Research HighlightsLong Juvenile Genetics Offer Potential Solution for Ultra-Late Soybeans
By Laura Temple
The long growing season in the Deep South permits farmers to raise two crops during the same growing season. They can plant a crop like sweet corn, field corn or watermelon in early spring and harvest it in late July or early August. They can then immediately plant soybeans to harvest in November or December, known as the ultra-late soybean production system. It’s an option for fields where peanuts are not part of the rotation.
“With this system, farmers are betting on a late frost, and soybean growth often ends suddenly due to a killing frost,” explains Michael Maw, assistant professor of agronomy at Abraham Baldwin Agricultural College in Tifton, Georgia. “Their other challenge is finding a soybean variety that will grow tall enough to produce a decent yield.”
He says the limited growing period for ultra-late soybeans often results in shorter plants with fewer, more compact nodes where flowers and then pods grow. Plus, the most mature pods with the largest soybeans are the first ones formed on the plant — and closest to the ground. These pods can be too low for the combine to effectively capture them during harvest.
Ultra-late soybeans on the college’s teaching farm provide a “field lab” for his fall semester crop production class. Maw uses this field for small-plot variety trials to help identify the best options for Coastal Plains farmers using this system, while also providing applied research experience for students. The Georgia Agricultural Commodity Commission for Soybeans provided soy checkoff funding in 2022 to support this effort.
Genetics Determine How Soybeans Mature
Soybeans carry determinate or indeterminate genetics that dictate their vegetative and reproductive growth patterns.
- Determinate soybeans tend to be shorter, grow many branches, and complete all their vegetative growth before switching completely to reproductive growth. These varieties typically fall in Maturity Group 6 or higher. They fit well in southern regions with longer growing seasons.
- Indeterminate soybeans typically grow taller, form all pods on the main stem, and continue to add vegetative growth while starting to flower and produce pods. These varieties classify as MG 4 and lower, and they fit well in northern areas with shorter growing seasons, where day lengths trigger the start of reproductive growth. Soybeans in MG 5 may be determinate or indeterminate, although most are indeterminate.
In the ultra-late soybean production system, determinate varieties don’t have time to complete vegetative growth, then switch to reproductive growth and mature. But the crop grows during shorter days in late summer, which also limits vegetative growth of indeterminate varieties. Currently, indeterminate varieties are the best option for this system.
“A third option for how soybeans mature is the long juvenile trait,” Maw says. “These recessive genetics delay flowering during shorter day lengths, allowing plants to put on vegetative growth first and extending the growing season. Varieties with this trait are not bred commercially in the United States, but they work well in lower-latitude growing regions like Mexico and Brazil.”
To learn how long juvenile varieties perform in the ultra-late production system, he started variety trials incorporating varieties developed through the soybean breeding program at Clemson. He also observed how different varieties with this trait put energy into vegetative and reproductive growth.
“Some long juvenile varieties are more indeterminate than others, putting on vegetative and reproductive growth at the same time,” he explains. “Others are more determinate, switching from vegetative to reproductive growth.”
In this system, many varieties don’t reach full maturity. Soybeans often require a desiccating harvest aid or killing frost at the late R6 or R7 growth stage to be ready to harvest.
New Options Show Potential
Maw’s ultra-late soybean variety trials include commercial determinate and indeterminate varieties alongside 10 long juvenile varieties for comparison. Conditions vary for this system every year. For example, the 2022 growing season was going well until a frost stunted plant growth in mid-November without killing the soybeans. Despite this challenge, he and his students have made general observations that encourage further investigation of long juvenile varieties.
“In general, commercial determinate and indeterminate varieties grow soybeans with better seed quality, but with fewer seeds and lower yields,” he reports. “Long juvenile varieties tend to have smaller soybean seeds, but a lot more of them.”
The long juvenile varieties usually grow taller than the commercial varieties, which correlates to higher yields. One of his students analyzed the protein and oil content of these varieties, finding they achieved the same ranges as commercial varieties. Late-planted soybeans in the Southeast often achieve a higher protein concentration at the cost of lower oil content, compared to full-season soybeans.
“I believe that with additional data, we will find the top three or four long juvenile varieties that farmers should consider as legitimate options for their ultra-late soybean production systems,” Maw says. “Taller varieties with seed quality and yield potential that match commercial varieties would fit well.”
Additional Resources:
Testing Varieties for Ultra-Late Soybean Production on the Coastal Plain – SRIN article
Development of Unique Soybean Varieties Fills Export Gap – SRIN article
Published: Jul 31, 2023
The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.
