Soybean seeding rates revisited

Concerns over higher seed costs have caused farmers, agronomists and seed suppliers to revisit the relationship between soybean plant population and soybean yield. To help answer the question — “How thick or thin should I plant my soybeans?” — two Iowa State University soybean research agronomists, Andy Lenssen and David Wright, recently wrote a new fact sheet titled “Understanding Soybean Plant Population Recommendations for Iowa.” A third author, Terry Basol, is listed on that publication.

Historically, soybean seeding rates have ranged from 180,000 to 240,000 seeds per acre. Today, many universities report 100,000 to 140,000 uniformly spaced plants per acre at harvest is optimal to achieve high yields. Soybeans planted at optimum populations may not achieve maximum yield but are likely to have a higher return per acre when seed cost is considered.

Research conducted between 2003 and 2006 at ISU by DeBruin and Pedersen concluded that soybean plant populations of more than 100,000 plants per acre at harvest did not result in significant increases in yield. They compared a wide range of seeding rates. Production of maximum yields required seeding rates of 175,000 to nearly 225,000 seeds per acre.

However, to obtain about 95% of maximum yield, only 75,000 to 125,000 seeds per acre were needed. Optimum planting rates depended on the field and were on the lower end of the range for more highly productive environments.

Soybean plants compensate for lower plant populations by increased branching, pod number and seed size. Studies have shown soybean plants produced 20% more branches per plant at lower seeding rates. At higher plant populations, yield per plant is decreased because of plant-to-plant competition, as plants are closer to each other within the rows. This competition can cause lower plant establishment, less branching, fewer pods per plant, fewer seeds per plant and increased lodging.

Similar to the Iowa findings, several studies in surrounding states found maximum soybean yields were achieved with plant populations ranging from 101,000 to 142,000 plants per acre. Nebraska trials reported 140,000 seeds per acre either maximized yield or had yields nearly equal to the highest yield. Minnesota trials also showed a seeding rate of 140,000 seeds per acre provided optimal yield in southern soybean growing areas of the state.

Determining the optimum plant population is one management practice that can improve soybean yield. But there are many factors that influence yield, including environment. Management practices and environment alone and in combination with each other can influence yield and change the profitability of producing a soybean crop. Using a management plan that includes optimum seeding rate and research-based best management practices will position the crop to achieve high yield within limitations of the environment.

The difference between harvest population and seeding rate will depend on such factors as seed quality, soil conditions after planting, temperature, moisture, soil crusting, and use of fungicide- and insecticide-treated seed. Studies have shown that low-yield locations required greater harvest populations than higher-yield potential locations, presumably because plants growing in lower-yielding environments are under more stress. Similarly, another study reported soybeans planted in high pH areas prone to iron deficiency chlorosis may benefit from higher seeding rates.

Seed treatment can help

Fungicide- and insecticide-treated seed is becoming more popular and may enable growers to use lower seeding rates than when planting untreated seed. ISU plant pathologist X.B. Yang reported seed treatments can protect seeds from infection by soilborne fungi, and the treatments can be beneficial when risk of damping off is high. Fields with heavy soil and no-till can have a relatively high incidence of damping off in the absence of seed fungicide treatment. He concluded that fields that had the disease in the past have a high probability of repeat infections.

Weeds compete with soybean plants for sunlight. Using seeding rates higher than the optimum rate can improve soybean competitiveness with weeds, due to more rapid canopy development. More rapid closure of the canopy can also be achieved by reducing row spacing and selecting varieties with traits that favor rapid canopy development.

Conclusions: ISU research over the years has found that soybean stands beyond 100,000 to 125,000 plants per acre at harvest typically don’t result in yield increases great enough to be economically important when the added seed cost is considered. Because soybean plants compensate well when grown at lower plant populations, fewer seeds are needed to achieve optimum yields.

A large number of factors influence yield on a field-by-field basis. Thus, you should evaluate a number of seeding rates in large-scale strips to determine the optimal seeding rate and optimal yield at which you are comfortable. Commodity grain prices and seed costs also greatly influence what is accepted as “optimal” seeding rates and yield for each production system. ISU agronomists recommend farmers re-evaluate seeding rates regularly and adjust “optimal” rates if necessary.

Basol is an ISU Extension field agronomist at Nashua in northeast Iowa. Contact him at tlbasol@iastate.edu.

ISU has 11 soybean management fact sheets at store.extension.iastate.edu.

This article published in the March, 2013 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2013.