Planting continuous corn or a long corn and short bean rotation is common and farmers have learned how to manage through the challenges. But a continuous soybean rotation with a long rotation in bean and short rotation in corn is almost unheard of.  However, some growers do it because it takes less investment and work and there are fewer bushels to handle at harvest. And favorable grain prices and lower input costs compared to corn have some growers considering planting more soybeans continuously.

Continuous soybeans require better management decisions:

• Growing soybeans year after year in the same field can lead to a buildup of diseases which can cause yield loss in the second and subsequent years.
• Growing soybeans requires paying more attention to soil pH and potassium levels. Growing continuous soybeans means the soil organic matter levels will drop over time when corn is no longer in the rotation.
• Adapting the best management practices can help mitigate the risk of growing continuous soybeans.

Challenges are many: Soybean stubble makes a great environment for precisely putting seed into the soil, so stand establishment issues related to soil are rarely ever an issue. However, when planting soybeans in a field for the second or subsequent year, a buildup of disease pathogens could have developed on the previous residue. By the third- or fourth-year soybeans, there are increased levels of disease and populations of nematodes.

Diseases such as soybean sudden death syndrome (SDS), brown stem rot (BSR), white mold, Phytophthora and Rhizoctonia root rots and soybean cyst nematode (SCN) build up and remain in the soil several years. The constant presence of soybeans will allow these pathogens to multiply and remain viable in the soil.

Continuous soybeans means fewer bushels to harvest, store and dry. However, your harvest window is shorter both in terms of the length of harvest in weeks and the hours you can harvest per day. Corn is wonderful in that regard. You can start combining early in the morning and combine into the night and corn will stand in the field until you get there. Soybeans need to be combined when ready and you have fewer hours in a day to harvest them.

Economics are the driver: For the last decade, many growers realized that corn yield gains made corn more profitable than soybeans. With increasing land costs and an upside opportunity to increase corn yield with better management, many growers opted to grow more corn in the rotation. They learned to make it work and sometimes grew truly continuous corn for a decade or more.

Today the economics seem to favor more soybeans in the rotation. Soybean yields are starting to increase now with better management, with the corn-to-soybean ratio approaching 3.2 – 3.5 to 1, down from 3.8 – 4.0 to 1 (think 200-bushel corn and only 50-bushel soybeans). And soybean prices have stayed higher relative to corn at a ratio of 2.75 – 3.0 to 1 instead of 2.0 – 2.5 to 1. This is making growing soybeans more attractive.

Variety selection and seed treatments are key: If planting continuous soybeans, choose a different genetic package every season. You do not have to necessarily change maturity group, but change genetics and make sure it has a good defensive package against diseases.

Seed treatments are a critical component in continuous soybean production. Soybean seed treated with a fungicide combination will help protect against soil borne diseases such as pythium, Phytophthora, Fusariumand Rhizoctonia. A neonicotinoid insecticide (Cruiser®, Poncho®) will protect against early season insects and today there are several biological nematicides on the market to protect plants against the soybean cyst nematode.

Soil condition is important: Most of the seedling diseases flourish when soil conditions are warm and wet or cold and wet. Two other strategies to consider are planting towards the end of the planting window into warmer soils with less risk of a cold spell. Also, make sure you have good drainage to remove excess moisture and condition the soil with tillage so that it warms and dries more quickly in the spring.

Let us know in our forum if you plant second-year or continuous soybeans and how you are managing them to minimize risks and protect yield potential.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.