Prior to 1) glyphosate-resistant [GR] soybean varieties and sole use of POST glyphosate herbicide, 2) glufosinate-resistant [GLR] soybean varieties and use of glufosinate herbicide, and 3) Xtend and Enlist soybean varieties [resistant to dicamba and 2,4-D auxin herbicides, respectively], numerous “conventional” [CONV] herbicides were applied both PRE and POST to control weeds that infested conventional [CONV–non-transgenic] soybeans. These herbicides were generally effective at managing the myriad broadleaf and grass weeds that are common in the Midsouthern U.S., and included both PRE and POST applications of products from more than one MOA group.
An online article titled “Weed Control in Soybean with Preemergence- and Postemergence-applied Herbicides“ by Oliveira et al. was published on June 1, 2017 in Crop, Forage, and Turfgrass Management journal. This article reports results from a study that was conducted in Nebraska to assess weed control in soybeans by herbicides that were available and used before herbicide-resistant [HR] soybean varieties were developed. In other words, this is a report of results from a herbicide weed control study where only CONV herbicides were applied [i.e., no herbicides (glyphosate, glufosinate, dicamba, 2,4-D) that are applied to current HR soybean varieties were used]. A summary of the results from this research are reported in a previous article titled “Is there a place for ‘public’ soybean varieties” that is posted on this website.
In a recent article titled “Conventional and transgenic herbicide-resistant soybean cultivars yield similarly across five site-years in Nebraska” by Mobli et al. [Crop Forage & Turfgrass Mgmt. 2022;8:e20189], the concept of using CONV soybean varieties vs. transgenic herbicide-resistant [HR] soybean varieties [both with CONV herbicides applied] is explored. A summary of the conduct of and results from this research are reported below.
• The high risk of injury to CONV soybean varieties from misapplications of glyphosate and auxin herbicides makes their use challenging.
• The objective of the research included the evaluation of the effect of herbicide-resistance traits on soybean yield.
• Field experiments were conducted in 2017 and 2018 at five sites [40°17' to 41°42' N lat.] in Nebraska.
• A single locally adapted and commonly adopted variety from CONV, glyphosate-resistant [RR2], and glyphosate- and dicamba-resistant [RR2 Xtend] HR trait groups were used.
• Valor [Group 14] herbicide was applied PRE, and Cobra [Group 14] plus Outlook [Group 15] herbicides were applied POST at the V3 to V4 soybean developmental stages. All plots were kept weed-free by hand-weeding to ensure no competition from weeds.
• HR traits did not affect soybean yield in this study that was kept weed-free. That is, the seed yield from an adapted CONV variety was similar to that from adapted transgenic HR soybean varieties when grown in these weed-free studies.
• The authors concluded that their results suggest that soybean producers can achieve yields from CONV varieties that are similar to yields from HR varieties assuming effective weed control can be achieved when using CONV herbicides in the absence of using glyphosate, glufosinate, and/or auxin herbicides for POST weed management.
The following points should be considered when making a decision about whether or not to grow CONV vs. HR soybean varieties.
• HR soybean varieties allow additional POST herbicide options.
• Using HR soybean varieties may be the only option for managing a specific problematic weed or weeds on a particular production site.
• The risk from injury related to off-target herbicide movement is high with CONV varieties.
• There may be a price premium for seed harvested from CONV varieties.
• Seed of public CONV varieties will likely cost less than seed of HR varieties.
• If future weed control in soybeans is to include application of many of the herbicides [especially fall and PRE herbicide applications] that were used before HR soybean varieties [i.e. RR, LL, Xtend, Enlist] were developed, then why not consider planting CONV soybean varieties that have desirable yield and agronomic traits to avoid the expense of purchasing seed with HR traits?
• When making a decision about planting a CONV variety vs. an HR variety, consider the difference in seed cost coupled with the cost of herbicides that will be applied to each system.
• Quantity of seed of CONV varieties [usually from public soybean breeding programs] may be limited. However, if there is demand, the amount of seed of a popular public CONV variety can/will be increased by the Foundation Seed programs of the various states.
• Seed of public CONV varieties can be saved by the producer for future plantings.
The above presentation is not intended to encourage or discourage the use of one soybean production system over another. Rather, it is an attempt to provide further insight into the use of CONV vs. HR soybean varieties in a production system where 1) adequate weed control can be achieved with CONV herbicides, 2) the production site is devoid of problematic HR weeds, and 3) there is no expectation of herbicide drift issues.
Composed by Larry G. Heatherly, Sep. 2022, email@example.com