Effects of Dicamba Micro-Rates on Yields of Non-Dicamba Soybeans

 

Control of glyphosate-resistant weeds with dicamba has been encouraging, resulting in increased adoption of dicamba-tolerant (DT) soybeans. However, the off-target movement of dicamba-based products to non-DT soybeans and other broadleaf crops (fruits, grapes, tomato) has become a significant concern. The negative impact of dicamba on non-DT soybean may vary with dicamba rate, soybean type, and soybean growth stage at the time of drift occurrence.

 

The majority of soybean acreage was (and next year will still be) planted to non-DT varieties. During the 2018 season, there were many dicamba drift complaints, many of which have led to litigation. For example, the Nebraska State Department of Agriculture received over 90 complaints of dicamba drift onto non-DT soybeans with an estimated impact on 60,000 affected acres. In addition, there were over a thousand complaints across the Midwest.

 

It is known that fine-sized dicamba spray droplets have tendency not only to drift with any air movement (even very slow wind), but also to move off target when fine aerosol droplets remain suspended during air temperature inversions (defined as air temperatures near the soil or canopy surfaces that are cooler than those higher up), high humidity, and little or no wind. These suspended tiny aerosol-size dicamba droplets may not evaporate for some time, and thus can drift from the target site, especially in the first 36 hours (in some cases even 98 hours) after application. This drift can travel long distances (two to three miles) before it is finally deposited onto nearby fields, where there may be dicamba-sensitive crops such as non-DT soybeans. Field studies were conducted in 2017 and 2018:

 

Field trials were conducted in 2017 and 2018 at the university’s Haskell Ag Lab near Concord. The study was laid out as a split-plot treatment design with six micro rates of dicamba products (Clarity, Engenia and XtendiMax), three application times (soybean growth stages) in an RCB experimental design with four replications. The dicamba rates included: 0, 1/10; 1/50; 1/100; 1/500; and 1/1000 of the standard product label rate for DT types. Plots had four rows of each soybean types (Round-up Ready, Liberty-Link, Conventional, and Dicamba-Tolerant serving as a check).

 

The three application times were second trifoliate (V2), just before flowering (V7/R1), and full flowering (R2). The V2 timing was to simulate potential drift at an early stage of soybean growth, the expected time for the earliest applications of dicamba products. The second and third timings were to simulate potential drift at the later stages of soybean growth, reflecting variable planting times. For, example some fields might be planted earlier, some later, thus these two timings would simulate potential drift among neighboring fields around flowering time. Visual evaluation of injuries was conducted at 7, 14, 21, and 28 days after treatment (DAT). Yields of all soybean types were harvested.

 

A four-parameter log-logistic regression equation was used to model the relationship between dicamba micro-rates and yield. The regression analyses helped to estimate the dicamba micro-rate at which a certain level of injury or yield loss could be predicted. Mean values were collected for variables which regression analyses were not possible.

 

 

Roundup Ready, Liberty Link, and Conventional soybeans were equally sensitive to all tested micro-rates of Clarity, Engenia and XtendiMax. When micro-rates were increased, soybean yields were significantly impacted. The levels of impact were dependent on the soybean growth stage of dicamba application with V7/R1 stage being the most dicamba sensitive.

 

 

All dicamba products injured non-DT soybean varieties in a similar fashion. The visual injuries ranged from 20%-80%, depending on the growth stage spray timing and dicamba rate. The visual injury was about 50% when Conventional, Liberty Link, and Roundup Ready soybeans were subjected to Clarity at a rate of 0.01-0.05 oz per acre, across three application times (Figure 1, Table 1).

 

Compared to the 12.8 oz/ac of the label rate, an Engenia dose of only 0.01-0.04 oz/ac caused 50% visual injury in Conventional soybean; 0.02-0.05 oz/ac in Liberty-Link soybean; and 0.01-0.05 oz/ac in Roundup Ready soybean across three application stages (Figure 1, Table 1).

