By Tom Barber, Extension Weed Scientist
Two weeks ago, I wrote an article on the effects of dicamba drift on soybean and potential yield reduction (click here for article). Basically to sum it up for yield loss, the most sensitive stages where yield loss can result from off-target movement of dicamba ranges from late vegetative through early reproductive (V6-R2). The graduate student that conducted this research used the same plots to evaluate the lasting effects of dicamba applications on soybean. If you remember from the earlier post, dicamba applications were made at 1/64th and 1/256th of an equivalent 16oz/A rate of Clarity (0.5 lb/A dicamba). These were 4 row plots and only the center 2 rows were sprayed to reduce spray drift onto neighboring plots. Applications were made at V4, V6, R1, R2, R3, R4, R5 and R6 growth stages to both an indeterminate Group IV and determinate Group V soybean cultivar. These research trials were conducted over a two year period at 2 different locations, so basically 4 site years of data are included in the figures below.
The interesting observation in regards to dicamba symptomology on soybean plants is that foliar symptoms are not apparent much past R3 or R4. In other words the dicamba in the plant is no longer being transported to the soybean leaves, but rather is all moving to the sink on the plant, which includes the pods and developing seed (Picture 1). Another interesting observation from late-season soybean exposure to drift rates of dicamba is the delay in plant maturity or ripening. Later applications, especially around R6 can greatly increase the amount of time for the crop to dry down and mature. In most cases, late applications have delayed maturity as much as 2-3 weeks over untreated plants.
Plots that were treated during the season were harvested to determine soybean yield reduction. A small 2 lb combine sample was collected from each plot at harvest to determine if any effects on seed quality or germination were apparent following the in-season dicamba applications. Seeds or progeny (off-spring) from these treated plants were sent to the Arkansas State Plant Board for germination and accelerated aging tests as well as planted in the greenhouse and field the next season to evaluate germination and potential injury. In Figures 1 and 2, germination through standard and accelerated aging tests was greatly reduced as low as 20% for standard and 5% accelerated aging in seeds from parent plants that were treated at R4-R6 growth stages.
Results were similar for both determinate and indeterminate cultivars. However, seed germination with the determinate cultivar (Figure 2) was not reduced as much as the indeterminate cultivar, especially when plants were sprayed at R6. When seed were planted in the greenhouse and field, germination and vigor were greatly reduced at these stages as was expected based on data from the lab tests. Figure 3 shows the percentage of injured plants, or plants showing dicamba-like symptomology once they emerged in the field the following season. Once applications of dicamba were made to parent plants at R4-R6, the number of plants showing visual injury increased significantly up to 94%, especially with the higher drift rate of 1/64X. When looking at injury to soybean off-spring, the amount or rate of dicamba drift plays a significant role. When the dicamba rate applied to parent plants was reduced to 1/256X, overall injury to subsequent off-spring decreased significantly from 50% to 35%. Based on these results, it is likely that rates lower than 1/256X will result in less injury to off-spring. Picture 2 shows the drastic difference in off-spring from parent plants sprayed in vegetative vs. reproductive stages. All plots above the red line are off-spring from plants sprayed in vegetative stages and below the red line are off-spring from plants sprayed in reproductive stages. Picture 3 shows the difference in the field between off-spring from parent plants sprayed with 1/64 and 1/256X rate. Even though off-spring from plants sprayed with the lower rate had higher germination percentages, foliar dicamba symptomology was still apparent in the plots (Picture 4).
These data shed some light on the effect of off-target dicamba injury to sensitive soybean in reproductive stages. It is important to note that regardless of the rate applied to parent plants, no dicamba residue could be identified in the seed when samples were submitted to multiple labs for analysis. This is just another indication of soybean sensitivity to dicamba which appears to be more sensitive than what is detectable in the lab through analytical techniques. These data back up similar research conducted in the late 60’s and early 70’s on dicamba effects on maturity, reduced seed quality, pod malformation and malformed progeny. The fact that late applications of dicamba can have an effect on seed quality the following season, seed producers should monitor production fields closely for late season dicamba symptomology on soybean pods.