Early season soybean replant decisions
click image to zoomSoybean Growth and Development, PM 1945, Iowa State University Extension and Outreach.Soybean seedling with plant structures labeled. Early planted soybean often needs visual assessments for stand quality. Nearly every year some soybean are replanted in Iowa. The decision to replant an area or entire field is necessitated due to saturated soils, ponding, hail, frost and disease incidence. This article provides a step-by-step process to use while making soybean replanting decisions.
Step 1: Stand assessment
Initial visual assessment of soybean is critical following frost, hail or herbicide drift events. This assessment is useful in determining the extent of the area affected, but is often a poor time to make a replanting decision. After severe weather or herbicide exposure, crop injury often looks worse than it really is and sometimes emotions can influence a replanting decision.
Conduct a thorough ‘regrowth’ stand assessment three to seven days following injury events. Examine the main stem for growth from the apical meristem. If the apical meristem is dead or broken off, look for new growth from the axillary buds on the remaining stem nodes (juncture of the leaf petiole and main stem).
Step 2: Stand counts
When estimating a stand count in soybean, make sure to accurately assess the remaining viable stand. An average stand count for the entire field or injured area should be determined. The number of locations needed to determine an average plant stand depends on the methodology used. For example, when using 1/1,000th of an acre or a hoop method, it is generally acceptable to stop at 6-10 random locations to accurately obtain an average. When estimating the number of plants per foot on row lengths less than six feet, consider using 15 to 20 locations to get an accurate average number of plants per foot.
While doing stand counts consider the uniformity of the soybean distribution within the row or area. Because of branching soybean can adequately compensate for one-foot gaps from early vegetative injury but the later injury occurs the less branching will occur. Plant gaps greater than a foot should be of concern in 30-inch row fields because that could negatively impact yield if there are less than 75,000 plants per acre.
Step 3: Yield potential
After an accurate assessment of the remaining stand is complete, determine what the yield potential of the remaining stand is for the population and planting date. If replanting is being considered, use available charts to assess the yield potential of a late-planted soybean crop with expected plant populations. Generally, replanting should be considered if uniform stands are less than 75,000 plants per acre when planted mid-May or earlier or less than 50,000 to 60,000 plants per acre when planted late May into June. Remember, crop production is a biological system that relies heavily on the weather during the growing season. Because of this, determining actual yield potential at various planting dates and plant populations can be extremely variable.
Step 4: Additional Factors