Response of Roundup Ready Sugar Beet to Dicamba Applied Pre- or Postemergence at Variable Rates

Contributed by Joel Felix, Oregon State University

On December 15, 2022, we published an article on the “regulated studies for sugar beet (Truvera® Triple-Stack Trait) resistant to glyphosate, glufosinate, and dicamba herbicides that are being conducted in several US regions including the Pacific Northwest”. I discussed results from the Oregon State University/Malheur Experiment Station including the effectiveness of pre-emergence (PRE) applied dicamba to control broadleaf weeds for several weeks. The Truvera trait has since been deregulated in the United States, but studies are still being conducted under regulated posture until global deregulation and clearance from the United States Food and Drug Administration is obtained. So, the disclaimer below regarding this forward-looking information still applies.

Once commercialized (probably towards the end of this decade), this trait will co-exist with current Roundup Ready sugar beet cultivars, which have been in existence since 2008. The question we are attempting to answer is, what happens if dicamba misapplication occurs immediately after seeding Roundup Ready cultivars or from residues in spray equipment or direct drift from spraying neighboring fields planted to tolerant Truvera sugar beet?

A few facts about dicamba herbicide. Dicamba is a Group 4 benzoic acid selective herbicide in the chlorophenoxy family of herbicides that control broadleaf plants in tolerant crops like corn (or genetically modified cotton, soybean, corn) and rangeland. Dicamba has soil residual properties that allows the herbicide to persist in the soil to injure susceptible plants. Dicamba translocates in both the xylem and phloem with symptoms most obvious in new tissue growth.

In this study, dicamba herbicide was applied PRE (immediately after seeding Roundup Ready cultivar  BTS 210N) at the rates of 0.5 lb ae/A (22 fl oz/A), 0.25 lb ae/A (11 fl oz/A), 0.125 lb ae/A (5.5 fl oz/A), or 0.062 lb ae/A (2.75 fl oz/A) representing 1x, 0.5x, 0.25x, or 0.125x, respectively. Separate post-emergence treatments were applied to sugar beet plants at the 2-leaf stage at 0.05 lb ae/A (2.2 fl oz/A), 0.02 lb ae/A (0.88 fl oz/a), 0.01 lb ae/A (0.44 fl oz/A), 0.005 lb ae/A (0.22 fl oz/A); representing 1/10x, 1/25x, 1/50x, and 1/100x, respectively. Glyphosate at 1.13 lb ae/A (32 floz/A) was included as a control. The study area was later sprayed with glyphosate at 32 fl oz/A to manage other weeds when plants were at 6-8 leaf stage. The study was conducted in a furrow-irrigated field.

Pre-emergence applied dicamba affected Roundup Ready sugar beet emergence as depicted in Figure 1 (A-E at 30 days after application). The injury was characterized by severe stunting and twisted seedlings, leaf cupping, wrinkled leaf margins, leaf strapping and ‘trumpeting’ typical of growth regulator herbicides (Figure 1 F-J at 62 days after spraying). PRE misapplication of dicamba at full rate (22 fl oz/A) resulted in the greatest injury and reduced plant stand by almost half (27,720 plants/A) compared to 44,055 plants/A for the untreated control. Similarly, PRE applied dicamba at 11 fl oz/A reduced stand by 19%. Plant stand was not affected when dicamba PRE was applied at 5.5 or 2.75 fl oz/A (43,065 and 42,669 plants/A, respectively). Injury from PRE applied dicamba is likely a result of its high solubility in water (4,500 mg/L at 25˚C), which aids mobility in the soil with rain or irrigation water to the seed zone to affect germination and control of susceptible weeds.

Post-emergence applied dicamba at rates ≤0.05 lb ae/A (0.22 fl oz/A) resulted in more prostrate leaf lying appearance compared to untreated plants (Figure 2 A-E). Plants also had leaf and vein strapping, and petiole twisting (Figure 2 F-J). The injury increased with an increase in application rate and persisted as plants continued to grow. The study will be carried to maturity to determine root appearance at harvest and yield. Follow up studies will conduct in-depth observations in sugar beet and other crops.

Circling back to the Truvera® triple-stack trait, the results from regulated studies so far suggest that once commercialized (probably towards the end of this decade), the technology will improve weed control in sugar beet. My prediction is that growers would benefit from pre-emergence applied dicamba. The million-dollar question, however, is how long will it take before the use selects weed-resistant biotypes? Dicamba resistant weeds already exist in the United States including waterhemp in Iowa and Illinois, kochia in Colorado, Idaho, Kansas, Montana, Nebraska, and North Dakota, and Palmer amaranth in Tennessee.  Hopefully, good stewardship including herbicide family rotation will help preserve this technology.

Disclaimer

This article may contain forward-looking statements based on current assumptions and forecasts made by Bayer® management. Various known and unknown risks, uncertainties and other factors could lead to material differences between the actual future results, financial situation, development or performance of the company and the estimates given here. These factors include those discussed in Bayer’s public reports which are available on the Bayer website. The company assumes no liability whatsoever to update these forward-looking statements or to conform them to future events or developments.