Soybeans:
Double-Cropping Soybeans
And Wheat in Mississippi

Many farmers double-crop soybeans and wheat to increase income, improve cash flow, spread risks, and better use land resources. Double-cropping also aids in reducing soil erosion by providing ground cover during the fall, winter, and spring.

Soybean yields are usually 15 to 30 percent lower when planted after a wheat crop, although this may range from no yield reduction up to a complete crop failure. With better planting equipment, better herbicides, and better varieties, you can keep potential yield reductions from double-cropping much lower today than you could just a few years ago.

No single method of double-cropping has proved superior. You should evaluate several approaches based on your management strategies, paying attention to soil type, weed species present, and planting equipment. The ability to irrigate makes double-cropping much less risky.

Early Wheat Harvest

A major reason double-cropped soybeans yield less than single-cropped is generally because of a later planting date and the lack of late-season soil moisture. Research has clearly shown that soybean yields decline rapidly when planted after mid-June. Anything that can contribute to early planting of soybeans behind wheat will help the potential yield.

One way to plant soybeans earlier is to harvest the wheat at 16 to 20 percent moisture. This requires artificial drying of the wheat, an added expense, but it may allow planting soybeans 5 to 7 days earlier.

Fertilization

Fertilization (P and K) is best accomplished by applying the recommended amount for both wheat and soybeans in the fall during wheat seedbed preparation. This is a good way to fertilize soybeans and eliminate the need for topdressing wheat with complete (N-P-K) fertilizers in the spring or applying the fertilizer after wheat harvest. When submitting soil samples for testing, ask for a double-crop recommendation. Without a soil test, a rate of 80 pounds of phosphate and 80 pounds of potash will provide adequate fertilizer based on average crop removal.

Wheat Straw Management

The wheat straw and stubble are a blessing and a curse. Leaving the straw in place and planting soybeans directly into it provides maximum moisture conservation and protection from soil erosion. The mulch also may slow the emergence and growth of weeds. Wheat residue may present a problem with planting and stand establishment, limit the use of herbicides and cultivation for weed control, may harbor diseases, tend to slow the early growth of soybeans, and may cause lower soybean yields.

Burning. Mississippi research at several locations has shown you get best soybean yields where you remove straw by burning. Burning does present some hazards from smoke and danger to surrounding property; therefore, burning is an individual decision and responsibility. If you can burn safely, you will kill weeds, reduce the amount of volunteer wheat, reduce the chance for seedling diseases, allow for a minimum of seedbed preparation, permit ease of cultivation and directed herbicide application, and eliminate the toxic effect of the wheat residue on soybean growth. You should plant soybeans in a prepared seedbed or with no-tillage immediately following burning. On clay soils it is generally better to plant without tillage.

Stubble Planting. Leaving the straw and stubble provides maximum soil conservation. It eliminates the hazard of burning and leaves straw mulch to conserve moisture and delay weed emergence. It does require planter modifications and special weed control considerations. The greatest problems with leaving the straw are stand establishment, weed control, and frequently lower soybean yields. However, in highly erosive situations this may be the only desirable method of double-cropping. Leaving about an 8-inch stubble and chopping and spreading the straw are suggested for stubble planting. Also, planting soybeans at a slight angle can prove to be beneficial in obtaining a soybean stand because it minimizes hairpinning of the wheat straw.

Incorporating Straw into Seedbed. Generally, this is the least desirable approach, since it requires considerable fuel and time to get a seedbed prepared for planting. Tillage often dries the soil, making it necessary to wait for rainfall or irrigation to obtain stands. Attempting to plow under the straw is particularly undesirable on clay soils. Additional disadvantages of this approach are the increased chance for seedling disease and the possible toxic effects of wheat residue on soybeans. It does let you use conventional planters and preplant tillage for weed control. Also, it permits incorporation of a preplant herbicide, although the straw may reduce its effectiveness.

Planting

Planters. You do not need special equipment on prepared seedbeds; however, no-till planting requires modifications for planters and drills. The major requirement for planting no-till is a planter equipped with some type of coulter to slice through the residue and to open the soil in front of the double-disk opener. Since no-till planting is often on hard or rough soil surfaces, you may need a heavy-duty planter with down pressure applied to the planter units.

Several types of coulters are available. The 2-inch-fluted coulter was the most common type used on early no-till planters. Today more narrow versions (3/4 to 1 inch), ripple-type, or straight coulters are available. The ripple or narrow-fluted coulters require less weight to penetrate the soil than the wider versions, do a better job of cutting through the trash, and generally do not throw as much soil out of the seed slot, especially at high speeds, under wet soil conditions, or on clay soils. The wide-fluted coulters disturb the soil more but are easier to align with the planter seed opener. In all cases the straw should be dry and the soil firm to enable the coulter to cut through the straw rather than press it into the soil.

Double-disk openers do a good job of placing the seed into the slot made by the coulter. Good seed-to-soil contact, proper depth, and complete seed coverage are essential to achieve uniform stands and prevent injuries from preemergence herbicides.

In the north end of the state as it gets later, use earlier maturing varieties. In the southern part of the state, the long growing season allows for use of fuller season varieties but may be subject to late-season problems (insects, drought, and diseases). Early varieties have been effective at aiding in increasing yields on later plantings when compared to fuller season varieties, but the full potential of this practice has yet to be evaluated adequately.

