Soybeans have the most flexible planting date of any row crop grown in North Carolina and are easily adapted to a number of different production systems and cultural practices. The flexibility results in the use of soybeans in diverse crop rotations across the state. Soybeans can be grown successfully under various crop rotations, cover crops, and tillage practices. What works best on one operation may not work for another, so it’s important to think about what will most benefit your farming system as a whole.

Questions arise about the impact of planting soybeans in rotation with other crops produced in North Carolina. While rotational advantages for soybean production can vary depending on a variety of factors, it is well established that planting any crop back-to-back year after year increases the potential for pest problems to intensify. Crop rotation is one of the most effective and profitable pest management tools available.

In two long-term rotation studies carried out by NC State University in the tidewater region (Washington County) from 1972 to 1993 (22 years) and in the piedmont region (Cleveland County) from 1985 to 1994 (10 years), soybean yields were increased by about 5 bu/acre following corn, compared to following soybeans (Figure 3-1). The yield advantage from rotating with another crop was not only observed at the end of the study period but often detected throughout the study period.

In addition to the 4 to 5 bu/acre 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. Following are some common problems.

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 threecornered 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 lets you 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 are critical.

Figure 3-1. 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 crops are soybeans for corn, and sorghum for corn for soybeans.

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Figure 3-1. 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 crops are soybeans for corn, and sorghum for corn for soybeans.

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Interest in using winter cover crops has increased significantly throughout the Southeast region for both the short-term and long-term benefits that can be gained from cover crop use. In 2017, 483,000 acres of cover crops were grown in North Carolina, whereas in 2012, only 393,000 acres of cover crops were grown (USDA-NASS, 2017). Short-term benefits of cover crop use include weed suppression, soil moisture conservation, disease suppression, nematode suppression, and suppression of some insects. Longer-term benefits of cover crop use can include reduction in soil erosion and nutrient leaching. Your desired benefits from using cover crops should guide your selection of species. Possible categories of cover crops include small-grain cover crops, legume cover crops, and brassica cover crops. Each of these categories of species can provide distinct advantages. For several reasons, growing a small-grain cover crop prior to soybeans is a good fit for many producers.

Cereal rye is a commonly grown winter cover crop planted before soybeans in the Southeast due to its resilience and high biomass production, which can be beneficial for weed control and soil moisture conservation. Soybeans can compensate at lower populations more effectively than other crops; therefore many have a better ability to compensate for the variable stand often seen following heavy residue cover crops. In addition, N tie-up in the initial stages of cereal rye decomposition is less problematic for soybeans than in other row crops due to soybeans' ability to fix atmospheric N. Studies have shown additional advantages of cereal rye, including winter hardiness, suitability across a large geographical area, early flowering, and rapid emergence. Recent on-farm research across North Carolina has explored the benefits and challenges of planting soybeans behind cereal rye.

Planting cover crop mixtures is becoming increasingly popular for the diverse benefits the crops provide, including government cost-sharing that often requires planting a mixture. Incorporating a winter cover crop mixture (including legumes) to substitute for fertilizer application helps reduce input costs. The aforementioned advantage would generally be more beneficial in crops with higher N-fertilizer demand than soybeans. Recent research conducted in North Carolina explored the biomass dynamics that occur in cover crops when a variety of legume cover crop species were mixed with several different small-grain species.

Cover crops can sometimes reduce problematic pests in soybean production; however, occasionally soybean pests may be enhanced due to a cover crop. For example, pea weevil can be devastating to soybeans following plantings of Austrian winter pea. Threecornered alfalfa hopper can be problematic in soybeans that were planted after a cover crop mix that was terminated in close proximity to soybean planting. In addition, cover crops increase the risk for cutworms. To ensure that problematic insects are identified, scouting strategies should capture pest dynamics in the cover crop mulch. Generally, increasing the time between cover crop termination and planting of the soybean crop can reduce risk. Ongoing small-plot research in North Carolina is investigating the impact of cereal and brassica cover crops on pest dynamics in soybean production. The research findings will be added to this production guide in 2023.