NC State Extension Publications

 

Soybeans are one of the most flexible crops grown in North Carolina and are easily adapted to a number of different production systems and cultural practices. Various tillage practices, crop rotations, cover crops, and seed beds can be successfully used to grow soybeans. What works best on one operation may not work for another, so it’s important to think about what will work best as part of your farming system as a whole.

Tillage

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Tillage is optional for soybeans. A number of tillage systems can be used to produce soybeans, including full, conservation, and no-tillage. Their relatively large seed do not require as fine a seed bed for successful germination as some of our smaller seeded crops do. Nevertheless, tillage can help improve stands, expand the root zone, help with weed control, allow you to incorporate fertilizers (or other chemicals), and break up a crust. The impacts of using tillage systems for pest management can be mixed. Sometimes they can enhance pest pressure, while other times they can reduce pest pressure.

Many North Carolina soybean producers use tillage because it is usually easier to get an acceptable stand of soybeans without crop or weed residue on the surface compared to where residue is present. Creating a furrow to drop the seed into was historically easier in a tilled field than in a no-till field, but with today’s planters and drills, no-till systems are becoming more popular.

Conservation Tillage

The Conservation Technology Information Center (CTIC) defines conservation tillage as any tillage and planting system that leaves as least 30 percent of the soil surface covered by residue after planting. The residue is important in reducing wind or water erosion of our soil, and in keeping more of our sometimes limited rainfall available for our crops, pastures, and forests. Recent data from a North Carolina piedmont field where three crops (corn, wheat, and soybeans) were grown in two years under conservation tillage demonstrated that almost no sediment was lost from this field.

Conservation tillage is not synonymous with no tillage, although most no tillage systems also fit the definition of conservation tillage. Some conservation tillage systems, however, involve some tillage, so they don’t fit the definition of no tillage. In a typical year, more than half of North Carolina’s soybeans are planted using no-till, including almost all of our double-crop acres.

In the piedmont, an experiment lasting more than 30 years provided soybean yield data over 10 years. Tillage treatments included no-till, chisel plow, in-row subsoiling, disk, chisel plow + disk, and plow. In seven of the ten years there was no statistical difference in soybean yield among any of the tillage treatments. In the three years when there were yield differences, all tillage treatment yields were greater than plowed plot yields. Overall, yields from no-till were 8 bu/A higher than yields from plowed plots.

In the mountains, two years of soybean data showed no difference in yield between no-till and conventional till (disk), although water quality data show greater soil loss under conventional tillage.

In seven years of testing, no-till yields were 2.1 bu/A higher than clean till yields across 287 tidewater variety/sites, and 1.6 bu/A higher averaged across 247 coastal plain variety/sites. In the lone piedmont site, with 35 varieties, the no-till yields were 5.4 bu/A higher than the clean till yields. The only times the no-till yields were not higher were when the growing conditions during the first 30 days after planting were wetter than normal. Yields from a third soil environment—burn and plant— in the same tests were 0.1 bu/A higher than no-till in the tidewater, 1.1 bu/a lower than no-till in the coastal plain, and 5.4 bu/A lower in the piedmont.

Figure 3-1. Soybean yield across various tillage treatments.

Figure 3-1. Soybean yield across various tillage treatments.

Crop Rotation

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Planting any crop back-to-back year after year increases the potential for pests and problems to build up.

In two long-term rotation studies carried out by NC State University in the blacklands (Washington County) from 1972 to 1993 (22 years) and in the piedmont (Cleveland County) from 1985 to 1994 (10 years), soybean yields were increased by about 5 bu/acre following corn compared to following soybeans (Figure 3-2). At both locations, these yield differences occurred most years, not just in the latter years of the test.

In addition to the 4 to 5 bu/A yield penalty for soybeans following soybeans, with continuous soybeans, sooner or later, a pest problem will get out of hand. If you do plant soybeans back-to-back, be prepared to scout your fields more often and potentially spend more money on pesticide applications. Common problems include:

  • Nematodes: Soybean cyst nematode populations increase in long-term soybean cropping systems. The only way to monitor the population is through a nematode assay.
  • Insects: You are more likely to see problems with three-cornered alfalfa hopper, Dectes stem borer, bean leaf beetle, and stink bugs. Be on the lookout for these when you scout.
  • Diseases: Diseases that overwinter in crop residue are likely to be a problem for back-to-back soybeans. Keep an eye out for stem canker, Cercospora, frogeye leaf spot, and Septoria brown spot.
  • Weeds: Rotating crops allows you to diversify your weed management strategy by allowing use of different herbicides and tillage practices. With soybeans back-to-back, a good pre-emergent with multiple modes of action and overlapping residuals is critical.

In the tidewater and coastal plain, the pest that gets out of control is most likely to be a nematode, and most likely soybean cyst nematode. Because most of our soybean varieties have resistance to only a few of the existing races of soybean cyst nematode, the emerging pest may be a race of soybean cyst nematode that we do not have variety resistance for. In the piedmont, the emerging pest is most likely to be a seedling or foliar disease. Crop rotation is one of the most effective and profitable pest management tools we have available.

Figure 3 2. Crop yields in North Carolina with and without rotat

Figure 3 2. Crop yields in North Carolina with and without rotation. There are 22 years of data for the blackland site and 10 years of data for the piedmont site. The rotated crop is soybeans for corn and sorghum. The rotated crop is corn for soybeans.

Cover Crops

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Cover crops may provide a more desirable cover of the soil between crops than the weeds that would likely grow up if a cover crop was not planted. Because most cover crop research in North Carolina is fairly recent and on-going, drawing many conclusions at this time would be considered premature.

Researchers are looking at both leguminous and grass cover crops, and the type selected will probably depend on how they will be used, the crop they are following or preceding, and the cost of establishment. Cover crop planting dates vary with the region and type of cover crop. Small grain cover crops should be planted by November 15 in the coastal plain, and by October 15 in the piedmont and mountains. It is possible, however, to plant as much as a month later if the weather cooperates. Because about half of our state’s soybeans are typically still in the field at Thanksgiving, it may be difficult to establish a cover crop after soybeans unless the cover crop can be planted before soybean harvest.

There are some cases where soybean pests are likely to be enhanced or reduced due to cover crop use. For example, pea weevil can be devastating to soybeans following plantings of Austrian winter field pea. In addition, cover crops increase the risk for cutworms. Generally, increasing the time between destruction of the cover crop and planting of the soybean crop can reduce risk.

Authors

Professor and Extension Soybean Specialist
Crop and Soil Sciences
Department Extension Leader (Nutrient Mgt and Water Quality)
Crop and Soil Sciences

Publication date: Nov. 21, 2017
AG-835

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