by Kevin R. Elmy, CPAg
Post Harvest Cover Crops
Fall is a busy time of the year on the farm. Time to capture the rewards of the growing season. But there is a missing opportunity. Need to get some cover crops growing, especially on irrigated land.
Our soils have been built with living plants in them, feeding the soil biology with root exudates, providing soil armour, cycling nutrients, and protecting the soil. An annual crop only provides root exudates for thirty days which feeds the soil microbes. The rest of the days of the year the soil biology is left to fend for themselves. For a carbon source, if the microbes do not have root exudates, they start to consume soil organic matter.
This is where cover crops come in. There are different strategies we can use to seed them, different species to use based on crop rotation, when to terminate them, and what the goals of the cover crop are.
Cover crops can be seeded as a relay crop, either before seeding the cash crop, with the cash crop, after the cash crop is established, or seeded after harvest. Relay cover cropping has the advantage of being active for a longer time to fix soil issues and build soil structure. Species will be selected also on herbicide systems used in the cash crop, or when planning to seed after weed control operations.
Crops like potato, root vegetables, sugar beet, dry beans, soybeans, peas and lentils will be best to use post-harvest cover crops due to the low cutting height or digging for harvest. Seeding should be done right after harvest, or if possible, right before harvest which would be preferable. When harvesting below ground crops, it is imperative to get something growing in the soil immediately to allow the plants and microbes to begin the healing process as soon as possible.
For crops that need to be cut low, seeding the cover crop prior to harvest gives the seedlings a chance to establish prior to the cash crop removal so they can gain more days of fall growth. But not too early of seeding so they grow too tall and interfere with harvest.
Vegetative plants growing in the fall are crucial to building soils. A plant in the vegetative stage will release up to 80% of the carbon it captures through photosynthesis as a root exudate. The vegetative plants feed the soil biology, so they will act as a delivery service for the cash crop if it is still growing. This helps the soil system suppress weeds by tying up excess soil nitrates, converts simple nitrogen sources to amino acids and proteins which saves energy for the cash crop, creates soil aggregates, improves water infiltration, helps prevent salinity build up, reduces erosion, cycles nutrients, builds mycorrhizal fungi, and helps maintain soil moisture. As the soil starts to cool, the microbes, in nature’s intelligence, take some of the root exudates and use the carbon to weatherproof the soil aggregates. This weatherproofing will increase soil aggregate stability so through the winter freeze-thaw cycles, helping them stay intact. In the spring, the soil biology will continue building more soil aggregates. That is assuming that we do not go in with high disturbance tillage, excess fertilizer applications, fungicides and early monoculture crops. Without fall cover crops, the soil aggregates will fall apart, and the soil biology must start from scratch in the spring.
Any time we can get a living root of a vegetative plant growing in the soil, our soils thank us. Aim for synergies when picking plant species, remember diversity is more important than the density of the cover crop. We are not going with a pure stand seeding rate, normally one to ten seeds per square foot is all that is needed, depending on plant architecture, ecological function, competitiveness, and your goals. This will feed our soil biology, creating a more natural system. Most of the problems we face in agriculture, we have created by our management. To fix the problems, we need to address the actual cause. Which means we need to address our management system. Cover crops would be one of the first steps to correct the issues.
Strategies for Thought Over Winter
For corn fields, annual clovers can be added early in the year, even before corn is planted if using conventional systems. In herbicide tolerant corn, add annual clovers after herbicide application. V8, add some winter wheat, winter triticale or Italian ryegrass, biennial cool season grasses to help keep the root exudates flowing, mycorrhizae fungi supported, soil covered, and will continue into freeze up. Do not work it in the fall. Use roots, not iron, to build soil. In the spring, terminate the over wintering cover crop before they use too much spring moisture and get too tall that will create a nitrogen tie up. If canola is not the next crop, using a drone to add some turnip, collards, and radish will help tie up unused soil nutrients to be released to the next year’s crop. If it is silage corn, the Brassicas can be added earlier to gain some feed value and potentially get some fall grazing or a great place to spread manure. Spreading manure on green plants increases manure utilization and reduces losses.
For cereals, adding a biennial grass at the time of seeding is a no brainer. No changes in management. Adding something like subterranean clover will create a low growing legume. There are limited herbicide options, so might be better added after herbicide application. Assuming canola is not the next crop, adding a Brassica two to three weeks before harvest will help loosen the soil.
Canola would benefit the most from a cover crop. It is non mycorrhizal, is heavily fertilized and has multiple applications of fungicide. Adding an annual clover, plantain, chicory, or biennial grass after herbicide application would help reverse the damage.
If you would like to discuss any further strategies unique to your operation or have any specific questions you would like answered, please feel free to reach out to Patrick at Fabian Seed Farms:
