With the low commodity prices and lower production costs with soybeans, growers may be inclined to plant some of their fields back to soybeans. While this should work okay, there are some dos and don’ts to this practice.
Do expect a small amount of yield loss when soybeans follow soybeans: Continuous soybeans over the past 11 years averaged 95 percent as much as soybeans rotated with corn at the Monmouth site, where yields have averaged above 65 bushels per acre. At Perry, where soils are lighter and yields have averaged a little less than 50 bushels, continuous soybeans have averaged 96 percent as much as soybeans rotated with corn.
There is no clear pattern of a higher penalty for continuous soybeans under stress conditions, as we sometimes see in corn. We also didn’t see the penalty changing over time, so it doesn’t seem that second-year soybeans are safer or riskier than soybeans grown more than two years continuously.
In a 20-year study in Wisconsin in which five years of corn were followed by five years of soybeans, the first year of soybeans yielded 6 bushels, or about 12 percent, more than continuous soybeans. It’s not clear why this is a larger penalty than we’ve been seeing in Illinois, but it was more common to have SCN-susceptible varieties 20 years ago than over the past decade, and that may be partly responsible.
The 4 – 5 percent yield penalty we’ve seen in Illinois trials should be our expectation if we follow soybeans in 2015 with soybeans in 2016. The drop could be larger than that if yields in 2015 were boosted by being grown following several years of corn. And, of course, with above-normal soybean yields in many fields in 2015, we can’t be sure that conditions will repeat to give us comparable yields in 2016, whether we rotate or not.
Don’t count on tillage to increase yields: Growing soybeans following soybeans was considered an unsound farming practice decades ago, but if it were necessary for some reason, the common advice was to do thorough tillage (moldboard plowing) to bury residue to keep diseases from carrying over to infect the next crop. In the studies in western Illinois mentioned above, chisel tillage increased yield by about 3 percent (2 bushels) at the Monmouth site, but produced slightly lower yields compared to no-till at the Perry site. So it’s unlikely that tillage will increase yield in most fields.
Tillage might have helped disrupt the soybean cyst nematode (SCN) back when there wasn’t much varietal resistance, but we wouldn’t expect to see that with most current varieties, at least in fields with moderate SCN numbers. The downside of tillage, beside cost, is that it buries already-scarce residue left by the previous soybean crop, and thereby increases the potential for soil erosion. In fact, sloping fields are not the best candidates for growing soybeans following soybeans due to increased potential for soil loss.
Don’t worry about adding extra fertilizer: Soybeans following soybeans respond to soil nutrients just like soybeans following corn, so if soil nutrients tested in the normal range in 2015, there’s little cause for concern in 2016. We don’t see rapid change from year to year in terms of phosphorus and potassium levels, but over time we need to replace what the crop removes.
We don’t have data on adding nitrogen on soybeans following soybeans, but with the ready availability of nodule-forming bacteria from the previous crop, lack of carryover N from the previous corn crop to inhibit N fixation, and somewhat warmer soils at planting expected due to the lower amount of surface residue following soybeans, nodulation should get off to a fast start. There should be no need to add N in the form of fertilizer.
Do pay attention, but there’s no need for extra concern about fungal diseases: While a major concern when growing soybeans following soybeans is an increase in diseases, there’s really not much cause for a great deal of concern, especially for fungal diseases. Fungal diseases like brown stem rot, sudden death syndrome (SDS), and white mold are as common when soybeans follow corn as when soybeans follow soybeans. The organisms that cause these diseases are not diminished by rotating to corn; in fact, some of them reproduce on corn plants or its residue.
Cool soils at planting can increase the incidence of diseases like SDS, and since soils in the spring are usually cooler following corn than following soybeans, rotated soybeans might even be more at risk than soybeans following soybeans. Because few of the most serious fungal diseases are controlled by fungicide, there is no reason to expect fungicides to produce more response in soybeans following soybeans than when soybeans follow corn.
Do be concerned about SCN: One soybean disease that is definitely of more concern following soybeans is the soybean cyst nematode. Many varieties today have some level of SCN resistance, but there are different biotypes of SCN in many fields, and so nematodes might develop cysts even on resistant varieties. If there are cysts in the soil following the 2015 crop, it’s best to use a variety with a different source of SCN resistance in 2016. Most varieties have the same source of resistance, so that’s not easy to do. But at minimum don’t use the same variety again, and if you are able to change to a variety resistant to a different set of SCN biotypes than the one you used in 2015, by all means do so.
Do remember that these are still soybeans: Soybeans grown following soybeans are not some different species with a different set of requirements—they are soybeans that should receive the same level of care that you provide for rotated soybeans. So adopt the same best management practices as you do for your rotated soybeans.
That means planting in late April or early May if possible, using adequate seeding rates (with warmer soils and better seed placement, soybeans following soybeans often emerge at a slightly higher rate), appropriate seed treatments, row spacing of 15 or 20 inches if possible, and controlling weeds early and well. Scouting should always be a priority, as with any crop we grow.
Growing continuous soybeans is not quite the special challenge that growing continuous corn is. Just use the same best management practices. And while there may be a slight yield penalty, the potential for net income may more than offset that.
Emerson Nafziger, Ph.D., is a professor of crop sciences and extension agronomist at the University of Illinois. Read more about him here.