• Early planting increases full-season canopy photosynthesis, which can improve yields.
• Early planting also carries some additional risks that need to be managed.
• Key risks include delayed germination, slow growth rates, herbicide injury, insect feeding, disease issues, imbibitional chill injury, and frost damage.

Benefits of Early Planting 
Farmers may be tempted to start planting soybeans as early as possible, and for good reason.  Long-term university studies have demonstrated that early planting provides the best opportunity for maximum soybean yield.  Early planting ensures that the soybean plant can build a canopy to fully capitalize on solar radiation to drive growth and yield.  Early-planted soybeans may canopy quicker to help control problem weeds, improve light interception, and retain soil moisture.  Early planting will also increase the opportunity for pod set and seed fill to occur under greatest sunlight levels, and extend the effective seed-filling period.  

The optimal soybean planting date varies by latitude and by season, but generally falls between late April and early May for the central soybean production areas in the Midwest (see Image 1).  When planting is delayed beyond the optimum, yield penalties accumulate at about 0.5 bu per day, equating to as much as 16% total yield loss by the end of May.  Daily losses tend to accelerate in June.  Early planting into cool, wet soil also carries some down-side production risks that will need to be managed to ensure success.  What follows are some early planting considerations for soybeans:

Maturity 
Variety selection should be driven by the highest yield potential and best defensive package within and across the normal maturity range for your geography.  However, recent research suggests that early planting may be of greater benefit to those fuller-season varieties within your seed portfolio (see Image 2).  Start planting with your longest maturity varieties for early planting, then stage adapted mid-season varieties for later planting dates.    

Soil Temperature 
Soybean germination begins by the seed absorbing 50% of its weight as water during the first 24 hours after planting, a process called imbibition.  If soybeans were planted into warm soils (greater than 54 degree F) at least 48 hours before a cold rain, then the risk for chilling injury is minimal.  If soil temperatures were cooler at planting (less than 50 degrees F) and a cold rain falls during the critical first 24 hours after planting, the risk increases for imbibitional chilling injury and cold stress.  Chilling injury from cold water absorption interferes with proper rehydration of the cellular membranes in the cotyledons and embryo.  Severe chilling injury may kill the seedling and reduce stand. 

Soil temperatures can vary quite a bit from field to field, depending on tillage and residue.  Before planting, be sure to check actual soil temperature and the forecast for the next several days.  Whenever possible, avoid 'beat the rain' planting scenarios and strive to plant when temperatures are expected to remain warm and stable after planting. 

Frost Injury 
In contrast to corn, soybean germination is 'epigeal', meaning that the cotyledons are pulled above the soil surface, thereby exposing the growing point to potential frost injury at emergence.  Assessing the risk of late frost for any specific region should be based on emergence, rather than planting date.  The degree of injury to soybean is a function of temperature and the amount of time the seedling must endure below 32 degrees F.  Temperatures below 28 F may be lethal.  Symptoms appear as water-soaked leaves and hypocotyls that eventually blacken and wither.  Entire fields are typically affected uniformly.

Seedling Diseases 
Cool, wet soils after planting are ideal conditions to promote infection by the common seedling diseases Phytophthora, Pythium, Fusarium, and Rhizoctonia.  Phytophthora and Pythium are oomycetes or 'water fungi' that produce motile zoospores that swim through the soil water to the root surface and infect the plant.  Infections cause seedling damping off, stunted plants, and reduced stands.  Seed treatment fungicides that include metalaxyl or mefenoxam should provide up to three weeks of protection after planting against Pythium and Phytophthora.  Other broad-spectrum fungicide seed treatments will protect against Fusarium and succinate dehydrogenase inhibitors like fluxapyroxad or sedaxane will protect against Rhizoctonia, which actually thrives in warmer conditions.  Another problematic soil-borne pathogen that infects the soybean root system during wet early conditions is sudden death syndrome.  Specialized seed treatments, like fluopyram, are very effective in fending off this disease.     

Early Season Insects 
Early planted soybeans that experience delayed germination and slow growth rates may be more subject to damage from below-ground feeders like wireworms and white grubs.  The small size and slow plant growth rate extends the potential window of feeding injury by these insects and makes the plant less capable of recovering or outgrowing the problem.  Another common early season pest includes over-wintering bean leaf beetles, which can also vector viral disease to the soybeans.  Systemic seed treatment insecticides will usually provide 3 weeks of protection from both above-ground and below-ground feeders.

Herbicide Injury 
Growing problems with resistant weeds has increased the need for residual pre-emergence herbicides.  Certain PPO-inhibitor (Group 14) herbicides may cause phytotoxic response in soybeans under cool, wet soil conditions.  Soybean growth rates slow down under these conditions, and the plant has lesser ability to metabolize the systemic herbicide, which can aggravate the injury.  The injury may appear as brown necrotic regions on the cotyledons or leaves, girdled hypocotyl, and stunted growth.  The symptoms may be difficult to distinguish from or even complicated by cold weather injury and disease (see Image 3).  Your FS Crop Specialist can help determine the best combination program of variety, seed treatment, and herbicides to best manage risks associated with early planting.

Summary of Early Planting Recommendations

• Plant as early as feasible within the optimal window. 
  
• Select a high-yielding, locally adapted, full-season variety to capitalize on solar radiation. 
  
• Use a premium seed treatment fungicide and insecticide to protect against early season threats.
  
• Select an effective herbicide program and application rate that considers the potential for phytotoxic injury and stand reduction.