In 2017, Roundup Ready 2 Xtend soybeans and approved formulations for over-the-top dicamba application technology became available to soybean producers in the U.S., providing a set of new genetics and another weed management tool.
However, some growers have considered planting conventional soybean varieties as part of their cropping systems given the higher seed prices of herbicide-tolerant soybean varieties (e.g., Roundup Ready 2 and RR2Xtend) compared to conventional varieties, widespread occurrence of glyphosate-resistant weeds in Nebraska, concerns with dicamba off-target movement when spraying RR2Xtend acres, and premiums for non-GMO soybeans.
Challenges exist, however, with growing conventional soybeans, including weed management without the use of glyphosate postemergence, and potential for misapplication and drift of glyphosate or dicamba to non-tolerant varieties.
A common question among producers is whether conventional varieties can provide yields similar to RR2 and RR2Xtend varieties.
University of Nebraska-Lincoln Extension conducted a study to explore the impact of herbicide tolerance trait selection, maturity group, and row spacing on soybean yield.
Key questions include:
• When treated the same, are the yields of conventional, RR2, and RR2Xtend varieties similar?
• Do these varieties respond similarly to different row spacing?
• Does maturity group play a role in the yield potential?
In an attempt to answer these research questions, a study was established at three of the 2017 Soybean Management Field Day locations. At Tekamah, Auburn and Ord, soybeans were planted on May 8, May 9 and May 15, respectively. Silty clay loam was the soil type at all sites. The study was conducted with a total of 12 treatments replicated four times arranged on a randomized complete block design. Plots were four rows wide (10 feet) and 30 feet long. Treatments consisted of:
• row spacing at 15-inch versus 30-inch
• maturity groups at early (2.2 to 2.4) versus late (3.2)
• herbicide-tolerance traits of conventional, RR2 and RR2Xtend
Varieties were all managed as conventional soybeans for weed management (i.e., no glyphosate or dicamba sprayed postemergence).
Yields, in bushels per acre, were determined with a small plot combine by harvesting two center rows of each plot after they were cut to a standard length of 30 feet. Yields were adjusted to 13% grain moisture for final reported values.
The experimental data were analyzed to evaluate interaction and main treatment effects on yield.
According to the statistical analysis, location by row spacing was the only significant interaction among all possible interactions evaluated in this study; therefore, simple effects for these two experimental factors were evaluated. The experimental factors’ herbicide tolerance trait and maturity group were significant, so their main effects were evaluated.
RESPONSE TO ROW SPACING: Soybean yield in response to study location and row spacing. Bars that do not share the same letter are significantly different.
Soybean yield response to row spacing varied across locations. The 15-inch row spacing soybeans yielded more than 30-inch at Auburn. At Ord and Tekamah, the yields were similar. Even though yield response to narrow row spacing was site-specific, 15-inch soybeans produced the same or more, never less, when compared to 30-inch soybeans. Higher yields in narrow-row spacing are likely due to faster canopy closure and greater light interception (e.g., plants “harvesting” more light for photosynthesis) due to a more even plant distribution in the field. Yields at Auburn were higher than Ord and Tekamah (average of 87, 79 and 77 bushels across treatments, respectively).
Difference among varieties, maturities
Conventional varieties yielded an average 2 to 3 bushels less per acre than the RR2 and RR2Xtend varieties. Yields from RR2 and RR2Xtend were similar. Even though the conventional varieties yielded less, the difference was relatively small. Depending on premiums offered to non-GMO soybeans and overall weed management costs, which are directly related to the herbicide tolerance trait selected, they may result in equal or higher profitability.
RESPONSE TO TRAIT: Soybean yield in response to herbicide tolerance trait. Bars that do not share the same letter are significantly different. Conventional = no herbicide tolerance trait; RR2 = Roundup Ready 2 (glyphosate-tolerant); RR2Xtend = Roundup Ready 2 Xtend (glyphosate and dicamba-tolerant).
The early-maturity varieties (RM 2.2 to 2.4) yielded 2 bushels more than the late-maturity varieties (RM 3.2; data pooled across herbicide tolerance trait, location and row spacing). Growers interested in cover crops could potentially use the early maturity varieties recommended for their regions to allow a wider window for cover crop establishment in the fall without hurting soybean yield potential.
This study was conducted at three Nebraska locations during the 2017 growing season. Row spacing at 15 inches yielded the same or more than the standard 30-inch row spacing. The results of this study indicate a yield advantage with RR2 and RR2Xtend soybean varieties when compared to conventional soybeans.
The early-maturity group tested in this study (RM2.2-2.4) had a yield advantage when compared to the late-maturity group (RM3.2).
To further validate these results, Nebraska Extension will replicate this study at the four locations of the 2018 Soybean Management Field Days.
This report comes from UNL CropWatch.