CROPS

Double-crop soybean considerations for 2006
By Dr. Paul Tracy

The USDA National Agricultural Statistics Service estimates that a million acres of winter wheat were planted in Missouri last fall. This compares to 590,000 acres planted the previous year. Increased wheat acreage has renewed interest in double-crop soybeans.

Unless double cropping was planned, evaluate all agronomic, economic and environmental factors before deciding to plant soybeans after wheat. Missouri Farm Facts from 1983 through 1999 showed that double-crop soybeans yielded 23 percent less than full-season soybeans. Using this average, a 77 percent soybean crop should produce good economic return this year.

The three most critical double cropping factors are: current and seasonal environmental conditions, stand establishment and early season weed control. These factors are interrelated with wheat residue management, variety selection, crop nutrition and pest management.

The greatest short-term concern with double cropping is soil moisture at planting. If the topsoil has no available moisture in the top 2 inches, some producers plant deep to reach soil moisture. My experience is that you are better off planting in the dust and hoping for rainfall, or not planting until it rains.

The most common long-term environmental concerns with double cropping are July/August rainfall and the first killing frost. While you cannot predict summer rainfall or the first autumn frost, you can play the odds. If you have decent soil moisture at planting, drought concerns will be lessened.

Most soybeans reach physiological maturity within 90 days of emergence. For example, if the average first killing frost occurs in mid to late October, the odds point to a successful soybean crop—if it has emerged by early July.

One advantage to double cropping in 2006 is the early wheat development we have seen. This year, wheat should be harvested 7 to 14 days earlier than normal, making double-cropping a more viable option.

Under moisture stress, wheat residue helps reduce evaporation losses. Wheat residue also helps control erosion when intense summer rainfall occurs. Straw should be chopped finely and, more importantly, evenly distributed. Bunched or windrowed straw left behind the combine often leads to poor seed placement at planting. Burning wheat straw allows easier planting. However, burning dries the soil during a time when moisture is critical. It also leaves the soil vulnerable to erosion and creates several social concerns. I don’t recommend burning except when fields are flat and soybean irrigation is planned.

For double cropping, choose a medium maturity variety normally used for full-season production in your area. Early season varieties flower early, are small statured, hard to harvest, slow to canopy and have low yield potential. Late-season varieties often do not have enough time to reach physiological maturity.

A few years ago, drills were promoted for double-crop soybeans. They are easy to use, fast and place seed in narrow rows that canopy quickly. However, optimum seed placement and stand uniformity have been very difficult to achieve using many drills. If possible, use a 15-inch planter for double-crop soybeans. This system provides the canopy closure attributes of a drill with the seed placement benefits of a planter. If you don’t have a 15-inch planter, splitting 30-inch middles with a second planting pass is a good alternative.

Since there is no replant margin of error, use higher seeding rates in double-crop compared to full-season soybeans. Standard seeding rates for double-crop soybeans range from 175,000 to 250,000 seeds per acre. This equates to 70 to 100 pounds of seed per acre at a 2,500 seed-per-pound seed size.

People ask about crop nutrition in soybeans following wheat. In planned double-crop systems, I recommend applying all the phosphorus and potassium needed for both crops prior to wheat planting. If you did not do this last fall, compensate for soybean phosphorus and potassium removal before planting soybeans. In high testing soils, fertilization compensation can be delayed until after fall soybean harvest.

Before glyphosate-tolerant soybean systems were developed, weed control in double-crop soybeans was hard to achieve. Pre-emergent grass materials often did not receive enough rainfall to “activate.” Don’t let the ease of glyphosate-tolerant systems lull you into a false sense of weed control security. Double-crop soybeans are especially susceptible to stand reduction and yield loss caused by “early” season weed pressure.

Most other insects and diseases are not severe in double-crop soybeans. However, this crop is not as forgiving as full-season soybeans. Treat pest problems promptly.

Double-crop soybeans may be at greater risk to soybean rust (if it becomes a major pest) because later growing dates coincide with potentially high rust inoculum levels that have had more time to disperse northward.

In forage areas, you may want to consider double-crop soybeans as a hay crop. This year offers an excellent opportunity for growing double-crop soybeans.