Averages help predict soybean stages during year of extremes

By Shaun Casteel, Purdue Extension Soybean Specialist

We live in a life of averages. GasBuddy, a crowd-sourced app, reported the highest national average price of gas was $3.70 in 2024. It also reported the top 10 percent of stations averaged $4.07 while the lowest 10 percent of stations averaged $2.46 per gallon in 2024.

We can follow similar extremes in the highs and the lows of most anything – the cost of eggs, fertilizer prices, trucks (don’t get me started), weather and so on. Indiana has an average rainfall of 40.6 inches each year with about 37 inches in northern Indiana and 47 inches in southern Indiana. This range of averages are based on 30 years of weather data, yet they do not describe the season that we are currently experiencing.

I think the best way to use averages is as a benchmark for comparison.

For instance, southern Indiana experienced a very wet spring (Figure 1) that delayed the planting progress for weeks to more than a month depending on the area. Let alone, our friends in Kentucky who have already received a year’s worth of rainfall (see dark blue on the map).

Even this interpolated map averages across the landscape making it seem like north-central and east-central Indiana are right on track for normal rainfall, when we know that areas have been wet most of the spring with small windows of planting in late May to June.

The other side of this extreme (and the map) is west-central and northwest Indiana (Figure 1). Dry spring conditions allowed for advanced planting progress with the hopes of great yield potential.

If we are to look at the planting progress of soybean at the state level, Indiana appeared to be a near-normal or average pace. Clearly, we know that we have a season of extremes in weather from over 20 inches of rain since April to drought conditions in northwest Indiana (D0-D1 for 23 percent of the state, droughtmonitor.unl.edu).

The 2025 soybean crop is variable to say the least. In early July, I have walked fields that are entering R3 (first pod with 12 trifoliate nodes) to those that are V4 (4 trifoliates) to discussing with farmers when to stop trying to plant soybeans.

Most people have the appreciation that the indeterminate soybeans we grow in the Midwest respond to photoperiod, but they also respond to heat units (thermal energy, growing degree days or GDDs, etc).

The catch is that soybeans have a sliding scale of influence depending on maturity group and planting date. The vegetative growth (e.g., trifoliate nodes, branches, internodes) and reproductive development (e.g., flowers, pods, seeds) are usually less as planting is delayed and/or maturity group is shortened.

Please use Table 1 to loosely predict when your fields will reach R1 and R8 maturity based on planting date and maturity group combinations. On average (remember there is a range and the time will be reduced as planting is delayed and/or maturity group is shorten), the duration of reproductive stages are approximately: