Cover crop seeding for corn and soybeans is just around the corner, so it’s time to give some consideration to how you will get them applied this fall. There are several ways to seed cover crops, each with its own benefits and drawbacks. You will need to learn what works best for you, your soil, climate, cropping system, and operation. For many, direct seeding with a drill or planter after harvest is the best method to ensure a good stand. But for others, overseeding cover crops before harvest may play a role on all or part of their acres.
Overseeding is applying a cover crop either by air or ground rig into a standing cash crop that is nearing the end of its lifecycle. Sometimes the term inter-seeding is used to describe this, but generally that is best reserved for early seeding into a living and vigorous cash crop with a long way until harvest. Overseeding is generally done by a broadcast method. Ground rigs such as high clearance fertilizer spreaders, or aerial applications via helicopter and plane are common methods. The use of drones is also starting to show up in certain conditions and will probably continue to grow as a possible option for many.
Advantages of Overseeding:
Overseeding cover crops into standing corn and soybeans prior to harvest is an effective way to get a jump start on cover crop seeding. By completing the task before harvest, we take away the need for labor to be dedicated to seeding during or after harvest when labor is generally in short supply. Often overseeding is contracted out to someone else, eliminating any need for significant labor on the farmer to get the cover crop applied. In late years when harvest drags on, having the cover crop already established as you harvest can be a huge benefit so as not to worry about getting seeding complete before the season ends.
Besides labor, there are two other big advantages to overseeding. The additional root growth of early seeded cover crops and the ability to use cover crop species that require earlier seeding dates for overwintering or fall growth make overseeding an appealing option. Clovers, annual ryegrass, and winterkill species, such as oats and radishes, can all benefit from overseeding due to earlier seeding dates. For farmers wanting to avoid cereal rye ahead of corn, overseeding can be great to get winter kill cover crops such as oats and radishes established.
Species Selection:
Keep in mind large seed cover crops (such as peas) do not do well when broadcast on the soil surface by any method. Leave them out of your overseeding applications.
When considering expanding to less traditional species such as clovers, brassicas, and annual ryegrass, do some research on how your herbicide program will affect the species. With the extensive use of residual herbicides to control water hemp and other weeds, we can see these chemistries persist into the fall and affect cover crop establishment and overwintering. The seed for these species is expensive, so we want them to be effective when applied.
Species that require lower seeding rates by weight can be more cost effective due to greater acres covered per application trip. This can be especially important with aerial applications when the plane requires a trip many miles back to base to refill. You will often see higher application costs per acre with higher seeding rates.
Our typical cereal cover crops such as cereal rye usually require higher seeding rates by weight than clovers, brassicas, and annual ryegrass. While we can’t cut the rates too much, we can account for additional tillering from these species and keep some of the effectiveness for higher rates by seeding earlier. We do need to keep in mind that some seed will be lost to predation and environmental conditions, so don’t reduce rates too much. If you are receiving a cost share payment for cover crops, always check the requirements for species and seeding rates.
Cash Crop Stage:
This is a tricky one and will depend on many factors including how tolerant the cover crop species is to shade. Corn is much more lenient on when you can apply, because even when our corn is still very green, some sunlight is always getting to the ground. However, in narrow row soybeans very little if any light is reaching the soil when all the leaves are intact and still green. The old advice of 50% light penetration in corn, and right before leaf drop in soybeans is still good basic advice. But I would be willing to go earlier in corn if the weather conditions are good. It should also be noted that overseeding applications into wide row soybeans tends to be more successful than narrow rows. Consult your seed supplier on the suitability of applying different species at different cash crop stages.
Germination and the Weather:
It is important to be in a reasonably wet weather pattern when overseeding. It is best to have moisture both in the ground and a good forecast for rain before an overseeding application. When the soil is dry, too often the 1 inch plus forecasted rain turns into a few tenths of an inch, and if there is not soil moisture present already, seeds can swell or sprout and not have enough moisture to continue and will die off.
You may think the seed will just sit on the ground until rain comes to germinate it. Sometimes this is the case, but often the seeds will be eaten by insects or even earthworms quickly. Yes, for those of you with good soil health practices who have built up a solid population of nightcrawlers, quick germination is key, or all the seeds will be pulled into the earth worm middens! Those worms are hungry by that time of year and are searching the soil surface for food.
So, what do you do if the soil is bone dry and there is no forecast for rain?
Keep the seed in the bag.
How to Measure Success:
You will need to recalibrate your idea of what a successful cover crop application looks like when overseeding. When compared to direct seeding methods (such as a drill), aerial applications will result in less uniform stands even under the most ideal situations. If you want (or need) a picket fence stand for your goals such as weed control, you will need to look at some sort of direct seeding post-harvest to achieve that. However, the benefits of added root growth and additional species options can make even a spottier stand worthwhile. Do not assess the success of an aerial application from the road. You need to get in the field and dig roots to tell the story of successful overseeding.
Some years no matter what you do, the success of overseeding will be limited. It is important to learn when to keep the seed in the bag and be willing to deal with the occasional year with less-than-ideal results. Understand that overseeding cover crops is a long-term approach. The good years resulting in substantial root growth and the boost to soil health from diverse species and nitrogen fixation from legumes can make it worthwhile. If you target your fields for aerial applications intentionally, manage them appropriately, work with a good applicator and seed supplier, and learn when not to move forward with an application, then overseeding cover crops can provide significant rewards.
ISAP recently published a Cover Crop Seeding Guide which gives an overview of nine cover crop seeding techniques. Explore the guide to find the seeding method that works best for you!