Row Spacing and Seeding Rates. When planting after late May, there is an advantage to narrowing the rows to 30 inches or less if you can control weeds. In a prepared seedbed, use the normal seeding rate; however, for no-till planting increasing the rate by 10 to 15 percent may be beneficial. Where the straw is not burned, apply a fungicide to the seed before planting. Extension Publication 1194 discusses seeding rates and plant populations.

Varieties

Plant high-yielding soybean varieties best suited to the area, soil types, and that address potential pest problems. Late-maturing varieties offer the advantage of extending the season of growth to capitalize on some fall rains and usually grow taller to provide more competition with weeds. Narrow rows and irrigation will help make up for the effects of late planting dates and allow early maturing varieties to approach the yields of those maturing later.

Weed Control in Wheat Stubble

Stubble planting is a double-cropping system that requires special consideration different from conventional planting. Do not attempt stubble planting where there is a problem with emerged weeds without applying a burndown herbicide. Also avoid stubble planting where the emerged weeds are so large and dense that chemical burndown cannot be successful.

Emerged Weeds. You need to kill weeds growing in the wheat before soybeans emerge. A contact herbicide is satisfactory for most annual weeds; Gramoxone is an option for this purpose. Adding a preemergent herbicide to Gramoxone will improve control. Base use rate on weed size and spectrum. Good spray coverage is the key to control with Gramoxone. To penetrate and cover weeds, use at least 15 gallons of water and adequate pressure. Up to 25 gallons may be needed where dense vegetation is present.

Roundup is another option in place of Gramoxone; however, it is a systemic material. It has an advantage over Gramoxone on horseweed (marestail) and johnsongrass; however, where johnsongrass plants have been cut off due to wheat harvest, the effectiveness of Roundup is reduced. Roundup is recommended to be applied by ground equipment at 10 gallons per acre.

Preemergence Herbicides. No-till planting in standing or burned stubble eliminates the use of soil-incorporated herbicides. You may need to do a surface application of preemergence herbicides after you plant the crop to reduce the amount of grass and broadleaf weeds likely to emerge unless you plan a total postemergence program. A total post program can be successful, but timing and narrow rows will be major factors. This is critical in standing stubble, where cultivation or directed sprays are more difficult.

For seedling grasses and pigweed, Dual, Lasso, Command, or Frontier are options. In standing stubble or following burning, these materials may be somewhat less effective than on a prepared seedbed because of inactivation. Adding Lexone/Sencor or Canopy as a tank mixture with Dual, Lasso, Command, or Frontier will improve control of hemp sesbania, ragweed, teaweed, and sicklepod and may also suppress cocklebur. Prowl is also labeled for a preemergence application with Lexone/Sencor or Canopy.

These preemergence herbicides may be tank mixed with Gramoxone and applied before, during, or after planting but before soybeans emerge. Rates and other information are given in Extension Information Sheet 963, available at your county Extension office.

Postemergence Herbicides. In stubble-planted beans, postemergence herbicides are generally applied over-the-top after crop emergence. This limits the choice of herbicides, but there are many options. Consult Extension Publication 1100, available at your county Extension office, for more specifics.

The main thing to remember in controlling weeds in stubble plantings is earliness. Application will be necessary about 14 to 21 days after emergence. Check fields closely about 10 days after emergence to determine when and what to spray. Failure to spray when weeds are small may result in poor control and significant yield reductions.

Examples of Double-Cropping Systems

Burn Straw and Shallow Seedbed Preparation (not suited for clay soils nor where there is a poor wheat stand):

• Spread straw; do not windrow.
• Burn immediately after wheat harvest.
• Shallow disk, do-all or harrow, and plant immediately into moist soil.
• Plant in rows (20 inches or less after late May) or drill.
• Apply preplant or preemergence herbicides.
• Cultivate shallow (row beans) and apply postemergence herbicides or direct spray as needed. This application may be banded or broadcast.

Burn Straw and Plant No-Till (not satisfactory where there is a poor wheat stand):

• Best suited for clay soils.
• Spread straw; do not windrow.
• Burn and plant immediately into moist soil with a no-till planter in rows (20 inches or less if after late May), and increase seed rate 10-15 percent.
• Apply preemergence herbicides or use a total post system.
• Cultivate shallow, direct spray, or apply broadcast postemergence spray as needed.

Plant No-Till in Wheat Stubble

• Leave about an 8-inch wheat stubble.
• Chop and spread straw.
• If there is adequate moisture, plant immediately with no-till planter equipped with coulters.
• Plant at a slight angle to wheat rows to avoid hairpinning wheat straw.
• Plant in row spacings less than 20 inches, using a 10-15 percent higher seed rate plus a fungicide.
• Apply a burndown herbicide.
• Apply a preemergence herbicide if desired.
• Apply postemergence herbicides over-the-top in 14 to 21 days, and repeat as needed.

Disk and Plant Conventionally

This is not a desirable option on clay soils. On other soils, use this option only if there is ideal moisture or irrigation is available.

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The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended of other products that may also be suitable and have label clearance. Always read and follow current label restrictions on pesticide use.

Publication 1380
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director