Source: Michael Staton, Michigan State University Extension
The primary risk of early-planting is emerged soybean plants will be damaged by freezing temperatures as the growing point is exposed and vulnerable when the cotyledons emerge from the soil. This risk is mitigated to some degree by the fact that germination and emergence may take up to 14 days in cool soil. Soybean seedlings in the VE to VC growth stages can tolerate temperatures around 29 to 30 degrees F for short periods of time. If the seedlings have hardened off through exposure to cool temperatures for several days, they can tolerate temperatures as low as 28 degrees F. Once the trifoliate leaves emerge, low temperature tolerance is reduced and temperatures below 32 degrees F will cause damage.
Another risk to consider when planting early is poor germination and emergence. As mentioned earlier, soybeans take longer to emerge in cool soils. The longer they are in the ground, the greater the exposure to soil-borne diseases such as pythium and insect feeding. If the seed was planted into extremely cold soils or a cold rain occurs right after planting, there is also the risk of imbibitional chilling injury. Researchers in Ontario demonstrated that soybean seed is the most vulnerable to injury from cold soil temperatures during the first six to 24 hours after planting.
Early-planted soybeans that emerge uniformly and escape frost or freeze injury also have a higher probability of experiencing damage from bean leaf beetles and sudden death syndrome (SDS) than soybeans planted later in the season.
If you decide to plant soybeans in April, consider the following recommendations:
- Don't plant unless the soil is dry enough to support equipment and allow planting equipment to operate properly. Shallow soil compaction and sidewall compaction will haunt you the remainder of the growing season.
- Treat the seed with fungicides to protect the seedlings from the soil-borne pathogen pythium.
- To reduce the potential for frost or freeze injury to emerged seedlings, plant in fields at higher elevations having good air drainage and till the field or clear the residue away from the row.
- Plant only the highest quality seed as overly dry seed or seed having damaged seed coats will take in soil moisture more rapidly, increasing the likelihood for imbibitional chilling injury to occur.
- If possible, plant when the soil temperatures are expected to be above 50 degrees F for the first six to 24 hours following planting. If you must plant into cold soils, wait until noon to begin planting to allow the soil to warm.
- Consider planting slightly shallower if soil moisture is available and planting equipment is providing uniform depth control and good seed-to-soil contact. This allows the seed to be placed into slightly warmer soil and also compensates for the fact that cold soils reduce hypocotyl elongation. Never plant less than 3/4 of an inch deep.
- Consider increasing seeding rates by up to 10 percent when planting early.
- Reduce the potential for sudden death syndrome by planting into well-drained soils that are free from compaction and using SDS tolerant varieties.
- Consider planting seed treated with an insecticide registered for bean leaf beetle if damage is expected.