Rules of Thumb for Late Planted Soybeans

This spring has been anything but normal. We are currently sitting at 67% of corn planted and 39% of soybeans planted. With the higher-than-average number of unplanted acres, we could be looking at a contender for 2013 prevent plant numbers. Although this has been tough for many of our farmers and dealers throughout the growing season, there has been a positive response for corn and soybean markets. With a few more days of drier weather, Latham Country could make a lot of #plant19 progress.

We have received many questions this spring about when it is time to switch soybean varieties. The good news is soybeans are not as picky about maturity and delayed planting as corn. The typical maturity range for your area can be planted up to Mid-June without any worries. For those in the northern part of the territory, that date is closer to Early June. Its not a good idea to jump down more than 0.5 maturity group for your area. Remember soybeans mature based on night-length and not heat units, so they will make it to maturity, but the full season maturity encourages them to accumulate more nodes (i.e. more pods) before fully switching to seedfill.

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”