 

The visual injury was about 50% when Conventional, Liberty Link and Roundup Ready soybeans were subjected to XtendiMax rate of 0.02-0.09 oz/ac, across three application times (Figure 1, Table 1). Across all tested products and soybean type, V7/R1 required the least micro-rate of any of the dicamba products to cause 50% injury, suggesting that V7/R1 was the most sensitive stage to dicamba.

 

Figure 1. Conventional, Liberty Link and Roundup Ready soybean injury caused by micro-rates of dicamba products at 28 days after treatment (DAT) for three growth stages.

 

 

Yield of all non-DT soybeans was significantly affected by the three tested dicamba products, irrespective of application time. However, the V7/R1 stage appears to be the most dicamba-sensitive stage, followed by the R2, and then the V2 stages. For example, Conventional, Liberty Link and Roundup Ready soybeans yielded 58, 60, 60 bu/ac in non-sprayed control plots. However, when the same soybeans were sprayed at V2 stage with 1/10 (1.6 oz/ac) of Clarity rate, they yielded significantly less, i.e., 29, 23, and 35 bu/ac, respectively. The same 1/10 rate of Clarity, when applied at R2, lowered yields of Conventional, Liberty Link and Roundup Ready soybeans much further to 17, 18 and 26 bu/ac, respectively. However, extremely low yields of only 2, 5 and 3 bu/ac were measured when the Clarity spraying occurred at V7/R1 stage (Table 2).

 

Engenia rate of 1/10 (1.28 oz/ac) lowered yields of Conventional, Liberty Link and Roundup Ready soybeans from 58, 60, 60 bu/ac in non-sprayed plots to 24, 22, and 27 bu/ac, respectively, in plots sprayed at V2 stage. The same rate of Engenia lowered yields of Conventional, Liberty Link and Roundup Ready soybeans to 18, 15 and 25 bu/ac, respectively, when sprayed at R2.  Spraying Engenia at the 1/10 rate further lowered yields to 3, 2 and 4 bu/ac for Conventional, Liberty Link and Roundup Ready soybeans respectively, when sprayed at V7/R1 soybean stage (Table 2).

 

An XtendiMax rate of 1/10 (2.2 oz/ac) lowered yields of Conventional, Liberty Link and Roundup Ready soybeans from 58, 61 and 63 bu/ac in non-sprayed plots to 25, 27 and 33 bu/ac, respectively, in plots sprayed at V2 stage. The same rate of XtendiMax lowered yields of Conventional, Liberty Link and Roundup Ready soybeans to 11, 20 and 26 bu/ac, respectively, when sprayed at R2. Spraying Engenia at the 1/10 rate further lowered yields to 3, 2 and 5 bu/ac for Conventional, Liberty Link and Roundup Ready soybeans, respectively, when sprayed at V7/R1 soybean stage (Table 2).

 

In most cases, the 1/50 and 1/100 of the labels rates reduced soybean yields by 13-16 bu/ac when applied at the V2 stage. For example, 1/100 of the label rate of Engenia reduced yields by about 14 bu/ac in Conventional, 16 bu/ac in Liberty Link, and 13 bu/ac in Roundup Ready soybean. The same rate applied at V7/R1 stage reduced yields by about 26 bu/ac in Conventional, 18 bu/ac in Liberty Link and 26 bu/ac in Roundup Ready soybeans.

 

Yields were also reduced even with “very low” exposures of 1/500 and 1/1000 of the label rate. For example, the 1/1000 of label rate of Engenia applied at V2 stage reduced yields by about 4 bu/ac in Conventional, 2 bu/ac in Liberty Link, and 4 bu/ac in Roundup Ready soybean. The same rates applied at V7/R1 stage reduced yields by 11 bu/ac in Conventional, 3 bu/ac in Liberty Link and 8 bu/ac in Roundup Ready soybean (Table 2).

