Organic Field Crop Production and Marketing

in North Carolina

North Carolina Organic Grain Production Guide

Chapter 5: Crop Production Management - Organic Soybeans

Jim Dunphy, Crop Science Extension Specialist, NC State University

John Van Duyn, Entomology Extension Specialist, NC State University

Production Management

Key management practices for organic soybean production:

• Choose varieties that perform well in your area (selecting earlier or mid-season maturity groups, if possible).
• Plant on time (not too late).
• Adjust equipment for a high plant population.
• Rotate crops.
• Plant in narrow rows.

Variety Selection

Choosing a soybean variety also means choosing a Maturity Group. In organic production, an earlier-maturing (Maturity Group V for most of North Carolina) or mid-season variety (Maturity Group VI) is preferred over late-maturing varieties (Group VII or later). Early-maturing beans can avoid hurricane winds and moisture and associated disease problems and yield losses. Because of soil type and more frequent rains, the blacklands of North Carolina can use an earlier-maturing bean (Group V or earlier) without yield loss. However, farther west and on sandier soils, a later-maturing variety (Group VI or later) may be needed to get adequate yields. In the coastal plain, a Group VI or late V (or an earlier planting) will help avoid corn earworm (CEW) infestation during flowering. CEW is seldom a problem in the piedmont. Variety selection is also an excellent way to deal with nematode problems. Selecting varieties that are resistant to the species and race of nematode present in the field can limit the yield loss caused by these pests. It is also a good idea to plant at least two different varieties because doing so will spread out the seasonal workload and risk. The Official Variety Test Report available at www.ovt.ncsu.edu or through your county Extension center is a good source of information on varieties. Unfortunately, there are fewer and fewer conventional or nontransgenic varieties available on the market. Organic farmers must be aware that transgenic beans are not allowed in certified organic production, and choose alternate varieties. Table 5-1 lists top-yielding non-GMO, feed-grade varieties in North Carolina through 2004, as noted by J. Dunphy in North Carolina Soybean Variety Information ( 2005, Extension publication CS-SB-15).

Table 5-1. Nontransgenic soybean varieties, relative yield over all locations, number of locations, and years in variety trial

* Percent above (+) or below (-) average yield of all varieties of the same maturity group at the same locations in N.C. Official Variety Tests (OVT) in 2000 through 2004.

Planting Date

Planting date and variety (or maturity group) selection go hand-in-hand. The key is to match planting date and variety maturity to the soil so that the row middles are lapped with soybean plants about 3 feet tall by flowering time. Planting earlier or planting a later-maturing variety can improve the likelihood of achieving this. In an organic farming system, avoiding pest problems is an important management technique. Planting early (by the end of May) with an early to mid-season variety can help the crop avoid insect and disease problems.

Row Spacing

Soybeans in row widths of 20-inches or less tend to have higher yields than soybeans in wider row widths. Narrow-row soybeans also lap the row middles sooner, making further weed control measures during the season unnecessary.

Plant Population

Weeds are the main pest that organic soybean producers face, and a thick plant population will compete with weeds more effectively. Thicker populations have denser, earlier-closing canopies that out-compete weeds and do not allow enough light to penetrate for weed seed germination. However, a thick plant stand also traps moisture in the canopy, which creates a good environment for disease. A good compromise is needed, but a closed canopy is preferred. Although it increases the risk of disease, it is also the best way to maximize yields. An ideal stand would have plants that are about 3 feet tall with row middles lapped by the time of flowering. Plant population in the field can vary widely and still achieve good yields. On 36-inch rows with a May planting date, an ideal plant population would be 6 to 8 plants per foot (about 100,000 plants per acre). In 20-inch rows, the recommended plant population is still 100,000 plants per acre, but the plant population per foot will be lower. If planting on 7-inch rows, 2 plants per foot (150,000 plants per acre) can achieve good yields. Seeding rate will depend on the planter calibration, seed germination, and soil condition. Proper calibration of the planter is important, as well as planting in ideal soil conditions (the soil should be warm and moist, but not wet). If planting in June, increase these seeding rates by 20 percent.

Soil Fertility

Soybeans yielding 50 bushels per acre will remove about 188 pounds of nitrogen per acre, 41 pounds of phosphate per acre, and 74 pounds of potash per acre from the soil. However, manure and compost applications are usually unnecessary because soybeans are nitrogen-fixing legumes and the crop can make use of any nutrients applied to, but not removed by, previous crops. If soybeans were not grown in previous years, soybeans should be inoculated with species of Bradyrhizobium bacteria specific for soybeans. Inoculums must not be genetically engineered. See Chapter 6 of this guide for more information about organic soil management.

