Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu
As you plan for this fall, cover crops can provide multiple benefits — from protecting soil and scavenging leftover nitrogen (N) to suppressing weeds and even fixing N for the following crop. To maximize these benefits, it’s important to choose the right species or mix, plant at the right time, and ensure good seed-to-soil contact.
- Planting Choices
- Rye remains the most reliable option for biomass production, N scavenging, and weed suppression. In Figure 1 below, the variation in rye growth likely represents differences in N left behind corn, which will be more important in fields that received manure and may continue to mineralize in the fall.
- Legumes such as crimson clover or hairy vetch add N to the system but need an earlier planting window and solid establishment to perform well.
- We have shown that mixes of rye with a legume (e.g., 40 lbs rye + 20 lbs clover or vetch) can build biomass while also providing N, making them a strong option for corn rotations.
Figure 1. A 2025 winter rye cover crop, where biomass varies with landscape (moisture) and potential N scavenging.
- Timing
- For best establishment, aim to seed cover crops by mid-October. Legumes should go in earlier (late September to early October) to allow enough fall growth.
- Later planting will reduce biomass potential and limit nitrogen or weed suppression benefits.
- Seeding Methods
- Drilling or incorporating seed generally leads to the best growth because it improves seed-to-soil contact. In past studies on Delmarva, fields that were broadcast and lightly tilled last year produced the most biomass.
- Although aerial seeding may allow for earlier planting, it is the least reliable, especially into standing soybean, as seed can be blocked from reaching the soil. If using this method, time applications ahead of rainfall for better germination. See this article from UMD on using drones for aerial seeding: https://blog.umd.edu/agronomynews/2020/11/10/evaluating-drone-seeded-cover-crops/.
- Surface broadcasting with no incorporation will also have limited results without seed to soil contact or good rainfall. While it can work where residue is light, establishment suffers under heavy residue, mostly associated with higher yielding corn crops. The high carbon to nitrogen ratio of corn residues means slower breakdown, which is great for soil cover, but limits effectiveness of surface broadcast seeding.
- Biomass Goals
- To suppress weeds, aim for at least 1.8 tons of biomass per acre by mid-April, which is best accomplished with earlier planting dates.
Bottom line: Prioritize early planting, good seed-to-soil contact, and consider adding a legume to rye for extra nitrogen and balanced growth. Choosing the right species and seeding method now will set up your cover crop for success this fall and the benefits that follow next spring.