By Strahinja Stepanovic, Laura Thompson and Keith Glewen

Continuous corn has been the most common, and in many cases, the most profitable irrigated crop sequence in southwest Nebraska. However, difficulties in managing resistant pests like western corn rootworm and western bean cutworm, as well as bacterial disease outbreaks like Goss’s wilt and bacterial leaf streak, have triggered the need for adding other crops, such as soybean, to irrigated crop rotations in southwest Nebraska.

Larger adoption of soybean, however, has not readily occurred in this area. For example, planted soybean acres in southwest Nebraska were 153,000 acres and 151,500 acres, and average soybean yield was 55.8 and 61.5 bushels per acre for 2010 and 2017, respectively, according to USDA National Agricultural Statistics Service data.

FIGURE 1: TerrAvion aerial imagery taken Aug 4, 2017, at the Chase County site shows less vigor and higher thermal stress in 30-inch-row soybeans than in 15-inch-row soybeans. 

15s versus 30s
Soybean row spacing research in eastern and central Nebraska often has shown potential to increase soybean yield with narrower rows (15-inch or drilled). Meanwhile, many farmers in western Nebraska have reported higher soybean yield with 15-inch rows due to faster rate of canopy closure, better weed suppression and reduced evaporative loss early in the season (see Figure 1). However, limited data are available to show what those yield differences may be.

In 2017, the Nebraska Soybean Board funded an on-farm research initiative to quantify the yield differences between irrigated soybeans planted in 15-inch versus 30-inch rows in southwest Nebraska.

Nebraska Extension conducted three replicated on-farm studies comparing soybean yields in 15-inch and 30-inch rows. Field experiments were carried out at one location in 2015 and two locations in 2017. Site descriptions; agronomic information and data on percent grain moisture at harvest and yield; and marginal net return in dollars per acre for these studies is summarized in the table.

TABLE 1: Site description, agronomic information and data on grain moisture at harvest (%), yield (bushels per acre) and marginal net return ($ per acre) for irrigated soybeans grown in 15-inch- and 30-inch-row spacings at three site-years in southwest Nebraska.

When averaged across site-years, soybean planted in 15-inch rows yielded 67 bushels per acre — 7 bushels more than soybeans planted in 30-inch rows (60 bushels per acre). Yield differences ranged from 4 bushels per acre in Chase County (2015, 2017) to 12 bushels in Perkins County (2017). Soybeans planted in 15-inch rows also had lower grain moisture at harvest (up to 0.9% less) and significantly greater marginal net return ($25 to $128 per acre) than soybeans planted in 30-inch rows.

Aerial imagery at the Chase County site in 2017 showed less vigor and higher thermal stress in 30-inch-row soybeans during early reproductive growth. The on-farm research cooperator at the site also observed better suppression of volunteer corn with 15-inch rows.

Worth consideration
There are several facts to take away from the research:

• Planting irrigated soybeans in 15-inch rather than 30-inch rows definitely showed potential for southwest Nebraska farmers to increase soybean yield and profit.

• Aerial imagery showed less thermal stress in 15-inch-row soybeans, which suggests that in cases where water may be limiting, like sandy soil with low water-holding capacity and higher evaporative losses, there may be an even greater benefit to 15-inch rows. More research is needed to evaluate soybean yield response to narrower rows in heavier soils compared to sandier soils.

• Although Nebraska Extension educators haven’t observed differences in disease pressure in these studies, it has been reported that planting soybean in 15-inch rows may increase the occurrence of white mold disease.

• Finally, switching from 30-inch to 15-inch rows would require either double planting or buying a 15-inch-row planter.

For more information about soybean row spacing, view the on-farm research results or search the results database. If interested in evaluating the impact of soybean row spacing on your farm in southwest Nebraska, contact Strahinja Stepanovic at 308-352-4340 or email [email protected].

Also check the latest UNL recommendations on soybean planting dates and seeding rates.

Stepanovic, Thompson and Glewen are Extension educators at the University of Nebraska-Lincoln. This report comes from UNL CropWatch.