Wednesday, February 19, 2014

Timing of Wheat Cover Crop Burndown

Chad Lee, Extension Agronomist, University of Kentucky, Lexington
Carrie Knott, Extension Agronomist, University of Kentucky, Princeton

Cover crops can be excellent systems to capture excess nutrients and reduce the risk of erosion over the winter months. The most common cover crop used is wheat but other species are gaining popularity. The timing of cover crop removal is important to the success of the primary crop. When possible, cover crops should be killed a minimum of two to three weeks before planting the primary crop. While killing the cover crop before planting the primary crop is preferred, there are some positive reasons for waiting to kill the cover crop after planting. Both options will be discussed.

Reasons to Burndown Before Planting
In general, burning down the cover crop before planting and planting into dead (brown or yellow) cover crop is the safest option for the primary crop. Early burndown reduces the risk that the cover crop will become a weed in the corn crop. Glyphosate and paraquat are the two most common herbicides used to kill a cover crop. Plants turn brown relatively quickly after paraquat. However, paraquat only kills what the chemical contacts. Glyphosate will move through the plant to the roots, but requires more time to result in yellow or brown plants. We know from field studies that small corn seedlings are negatively influenced when other green plants are nearby. The nearby green plants change the red/far red light ratio available to corn seedlings. The altered light ratio results in taller corn seedlings with narrower stems.

There is evidence that wheat roots have chemical compounds that negatively affect corn germination and seedling growth. Burning down the wheat two or three weeks before planting will stop production of these chemical compounds and may allow them to begin degrading before the corn seeds germinate.

Burning down prior to planting will eliminate problematic weeds present in the field. It can reduce insect and vole populations by destroying their habitat. Depending on the planter, better seed placement may be achieved when wheat cover crop is dead. Some units do not cut through the green residue very well. The green wheat will get tangled up on these planters and may prevent proper seed placement. Certain row cleaners generally do not work as well in these fields. There are other planters that have difficulty getting through a mat of dead plant material.

Reasons to Plant Before Burndown
Delaying wheat cover crop burndown until after corn planting may reduce erosion on extremely sloped fields. It may also be the best option if the wheat is already waist-high or taller. Excessively dead tissue may mat-up and make it difficult to achieve proper seed placement.

Considerations for those that plant before burndown are that insect and vole populations may be higher due to living tissue providing habitats for longer periods. Therefore a high rate of seed treatment or an insecticide applied in the furrow is a good idea to help manage against insects. Planting before burndown may also increase the possibility that the wheat cover will deplete available water in the soil that will be needed for the developing corn crop if the growing season becomes dry with little rainfall.

Some may prefer to delay burndown until after planting to allow fields to dry up and facilitate quicker corn planting. However, researchers at the University of Kentucky observed that allowing wheat cover crop to remain living did not dry out the field and allow for earlier corn planting.

Some federal programs encourage a late burndown to allow more growth of the cover crop. If you are involved in one of those programs, then you need to follow their guidelines.

Final Thoughts
Strictly looking at the potential success of the corn crop, burning down the cover crop several weeks before planting corn is the safest option for most years and in most situations.

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