Does early equal better when planting soybeans?

Interest in planting soybeans early to maximize yield potential has increased in recent years.

To understand why early-planted soybeans may increase yield, we must look at two very different disciplines: plant physiology and astronomy.

A soybean plant’s life cycle, including the onset of reproduction or flower initiation, is regulated by night length. Soybeans with shorter relative maturities need less darkness to begin flowering than longer-maturing soybeans. This is why longer maturities are selected for later planting dates, such as double-crop soybeans following wheat. The longer maturity allows the plant to grow more vegetatively before flowering, increasing sites for flowering and yield potential.

That’s the plant physiology portion of the discussion. Now for the astronomy. The first day of summer occurs around June 20-21. This day, the summer solstice, has the longest period of daylight and shortest period of darkness. Soybeans “know” this due to the chemical phytochrome, which is present in plants that have developed trifoliates. As long as soybeans are large enough to contain phytochrome, they will begin flowering at a certain point that’s triggered by length of darkness. That point depends on relative maturity. There are about 10 days between maturity groups, so if Group 3 starts flowering a few days after the solstice, with all else being equal, you can expect Group 4 to start around 10 days later.

Where does the increased yield potential from early-planted soybeans come from? Let’s use an illustration to help explain it. Imagine that the growing season is not measured by days but by a trip up a mountain and down the other side. The peak represents the summer solstice and the least amount of darkness for the year. Every night on the other side of that peak will be a little longer until the winter solstice in December, when the process reverses.

Using the mountain analogy, a soybean passing through a certain elevation that triggers flowering on the back side of the peak will have also traveled through that elevation on the front side. If there is enough growth to produce phytochrome, the plant will start flowering at the earlier “elevation” or period of darkness. Plants that begin to flower prior to the solstice will eventually sense that the nights are still getting shorter and will stop flowering until that same period after the solstice and then flower again. If retained, the flowers produced during the early period provide the opportunity for increased yield deep in the canopy before the rest of the plant shades those flowers.

Understanding these dynamics helps to inform our variety selection. Remember, a Group 3 soybean flowers closer to the solstice than a Group 4. If we want early-planted soybeans, we must plant Group 4 varieties first because their photoperiod will not only occur farther behind the solstice but also that same distance before the solstice.

There are some cautions associated with this practice. Late frost is an obvious one. I spent several early mornings last April exchanging texts with nervous growers who were monitoring freezing temperatures around their bean fields. Not only was the stand at risk, but if a replant was needed, that expense would most likely fall on the grower.

Early flowering also decreases the window for post herbicide applications. Liberty and Xtend may not be applied after R1 (flower on any node), and Enlist may not be applied after R2 (open flower at one of the two uppermost nodes). Changing practices to encourage early flowers only to lose them with an off-label application doesn’t make much sense, so residual herbicides and, in my opinion, narrow rows that shade the ground sooner are key to making this practice work.

In addition to narrow rows, seed treatments are table stakes for early soybeans to manage stand-
robbing fungi and the effects of sudden death syndrome later in the season.

Early-planted soybeans may also benefit from sulfur at planting. Cool soils do not provide as much sulfur through mineralization as warmer soils do, so an application near planting may be beneficial.

Most of us live by the adage of not putting all of our eggs in one basket. We do that with seed varieties, investments and, for some of us, actual eggs. Early-planted soybeans may be a good fit for some of your acres, but date diversification may be prudent to manage the challenges that come with the practice.