How long do we have for late planted soybeans to mature?

By Dusty Sonnenberg, CCA, Ohio Field Leader

In a typical year, by the time Farm Science Review rolls around, many of the soybeans in the state of Ohio have started to turn a bright fall yellow color and are quickly drying down. Some fields in the southern regions have even been harvested. But it has not been a typical year. In the southern part of the state much of the crop is on schedule, and the yield has already been determined. In the northern regions, many beans are still very green and filling pods. Soybeans found in the R4 to R5 growth stage not uncommon in much of northern Ohio. The question among many growers is, “How long do we have to complete the grain fill period?”

According to the University of Missouri and Missouri Soybean Center, soybean yield is a product of the number of days of seed fill and rate at which the seeds fill. The vegetative growth that occurs before and after flowering begins, builds the photosynthetic factory with more leaves creating a greater rate of seed fill. Later maturing varieties will produce more leaves before flowering starts and will also have a greater number of days for seed filling. These soybeans may have a faster rate of fill for more days, so they may yield more. Soybeans planted very late will be induced to flower almost immediately after emergence.

David Holshouser, an agronomist with Virginia Cooperative Extension said soybean plants sense changes in night length and initiate flowering only after the night is longer (and days grow shorter) than a critical length.

“This is the concept of photo determinism. The length of the day, and thus the length of the photo period available ties directly to the summer solstice. The 2019 summer solstice in the Northern Hemisphere was Friday, June 21. Every day after this date has fewer hours and minutes of sunlight. Although technically incorrect, such plants are termed short-day plants. The length of required dark period depends on the species and variety of species,” Holshouser said. “Soybeans are a short-day crop and their development is determined largely by variety-specific daylength requirements that initiate floral development. In other words, as the days grow shorter, soybeans will flower and enter into reproductive development stages. Due to this photoperiod requirement, days from planting until maturity cannot be accurately estimated for soybeans due to variation in planting date and other environmental variations.”

This, however, changes once the soybeans flower.

“After flowering, temperature drives development and the days until maturity can be estimated. The number of days from floral initiation at R1 until physiological maturity at R7 is usually independent of variety, but will vary slightly from year to year due to temperature differences between years,” he said. “Late planted soybeans initiate flowering during a warmer time of the year; therefore, post-flower development speeds up.”

With this in mind, the concerns about frost limiting soybean potential in northern Ohio this year are warranted, said Greg LaBarge, Ohio State University Extension field crops specialist.

“Soybeans need both a certain number of days and temperature in order to mature,” LaBarge said. “Typically, in northern Ohio it takes soybeans that are at the R4 growth stage approximately 45 days to reach physiological maturity at R7. Soybeans that are at R5 will take about 35 days. Given that we are past the middle of September, this will push development into early November. That could be a challenge for many of the fields.”

The Ohio Agronomy Guide, using data from 1980-2010, lists a median climatological date (50% chance of the first freeze at 32 degrees F), occurring between Oct. 1 and Oct. 20. The Old Farmer’s Almanac has determined Oct. 13, 2019 as the date that has a 30% probability of frost, based on 1981-2010 climate norms.

Ohio Field Leader is a project of the Ohio Soybean Council. For more, visit ohiofieldleader.com.