Soybean maturity and harvest: When is it ready?

By David Nicolai, University of Minnesota Extension

As area soybean fields approach maturity the question that is often asked is “When are soybeans mature and when should harvest begin?”

Soybeans are photo-period sensitive therefore the date of physiological maturity is due to both day length and the stage of seed development in the uppermost pods on the plants. Full maturity (R8) of the soybean plant is defined as when 95 percent of the pods have reached their mature color which means that typically after five to 10 days of good drying weather after this stage, soybeans should have less than 15 percent moisture or harvest moisture. Soybeans will lose moisture rapidly with warm and dry weather at this point but should be harvested soon to prevent losses.

Try to harvest as much of your crop as possible before the moisture level falls below 12 percent to reduce splits and cracked seed coats. Shatter losses have been shown to increase significantly when seed moisture falls below 11 percent and when mature beans undergo multiple wetting and drying cycles. Shatter losses can be reduced by harvesting in the morning or evening when relative humidity is higher.

Keep in mind that storage of soybeans that are wetter than 13 percent moisture are likely to mold under warm conditions and buyers usually apply shrink factors and drying charges when wet beans are delivered. On the other hand, soybeans that are drier than 13 percent moisture are more likely to split during handling and since they weigh less, fewer bushels are available for sale. If the storage temperature is kept below about 60 degrees, soybeans can usually be held for at least six months at 13 percent moisture without mold problems. For storage under warmer temperatures or for storage times longer than six months, however, the recommended moisture content is 11 percent.

The following harvest guidelines are adapted from Iowa State University’s Soybean Extension and Research web page for 2008:

Reducing harvest loss

Harvest loss results from a combination of gathering loss, pre-harvest loss and machine loss. Gathering loss is the soybean harvest that does not get inside the combine, and accounts for around 80 percent of the total harvest loss. It is important to remember that ground speed, cutter bar conditions, and the location and speed of the pickup reel have an important influence on gathering loss.

How to estimate harvest loss

To get an estimate of your soybean harvest loss, check an area of 10 square feet. Approximately 40 soybeans lost in this area will add up to one bushel per acre. Make loss determinations at several locations and calculate an average. If losses are greater in this area, machine adjustments need to be made. A goal for good field conditions is to keep losses under one bushel per acre. The key to minimum loss is to adjust the combine for the crop conditions. Check losses periodically in the field, making the adjustments are well worth the time.

To keep harvest loss to a minimum:
• Keep a level seedbed.
• Keep the cutter bar as close to the ground as possible.
• Don't drive to fast. Most combines today with platform headers are able to operate without problems at three to four miles per hour. If you use a row-crop header you can go a little bit faster. However, if you use a rigid platform you can't go as fast as with a platform header or a row-crop header.
• Use a reel speed about 10- to 25-percent faster than ground speed (up to 50 percent faster if the crop is lodged).
• Keep the reel axle 6 to 12 inches ahead of the cutter bar and as low as possible. Reel bats should leave beans just as they are cut and the reel depth should be just enough to control the beans.
• Complete the harvest as quickly as possible after beans reach 13 percent moisture content. Damage increases at lower moisture content (less than 10 percent) as well as at excessive moisture (greater than 14 percent).

(David Nicolai is a crops educator with the University of Minnesota Regional Extension in Hutchinson.)