After dicamba-resistant soybean came to the market in 2017, there were a number of off-target injury complaints in US soybean production areas, including eastern Nebraska, in the 2017 and 2018 growing seasons. To learn more about possible causes, we established a two-year research project.
We evaluated the influence of six micro-rates of two dicamba products (Engenia and XtendiMax) on growth, development, and yield of three non-dicamba-resistant soybeans (Roundup Ready, Liberty-Link and conventional) at three growth stages of application: second trifoliate (V2), beginning flowering (V7/R1), and full flowering (R2). We also used time-lapse photography to document injury symptoms and compiled the photos into videos. The objective of this article is to provide a brief summary of the project results and share the 18 short videos.
Brief Study Procedures
As previously reported in CropWatch, our field trials were conducted in 2017 and 2018 at the Haskell Agricultural Laboratory near Concord. Each study was laid out as a split-plot design with six micro-rates of dicamba products (Engenia and XtendiMax) and three application times (soybean growth stages) in a randomized complete block design with four replications.
In 2017, the fractions of the dicamba rates studied were: 0, 1/10; 1/50; 1/100; 1/500; and 1/1,000 of the product labeled rate in dicamba-resistant soybean. (The labeled rates are: Engenia, 12.8 fl oz/acre, and XtendiMax, 22 fl oz/acre.) In 2018, the fractions of the dicamba rates studied were: 0, 1/10; 1/100; 1/1,000; 1/5,000; and 1/10,000 of the standard product label rate.
To simplify this, these fractions of the XtendiMax label rates are equivalent to: 1/10 = 2.2 oz/acre; 1/100 = 0.22 oz; 1/1000 = 0.022 oz and 1/10,000 = 0.0022 oz/acre. To better visualize these amounts consider that 1/10th of the label rate is equivalent to 3 tablespoons, 1/100th is a 1 teaspoon, 1/500 is 1/5th of a teaspoon and 1/1000 is 1/10th of a teaspoon applied over an area the size of a football field (one acre).
To develop the short videos, time-lapse cameras took hourly photos from 10 a.m. to 3 p.m. for 30-40 days after treatment (DAT) to record soybean injury. The photos for each of the treatments were then combined into 30- to 60-second videos.
Brief Summary of Results
As we previously reported, the results from our project clearly showed that all non-dicamba-tolerant soybeans types (conventional, Liberty-Link, and Roundup-Ready) were similarly sensitive to micro-rates of Engenia or XtendiMax.
Visual injury caused by both dicamba products ranged from 20% to 80%. Soybean exposure to dicamba just before flowering resulted in the greatest soybean injury and greatest yield losses. Types of soybean injury included:
- leaf cupping at V2 and V7/R1 spraying;
- epinasty (curly stems) at V2, V7/R1, and R2 timings;
- abortion of flowers at V7/R1; and
- swollen nodes and curly pods at R2 timing.
For visuals and symptomatology details view the videos.
Soybean height was reduced by 10%-85% across all dicamba micro-rates and growth stages of dicamba application. Delayed flowering and maturity delayed harvest, potentially subjecting the soybeans to early frost damage.
The yield losses varied with the doses and application time. The 5% yield loss is used to illustrate the potential for yield losses, which is equivalent to about 3 bushels/acre assuming a 60-bushel/acre average yield. It is reasonable to determine this level of yield difference consistently across years and locations despite the natural variability in field experimentations.
The range of doses that reduced yields by 5% varied from 0.02 to 0.08 oz/acre across all herbicides, application times, and soybean types, and represented approximately 1/1,000 to 1/300 of the label rates.
Other researchers have reported similar soybean responses to dicamba in their studies. Robinson et al. (2013) estimated that a dicamba rate less than 1/1000 of the label rate caused a 5% yield loss across growth stages. Foster and Griffin (2018) showed that 0.020-0.20 oz/acre (1/1,000 – 1/100) of dicamba caused 1%-9% yield loss when applied at V3/V4 stage and 2%-17% yield loss when applied at R1/R2 stage. Egan et al. (2014) compared six studies and concluded that approximately 4% yield loss was caused by 1/100 of label rate dicamba when applied at vegetative stages. Similarly, Kniss (2018) in his meta-analysis reported that the 5% yield loss threshold was evident at early vegetative stages (V1-V3) by 1/295 of the label rate compared to 1/98 of the label rate at late vegetative (V4-V7). The reproductive stage (R1-R2) required the least amount (1/629) of the label rate of dicamba to cause a 5% yield loss.
These examples are similar to the doses that caused a 5% yield loss in our study (0.02-0.08 oz). To help visualize such small amounts of dicamba, those rates are equivalent to about 1/10 or 1/2 of a teaspoon (1 oz = 6 teaspoons). In conclusion, Engenia and XtendiMax had similar effects on the growth and development of all non-dicamba-tolerant soybeans. This was evident by measuring various growth and development parameters, including final yields.
Soybean yields were affected by exposure to dicamba at all three growth stages, while the most sensitive stage was V7/R1. These results and accompanied videos clearly demonstrated that non-dicamba-tolerant soybeans were sensitive to low-rates of Engenia and XtendiMax, hence, efforts must be made to avoid drift of dicamba onto non-dicamba-tolerant soybeans or any sensitive crops.