Research Highlights

Figuring Out Ultra-Late Soybean Production Fit

Highlights:

• In the southeastern Coastal Plain, farmers can double-crop corn and soybeans in the same calendar year.
• Clemson research broke down elements of ultra-late soybean production to develop best practices and guidelines for farmers to determine if the system makes sense for them.
• Soybean variety selection, crop management timing and logistics and nematode control all impact the success of ultra-late planted soybeans.

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By Laura Temple

A two-for-one deal is hard to pass up. 

Farmers in the southeastern Coastal Plain can raise two crops in one season. The long growing season allows them to grow corn under irrigation, harvest in mid- to late July, and immediately plant soybeans. This ultra-late soybean production system can yield 20 to 35 bushels per acre as a bonus within the same calendar year.

“Progressive farmers in the region have been experimenting with this system for over a decade,” says Michael Plumblee, extension corn and soybean specialist and assistant professor of agronomy at Clemson University. “More farmers started asking questions about ultra-late soybeans.”

A survey of farmers with more experience with the system found that they all do something different. 

Plumblee led research to gather information to answer common questions about ultra-late soybean production and develop best management practices for South Carolina farmers. Soy Checkoff investments from the South Carolina Soybean Board funded trials exploring system timing, variety selection, nematode management and more. 

“Our trials picked the system apart to explore all aspects of it, from corn planting in March or April through soybean harvest in November,” he says. “However, we kept in mind that corn is the driving commodity here. In these trials, we raised 250-plus bushel corn every year under irrigation.”

Their results help farmers determine if ultra-late planted soybeans make sense for their operation, and if so, how to get the most value from the two-for-one approach.

Variety Selection Critical

Plumblee says the most important factor in ultra-late soybean production is choosing the right variety. Research trials compared varieties from Maturity Groups 4, 5, 6 and 7, with various seeding rates, row spacings and at-plant fertilizer applications.

“Indeterminant MG 5 varieties seem to be the sweet spot,” he reports. “We learned varieties need both yield and plant height at harvest.”

Combines can’t effectively harvest soybeans that set and fill pods too close to the ground, but the compressed growing season limits vertical growth. 

Other agronomic factors studied did not impact ultra-late planted soybean yield, likely because of the short growing season. However, farmers can consider trends his team observed to adapt planting to fit their operation.

• Seeding rates from 120,000 to 240,000 seeds per acre didn’t impact soybean yield, though the rate of 150,000 seeds per acre produced the best stand counts.
• Soybeans planted in 15-inch and 30-inch rows yielded the same. Ultra-late planted soybeans don’t necessarily grow enough to canopy rows, regardless of spacing.
• Adding nitrogen fertilizer when planting ultra-late soybeans provided no yield, economic or harvestability benefits. Trials compared 0 to 90 pounds of N per acre.

“Our trials showed that choosing the best variety matters more than maturity group or any other factor,” Plumblee adds. “Soybean varieties that yield well and grow tall, rank and aggressively fit the ultra-late production system well. If they have resistance to southern root-knot nematode, that’s even better.”

Time to Plant, Harvest and Plant Again

Timing for ultra-late soybean production starts with spring corn planting. Plumblee’s team studied how manipulating corn management impacted the following soybean yield. 

“To give soybeans a reasonable chance to yield, we found it’s ideal to plant them by the first week of August,” he says. 

With that in mind, the team compared two corn planting dates in March and April, two corn hybrids, 113-day and 120-day corn, and harvesting it at three different moisture levels, 25%, 20% and 15.5%. 

March corn planting provided more flexibility for ultra-late soybeans. However, some April plantings worked when using the earlier-maturing hybrid and harvesting at a higher moisture level. 

Irrigated South Carolina corn dries down relatively quickly from 25% to 20% in the field, typically in one to two weeks. However, that corn needs about one month to dry down to the storage moisture of 15.5%.

“Because of this, ultra-late soybeans make sense following silage or for farmers set up to dry corn,” Plumblee says. “System timing is mostly about logistics.”

He notes that residue management following corn harvest impacts seed-to-soil contact for soybeans. He recommends planting soybeans between corn rows. Planters equipped with no-till coulters and row cleaners help manage heavy corn residue.

“Remember that soybeans lose over half a bushel of yield per day of later planting,” he adds. “That can help farmers determine when to stop planting ultra-late soybeans.”

Nematode Management

Plumblee cites nematode management as another key factor in ultra-late soybean production. Many fields experience southern root-knot nematode pressure, with Columbia lance nematodes also common. Stubby, lesion and other nematodes species can also cut crop yields.

“Southern root-knot nematodes feed on both corn and soybeans,” he says. “But we’ve seen populations explode when planting soybeans.”

Trials tested nematicides on corn, nematicides on soybeans and soybean varieties with resistance as management options. 

“Using a nematicide on corn picked up 12 bushels per acre, even with high corn yields,” Plumblee reports. “Soybean nematicide or resistant varieties reduced nematode counts, which could reduce pressure next season.”

However, lower nematode populations don’t necessarily equate to soybean yield that season. The trials showed that other soybean variety factors mattered more than nematode resistance, as some susceptible varieties yielded better than those carrying resistance. 

Plumblee recommends choosing nematode management based on the species present in the field. 

Weighing Costs and Benefits of Ultra-Late Planted Soybeans

Much like full-season soybeans in the region, farmers want to raise ultra-late soybeans as cheaply as possible. Plumblee says farmer should plan for one herbicide application and maybe one insecticide treatment. 

Even if ultra-late soybean production breaks even, he points out benefits like interrupting the continuous corn cycle in irrigated fields, knocking back pressure from weeds and nematodes. 

“In this area, farmers typically grow corn in their best soils, under irrigation,” Plumblee says. “They plant cotton and peanuts in other fields, while soybeans typically go on marginal land with lots of variability and heavy deer pressure. This system moves soybeans into the best fields.” 

He doesn’t recommend ultra-late planted soybeans to every farmer, but he believes the system has a fit in some operations.

“If the labor and logistics are in place, there is money to be made in this system,” Plumblee says. “Do the ‘napkin math’ to see if it might pay.”  

Additional Resources

Testing Varieties for Ultra-Late Soybean Production on the Coastal Plain – SRIN article 

Long Juvenile Genetics Offer Potential Solution for Ultra-Late Soybeans – SRIN article 

Double Cropping Soybeans Behind Corn – YouTube video 

Ultra Late Soybean Planting – Double Crop Corn/Soybean – Clemson University blog article 

Meet the Researcher: Michael Plumblee SRIN profile | University profile

Published: Sep 15, 2025