Risk management for early planted soybean

 Anibal Cerrudo, Researcher, and Seth Naeve, Extension soybean agronomist

Key points

Photo: Dave Nicolai

Early soybean planting can lead to higher yields: Early planting offers advantages in resource utilization, potentially resulting in increased yields compared to delayed planting.

Consideration of frost dates and plant-to-emergence stage is essential: When planning early planting, it's crucial to assess the local risk of late frosts after emergence, germination chilling injuries, and the duration of the plant-to-emergence stage to determine optimal planting windows.

Seed quality and protection are imperative: Ensuring seed vigor and employing technologies like fungicides and insecticides are essential for early plantings to enhance crop resilience against environmental challenges.

Utilize available data and technologies: weather records, leveraging local monitoring, weather forecasts, and seed technologies can aid farmers in making informed decisions to reduce risks while searching for the potential benefits of early plantings.

Research consistently indicates that the timing of soybean planting significantly impacts soybean yield. Early planting could offer an advantage, resulting in potential higher yields compared to delayed planting. This advantage stems from improved resource capture and utilization. However, while early planting presents potential yield benefits, it's crucial to evaluate risks due to adverse conditions affecting crop establishment that could depress yield potential, and which may even lead to replanting.

Figure 1. Conservative last frost date (daily minimum
 temperature below 30°F) at a county level (90th
quantile). Nine out of 10 years the last frost should
 be earlier than the indicated date. Estimated using
DAYMET database for the last 21 years.

A key consideration when adjusting planting dates earlier is the likelihood of late frosts occurring after crop emergence. Predicted conservative last frost dates at the county level can be observed in Figure 1. As expected, the timing of the last frost and its risk distribution across Minnesota is heavily dependent on latitude. Moreover, other factors such as altitude, position in the landscape, and moisture levels also contribute to the observed patterns among counties and would contribute to the variability we should find within counties. Additionally, it's worth noting that while our estimates are conservative, using 30°F as a frost limit, in many situations, crops can withstand lower temperatures without significant damage. However, it's essential to recognize that the objective of the report is not to provide recommended localized planting dates but to initiate a discussion about the different risks associated with early plantings.

To inflict damage on soybean tissue, temperatures typically need to reach or fall below 28-30°F for an extended period. However, prior to emergence, the soil serves as a protective barrier against freezing temperatures. The duration from planting to the emergence stage varies based on the average soil temperature. Considering that soil temperature is influenced by ambient conditions, soybeans typically require approximately 200 growing degree days (above a base temperature of 50°F) after planting before emerging above the surface.

Figure 2. First possible planting date for an emergence after
the last frost at a county level (90th quantile). If planting
 soybeans on these dates, nine out of 10 years the crops should
 emerge after the last frost. Estimated using DAYMET database
 for the last 21 years considering the duration of the
 planting-to-emergence to be 200 growing degree days
above a base temperature of 50°F.

Incorporating this plant-to-emergence period into the equation, and considering both the spatial data on frost dates and the duration of this stage based on daily average temperature, we can determine an indicative earliest planting date that will prevent soybeans from emerging before the last frost, as illustrated in Figure 2. This figure displays the first planting date that would allow the emergence to occur after the last frost in 9 out of 10 years (shown in Figure 1), with an imposed earliest planting limit of 20 days before that date.

As anticipated, the potential earliest planting date precedes the last frost date. A key point to consider is that after planting the seed there is a very fast uptake of water (the imbibitional phase typically is not longer than 24 hours). Chilling during the first phase can cause problems because cold temperatures (conservatively, less than 50°F) interfere with proper hydration of the membranes. Thus, getting a cold snap 0-24 hours after planting can lead to chilling injury in soybean. Chilling injury is likely to be lower if soil temperatures become cold 24 or more hours after sowing. Checking the average soil temperatures and the temperature forecast are useful tools to manage chilling injuries during germination. However, even managing the seed chilling damage, what we want to emphasize here is the long duration for the plant-to-emergence stage under these earlier planting date strategies. By comparing Figure 2 and Figure 1, we are able to deduce that for these early planting dates, the plant-to-emergence stage would extend well over two weeks (the difference in between planting date (Figure 2) and the frost date (Figure 1). In fact, the average estimated difference between the last frost and the earliest planting date for the entire state was above 17 days.

Figure 3. First planting date that allows the crop to emerge
 in less than 15 days at a county level (90th quantile). Nine
out of 10 years the crop would emerge in 15 days or less
 if planted on the indicated date. Estimated using DAYMET
 database for the last 21 years.

The duration of the planting-to-emergence stage is relevant for germination, seedling emergence and crop establishment. In general, the longer this stage lasts, the greater the chances of encountering disease and insect-related issues, and the more uneven the crop is likely to be. A different approach to analyze the same issue is to assess the first planting date that would allow the crop to emerge in a determined period of time. Accordingly, Figure 3 illustrates the first planting that would allow the crop to emerge in less than 15 days in 9 out of 10 years. If we compare this with Figure 2 we see that these dates are later compared to the earliest planting date targeted to avoiding frost. Therefore, the length of the planting-to-emergence stage is a significant consideration for early soybean plantings.

The noteworthy aspect here is that while we can set an early planting that implies a low frost risk (the idea behind Figure 2), it seems difficult to escape from a prolonged duration of the plant-to-emergence stage for early plantings. Let's take the data for Le Sueur as an example. For this county, the earliest possible planting was Apr 28th, which means that if we plant after this date we should expect a low risk of frost damage for our crops (1 out of 10 years). However, for Le Sueur, the first planting date that allows the crop to emerge within 15 days is May 4th. Therefore, there is a low frost risk for plantings at the beginning of May, but what is certain is that for these earlier planting dates, the period from planting to emergence will exceed 15 days. Consequently, while the last frost generally raises the major concerns, the prolonged plant-to-emergence period consistently impacts early planted crops.

Fortunately, we have tools at our disposal to address these situations. Seed quality, such as vigor, is crucial. A pertinent recommendation for such cases is also to evaluate and leverage technologies that aid in seed protection, such as fungicides and insecticides.

In conclusion, early soybean planting offers the potential for higher yields due to improved resource utilization. However, it's essential to carefully manage the associated risks, particularly regarding adverse conditions that may affect crop establishment. Understanding the relationship between planting timing, frost occurrence, and the duration of the plant-to-emergence stage is crucial for attaining yield potential while mitigating risks. By leveraging available data and technologies, farmers can make informed decisions oriented to reduce frost risk, protect seed quality, and enhance crop resilience against environmental challenges presented in early plantings.