Compact Soybean Plants and Reproductive Development

Indiana planting pace in 2023 was one of the fastest on record, which lined up with drought years as well as yield-breaking years. For most of our fields, soybean development in the month of June was summarized in one word – stagnant. Well, at least the aboveground growth seemed to stall out with the dry conditions. Fortunately, these soybeans were rooting down deep rather than expending energy into aboveground growth. If we have our choice of dry June or dry August, we will choose a dry June every time (assuming the roots have some access to moisture).

The combination of timely planted soybean with good stand establishment and a dry June sets us up for a nice compact plant. We would rather have a compact plant that has good trifoliate node development and reproductive branches so the water use and photosynthetic efficiencies are optimized during pod development (July-August) and seed fill (August-September). Otherwise, if we get the large plant that has a lot of biomass, we need more water to support that biomass in addition to the developing pods and seeds. Secondly, the large amount of biomass and taller/rank plants can create a situation that is more conducive to foliar disease development (e.g., Septoria Brown Spot, Frogeye Leaf Spot).

The pace of soybean flowering has not followed the timeline of past years of advanced plantings (Figure 1). In fact, flowering has been even delayed compared to the 5-year average. Some might be concerned with this delay, but soybean typically flower over 4 to 6 weeks and often abort nearly 75% of these flowers. Our soybeans have not invested into many flowers and pods as of yet, but again they have established solid root system to access soil moisture once pods develop and seeds begin to fill. In fact, I have found many fields that have developed more vegetatively over the last couple of weeks as the rains have returned. We think of soybean switching from vegetative growth to flowering around V6 (give or take a node), but I have found some fields upwards of V8 to V9 (matching the statewide delay in development).

Figure 1. Indiana progress of soybean flowering in the years with advanced plantings. Years with drought were 1988, 1991, and 2012. State record yields were in 2018, 2020, and 2021. The current progress of soybean flowering is delayed compared to 5-year average. (Adapted from USDA-NASS, 2023)

Soybean planted in April and early May are entering late R2 (full bloom) to early R3 (first pod, 3/16” long pod on one of upper 4 nodes, Figure 2). Many farmers and ag retailers target R3, for a foliar fungicide and/or insecticide application. In a given year, I actually lean toward an application at R4 (full pod, ¾” long pod on one of the upper 4 nodes) if it is needed. Today’s soybeans have more pod development and seed fill later in the season than the varieties of old.

Figure 2. Soybean in R3 (Beginning Pod)

In a season like this one, I am even more on the side of R4 application than R3. This year’s soybeans are compact (i.e., smaller leaves, stacked internodes, shorter stature), so the present amount of vegetative biomass is relatively limited. If foliar fungicide and/or insecticide is on your planned program, I highly suggest holding off until soybean develop more. Soybean typically are in R3 about 10 days (Table 1), so we have time to scout to see if foliar diseases and insect pressures are building and warrant an application. Soybean development of flowers, pods, and seeds are dictated by the growing conditions (photoperiod, heat unit accumulation, and of course water supply). Table 1 provides a good rule of thumb the duration of soybean reproductive stages, but it can fluctuate. For instance, R1 can be as short as a couple days if growing conditions are favorable for active development (e.g., good soil moisture, temperature, and sunlight).

Table 1. Soybean reproductive stage and average duration (days).