Sunlight and Soybeans

July is an interesting month for soybean growers. The crop has been planted and most of the post-emergence herbicide applications have been completed. With the exception of potential fungicide application, most of the factors that are within the farmer’s control are past. Weather will be the biggest factor in determining the size of the soybean crop. To understand the role that weather plays in determining yield, I spoke with Dr. Emerson Nafziger from the University of Illinois. Dr. Nafziger points out that soybean yield potential is determined in July and yield is created in August.

Setting Pods
The number of pods per square foot or acre is the single most important factor that correlates to yield. Soybean nodes are being created at a rate of about one every 4 days, regardless of weather conditions (Bastidas, et al. 2008). Earlier planting usually results in more nodes, which is why planting date is so important for maximizing node formation and yield. Although nodes are a prerequisite for ultimately producing a harvestable crop, there are other factors that affect how many pods will be set and retained on the plant to produce seed.

The first couple of months of the growing season set the foundation for potential yield. Conditions in May and June determine leaf area, which is related to growth rates and nitrogen uptake. This sets up how well-prepared the plant is to put on and keep a lot of pods during the podsetting stages, usually about the last third of July and first third of August. Soybean plants set a lot of flowers, and each flower has the potential to produce a pod, but only small percentages of flowers form pods. Ultimately only 20-40% of pods that are created go on to produce seeds (Nonokawa, et al. 2007).

While it’s common for people to wish more flowers turned into pods and seeds, soybean plants have been bred for high yields, and that means they have a lot of flowers to assure setting enough pods so they can fill them. Today’s varieties have high yield potential because they hold on to pods much better than the varieties of the past.

Photosynthesis Key in July
The most important factor determining how well soybean plants set and keep pods is how much photosynthesis occurs in the crop on a daily basis, especially this time of year. Maintaining high photosynthesis takes a good canopy with good leaf color, enough soil moisture, temperature and sunlight. Soybeans with enough water grow well at 85°F or so; higher temperatures can create heat stress (lower photosynthetic rates) at any growth stage, especially if soil water starts to run short.

Sunlight quality and timing are both important. Maximum sunlight usage requires a full canopy. A high-yielding soybean crop uses about 5% of the energy from sunlight to create yield, but the amount of sunlight energy that falls on a field is so high that this is enough to produce 100-bushel yields. Having a lot of cloudy days can limit yield potential, but will lower rates of water loss, so might not lower yields if water is the limiting factor.

Under perfect conditions after seeds start to develop, soybean plants can fill seeds at rates as high as four bushels/acre per day, according to research conducted at the University of Illinois. Early planting and favorable early season growing conditions prepare the foundation for reaching such levels of yield accumulation, but we have had very high yields with less-than-ideal early seasons.

In 2020 growers had the early planting down pat, but growing conditions across most of the state have been less than ideal. Soybeans have shown an incredible ability to thrive, even in adverse conditions. As an example, the Illinois soybean crop ratings on July 1 in 2014 was about 75% good or excellent. In 2015, that number was only 52% but yields in both 2014 and 2015 ended up at the same 56 bushels per acre, which set and then tied the record high for Illinois. So as long as the canopy is in place with good color in late July, there’s no reason to give up on this amazing crop.

Sources:
Bastidas AM, Setiyono TD, Dobermann A, Cassman KG, Elmore RW, Graef GL, Specht JE. “Soybean Sowing Date: The Vegetative, Reproductive, and Agronomic Impacts” Agronomy & Horticulture — Faculty Publications. 99.2008.
Nafziger ED. Personal communications (May 29 & July 1, 2020).
Nonokawa K, Kokubun M, Nakajima T, Nakamura T, Yoshida R. “Roles of Auxin and Cytokinin in Soybean Pod Setting” Plant Production Science 2008;10(2):199-206. DOI: 10.1626/pps.10.199