By Delbert Voight, Educator

Over the last several years of working with what I consider the top soybean producers in Lebanon County, I have learned the importance of timely harvest of soybeans. It has been my experience that once 95% of the pods turn brown, it’s time to combine about a week later. 

Once moistures dip below 13%, a grower is essentially giving the mill soybean dry matter since they will correct the moisture to 13%. I still remember John Yocum referring to the fact that after the plants first reach harvestable moisture content, dry matter losses occur simply by the alternating day/night and heavy dew.

It is important to consider the variety since some varieties will have slight differences in the pod integrity and not tend to split, as the heavy dew at night can speed up this process. There are also impacts of erect varieties that might dry quickly, and delays in harvest may impact those vs. varieties that that tend to lay over and nestle protecting large fluctuations in dry down.

The University of Wisconsin conducted a 3-year study on field losses and found that the average loss by delaying harvest 2 weeks was 2% higher than harvesting at the proper timing. This loss continues to increase as harvest is delayed (See Table 1). 

Table 1. Effect of Harvest Delay on Soybean Field Losses 
Source: University of Wisconsin
Harvest Delay Year 1% of Yield Lost Year 2% of Yield Lost Year 3% of Yield Lost  3-Year Average % of Yield Lost
None 4.1 6.7 7.5 6.1
2 Weeks 5.0 9.9 9.2 8.1
4 Weeks 6.3 16.1 12.1 11.5
6 Weeks 6.8 18.1 19.9 13.9
Average 5.6 12.7 11.4 9.9

Numerous tests of soybean combine losses show that up to 12% of the soybean crop is lost during harvest. Harvesting losses cannot be reduced to zero, but they can be reduced to about 5%. Combines can be operated to reduce losses without affecting the harvesting rate.

I found the following excerpts from an article by the Missouri Department of Agricultural Engineering useful during harvest to capture the losses that may occur during harvest:

Your best guide for correct combine adjustment is your operator's manual.

Remember that more than 80% of the machine loss usually occurs at the gathering unit. The height of the cutter bar directly impacts what beans get into the bin. If I were to harvest pods by hand vs. as little as a 3.5-inch height of cut would equate to a 5% loss just from the cutter bar height. If I go to 5-inch height of cut, the loss jumps to 10%.

Here are some suggestions to keep your losses to a minimum:

  • Make sure that knife sections, guards, wear plates and hold-down clips are in good condition and properly adjusted.
  • Use a ground speed of 2.8-3.0 mph. To determine ground speed, count the number of 3-foot steps taken in 20 seconds while walking beside the combine. Divide this number by 10 to get the ground speed in mph.
  • Use a reel speed about 25% faster than ground speed. For 42-inch-diameter reels, use a reel speed of 11 revolutions per minute for each 1 mph ground speed.
  • Reel axle should be 6-12 inches ahead of the cutter bar. Reel bats should leave beans just as they are cut. Reel depth should be just enough to control the beans.
  • A six-bat reel will give more uniform feeding than a four-bat reel.
  • Complete the harvest as quickly as possible after beans reach 15% moisture content.
  • A pick-up type reel with pick-up guards on the cutter bar is recommended when beans are lodged and tangled.

For specific methods and a sheet to take to the field, refer to this publication and enter your own information.

Finally, if you would like more reading on this, I would direct you to navigate to the Agronomy Journal Publication B.D. Philbrook 1983 that addresses this issue in more detail: B.D. Philbrook and E.S. Oplinger Harvest Loss in Soybeans.