Searching for the optimum soybean seeding rate

6:17 PM, Feb. 28, 2013  |  Comments
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Soybean seed was once a relatively minor input expense. Those days are gone.

Several factors must be considered when determining how many seeds to purchase and drop out of the drill. Because it is not uncommon to pay $50 or more per bag for soybean seed, overseeding without getting a yield return doesn't make a whole lot of sense.

Soybean plants have the innate ability to drastically change their growth habit based on plant density. High density stands will result in plants that are taller but with less branching, fewer pods and smaller seeds per plant. Conversely, low density stands will be characterized by plants that are shorter with more branching, while also having more pods and larger seeds.

The goal is to find the point where more plants no longer contribute to an economic increase in yield. Researchers generally identify this point at somewhere between 100,000 and 120,000 plants per acre. I'd suggest aiming for the high end of this range.

To determine optimum planting rates, producers need to know what the difference is between the number of seeds that get planted and what actually develops into a productive plant. This of course will vary from year to year, farm to farm, and sometimes field to field. Nevertheless, you need to start monitoring plant emergence and keep records over time if you plan to make reasonable assumptions of final stands based on planting rate.

Generally speaking, higher planting rates result in a higher percentage of plant loss.

It's always a good idea to take into account germination percentage and the emergence rate when determining seeding rate. For example, if germination rate (located on the seed tag) is 95 percent and emergence rate (determined from your own records) is 85 percent, then planting 180,000 seeds per acre will result in a stand of 145,350 plants per acre (180,000 x .95 x .85).

Finally, consider and monitor seed planting depth. Soybeans need to be planted between 0.75 and 1.25 inches deep. Either shallower or deeper can reduce stands.


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