture retention, weed suppression, and protection against soil erosion.
Planting into heavy residue can present a challenge, as specialized equipment is needed to cut through the thick mat of material without hair-pinning it into the seed trench. Alternatively, incorporating the residue through tillage speeds up decomposition by increasing the contact between the plant material and soil microbes. This method hastens nutrient availability but sacrifices the protective benefits of surface mulch and increases soil organic matter loss.
The decomposition rate of the residue is heavily influenced by its carbon-to-nitrogen ratio (C:N). Residue with a high C:N ratio, such as mature cereal rye, decomposes slowly because microbes require extra nitrogen to break down the carbon-rich material. This potentially causes a temporary “nitrogen tie-up” or immobilization that can starve the new cash crop of nitrogen. Conversely, leguminous cover crops like vetch have a low C:N ratio, leading to rapid breakdown and a quick release of nitrogen into the soil.