Final Soybean Populations to Expected Yields

Figure 1. Final Soybeans Populations To Expected Yield

While poor germination can be caused by a great variety of factors, flooding of our heavy clay soils is a common one. Many soybean fields were left with poor stands in terrace channels and on the edges of this year, much like last year, we had just too much rain after many of the long season soybeans were planted fields where water collects. Saturated conditions for 48 hours can reduce germination by 30 to 70 percent. The seedlings that do make it through can be damaged and slowed as well. For the fields that were already germinated and had a little growth, young seedlings at V2 to V4 can survive 3 to 4 days of waterlogging, depending on variety.

Over-planting soybeans is not ideal but an area of the field can have such a poor stand that there could be few other options. The first step in over-planting…don’t underestimate soybeans ability to compensate for thin stands. As can be seen in Table 1 from Purdue Extension, a soybean population between 60,000 to 80,000 plants per acre can still result in nearly full yields. Similar results have been shown in K-State Research and Extension research as well. It isn’t until plant populations get into 30s to 40s, often only a third of planting rates, which yield effects start to show. Another thing to keep in mind is that replanting is going to damage some of the current stand of seedlings. Usually soybeans at the V3 to V4 stage are pretty flexible but not from a direct hit from a coulter and closer wheel.

The second step is to actually calculate what the current stand is. One method is to count the number of plants in a row for a certain length. It is easiest to do this in 1/1000 of an acre row lengths; 15” rows:  Number of plants in row for 34 ft and 10 inches X 1000 = population per acre 30” rows: Number of plants in row for 17 ft and 5 inches x  1000= population per acre

In thin stands, the hulu hoop method is often the easiest. Yes, this does mean using a kids hula hoop, extra points for actually being able to hula hoop while counting with the hula hoop method. 24” diameter hoop: Number of plants in hoop X 13,872 = population per acre

30” diameter hoop: Number of plants in hoop X 8,878 = population per acre.

For either method, make at minimum, five averaged checks throughout the area in question, and attempt to be as random in sampling as possible. Often, the real problem is deciding the line between the good part of the field and that which might be replanted. Usually that decision depends on the length of the planter.

Without giving the full charts from Missouri and Purdue Extension, the expected yield from soybeans planted in mid to late June are around 80% of those planted as full season in mid-May. To determine if the flooded area is even large enough to be worth the effort, Google Maps can calculate area with some estimating from a satellite picture. In the field, there are some phone apps that can help. Personally, I use Field Area Measure but there are a number of good others.

The third step is putting it all together and determining if the expected yield increase from over-planting is worth the cost of seed and fuel. Figure 2 from Purdue Extension lines out many of the expected costs and returns within the replanting calculation. It is likely that unless farmers have large areas of the field, at least a planter length wide that have been completely wiped out, it is going to be hard to justify replanting. Please contact me for help in finding publications to guide the replanting or overplanting question.

For more information, please contact James Coover, Crop Production Agent, jcoover@ksu.edu or (620) 724-8233.

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