Double-cropping is a practice that offers growers the opportunity to manage what grows in wheat stubble instead of letting weeds take over. By growing a crop on the land year-round, growers not only have the opportunity to increase profits per acre, but can also help control soil erosion. A successful wheat-soybean double-crop requires proper management and adequate environmental conditions.
Wheat Crop Considerations
Harvesting the wheat crop early can allow more flexibility for the timing of soybean planting. If drying facilities are available and wheat prices permit drying costs, you may consider cutting wheat a little early at a higher moisture content and drying it. According to data from Purdue University, harvesting wheat at 18 to 20% moisture does not appear to affect the milling or baking quality.1 In addition to permitting earlier planting of a second crop, early harvest also results in less field loss from shattering and lodging.
No-till is a valuable practice that can help retain soil moisture after wheat harvest. Although residue can complicate planting and herbicide application, proper management can help alleviate these problems. In a no-till system, residue should not be bunched or windrowed unless it will be baled and removed. An easy way to manage wheat straw is to shred the straw and distribute it evenly across the field which can be done with a shredder attachment on the combine or a rotary chopper.
Choosing the Right Soybean Product
Determining the best soybean product to follow your wheat crop depends on several factors. The most important things to consider are that the product fits well in your location and will use the remaining growing season to mature. This will help avoid the potential for yield loss due to frost. Generally, early products may be shorter in stature, lower yielding, and more difficult to harvest. Late products may grow well, but could still be green at first frost. Therefore, medium season products will likely be your best bet in order to meet the season and yield requirements and should have sufficient canopy growth to shade out weeds.2
Double-crop soybeans may experience more disease and insect pressure because they are planted later in the season. To help control this, consider planting products with good disease resistance that have been treated with Acceleron® Seed Treatment Products.
A good wheat stand can usually suppress weeds until harvest time. Any weeds remaining in the field after wheat harvest should be controlled before soybean planting. Tall weeds that may have gotten an early start in thin wheat may have been topped by the combine sickle and may require time to grow new tops and leaves in order to be controlled with herbicides.3 Smaller annual weeds, however, should be easily controlled with a burndown herbicide after the straw settles enough to get even coverage. Be sure to observe any plant back restrictions that may exist with your herbicide.
Soil Environmental Considerations
Rapid germination and emergence are very important for double-crop soybean. Sufficient soil moisture is the key environmental factor that determines whether a good stand can be obtained or not. Ideally, soybean products should be planted as soon after wheat harvest as possible. However, for best results, make sure that there is enough existing soil moisture or that rain is forecasted within a week of planting.
Also keep in mind that at least 90 frost-free days are needed for double crop soybeans to reach maturity. To determine the last safe date to plant soybeans, subtract 90 days from the average date of the first killing frost.2
Adequate soil fertility levels are also important when double-cropping soybean following wheat. It is practical to fertilize for both crops when planting wheat to save both time and money. Even if you are unable to get the second crop planted, the extra phosphorus and potassium will remain in the soil to be used by following crops.
Planting into Wheat Stubble
Some important considerations when planting double-crop soybeans are planting through the wheat residue to ensure consistent seeding depth and using a row spacing that promotes thorough canopy closure. First, the planter must be able to cut through or move the surface straw from the wheat crop. Planters may be equipped with row cleaners or coulters to move or cut residue. Proper planter adjustment and operation can help achieve good seed-to-soil contact and the consistent seeding depth needed to promote rapid germination. Row spacing is also an important factor to consider because when soybeans are planted late, there is less time available for vegetative growth. Because less branching occurs in double-crop soybean, their canopy development has a hard time covering the space between wide rows. Research in Missouri has shown that drilled or narrow-rows (less than 20 inches) can result in a 15 to 20 percent increase in yield over wider rows.
Ensuring a successful soybean double-crop begins before the wheat even leaves the ground. Planning for appropriate residue management at wheat harvest is the first step. Choosing the right soybean product and reducing its competition from weed pressure can help achieve maximum yield potential. Finally, growers should make sure that environmental conditions, especially soil moisture and fertility, can support the second crop.