How low can you go with seeding rates on indeterminate varieties of soybeans? Long-time North Carolina State Soybean Specialist Jim Dunphy wrote the book on seeding rates for determinant soybeans, but he says the earlier maturing indeterminate varieties are a little different.

Soybeans are classified as indeterminate, semi-determinate or determinate in growth in the United States. Many southern varieties are determinate in growth and cease vegetative growth when the main stem terminates in a cluster of mature pods.

Maturity Group III and Group IV beans have gained some popularity in recent years in the upper Southeast. These indeterminate varieties develop leaves and flowers simultaneously throughout a portion of their reproductive period, with one to three pods at the terminal apex.

With soybean development being driven by photoperiod, northern varieties have vegetative growth limited by the season length. Semi-dwarf, determinate varieties that are usually only 40 percent-50 percent as tall as indeterminate varieties, are commonly grown in the Midwest, primarily to avoid lodging.

Dunphy planted one late Maturity Group IV and one mid-Maturity Group IV soybean variety at eight different plant population levels in on-farm test plots near Elizabeth City, N.C.

“These indeterminate varieties are a little different animal than our more popular determinate varieties. We know what we will get with different seeding rates with most of the determinate varieties we plant in North Carolina. The tests at Elizabeth City were done to help us determine whether we need to change our thinking on seeding rates with indeterminate varieties,” Dunphy says.

Visual evidence

Dunphy showed visitors to the recent Northeast Ag Expo the test plots. Standing in the plot with 125,000 seed per acre, or a little more than 100,000 plants per acre, he asked the growers, “is this enough seed.”

The consensus answer was yes. Dunphy explained the two varieties at the 125,000 seed per acre rate were planted with a planter, rather than a drill, on 15-inch rows. As expected, Dunphy says a grower would be hard pressed to find a place to put another plant in the field.

At the 150,000 rate, drilled in 15-inch rows planting provides four seeds per foot. “I talk to growers a lot about ending up with two plants per foot. I don't think I can talk growers into less than two plants per foot. When the plants are waist high two plants per foot look fine. When the plants are small, it's tough to convince farmers two plants per foot is enough,” Dunphy says.

The difference between 125,000 seed per acre and 150,000 seed per acre is hardly visible in the side-by-side plantings. “The higher rate may be a little thicker, but I doubt it will out-yield the lower rate,” the venerable North Carolina State researcher contends.

Not much difference

At 175,000 seed per acre, there was still not much difference than the 125,000 rate. The take home message, Dunphy stresses, is that the lower seeding rate costs less to grow and yield will not be negatively affected by having 125,000 versus 150,000 or even 200,000 seed per acre.

“When you get up to 200,000 seed per acre, the soybean plants begin to be noticeably taller. The thicker the seeding, the taller the plants and the more likely they are to blow over. Plants are competing for sunlight and tend to grow taller and more spindly at these higher seeding rates, regardless of whether they are determinate or indeterminate varieties.

“The lower seeding rate produces plants with the same number of fruiting nodes and the same leaf surface area as the higher seeding rates,” Dunphy says. “If a grower can get four layers of leaves, or four acres of leaves per acre of ground, he is getting about all the growth potential the soybean plant has to offer.

“The top two layers of leaves get plenty of sunlight, the third layer gets enough sunlight to be productive, and the fourth layer of leaves gets enough sunlight to produce more photosynthate than it uses. The fifth layer of leaves don't get enough light to be productive,” he explains.

Lower seeding rates

Going the other direction with lower seeding rates, Dunphy points out soybeans planted at 100,000 seeds per acre. Though shorter, there doesn't appear to be any room for more plants.

At 75,000 plants per acre, in a drill, this plant population is one plant per foot. It's not enough for most growers, but if you are in a re-plant situation, I would advise a grower to not re-plant, if he has one plant per foot, Dunphy says.

By comparison, determinant varieties planted in May begin to lose profitability at seeding rates over 50,000 seed per acre. Even in June and July plantings, 75,000 seed per acre was as profitable as 100,000 seed per acre and higher.

“At the 50,000 seed per acre rate the savings in cost of seed was higher than the value of yield losses. That's the bottom line growers need to shoot for,” Dunphy says.

With the indeterminate varieties, the break even line was at 100,000 seed per acre. So, for Group IV indeterminate varieties, growers need to plant more seed per acre than determinate varieties planted at the same time.

Even to get high yields, in the 70 bushel per acre range, 50,000 seed per acre for determinate varieties and 100,000 seed per acre for indeterminate varieties planted in May is adequate. “If you're not getting a bushel and a half per pound of seed you plant, or 60 bushels from 40 pounds of seed, plant population is not what's holding yields back,” Dunphy says.

Time of planting is also critical for growers thinking about planting Group IV varieties. Later planted indeterminate varieties planted at lower seeding rates can be difficult to combine. The plants are shorter and stronger and beans are set closer to the ground.

“There is also no doubt that indeterminate varieties can't sit in the field after maturing and wait to be harvested. There is not doubt North Carolina growers can grow Group IV varieties, but they need to be careful to not get too early.

“The Elizabeth City tests don't necessarily provide a definitive answer as to how many indeterminate seed to plant. These results do indicate the difference between seeding rates needed to reach optimum plant populations — whatever that number is,” Dunphy says.

“Perhaps most importantly, these tests show us that lower plant populations of indeterminate beans can produce a good yield and even at these ultra-low seeding rates, the economics are better to grow the crop, rather than replant it,” he adds.