Soybean dry down rate before harvest
The latest USDA-National Agricultural Statistics Service crop progress and condition report classified 56% of the soybean crop to be in good or excellent condition. Overall, 28% of all soybeans in Kansas are dropping leaves.
The weather conditions expected for mid-September to mid-October will be critical for soybeans as related to seed filling and determining final seed weight.
While current conditions have been warmer-than-average, temperatures are expected to trend towards cooler-than-normal during the next 8 to 14 day period. This pattern is expected to continue for October. There is a slight chance for wetter-than-normal conditions in the short term (6-10 days), but the outlook for October favors equal chances for wetter-than-normal conditions in the southwestern third of the state, and equal chances for the rest of the state. Keep in mind that there is a significant drop in normal rainfall amounts from September to October as we move into what is typically the drier part of the year.
Soybeans will reach final maturity with high seed water content, moving from 90% to around 60% from beginning of seed filling until final maturity. Final maturity is defined as the formation of the black layer in the seeds. The dry down rate will depend on the maturity group (affecting the length of the season), planting date, and weather conditions experienced during the latter part of the reproductive phase.
Changes in the water content during the seed-filling process (Figure 1) were previously described in our “Soybean Growth and Development” poster. As described for corn, seed water loss for soybeans can also divided in two phases: 1) before “black layer” or maturity, and 2) after black layer.
Figure 1. Soybean seed filling process from full seed to full maturity. Photo and infographic prepared by Ignacio Ciampitti, K-State Research and Extension. Taken from Soybean Growth and Development.
To address the question related to the dry down rate for soybeans, a study was conducted to investigate the changes in water content from black layer formation (maturity) until harvest time (Figure 2). During the last days of September and mid-October 2016, the overall dry down rate was around 3% per day (from 58% to 12% seed moisture) – taking an overall period of 15 days.
Figure 2. Grain moisture dry down (orange line) across three hybrids and different N rates near Manhattan, KS. Horizontal dashed lines marked the 58% seed moisture at black layer formation.* Graph prepared by Ignacio Ciampitti, K-State Research and Extension.
*Note: It is desired to reach harvest with 13% seed moisture to maximize the final seed volume to be sold, thus the importance of timing harvest with the right seed moisture content.
Soybean dry down rate was three-time faster, 3% per day, relative to corn at 1% per day. These dry down rates for corn and soybeans are primarily affected by temperature, humidity, and overall water content at the point of black layer formation (maturity). These main factors should be considered when the time comes to schedule soybean harvest.
For more information on dry down rates for corn, see the eUpdate article, “Rate of dry down in corn before harvest”, in the September 7, 2018 issue.
Ignacio A. Ciampitti, Crop Production and Cropping Systems Specialist
Santiago Tamagno, KSUCROPS Production, Dr. Ciampitti’s Lab
Damaris Hansel, KSUCROPS Production, Dr. Ciampitti’s Lab
Mary Knapp, Weather Data Library