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Soybean Seed Rates: Should They Change With Row Spacing?

Eric Adee Eric Adee
Senior Research Specialist
Department of Crop Sciences
(309) 734-7459; adee@illinois.edu
Gary Pepper
  Gary Pepper
Assoc. Professor & Extension Agronomist
Department of Crop Sciences
(217) 333-4424; g-pepper@illinois.edu

In the last two decades, many soybean producers in Illinois have purchased equipment specifically to plant soybean in rows narrower than the 30 inches most commonly used for corn. As a result, more than half of the soybean acres in Illinois are planted in rows less than 10 inches apart, most of which are drilled in rows 7 to 8 inches apart (Figure 1). During the 1990s, the percentage of rows between 10 and 18.5 inches has also increased, so that these and 30-inch rows now occupy similar acreage. Most of this increase is in 15-inch rows, which are often produced by splitting rows of 30-inch planters. This allows the same piece of equipment to be used for both corn and soybean, with the row splitters usually used only in soybean.

Trends in soybean row spacing, 1989-98

Figure 1. Trends in soybean row spacing, 1989-98.

Much of the change to drilled rows resulted from perceptions that drilled rows yield more, are easier to harvest, and have improved weed control due to more rapid canopy closure. The development of split-row planters and the rising cost of soybean seed has caused producers to re-examine how to get the most return from their soybean seed investment. Fifteen-inch rows may offer a way to capture high yields from narrower rows and to lower seed rates due to better seed placement and higher emergence percentages.

Three row spacing/equipment combinations were compared in this study: 7- or 7.5-inch/drill, 15-inch/split-row planter, and 30-inch/planter. For each row spacing, soybeans were planted at three populations: 125,000, 175,000, and 225,000 seeds per acre. Data were collected on canopy closure date, harvest population, and grain yield. Profitability was calculated by multiplying the yield by the price and subtracting Roundup Ready® seed costs. This study was conducted at Urbana (1997-99), Monmouth (1998-99), and DeKalb (1999).

At the two higher seeding rates, 15-inch rows canopied within one to two days of the drilled soybeans, while the 30-inch rows took 18 to 20 more days to canopy. At the low (125,000/acre) seeding rate, drilled rows took 10 days longer to form a canopy than at 225,000 seeds/acre. Seeding efficiency, calculated by dividing the harvest plant population by the seeding rate, averaged 57 percent for the drill, or 20 to 25 percent lower than for the 15- and 30-inch rows, which were similar.

Yields of soybean planted with different planters using different seeding rates

Figure 2. Yields of soybean planted with different planters using different seeding rates.

Drilled and 15-inch rows produced about the same yield at the higher seeding rates (Figure 2). Reducing the seeding rate to 125,000 decreased the yield of drilled soybeans by four to five bushels per acre in drilled rows, but by only one to two bushels per acre in 15-inch and 30-inch rows, due to the higher plant populations from the row units. The 30-inch rows yielded three fewer bushels per acre than the drilled or 15-inch rows at the higher seeding rates. At the lowest seeding rate, 15-inch rows yielded two to three more bushels per acre than the drilled or 30-inch rows, even though plant populations for 15- and 30-inch rows were the same.

The most profitable row spacing/planter and seeding rate combinations were the drill at the two higher populations and 15-inch rows at the two lower seeding rates (Figure 3). The 30-inch rows averaged $15 less return per acre than the drilled or 15-inch soybeans. Note that all of these numbers are based on the assumption that costs to own and operate planting equipment are the same for all three planter types.

These data show that, providing equipment and operating costs are similar, the drill is still one of the most profitable methods of producing soybeans in the northern half of Illinois. However, the increased seeding efficiency of the split-row planter, with yields very similar to those of drilled soybeans, makes it as profitable as the drill, and more profitable than planting in 30-inch rows.

Returns from different planters and different seeding rates

Figure 3. Returns (yield times price of soybeans minus seed costs) from different planters
and different seeding rates. Returns assume equal planter costs for the three planters.

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