Let me make the most obvious statement first: When you swap from your normal variety to an earlier variety, you will lose yield potential and that can’t be made up. An example of this would be dropping from a 2.5 RM to a 1.5 RM. You chose the 2.5 RM for a reason—stick with your plan. There are many factors, other than the date on the calendar, to consider when swapping soybean seed maturities.
1. Soybeans are better than corn for stretching maturity, since soybean development is based on the length of the nighttime, or photoperiod. In contrast, corn relies on Growing Degree Days to mature, initiate flowering and eventual physiological maturity.
2. Don’t consider a seed swap until June 16, as long as your initial selection wasn’t pushed too far to the north out of zone. For example, if your initial planned variety was a 4.0 RM and you live in a 3.0 RM area, then you should be swapping for a 3.0 RM and keep the 3.0 RM through June. If you choose to go with an earlier variety than a 3.0 RM, for example a 2.6 to 2.9 RM, then you can keep the new variety well into July.
3. If you initially selected a maturity in the correct zone and a swap is needed in late June to early July, consider only a 0.5 to 0.6 maturity change. This is once again related to the length of nighttime. We expect to see shorter plants due to late planting even within our normal maturity group, but planting an early maturity out of zone will impact that even more. Pods will cluster close to the ground, making them difficult to harvest.
4. Temper expectations for chest-high soybeans—this likely won’t happen in late-planted soybeans. Manage your expectations for yield as well. If you plant the correct maturity in your area without swapping, plan to on average make 80% of expected yield if planted by June 15 and 60% if planted by July 1. A larger reduction in yield may occur with late planting. Early varieties moved South out of zone will mature faster, causing seedfill to occur during the hot and dry periods in July and August. This puts additional stress on the plant and lowers yield expectations.
5. Increase planting rates. Since the soybeans will not get as tall due to late planting, we may need to increase seeding rates to achieve a quicker canopy. Thirty-inch rows planted in late June will struggle to reach canopy; 7-inch, 15-inch and 20-inch rows can still fill the canopy when planted late. On average, consider increasing planting rates by 10-15% in the first half of June and up to 15-20% when planting in the last half of June. This population increase will help to reach canopy and give you more yield potential.
6. There are subgroups within each maturity group. For example, Maturity Group 2 ranges from 2.0 to 2.9. Swapping from a 2.5 RM to a 2.4 RM makes no sense, because there is no standard method to rating soybean maturities. One company may call a variety a 2.5 RM, but another may call it a 2.3 RM or a 2.5 RM, depending on the holes in their line-up. If a swap is warranted, swap varieties for a 0.5-0.6 RM lower.
7. Soybeans speed up when planted late. Trials conducted at seven sites over five years with the same soybean variety planted 40 to 60 days apart reached physiological maturity within 7-10 days of each other. Even more noteworthy, a 0.5 to 1.0 maturity group spread resulted in a difference of only 3-5 days to reach maturity—another good reason for not drifting too far out of your normal zone.
8. Delayed planting days are not comparable to days in the Fall. For example, when a late-planted soybean has a four day delay in planting, this is equal to approximately one day in the Fall. Consider this discrepancy when looking at the average freeze dates in your region.
Understanding the potential pitfalls of swapping out your soybean seed maturities will help you lay out a plan for moving forward in a challenging planting season. Check with your expert crop insurance provider, as they can help you navigate your coverage options. Above all, be safe when you do finally get a chance to plant.
Pro Tip: Check out this soybean planting decision tool for Iowa (which is also helpful in Minnesota and Missouri. The goal is to help farmers understand planting date by maturity group and location interactions on yield. This is only a single tool in the tool box to you make informed seed-swap decisions.
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Sources:
https://crops.extension.iastate.edu/cropnews/2019/05/late-soybean-planting-options