A review of soybean row spacing studies published within the past 10 years generally confirms previous results comparing 30-inch rows and drilled narrow rows. In five studies, drilled soybeans outyielded 30-inch row soybeans by an average of 4.1 bu/acre (Figure 1, Table 1). Nine studies that compared 30-inch rows and 15-inch rows found similar results, with 15-inch rows holding a 4.0 bu/acre yield advantage. Yields were similar between 15-inch row and drilled narrow-row soybeans in these studies.
Because many of these studies used higher seeding rates with narrower row spacings, increased seed costs partially offset the yield benefit associated with narrow rows. Higher seeding rates with narrower rows have been a common practice, particularly with drilled soybean; however, not all research supports this practice. A study conducted in 2008-2009 (Cox and Cherney, 2011) found no row spacing by seeding rate interaction for soybeans planted in 7.5-inch, 15- inch, and 30-inch spacings. Recent research conducted in Iowa had similar results, indicating that narrow-row systems do not necessarily require a greater harvest stand to maximize yield (Pedersen, 2008). Historically, less accurate seed placement made higher seeding rates necessary with drills; however, improved seed placement with newer precision drills has reduced this need. In light of these findings, seed cost may not be a requisite consideration for row spacing decisions.
Conditions favoring narrow rows
Research has shown that soybeans need to attain 95% light interception by early reproductive growth in order to maximize yield, which requires a leaf area index of 3.5 to 4.0 (Board and Harville, 1992). Narrower rows spacings are likely to provide a greater yield benefit in systems where soybeans have a limited time frame for vegetative growth prior to flowering. Such scenarios include northern soybean producing regions where the growing season is shorter (Lee, 2006), early soybean production systems where short maturity varieties are planted early to avoid drought (Holshouser and Whittaker, 2002), delayed planting situations (Lee, 2006), and double-crop systems (Minor and Wiebold, 1998; Holshouser et al., 2006).
Conditions that may not favor narrow rows
Research has also shown that narrow rows may have reduced or no yield advantage under some conditions. Several experiments over the years have shown that moisture stress can reduce the yield benefit of narrow rows (DeBruin and Pedersen, 2008). Brown stem rot, white mold, nitrogen stress, soybean cyst nematode, and Sudden death syndrome may also tend to negate the benefit of narrow rows (Cooper and Jeffers, 1984; Pedersen and Lauer, 2003; Swoboda et al., 2011).
Row spacing research in corn has generally shown that the yield advantage with narrow rows diminishes outside of northern Corn Belt latitudes, since corn grown in the central Corn Belt and south is better able to attain maximum light interception prior to flowering (Butzen and Paszkiewicz, 2008). No such trend has been consistently observed in soybean when planting at optimum timings, although narrow rows have proven advantageous with late planting regardless of latitude (Lee, 2006).
In recent years, 15-inch and 30-inch row spacings have been the most common planting configurations in North American production, each accounting for over 1/3 of total acreage (Figure 2). Drilled soybeans, in row spacings of 12 inches or less, account for less than 20% of total acreage.
However, row spacing practices vary widely across different areas. Among the largest soybean-producing states there are substantial differences in row spacing practices, with a majority of growers in Illinois, Indiana, Ohio, and Missouri favoring 15-inch spacings, compared to Iowa and Minnesota where soybeans planted in 30-inch rows are much more common (Figure 3). Row spacings of 36 inches and wider are rare in the northern and central Corn Belt, but more common in southern raised-bed systems. Similarly, 22-inch rows are common in sugar beet producing areas such as Minnesota, but are not generally found elsewhere.
In many cases, this decline in drilled soybeans has been accompanied by an increase in acres planted to 15-inch rows, which is now the most common row spacing for soybean. However, acreage planted to 30-inch rows has also increased in almost all regions of North America over the last several years, reversing the long-term trend away from wider rows. In some areas this increase has been substantial. For example, Illinois went from 18% to 32% of soybean acres planted to 30-inch rows over the last ten years (USDA-NASS survey). This recent shift toward wider row spacings runs counter to the higher yields often associated with narrower rows, which indicates that other factors beyond yield are driving grower decisions in this area.
Equipment and Time Management
Other than yield, the most important factor driving soybean row spacing practices is equipment and time management during the planting season. One of the key issues growers must consider is whether the economics of their farm justify having a machine dedicated specifically to planting soybeans.
Larger farms are more able to justify the expense of a dedicated soybean planter and provide an operator for it. Thus, they are more likely to be planting soybeans in 15-inch rows (Figure 5). For smaller farms, it may be more practical to share a soybean planter with another crop, such as a drill with wheat or a 30-inch planter with corn. This often results in more 30-inch or drilled soybeans for smaller farms (Figure 5).
As farms get larger, more acres must be planted in a shorter amount of time. Wet conditions in many areas during the last few planting seasons have exacerbated this situation by creating very short and intermittent planting windows. To plant more acres during the available window, some growers have opted to use their 30-inch planter for soybeans. Because 30-inch planters are typically wider than 15-inch planters, they can cover the ground more quickly. Another option – owning a second planter specifically for soybeans – allows both crops to be planted at the same time, resulting in earlier completion of soybean planting. However, the total number of operator hours spent planting would be greater and the second planter would require a second operator, which may not always be feasible.
It is difficult to weigh the potential yield benefit of narrow-row soybeans against equipment costs, time constraints and operator availability required. Equipment and workload considerations are unique for every farm operation and ultimately come down to the needs of each individual grower.
A factor that has likely played a role in the recent increase in soybeans planted in 30-inch rows in some areas is Sclerotinia stem rot (Sclerotinia sclerotiorum), or white mold. White mold development is favored by cool and wet conditions during soybean flowering. A dense soybean canopy can enhance these conditions and increase white mold severity. The rationale behind increasing row spacing is to increase light penetration and air movement in the lower canopy, thereby making conditions less favorable for white mold development.
Soybean variety selection, row spacing and seeding rate are important factors influencing white mold development and a good management strategy should address all three. Seeding rate generally appears to have a greater effect on white mold severity than row spacing (Lee et al., 2005). Changing from drilled narrow-row soybeans to 15-inch row spacing in areas where white mold is prevalent is likely a good move, particularly when accompanied by a reduction in seeding rate. The benefit of moving to a 30-inch spacing is less clear and is not generally recommended by university pathologists for reducing white mold, particularly given the likely reduction in yield potential. However, in areas with frequent white mold incidence, wide rows may provide some benefit.
Foliar Fungicide and Insecticide Applications
The need for fungicide and/or insecticide applications may also impact row spacing decisions. When an application is made during vegetative growth, plants are generally able to compensate for damage caused by the sprayer wheels with little reduction in yield. For applications made following the R1 growth stage, which would include most foliar fungicide and insecticide applications, wheel damaged areas will have lower yield. A research study conducted in Delaware and Virginia found significant yield reductions due to sprayer wheel damage in R4 soybeans planted in 7.5-inch and 15-inch row spacings, whereas soybeans planted in 30-inch and wider row spacings did not sustain any sprayer wheel damage (Holshouser and Taylor, 2008). Actual yield loss due to wheel traffic will vary according to boom width (Table 2).
Table 2. Soybean yield loss due to sprayer wheel damage in 7.5-inch and 15-inch row spacings with four different boom widths (Holshouser and Taylor, 2008).
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USDA-NASS Objective Yield Surveys
¹ DuPont Pioneer Agronomy Information Manager
² DuPont Pioneer Agronomy Sciences Intern