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Management changes with double-cropped wheat, soybeans

Double cropping drastically reduces the elapsed time between successive crops and therefore can greatly increase the disease pressure for both crops. Where double cropping is practiced, the beneficial effects of crop rotation (weed, insect, and disease control) are totally negated.

There are two primary requirements for profitable multiple cropping:

  • There must be adequate time for the production of a second crop.
  • There must be adequate water to produce two crops, whether from stored soil moisture, rainfall, or irrigation.

Because the soybean crop is photoperiod sensitive and matures in response to day length, it is ideally suited for multiple cropping systems where planting dates for the second crop are later in the season and can be variable due to weather.

Currently, most double cropping systems depend solely on a combination of rainfall and stored soil moisture to supply adequate water for two crops. Irrigation can be used as a supplement for soils with a less than adequate water supplying capacity and/or inadequate rainfall. While irrigation can greatly increase the consistency of crop yields, it also increases the cost of production. If the top three inches of soil is dry when the second crop is planted, germination is greatly slowed until the receipt of adequate rainfall.

There also must be adequate soil moisture to enable the root system to grow into moist soil where water availability is more consistent. Because the water requirement is so large for double cropping, it is generally most successful on soils with large water supplying capacities that are sometimes referred to as "good corn soils."

Early planting of the second crop is essential for success, which requires harvest of the wheat as early as possible. Potential double crop soybean yield decreases by one bushel for each day that planting is delayed after June 20. Early wheat harvest can be accomplished by planting an early to mid-maturity wheat variety soon after the fly-free date in the fall and harvesting when the grain moisture decreases to 18% to 20% and then using air with or without supplemental heat to dry the grain. These actions combined can save several days that would normally be used to field-dry wheat to 10% to 14% moisture. If a grower wishes to maximize wheat production because of the high value of wheat relative to a following crop, early harvest may be less important.

Below-normal temperatures in June delay wheat maturation, which may require growers to reconsider planting the second crop. For someone considering double cropping, it may be necessary to have two wheat varieties differing in maturity available in the event that wheat maturation is delayed. In southern Ohio, soybean varieties with maturity ratings of 3.4 to 3.9 will usually mature before the first freezing temperature if planted in June. Other than selecting a variety that matures before the first freeze, variety selection is not as important for double cropping as it is for a full season crop. Ohio studies have shown that early planting and July-August rainfall have a much greater impact on double crop soybean yield than does variety.

Straw remaining after grain harvest must be managed so as not to interfere with planting the second crop. Some stubble may be left to provide mulch cover. Leaving an 8 to 12-inch stubble with the combine and baling the cut straw is an efficient practice and marketing the straw adds income from winter wheat. Alternatively, the straw can be chopped and spread evenly on the field. Usually, a no-till planter or no-till drill can plant through chopped straw, if the soil is not excessively wet or dry and hard.

Soil moisture at the time of planting the second crop is critical for its success, because average rainfall in July and August often does not replace the moisture used by the second crop. In most years, moisture used by wheat in May and June is replaced by rainfall, but in dry seasons some subsoil moisture may be used, leaving an inadequate amount of water for the second crop. Soils with low available water holding capacity are not suitable for double cropping soybeans.

Generally, such soils are poorly drained, somewhat poorly drained without tile, eroded, or sandy. Growers should also be aware of the water holding capacity of their soil, and rainfall in May and June when planning to double crop soybeans after wheat. An important rule of thumb to consider is: "If June is dry, don't try to double crop." Increased nitrogen application for the small grain produces more vegetation, which increases soil moisture use. Because wheat uses moisture from the upper eight to 12 inches of soil, growers should be aware of the moisture remaining below that depth.

Because of the short growing season remaining after wheat harvest and other time constraints, double crop soybeans should be planted no-till. The surface residue associated with no-tillage planting helps reduce moisture lost by evaporation and increases rainfall infiltration. In dry years, no-tillage planting can make a difference between satisfactory and unsatisfactory seed germination resulting from the moisture saved.

A goal should be to plant the second crop on the same day the first is harvested. Narrow row, no-tillage planters equipped with residue cutting coulters and double disk openers have performed well for double cropping, but modern no-till drills are excellent implements also. Because double crop soybeans do not grow very tall, they should be planted in narrow rows (7.5") and planted at high seeding rates (minimum of 250,000 seeds per acre) to obtain maximum leaf canopy and yield.

Weed control is not especially difficult with ordinary double cropping. The break between crops allows the use of a nonselective herbicide, such as Gramoxone Extra or glyphosate, to remove established weeds and the use of glyphosate and glyphosate tolerant soybeans can be used to control weeds that develop following soybean planting. Occasionally, broadleaf weeds, such as Canada thistle or ragweed, become established in winter wheat fields and interfere with grain harvest or with the following soybean crop. These weeds can be controlled in the wheat crop with application of 2,4-D amine; 2,4-D ester; MCPA; dicamba; or Buctril.

At the wheat hard-dough stage, 2,4-D amine can be used to control some weeds that might be a problem for the following soybeans. Always read the herbicide label to ensure compliance with requirements for use.

Double cropping drastically reduces the elapsed time between successive crops and therefore can greatly increase the disease pressure for both crops. Where double cropping is practiced, the beneficial effects of crop rotation (weed, insect, and disease control) are totally negated.

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