Weed Management

Organic weed management is more challenging in soybeans than in corn since the soybean foliage does not generally overlap and shade the row middles until later in the season. Generally, narrow rows and increased plant population can help the crop compete more effectively against weeds. When managing weeds in soybeans, consider also that different planting times for soybeans result in the plants competing against different sets of weed species. Weeds that emerge during the first four to five weeks after planting will cause the most damage in terms of yield reductions. Weeds that emerge after this time will have little effect on yield, although they may make harvest more difficult and will set seed. The goal should be to keep the field clean through the first four to five weeks after planting. See Chapter 7 of this guide for more information on managing weeds in organic production.

Insect Pest Management

Differences caused by variety selection, planting date, cultural techniques, site, and season cause great variations in its attractiveness to insect pests. If organic soybean farmers recognize these differences, they can manage the crop for reduced insect pest numbers or, when this is not possible, predict which fields are attractive and may need more attention to prevent yield loss. The organic soybean grower can normally rely on three factors to limit insect damage: reducing soybean attractiveness to pests, attracting beneficial insects that reduce pest numbers, and the plant’s ability to compensate for insect damage (tolerance). Important tactics used to reduce insect damage include the following five strategies:

Rotation

Rotation helps reduce levels of pests like soybean colaspis and cyst nematode and often improves crop health. Avoiding pests through a rotation of at least two years allows soybeans to tolerate the feeding of pests that later move into the field.

Soil fertility and pH maintenance

Thin plant stands often have more corn earworms, but good growth reduces attractiveness. Reducing plant stress from low pH, poor fertility, or inadequate moisture will enable plants to better tolerate insect feeding.

Variety selection and early planting

High caterpillar populations can often be avoided by early planting of an early-maturing variety (such as varieties from Maturity Groups III, IV, or V). These plantings will bloom and harden-off before the corn earworm moth flight from cornfields, and the plants will be unattractive to the moths. Also, early maturity can greatly reduce soybean looper, velvetbean caterpillar, and late stink bug infestations. In rare situations, stink bugs can be trap-cropped by early-maturity fields, leading to greater damage.

Narrow rows

A complete canopy allows a higher level of biological control by insect predators, parasites, and diseases. Also, narrow-row soybeans seem to be less attractive to egg-laying corn earworm moths.

Remedial control

Group V or later-maturing varieties that are planted after late May can become infested by corn earworm moths moving from corn. These moths produce pod-feeding corn earworm larvae, and a high infestation may reduce yield by as much as 50 percent. Also, populations of leaf-feeding caterpillars (green cloverworm, soybean looper, and velvetbean caterpillars) may occasionally damage soybeans to above threshold levels. These worms are usually very late-season pests. In instances where caterpillar pests are not avoidable, insecticides approved for organic production, such as spinosads or Bacillus thuringiensis (Bt), may be successfully used. Scouting and the use of thresholds will indicate which fields are at risk. For scouting procedures for corn earworm see this Web site: http://www.ces.ncsu.edu/plymouth/pubs/ent/index3.html.

Disease Management

Soybeans have very few disease problems. This makes disease management in organic soybeans relatively easy. Nematodes are the main soybean disease agent in North Carolina. However, Asian soybean rust is a possible problem, and, if present, will require much more intensive management to make organic soybean production viable.

Nematodes

The best way to avoid nematode damage is to plant varieties that are resistant to the nematode (and race) present in the field. These varieties can be found on the Web site www.soybeans.ncsu.edu/soyvar or from county Extension agents. Conventional nematicides are prohibited in organic agriculture. Crop rotation of at least two years will probably help reduce soybean cyst nematode populations, but is not as useful when dealing with root knot nematode because it has multiple host plants. If you suspect nematode damage, you can take samples from your fields (fall is the best time) and send them to the NCDA&CS laboratory (1040 Mail Service Center, Raleigh, NC 27699-1040, 919-733-2655) for nematode assays. They will identify a nematode population and species, if it is present. The Agronomic Division of NCDA&CS also has nematode management and assay information on their Web site: www.ncagr.com/agronomi/nemhome.htm.

Asian soybean rust

Asian soybean rust is a disease that has the potential for causing severe economic damage in North Carolina soybean crops. It must be considered when managing for soybean disease. To manage soybean rust potential in organic soybeans in North Carolina, select early-maturity groups, plant early to get the plants out of the fields in time to avoid the rust inoculum, or do both. Do not, however, create such an early-maturing soybean crop that yields are reduced substantially.

For more information on soybean rust, go to one of these Web sites:

• www.usda.gov/soybeanrust/ (USDA site on Asian soybean rust)

• www.ces.ncsu.edu/depts/pp/soybeanrust/ ( Soybean Rust Forecast Center at NC State University)

• www.sbrusa.net (USDA online soybean rust tracking site)

• www.attra.ncat.org/attra-pub/asian_soy_rust.html (Appropriate Technology Transfer for Rural Areas document on possible organic treatments for Asian soybean rust)