Image 3. Soybeans expressing symptoms related to the combination of cold weather, herbicide injury, and soil-borne disease. (Source: Ohio State University).
- Planting soybeans early increases full-season canopy photosynthesis (carbohydrate production), which results in higher yield potential.
- Early planted fields are exposed to various stressors that can result in emergence, stand establishment and crop injury issues.
- Selecting a well-adapted soybean variety, effective seed treatments, and appropriate herbicide programs can help manage some of the risks associated with early planting.
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 up to 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 downside 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 to common diseases and pests within and across the normal maturity range for your geography. However, recent research suggests that early planting may be of greater benefit to 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.
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
Wet soils at planting are favorable for many species of Pythium and the fungus that causes Sudden Death Syndrome (SDS), while wet and warm conditions favor Phytophthora, Red Crown Rot (RCR), and Rhizoctonia infection on soybeans. Consider field history and select soybeans that display tolerance to pathogens of concern when possible. As soybean cyst nematode (SCN) has been shown to influence SDS, selecting a variety with tolerance to SCN may subsequently decrease risk of SDS. 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 (roughly 3 weeks) from these fungal pathogens. Any factor that slows seedling emergence and growth will make seedlings more prone to seedling diseases, so planting into conditions that favor rapid emergence and growth are essential. For more information on soybean seed treatments click here.
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 pests 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. Slugs can cause significant stand reduction, particularly in fields with terminated cover crops. Slugs are not insects, they are mollusks. Insecticides are not effective control measures but there are baits labeled in some states that provide slug control.
Herbicide Injury
Growing problems with herbicide resistant weeds have increased the need for residual pre-emergence herbicides. However, caution is advised when planting early as many soil-applied herbicides may cause a phytotoxic response in soybeans planted in cool, wet soils as the slow growth of the plant reduces the ability to metabolize the herbicide. The symptoms may be difficult to distinguish from cold injury or disease (Image 3), and they may vary between herbicide groups. For example, while injury from ALS inhibitors may manifest as overall stunting and yellowing, a rainfall following the application of a PPO inhibitor may also include brown, necrotic spots on leaves and stems. 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 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 (only 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)
Other Sources: Iowa State University
Related categories: early planting, soybeans, maturity, disease, insects, temperature