Rules of Thumb for Late Planted Soybeans

by Bill Latham

Last fall climatologists were saying it would take approximately two seasons to recharge soils after the 2012 Drought. I, for one, sure didn’t think we’d find ourselves struggling to plant soybeans due to excessively wet soils.

Across North Iowa, many farmers are still waiting to begin planting soybeans. Recent downpours, like the deluge of 2.75 inches that fell in Mason City during a two-hour time span on Sunday afternoon, are creating a need for some fields to be replanted before many farmers are even finished planting the first time. Based on the June 9 crop report from the USDA National Agricultural Statistics Service, 60 percent of Iowa’s soybean crop is in the ground.

We obviously have a lot of ground to cover, but there is still hope of making good yields! Research by Iowa State University, and backed by our internal data, shows the highest yields are most consistently produced using full-season varieties planted from late April to late June.

June 20-25 is a target date for switching soybean maturities, but we can continue planting soybeans until early July. As a rule of thumb, we haven’t recommended that soybeans be planted here after the Fourth of July because there just isn’t enough sunlight conversion time to escape an early or normal frost and we typically run short of moisture in late July and early August. We have seen July planted soybeans range from normal yields to extremely low yields. As we know, this year is not typical!

Rules of Thumb on Later Planting:

  1. Each 4 days later in planting date causes 1 day later maturity. One month later planting causes 1 week later maturity.
  2. Farmers should not switch to a soybean variety that is out of their maturity zone.
  3. For later planting, use narrower rows and slightly higher seeding rates whenever possible. We need to get canopy coverage as soon as possible.
  4. When planting between June 1-10, expect a yield loss of one half bushel per day. When planting between June 11-20, expect an additional yield loss of one bushel per day.
  5. Yield predictions for planting July 1 or later are extremely variable depending on growing conditions and frost date.

Soybean Yield Expectations & Physiological Maturity

Relative yield percent and Bushels Per Acre based on a 60 bpa potential yield

Planting Date Northern Iowa Central Iowa Southern Iowa
Late April 100% 60.0 bpa 96% 57.6 bpa 98% 58.8 bpa
Early May 96% 57.6 100% 60.0 100% 60.0
Mid May 99% 59.4 96% 57.6 98% 58.8
Early June 81% 48.6 93% 55.8 89% 53.4
Mid June 61% 36.6 59% 35.4 82% 49.2
Early July 33% 19.8 45% 27.0 47% 28.2
NOTE: All yields through mid-May are not statistically different from 100% yield.
SOURCE: ISU Extension publication PM 1851, “Soybean Replant Decisions”