 

 

Low doses of less than 1/10 of the label rates of all tested dicamba products caused yield loss. For example, a Clarity dose of 0.13-0.49, 0.25-0.53 and 0.18-0.49 oz/ac reduced yield by 50% in Conventional, Liberty Link and Roundup Ready soybean respectively, depending on the crop growth stage of Clarity application. The V7/R1 was the most sensitive stage to Clarity in Liberty Link and Roundup Ready soybeans as 0.18-0.25 oz/ac reduced yield by 50% (Figure 2, Table 3).

 

Engenia doses of 0.12-0.33, 0.16-0.41 and 0.18-0.37 oz/ac reduced yield by 50% in Conventional, Liberty Link and Roundup Ready soybean respectively, depending on the crop growth stage of Engenia application. The V7/R1 was the most sensitive stage to Engenia in Conventional and Roundup Ready soybeans as 0.12 oz/ac reduced yield by 50% (Figure 2, Table 3).

 

XtendiMax dose of 0.16-0.69, 0.27-0.68 and 0.25-0.77 oz/ac reduced yield by 50% in Conventional, Liberty Link and Roundup Ready soybean respectively, depending on the crop growth stage of XtendiMax application. The V7/R1 was the most sensitive stage to XtendiMax in Liberty Link and Roundup Ready soybeans as 0.18-0.25 oz/ac reduced yield by 50% (Figure 2, Table 3).

 

Figure 2. Yield loss in Conventional, Liberty Link and Roundup Ready soybeans as caused by micro-rates of dicamba products at 28 days after treatment (DAT) for three growth stages.

 

 

 

The results from this study clearly showed that non-DT soybean types (Conventional, Liberty Link, and Roundup Ready) were very sensitive to micro-rate applications of Clarity, Engenia, and XtendiMax. Soybean heights were reduced by 10%-70% depending on the dicamba micro-rate and the growth stage of dicamba application. Unreported data indicated that soybean height was reduced by the dicamba micro-rates which later resulted in delayed canopy closure when sprayed early in the season, and inability to attain canopy closure when sprayed just around flowering stage, even with the lowest dicamba rate (1/1000 of label rate).

 

From a practical standpoint, the reduced soybean height and delayed (or no) canopy closure could potentially reduce soybean competitiveness against weeds, resulting in a significant reduction in growth and yield. Soybean height was not only reduced by the dicamba products, an early season exposure (for example, V2 growth stage) to the products as low as 1/100 of the label rate terminated apical meristem growth, thereby promoting a “fork-like” pattern of plant branching. The agronomic implication of this change in plant branching morphology is yet unclear, however, the fork-like pattern of branching could be an important symptom of early season dicamba drift in soybean. There was also a delayed flowering and maturity caused by the micro-rates of dicamba (data not shown). A delay in maturity may delay harvest, increasing the possibility of early frost damage to soybeans.

 

Injury caused by the dicamba products ranged from 20%-80%, depending on the dicamba rate and growth stage of dicamba spray in the sensitive soybeans. Generally, injury increased with increases in the tested dicamba micro-rates. Dicamba application just before flowering resulted in the greatest soybean injury. Injury symptoms largely depend on the growth stage of dicamba application and included, for example, cupping of leaves in V2 and V7/R1 timings; epinasty in V2, V7/R1, and R2 timings; abortion of flowers in V7/R1 timing; and swollen nodes and curly pods in R2 timing.

 

The take-home message is that Clarity, Engenia and XtendiMax had similar effects on the growth and development of all non-DT soybeans. This was evident by measuring various growth and development parameters, including final yields. Soybean yields were most sensitive to dicamba at V7/R1 growth stage compared to R2 and V2 stages. The high sensitivity of soybean yield at V7/R1 may be attributed to relatively high injury levels and the abortion of flowers recorded after application of dicamba. These results clearly showed that non-DT soybeans were sensitive to even very low micro-rates of Clarity, Engenia and XtendiMax, hence, efforts must be made to avoid drift or any kind of off-target movement of dicamba onto non-DT soybeans.