Replant decision requires careful consideration

Soybean Corner: Hailstorms and other calamities can reduce soybean stands.
Apr 29, 2019
A hailstorm thinned my soybean population. How do I determine if there are enough healthy plants remaining? What should factor into a replanting decision?
The Indiana certified crop advisers answering this question are Jamie Bultemeier, A&L Great Lakes Labs, Fort Wayne; Troy Jenkins, agronomist with Ceres Solutions, Rochester; and Marty Park, Great Lakes Seed Service, Rensselaer.
Bultemeier: Keep emotion out of the decision. Anytime a soybean field is hit with hail, it will visually look rough. Look at the facts in the field. Soybean replant decisions early in the growing season focus on population and probability of yield based on planting date. Then determine how many existing plants will survive and become productive plants.
For a young plant to survive it needs leaf tissue and an auxiliary bud located in a leaf axis. If the main stem is healthy above the node where the cotyledons attach, and there is some green leaf or cotyledon tissue remaining, the plant should survive. Regrowth three to five days after the hail event is the best indicator of plant survival.
Plant populations can be determined by counting number of plants likely to survive in 10 feet of row in several locations. For 30-inch rows, multiply the count by 1,750; for 15-inch rows, multiply by 3,500; for 7.5-inch rows, multiply by 7,000.
Determine percent of yield remaining based on population of healthy plants. Multiply that percentage as a decimal by the decimal percentage of normal yield based on planting date. If the resulting percentage is higher than the percent of normal yield for the replant date, it isn’t financially wise to replant. The increase in yield from replanting needs to cover costs associated with replanting.
Jenkins: Only counting plants that are expected to survive is advised. These would be plants that are:
  • missing only one cotyledon — not plants cut below the cotyledons
  • missing both cotyledons but have a living growing point
  • cut off above the unifoliate leaves, with living tissue remaining
  • lightly bruised on the stems — not heavily bruised, folded or broken off
If you use the hula hoop method, multiply number of plants per hoop by the factor for hoop diameter. Repeat the counting in five different areas.
Here are the multipliers to use, based on the hoop diameter in inches:
  • 18, multiply by 24,662
  • 21, multiply by 18,119
  • 24, multiply by 13,872
  • 27, multiply by 10,961
  • 30, multiply by 8,878
  • 33, multiply by 7,337
  • 36, multiply by 6,165
The multiplier is found using this equation: 43,560 divided by ((hoop radius2 times 3.14) divided by 144)
The threshold for plant population is about 100,000 plants per acre. Plant stands above 100,000 will most likely not see yield benefits from replant. Growers may choose to replant to reduce weed competition. Where plant populations are below 100,000, replanting into existing stands to bring stands above 100,000 has shown significant yield increase. Replanting into an existing stand allows for improved cumulative sunlight interception that would be lost in the till and replant method.
Initial plant date delays after mid-May greatly reduce soybean yield potential. Late-planted soybeans would see less of a positive yield response from replant.
Park: Hail damage always looks worse than it is, and recovery can be remarkable — with limited to no yield loss, especially prior to reproductive stages. Notify your crop insurance agent before you evaluate the crop. Waiting several days to evaluate the field will make this process much easier.
You should see new trifoliate leaves developing and possibly axillary buds starting if stems have been cut off or broken. The focus should be on evaluating the “surviving” population and comparing that to population and yield charts. Unless population loss is extreme — roughly 50% — I wouldn’t expect an advantage for replanting in June.