Shifting soybean planting dates earlier has been recognized as an opportunity to increase soybean yield (Mourtzinis et al., 2019; Salmeron et al., 2014). Early planting can result in more light interception and faster early-season canopy closure; this can lessen moisture stress during the reproductive periods, lower the risk of wind and water erosion, and decrease weed competition, thereby increasing yield potential while protecting the soil. The main consideration when shifting planting dates earlier is the probability of occurrence of late frosts after crop emergence. In soybean, the growing points are aboveground and exposed after the cotyledons open. It’s fatal when all growing points are frozen. To kill soybean tissue, temperatures must be at or below 28-30 oF for several hours.
Early planting date decisions should consider the probability of late frost events. The frost risk distribution for different planting dates will be dependent on the location (latitude, position in the landscape, etc.). There are tools such as the Freeze Date Tool (Midwestern Regional Climate Center-USDA ) to provide information about the last spring freeze dates across the northcentral and northeastern United States. This kind of tool can be useful, but it doesn't take into account that from planting date to emergence the warm soil protects the plant. Thus, it seems that a precise tool to assist planting management decisions, based on frost risk in early stages, should incorporate this into the model.
The time from planting to emergence cannot be defined as a fixed period. The duration of this crop developmental stage depends on the average soil temperature. Since soil temperature is driven by ambient conditions, soybean is known to require 90 °C days (over a base temperature of 10 °C) after planting before it emerges (Castell, 2010). Therefore, we propose to combine i) this developmental predictor based on daily average temperature, and ii) data on frost date to define the probability of frost occurrence for soybean at a county level.