Soybean Row Spacing
Soybean row width preferences vary greatly across North America, with the 4 largest soybean-producing states showing substantial differences. The majority of growers in Illinois and Indiana currently favor 15-inch and narrower spacings, while growers in Iowa and Minnesota favor 30-inch rows (Figure 1).
Figure 1. Soybean row spacings (inches) in the 4 largest soybean-producing states in 2012 as a percent of total acres (USDA-NASS.
Numerous research studies over the past 40 years have evaluated soybean row spacing. In the last 10 years, studies show a 3 to 4 bu/acre yield advantage for soybeans planted in drilled narrow rows and 15-inch rows over 30-inch rows (Figure 2).
Figure 2. Average yield results from 7 soybean row spacing studies published during the last 10 years.
Other than yield, important factors driving soybean row spacing decisions include equipment and time management issues during the planting season. Larger enterprises are more likely to have a dedicated soybean planter; whereas for smaller farms, it may be more practical to share a planter with another crop, such as a drill with wheat or a 30-inch planter with corn.
Although row width preferences vary, a consistent trend across North America is a decline in drilled soybean acres (Figure 3). In many cases, the decline in drilled soybeans has been accompanied by an increase in acres converted to 15-inch rows.
Figure 3. Changes in common soybean row spacing shown as a percentage of total soybean acres from 2006 to 2013 in North America. Source: DuPont Pioneer Brand Concentration Survey.
Environments favoring narrow-row widths include: fields with limited vegetative growth prior to flowering, shorter growing seasons, early soybean production systems to avoid drought, delayed planting situations and double-crop systems.
Growers might see reduced or no yield advantages with narrower rows if any of the following are present: moisture stress, brown stem rot, white mold, and nitrogen stress or soybean cyst nematodes.
In recent years, soybean acres planted in 30-inch rows have increased in some areas. This shift is likely due to white mold, also known as Sclerotina stem rot (Sclerotinia sclerotiorum) (Figure 4). White mold development favors cool and wet conditions during soybean flowering. A dense soybean canopy enhances these conditions, potentially increasing the severity of the white mold. Alternatively, wider row spacings increase light penetration and air movement in the lower canopy, making conditions less favorable for white mold development.
Figure 4. A dense soybean canopy enhances conditions favoring the development of white mold.