Q: I will be planting popcorn into a cereal rye cover crop shortly after killing the cover crop. What kind of toxic reaction might occur?
A: It’s the decaying green “ooze” from cereal rye that can create problems for germinating grasses (like corn), usually not the herbicide used to kill the rye. This toxic effect of the dying rye is what makes it a good cover crop choice for organic producers. A good rye stand reduces weed problems while the rye is growing. Producers then roll down the rye to terminate it — no herbicide is used — and then plant their broadleaf crop.
The decaying rye affects the germinating grasses, greatly reducing the grass weed pressure while the crop is becoming established. The cover provided by the rye also reduces weed pressure by providing a mulch and by keeping the sun off the soil surface until the crop canopy forms.
When no-tillers are using a cereal rye cover crop, it’s best to spray out the rye two to three weeks before planting corn, so that the rye is brown before planting. The worst time to spray out the rye is 5 to 10 days before planting, assuming that you are using glyphosate to kill the rye as it will be decaying as the corn is germinating. For a shorter window before spraying and planting, use a different herbicide to kill the cereal rye faster so that it is dead brown sooner.
Some producers say that the germination problems are minimal if there is no rain to leach the “ooze” down to the germinating seed during this establishment time. (Make sure that the seed-vee is properly closed.) Others report that the germination problems can be reduced by planting the corn deeper so that not as much of the “ooze” makes it down to the germinating seed. However, I don’t think you want to plant popcorn very deep.
Back east, where rainfall is more plentiful and moisture conservation isn’t much of a concern, many no-tillers spray out the rye after the corn is planted and already germinated. However, this is not permitted in Nebraska if you want crop insurance on the corn as RMA doesn’t allow planting an insurable crop into a growing cover crop.
In Nebraska, the growing cover crop will use valuable soil moisture and should be killed early. While that doesn't provide much time for the development of top growth, the roots help feed the soil system.
Check with your crop insurer for further information regarding insurance coverage following a cover crop.