Accelerating soybean yields

By Alexandra Stinemetz and Kyle Poling, Pioneer field agronomists

Soybean was brought to the United States in the late 1800s or early 1900s, first as a forage crop. Farmers soon learned that the protein from the seed was a much better feed supplement for livestock than feeding the whole plant. Growing soybean gained in popularity in the 1940s and is now the second largest row crop (based on acreage) in the country.

Plant breeding has significantly increased the yield potential in modern-day soybean varieties. Yield improvements in soybean is focused on (1) producing more seeds per acre and (2) larger seeds on each plant. In the process of selecting higher yielding varieties, soybean breeders have improved disease tolerance, stress tolerance, and altered growth patterns compared to older varieties. 

Today’s varieties spend 7 to 10 less days in vegetative growth and nearly 2 weeks more in the reproductive stages. This change in soybean growth habit has provided huge opportunities for increased yield compared to “the varieties that Grandpa grew.” Newer varieties will respond more favorably compared to older varieties when growers adopt crop management strategies that take advantage of the entire growing season, increase nutrient availability, and promote improved plant health.

The number of seeds produced by a soybean plant is directly tied to the total amount of sugar produced by the plant during photosynthesis. Because photosynthesis is driven by solar radiation, adopting practices to capture more sunlight often results in higher yields. Grower practices that have shown to most consistently enable soybean plants to maximize photosynthate production include: planting full-season varieties, targeting an early planting date, eliminating weed competition, improved nutrient availability (especially potassium), water management, and protection against diseases and insects. 

Foliar fungicides protect plants from leaf diseases that can decrease sunlight interception. In soybeans, research has shown the ideal timing for a fungicide application to be the R3 growth stage (when pods are 3/16-inch long near the top of the plant). In trials by Pioneer Hi-Bred, foliar fungicide applied to soybeans at R3 has resulted in an average increase of +3.7 bushels per acre. Delaying fungicide application into late R3 or early R4 will be more beneficial than applying during the flowering stages (R1-R2). 

Fungicide products containing multiple modes of action (MOA) offer the best consistency versus a single MOA product. While disease protection can be provided from multiple families, the strobilurin (QoI) fungicide family will provide additional plant health benefit that can help to preserve yield. The addition of insecticide can lead to an increased yield response in soybeans from protection against leaf and pod feeding as well as due to the additive effect of a fungicide with insecticide.

Some growers dislike the fact that applications foliar fungicide delay harvest. However, the trade-off for a later harvest date is opportunity to extend the seed-fill duration. The length of the seed fill period from R6 to R7 has a major impact on seed weight, that directly influences yield.

The use of plant growth hormones (PGHs) in soybean production has become more common. While there are many known growth hormones, the most common include: auxins, cytokinins, gibberellins, ethylene, abscisic acid, and synthetic substances. Below are three common plant growth hormones important for soybean growth and development. 

Auxins: PGHs known for involvement in the development of shoot architecture and apical dominance, enhancing nitrogen utilization, improving nodulation, and increasing dry matter accumulation/seed yield.

Cytokinins: Cytokinins are formed primarily in root tissue and small quantities in newly developing leaves. These PGHs promote cellular division and lateral bud formation. Nutrients will accumulate in tissue where larger levels of cytokinins are expressed. Cytokinins promote root and shoot development and stimulate plant responses to stresses. 

Gibberellins: This class of PGHs are known for enhancing water use efficiency in a plant, inducing nodulation, improving tolerance to abiotic stresses (such as drought) and increasing flowering/pod setting/seed yield. 

It has been documented that it is not uncommon for 60% to 75% of flowers on a soybean plant to abort. Application of PGHs could be an opportunity to retain more flowers and achieve more pods per plant. Current research data on PGHs is inconclusive. However, soybean research does indicate that promoting flower and pod retention is a key area to focus on in in the goal to achieving high yield soybeans.

In the journey to reaching higher yields in any crop, the road traveled is a season-long, multi-year endeavor. Finding the right combination of management practices, specific products, application timing, and environment can all impact the success rate.