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Conference attendees learn how to boost yield for double crop soybeans

By DOROTHY NOBLE
AFP Correspondent

GRANTVILLE, Pa. (Nov. 15, 2016) — Professor David L. Holshouser, of Virginia Tech, discussed ways to close the gap between double crop and full season soybeans at the Keystone Crops and Soils Conference.
Double crop soybeans yield less. Holshouser shared his research at the Tidewater Agricultural Research & Extension Center, Suffolk, Va., in addition to his observations on double crop systems in the Mid-Atlantic and other areas.
Holshouser attributed the yield loss primarily to less time for the plant to develop adequate leaves — a full canopy — to capture 90 to 95 percent of sunlight by early pod development.
Yet several strategies can minimize a yield decrease. First, he urged the growers, “You can’t manage double cropping like a full season system.” Furthermore, double cropping takes more management.
The main reason for reduced yield with double cropping, he noted, is the late planting date.
In Virginia, soybean yield will be reduced by a half bushel per acre for each day planted after mid-June.
Using a leaf area index, which is the area of leaves per area of ground, previous research showed than a LAI of 3.5 to 4.0 is needed for early pod formation to attain maximum yield.
In other words, soybeans must accumulate 3.5 to 4.0 square feet of leaves for every square foot of ground to capture enough sunlight during their reproductive stages.
Without that leaf area, yields will suffer.
In addition to the leaf area, planting late means the pod- and seed-fill stages occur in the shorter days of August and September.
That means even less light for their existing leaves to absorb.
One strategy would be planting into unharvested wheat, although the wheat may be damaged.
Holshouser’s research indicated a better solution can be using an early-maturing, high-yielding wheat variety. Barley rather than wheat also allows earlier planting.
Soybean variety selection, too, plays a part.
Holshouser suggests planting as late a maturity group as possible that will mature before the first frost to allow more time for growing.
Also, he noted that the soybeans should be planted on the same day the small grain is harvested. Follow the combine in the field with the planter or drill that day.
Narrow rows, according to Holshouser, always obtain complete ground cover sooner and capture more light for a longer period of time. Hence, that is another strategy to produce more leaf area.
Uniform stands, or nearly equal spacing between plants, also make a difference.
Holshouser adds that is the reason there is usually no yield advantage to using a drill over a narrow row planter.
For maximum profit, the seeding rate in double cropping should more than double that of full season soybeans.
In Virginia, with 75 to 80 percent emergence, the recommendations are 80,000 to 100,000 plants per acre and from 100,000 to 133,000 seeds per acre for full season soybeans. For double crop the recommendation is 180,000 to 220,000 plants per acre and from 225,000 to 295,000 seeds per acre.
However, the seeding rate depends on the field location, its fertility, and the farm, Holshouser said.
He also noted that the further south, the less seed is needed.
The proper depth depends on both soil moisture and the rain forecast. Moist soil for planting is preferred, of course.
Although soybeans should not normally be planted deeper than 1 inch in a full soybean situation, Holshouser noted that since the soil is much warmer in June and July, 1 1/2 to 2 inches deep can be used to reach soil moisture.
Fertilizer and inoculants must also be evaluated with the agronomy guides.
Weed control can be easier due to the higher amount of small grain residue. However, Holshouser noted some herbicide resistance problems from large weeds. Also, he indicated that excluding a burndown herbicide can be a problem. He urged, “Start clean and stay clean!”
Early season emergence can reduce pests such as thrips or bean leaf beetles.
However, a smaller canopy means less tolerance to defoliating insects. Keep the 3.5 to 4.0 LAI in mind, Holshouser warns.
Regular scouting for insects is recommended.
However, spraying when insect pests are lower than economic threshold disrupts biological control and leads to resistance issues.
Because seedling disease problems are lessened, fungicide seed treatments are usually not needed. In addition, anthracnose or Phomopsis seed decay are less prominent because of lower temperatures. But frogeye leaf spot and Cercospora blight and leaf spot could occur in pod and seed development.
Holshouser indicates that foliar fungicides would pay in cases of high levels of the pathogen, a susceptible variety, and optimum temperature and humidity conditions for spreading.
With good management and favorable conditions, double cropping can be profitable. In the Mid-Atlantic, soybeans are frequently double cropped after wheat, but grain sorghum or even corn can fit into a system with small grains.
Holshouser reports that he dislikes the environmental effects and food loss of crop kill. “That’s where double cropping fits in,” he says, noting the benefits of increasing cash flow and profits; improving soil quality; providing greater usage of land, equipment, labor and capital; and fostering more food and feed for world population.