Benefits of Early Planting. Farmers may be tempted to start planting soybeans as early as possible, and for a 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 periods and extend the effective seed-filling period.
The optimal soybean planting date varies by latitude and by season, but generally falls between mid-April and early May for the central soybean production areas in the Midwest (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 due to a few factors such as less time for vegetative growth, decreased day length and unpredictable timely rains. Nonetheless, 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 (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 (> 54° F) at least 48 hours before a cold rain, then the risk for chilling injury is minimal. If soil temperatures were cooler at planting (< 50°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 considerably from field to field, depending on tillage and residue. Before planting, be sure to check actual soil temperature and the forecast for the next 7 to 10 days. Whenever possible, avoid ‘beating the rain’ scenarios and strive to plant when temperatures are expected to remain warm and stable after planting AND soil conditions permit.
Frost. 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 from emergence on into the early vegetative stages. 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 temperatures below 32° F. Temperatures below 28° F may be lethal. Frost injury symptoms will appear as water-soaked leaves and hypocotyls that eventually blacken and wither. These symptoms can develop in a matter of 24-36 hours after a frosting event and entire fields are typically affected uniformly.
Seedling Diseases. Cool, wet soils at planting are favorable for Pythium and Sudden Death Syndrome (SDS) infection, while wet and warm conditions favor Phytophthora infection on soybeans. Pythium and Phytophthora are oomycetes or ‘water fungi’, that produce zoospores able to swim through the soil water to the root surface and infect the plant. Avoid planting in wet soils with compaction and drainage issues, as these conditions will favor infection by the above pathogens. In early planting situations, fungicide seed treatments can provide early season protection from these fungal pathogens. For more information on soybean seed treatments visit: https://www.fssystem.com/Products-Services/Resource-Center/Resource-Detail/seed-treatments-for-soybeans
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, seedcorn maggots and white grubs. The small size and slow plant growth rate extend the potential window of feeding injury by these insects and makes the plant less capable of recovering or outgrowing the damage. In fields with a history of these pests or those with high organic matter or recent manure applications, an insecticide seed treatment can help protect seeds and seedlings from damage. For more information visit: https://www.fssystem.com/Products-Services/Resource-Center/Resource-Detail/seed-treatments-for-soybeans
Herbicide Injury. Growing problems with herbicide resistant weeds have increased the need for residual pre-emergence herbicides. Certain PPO-inhibitor (Group 14) herbicides may cause a phytotoxic response in soybeans under cool, wet soil conditions. Soybean growth rates slow down drastically under these conditions, and the plant has a lower ability to metabolize the systemic herbicide, which can aggravate the response. The injury may appear as brown necrotic regions on the cotyledons or leaves, girdled hypocotyls, and stunted growth. The symptoms may be difficult to distinguish from cold injury or disease (Image 3). Herbicide injury and disease symptoms may not always be uniform but may be more scattered and random in fields, whereas frost injury will be a more uniform pattern.
Summary 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 (where warranted), to protect against early season seedling diseases and insect pests.
- Select an effective herbicide program minding the potential for crop injury under early planting conditions.
Image 1. Soybean planting date response over 19 central and northern Illinois site year 2010-2014 (Source:Nafziger and Vossenkemper, 2015. Link: http://bulletin.ipm.illinois.edu/?p=2931)
Image 2. Interaction between varietal maturity and planting date across 12 site-years in central and northern Illinois and central Iowa, 2012-2014. (Source:Nafziger and Vossenkemper, 2015. Link: http://bulletin.ipm.illinois.edu/?p=2931)
Image 3. (Top right) Soybeans expressing symptoms related to the combination of cold weather, herbicide injury, and soil-borne disease. (Source:Ohio State University. Link: https://agcrops.osu.edu/sites/agcrops/files/newsletter_article/image/Soybean%20seedlings.jpg)
Other Sources: Iowa State University