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Rethinking soybean seeding rates

For soybean planting rates, what’s optimum, considering the increase in seed costs? Looking back on historical practices regarding planting soybeans, there long has been a tendency to overplant this particular crop in the Midwest to ensure enough plants per acre for optimal yield.

Soybean seed costs have risen dramatically since we started producing this crop in Iowa. In the early 1990s, a 50-pound bag of soybean seed typically cost a producer from $16 to $18. Now, after discounts, a bag of soybean seed can cost an average of $50, with a seed count of 140,000. If planting 160,000 to 170,000 seeds per acre, growers will spend from $57 to $61 per acre for seed alone for the 2012 growing season.

Considering current costs for soybean seed, does it make economic sense to continue planting an excessive number of soybean seeds per acre as insurance? Will the extra seed cost due to planting higher rates provide a positive return on your investment with increased yield?

Farmers should use soybean planting rates based on desired stand density to maximize profit potential. In research conducted in Iowa, De Bruin and Pedersen (2008) found a linear relationship between soybean planting rate and stand density at harvest.

Researchers with Iowa State University have conducted soybean seeding rate studies over many years. Weber and colleagues (1966) compared four plant populations in each of four row spacings. Plant populations ranged from less than 50,000 plants to more than 200,000 plants per acre. In this study, which was conducted in Iowa, yield did not increase for soybean density higher than 64,000 plants per acre.

Row spacing’s effect

More recently, De Bruin and Pedersen conducted studies with soybean comparing a wide range of planting rates in both 15-inch and 30-inch row spacing. They reported that production of maximum soybean yields, which averaged 65.9 bushels per acre, required planting rates of 171,000 to nearly 220,000 pure live seed, or PLS, per acre (See Table 1). However, to obtain 95% of maximum yield, 62.6 bushels per acre — on three of six sites, Crawfordsville, DeWitt and Whiting — only required seeding rates at about 37% of the rate necessary for maximum yield. The highest seeding rate among the six locations necessary to produce 95% of maximum yield was 140,000 PLS per acre.

In an on-farm study comparing soybean seeding rates in northwest Iowa, DeJong and Sievers (2006) reported that growers did not find any yield advantage by increasing seeding rate from 125,000 to 175,000 seeds per acre (See Table 2). Soybean stands averaged just over 100,000 plants per acre in August for the lower seeding rate, while the higher seeding rate averaged just over 139,200 plants per acre. However, yields averaged only 0.7 bushel per acre more for the higher seeding rate.

Soybeans can compensate

Populations at harvest higher than 100,000 plants per acre generally do not result in increased yield of soybeans. Soybean plants can compensate well for lower stand densities by increased branching, resulting in higher yields on a per plant basis. At higher densities, yield per plant is decreased. However, populations at less than 100,000 plants per acre may suffer yield loss under adverse conditions.

The results from De Bruin and Pedersen show a 23% average mortality at the high seeding rates were required to maximize yield, presumably due to intraspecific competition, or competition between neighboring soybean plants.

Average predicted mortality rate was reduced to slightly less than 12% when planting for 95% of maximum yield.

Interestingly, in a study conducted in New York, Cox and Cherney (2011) found that maximum partial returns for soybean occurred at early-season densities of 100,600 plants per acre, obtained by planting treated seed at 138,400 seeds per acre. Iowa is not New York, but the results of Cherney and Cox are very similar to recommendations developed by Palle Pedersen for Iowa soybean production. Soybean plants have tremendous capability to compensate for differences in stand density, so precision planting is not critical to obtain satisfactory stands.

In summary, populations at harvest higher than 100,000 plants per acre generally do not result in increased yield of soybean. Remember, always plant good-quality seed with a high percentage of pure live seed.

Basol is an ISU Extension field agronomist at Nashua in northeast Iowa. Lenssen is the ISU Extension soybean systems agronomist at Ames.

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De Bruin, J.L. and P. Pedersen, 2008a. Agronomy Journal 100:696703.

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DeJong, J., and J. Sievers. Publication ISRF06-29,31

This article published in the March, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.