Row width trends in soybeans
Factors Driving Row Spacing Trends
click image to zoomFigure 4. Soybean row spacing utilization according to farm size in 2011 (Small = 100-249 soybean acres, medium = 250-499 soybean acres, large = 500+ soybean acres). Source: Pioneer Brand Concentration Survey. 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 4). 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.
As farms get larger, more acres must be planted in a shorter amount of time. 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 key factor driving the recent increase in soybeans planted in 30-inch rows is Sclerotinia stem rot (Sclerotinia sclerotiorum), or white mold. White mold development is favored by cool and wet conditions during soybean flowering, conditions that were widespread in the Midwestern U.S. in 2009. 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.