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When is a good time to plant soybeans, and is late winter too early? One thing that describes winter in Illinois—year in and year out—is it’s just plain cold. Hardly a perfect time to plant soybeans, right?

According to a study conducted this year that was just completed, plant soybeans in winter. This planting date study began March 19 (Figure 1); still technically winter, in Montgomery County. The opportunity presented itself because it was considerably dry during this time and field conditions were ideal for spring field operations. When planting occurred March 19, the soil temperature at 4” was 45˚ F and soil was in fit condition for planting.

The soybean seeds received a full seed treatment with fungicide and insecticide. The planting population was 140,000 seeds per acre. During the next few days after planting, the soil temperature didn’t reach 50˚ F, but finally by March 23 the soil temperature rose to 53.3˚ F.

On March 25, we noted seedling growth below ground (Figure 2), with the radical—the first root of the soybeans plant—starting to develop. During the next few weeks the plot experienced three frost events, but luckily for the soybeans, the hypocotyls and growing points were still below the surface and safe from the frost.

On April 4 we could see the continued growth and development of the soybean seedlings (Figure 3). The radical has continued to grow and the hypocotyl—the stem of your soybean seedling—had started to elongate and began moving towards the surface, pulling along the cotyledons or seed leaves.

On April 17, 26 days after planting, the soil temperature was consistently above 50˚ F, making conditions more ideal for soybean growth. Finally, on April 19, the soybeans began emergence—31 days after planting.

Once the seedlings started to emerge, upon further inspection of the plants, one could see lesions on hypocotyl, which were most likely soil pathogens starting to infect the stem. Even though the seed did have full fungicide seed treatment, seed treatments are only typically effective up to 28 days and their efficacy wanes with time. Since the soybeans were in the ground for 31 days, this means the seedlings were vulnerable to these seedling diseases. In addition, soil crusting impacted emergence. The final stand was around 50,000 plants per acre, enough to warrant a replant.

The second round of early planting was conducted April 15. The soybeans planted April 15 began to emerge April 23, only 8 days after planting. Looking at Figure 4, we can see the difference in growth between the two planting dates. In Figure 4, the soybeans on the left were planted April 15 and soybeans on the right were planted March 19.

From vegetative stage (VC) until flowering (R1), the March planted soybeans were one vegetative stage ahead of the April planted soybeans even though they were planted nearly a month sooner. In addition, soybeans planted March 19 begin flowering June 1 while those planted in April begin to flower 5 days later (June 6). The early flowering date can translate into a longer flowering period, which could lead to more pods at harvest.

Due to poor emergence and the reduced final stand of the March 19 planting date, no yield data was collected from this trial. From my observations, the poor stand was due to adverse emergence conditions and not due to the cold soils. From current observations, it appears that soybeans can handle cold soils for long periods of time and not have major vigor or emergence issues once soils warm up. More studies are needed to understand the challenges and obstacles of soybeans that will be planted ultra-early.

Nick Marley is an independent consultant. He has an associates degree from Lincoln Land Community College, 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 via their online program.