Selecting soybean maturities when facing delayed- or re-plant situations

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

Every spring is different in Minnesota, but the spring of 2024 seams unusually -- well, unusual. The spring started extremely warm and dry with some farmers planting corn and soybeans in early April. Nagging rains slowed planting throughout the state for a few days to several weeks throughout the state early in the spring. By the end of May, the entire state had received enough rainfall, planting ground to a halt, and water began ponding in fields. As of June 2, USDA-NASS reported that 80% of the state’s soybeans were in the ground. But some fraction of those that have been planted will require replanting.

So, here we are in early June with some soybeans that either need to be planted for the first time, or perhaps need to be replanted. Conventional agronomic wisdom (and University of Minnesota recommendations) says that farmers should hold their current varieties when planting is delayed until June 10, then relative maturities (RM’s) should be reduced by 0.5 points. This recommendation is intended to support maximum yields given the reduced yield potential due to planting delays, without increasing the risk of frost injury. This very basic recommendation also represents a solid starting point for making more nuanced decisions about planting and or replanting soybeans in June. There are several factors that will affect how a farmer might utilize this rule of thumb. These include:

1. Is this a delayed planting or a re-planting situation?
2. Does the farmer have remaining seed, or will new seed be required?
3. Does the final maturity need to match earlier planted soybeans?
4. What is the farmer’s level of risk aversion for frost damage?

Our work has shown that when starting with locally adapted, full-season soybeans, RM shifts of +/- 0.5 units have very little effect on yield potential. Choosing high yielding varieties is more important that dialing in on very specific RM’s, from a yield perspective. Therefore, we will not focus on yield potential here, but we will instead take a deep dive into the effects of delayed planting and RM on final maturity of soybean in Minnesota. This will help producers to choose varieties that either match maturities of earlier planted soybeans, mature within a reasonable timeframe of the early planted soybeans, or avoid early frost events.

The data used to illustrate the effects of delayed planting date and RM on soybean maturity here are derived from a Minnesota Soybean Research & Promotion Council supported soybean modeling project, Soybean phenology predictor tool for Minnesota. This project has adapted, calibrated, and validated the sub-routine for crop development estimation from the CROPGRO model using local data previously generated by the Naeve lab and data from the literature. Once calibrated and validated to local conditions, the model was coupled to the DAYMET database to obtain the weather variables to estimate phenology dates for a determined combination of variety, planting date and location. We used local weather information for each region from 2000 to 2022 (22 years). As a first step, we generated information for each of the primary soybean production regions of Minnesota.

Here we provide model runs for six soybean producing regions of Minnesota and are presented as phenology charts. Within each region we provide four soybean RM’s from full season to very short. Models were run to predict R7 (physiological maturity – one pod with mature color) across planting dates for each of the four RM’s. Note that harvest maturity, or R8, may lag behind R7 by 10 days.

In each chart below, we have identified two theoretical planting dates for illustrative purposes. We have identified May 10 as the initial planting date (or target planting date) and June 10 as the replant date (or delayed planting date if early planting was not possible). The solid curved lines represent predicted (average) R7 dates for each planting date and RM combination. Dotted lines indicate the range within which R7 is expected to fall in 50% of years

Using the Southwest region as an example, one can see that the expected R7 date for a RM 2.0 soybean planted on May 10 would be close to September 27. If a farmer was delayed in planting by one moth or replanted on June 10, they would expect that same soybean variety to mature 12 days later - October 10. If that same farmer were to switch to a soybean that is 0.5RM shorter, or a 1.5RM, maturity would be delayed by 7 days. If the farmer wanted to maintain the same maturity date for the delayed planted soybean, they would need to reduce their RM by one full unit.

Under what circumstances might a farmer make each of these choices? Here are three theoretical scenarios.

1.  Farmer #1 was able to plant all of their soybeans except for one field (10% of their acreage) during the early planting window around May 10. All of their planted fields have good stands without drown out areas. All of their seed order is on-hand. Their seed dealer does not have the very best soybean genetics available in earlier RM varieties, only some returned seed from up North.
1. Decision: Farmer #1 decides to hold with their current seed.
2. Reasoning: Farmer #1 is willing to accept delays in harvest due to their own personal harvest logistics. They have the capacity to switch between corn and soybeans during harvest. They are willing to accept some frost injury to their soybean based on the fraction of their soybean crop and because they are still within a reasonable frost-free period. For instance, 90% of years frost occurs after September 22 in their region (based on a very conservative 30F minimum temp).
2. Farmer #2 planted all of their soybeans on around May 10 but lost 10% of their crop to drown out areas within each of their fields.
1. Decision: Farmer #2 decides to switch RM by 0.5 earlier to replant down out spots
2. Reasoning: Farmer #2 wanted to maximize yields without delaying maturity deep into October. By replanting areas with a RM 0.5 unit shorter than the surrounding beans, most drown out spots will still require a slightly later harvest, but the delay is going to be shorter than with a full season variety. The farmer plans to clean up all of the drown out spots just prior to corn harvest or possibly delay harvest of selected fields a couple of days to allow all soybeans to reach harvest moisture levels.
3. Farmer #3 planted all of their soybeans on around May 10, but lost less than 5% of their crop to a few drown out areas within some of their fields.
1. Decision: Farmer #3 decides to switch to a soybean variety one full RM shorter than the original planted soybeans
2. Reasoning: Farmer #3 prioritizes harvest logistics. They do not want to delay soybean harvest or harvest around low areas, yet they would like to have a soybean crop in low areas for weed suppression, and to support their overall APH. They want a crop that will mature with a high level of synchrony with their surrounding early planted soybeans. Therefore, they chose a much shorter variety that will mature at the same time as the surrounding early planted soybeans.

These three hypothetical farmers are provided as examples of different scenarios resulting in different decisions about delayed or replanted soybeans. This illustrates that there is not one correct answer for all situations and there is relatively little downside even when the optimum decision is not made. In fact, there is no wrong answer here. Changes in soybean RM’s will change maturity date and affect harvestability significantly, but it has relatively little effect on yield potential. We have found that short RM varieties yield as well or nearly so with full season varieties, and frost injury near R7 has little effect on yield.