Some growers will decide to plant back to back soybeans because of recent grain market outlooks. Most sources say that if a grower chooses to go with a continuous soybean rotation, they should expect potential yield losses between 5 to 20% compared to a corn after soybean rotation. This yield loss gradually increases, the longer a field is planted back to only soybeans.
However, some Illinois research revealed that yield loss was not as high as expected in some field situations. If you choose to grow soybeans after soybeans, the days of “planting soybeans and forgetting them” are over. A continuous soybean crop will need to be highly managed. Field placement, fertility, variety selection, weed management, and disease will all be factors that could determine potential yield loss.
The biggest disadvantage of a continuous soybean crop is the potential for an increase in disease as well as soybean cyst nematode populations. Many are unaware of just how many disease management benefits are gained with crop rotation. Diseases that overwinter in soybean residue, such as bacterial blight, stem canker, pod and stem blight, Septoria brown spot and frogeye leaf spot will increase. If survival structures of SDS (Sudden Death Syndrome) or white mold are found in the soil, a continuous soybean crop will truly face challenging or disastrous results. The greatest threat to soybeans is soybean cyst nematodes (SCN); therefore, it is strongly recommended to conduct an SCN test before planting soybeans after soybeans to assess populations and make sure the egg count is low.
Not just any field will do if planting soybeans after soybeans. This is because the yield penalty will only become greater in environments less suited for soybean growth. Fields that more easily stress a crop are more prone to have disease issues. Avoid fields that are low lying, compacted or have poor drainage as these are the fields that will be more vulnerable to root rot diseases. The addition of seed treatments for early season diseases such as Pythium, Phytophthora, or SDS are strongly encouraged. And make sure your seed treatment of choice has two or three modes of action to strengthen control.
Selection of soybean varieties with disease resistance or tolerance is critical, especially if there is a field history of soybean disease. Pay attention to soybean variety disease ratings and rotate between soybean varieties to reduce the selection pressure on the same sources of resistance. Ask your seed dealer for specific details on disease resistance or tolerance and whether a variety will perform in a continuous rotation.
Oftentimes, planting soybeans with a shorter relative maturity may help avoid diseases that tend to make their debut later in the season. Some sources encourage a slight plant population increase with continuous soybeans; however, lower planting populations as well as an increase in row width may be needed to reduce the onset of diseases such as white mold or stem canker. Later planting dates may be needed for some diseases—such as SDS that are more likely to infect soybeans in cooler or wetter soil conditions. However, planting shorter season varieties later could also increase your potential yield loss depending on environmental conditions and location.
If you are considering planting a field back to soybeans, remember to be ‘on your game.’ This rotation can work if you keep it short, select the right field and variety, and protect from potential diseases that could be lurking in residue or soil.
Stephanie Porter is a Burrus Seed Sales Agronomist. Read more about Stephanie here.