What to consider if rain delays soybean planting past May 15.

As we progress later into the soybean growing season, you may be asking yourself how to reach the full yield potential for late-planted soybeans. A University of Illinois soybean planting date study discovered that the soybean crop is not able to reach 100 percent yield potential if planting moves past May 10 (Figure 1). However, this is not the time to give up on the crop. Soybeans are amazing plants and can make the best out of bad situation. Here are a few tips to help reach full potential of your late-planted soybeans.

Figure 1. Impact of planting date on soybean yield. University of Illinois

Population: Planting soybeans later than normal is a numbers game. Soybeans planted later are not able to compensate as much compared with early plantings and yield will be driven by the soybean population.

Looking at Figure 2, in a study conducted by the University of Missouri we can see that the same population of soybeans (150,000) planted at later dates will result in a lower yield. This is partly due to the lack of sunlight the plants can capture during the growing season. Later-planted soybeans will also be shorter and have fewer nodes per plant, so the plants will have less opportunity to develop pods. To overcome this problem producers will need to plant more seeds, but how many more?

Figure 2. Effect of planting date on yield at the same population. University of Missouri

Seeding rates: As we move into June for planting soybeans, consider increasing populations. If planting is delayed until June 10, increase populations by 15 percent. For example, if normal seeding rate is 140,000 seeds per acre, increase to roughly 160,000. If delayed until June 20, populations need to increase roughly 25 percent. If delayed until July 1, or when considering double-crop soybeans, seeding rate needs to be 50 percent higher. I don’t recommend populations over 250,000 plants per acre due to the risk of lodging.

Narrow rows: The use of narrow rows (less than 30 inches) will allow the plants to capture more available light compared to wide rows and help them to close over the bare soil underneath. Soybeans that capture more light have a higher chance of producing greater yield. In addition, narrow rows will allow for faster canopy development and help reduce weed pressure.

Full season variety: Being delayed in the spring does not mean the same delay in the fall. For every three-days delay in planting we can expect a one-day delay in maturity for the same variety (3:1 ratio). This allows us to plant the same varieties while not incurring frost damage. Planting a full season variety will allow the plant to capture more light in the last part of the growing season. For example, if the normal maturity in your region is 3.5 to 4.5; the 4.5 maturity group is recommended.

Frost date: At the same time, don’t plant a variety beyond the full maturity variety for your area due to the risk of a frost event. Figure 3 shows that Illinois has a wide range of fall freeze dates. Based on your location, pick the correct maturity to gather the maximum amount of sunlight while trying to beat the first killing frost of the fall season.

Figure 3. Median fall frost date in Illinois. Illinois State Water Survey

Nick Marley is an agronomist and Certified Crop Adviser. He has a B.S. in agronomy and crop science from the University of Illinois and is working on a Master’s in agronomy and crop science at Iowa State University. He is a 2017 Illinois Soybean Association CCA Soy Envoy.