EVALUATION OF TRIFLUDIMOXAZIN, A NEW PROTOPORPHYRINOGEN OXIDASE-INHIBITING HERBICIDE, FOR USE IN SOYBEAN
In Midwestern soybean [Glycine max (L.) Merr.] systems, especially in Indiana, three summer annual weed species are among the most common and troublesome for soybean producers: tall waterhemp (Amaranthus tuberculatus), giant ragweed (Ambrosia trifida), and horseweed (Conyza canadensis). Evolved resistance to current herbicides [e.g. glyphosate and acetolactate synthase (ALS) ihibitors], coupled with a dearth of new herbicide active ingredients being commercialized in the last two decades, has made controlling these problematic weeds particularly challenging. Trifludimoxazin is a novel protoporphyrinogen oxidase (PPO)-inhibiting herbicide that is currently under development for use in soybean and is likely to be commercially applied either alone or in combination with the herbicide saflufenacil. Research herein was conducted to investigate foliar control of tall waterhemp (including genotypes that are resistant to applications of other PPO inhibitors), giant ragweed, and horseweed following applications of trifludimoxazin alone and in combination with other herbicides. Additionally, the efficacy of soil-residual applications of trifludimoxazin and trifludimoxazin plus saflufenacil was evaluated for tall waterhemp and compared to other preemergence herbicides commonly used in soybean. Finally, soybean response to preplant applications of trifludimoxazin and trifludimoxazin plus saflufenacil at various preplant timings was investigated along with impact of adding the WSSA Group 15 herbicides acetochlor, pyroxasulfone, and S-metolachlor to preemergence applications of trifludimoxazin plus saflufenacil.
Applications of 12.5 g ha-1 trifludimoxazin were highly efficacious in foliar applications on tall waterhemp (94% control) at 28 days after application (DAA), less effective when applied to giant ragweed (78% control, 21 DAA), and ineffective on horseweed (9% control, 28 DAA). When applied in combination with glufosinate, glyphosate, paraquat, or saflufenacil, foliar control for these species was 91% to 100%, except for trifludimoxazin plus glyphosate applied to a glyphosate-resistant population of horseweed (17%). Furthermore, foliar efficacy of trifludimoxazin applied to tall waterhemp or Palmer amaranth (Amaranthus palmeri) was not impacted by the presence of target-site mutations (ΔG210 or R128 in waterhemp, ΔG210 or V361A in Palmer amaranth) that confered resistance to saflufenacil and fomesafen.
Near complete soil residual control [≥ 98% at 2 weeks after application (WAA)] of tall waterhemp was initially observed with 12.5 to 50 g ha-1 of trifludimoxazin but were less effective (39% to 69%) relative to commercial standards of pyroxasulfone (91%) or sulfentrazone (95%) by 6 WAA. Combining saflufenacil at 25 or 50 g ha-1 with soil-residual applications of trifludimoxazin improved efficacy on tall waterhemp at 6 WAA relative to trifludimoxazin alone. With the exception of 12.5 + 25 g ha-1 (74%), applications of trifludimoxazin plus saflufenacil, respectively, resulted in comparable residual tall waterhemp control (84% to 92%) as the commercial standards.
Soybean injury following applications of trifludimoxazin was relatively low (< 10%), regardless of preplant application timing [0 to 28 days before planting (DBP)] or rate (6.25 to 25 g ha-1). However, the addition of saflufenacil increased soybean injury, especially when environmental conditions were more conducive to soybean response. For instance, at Pinney Purdue Agriculture Center (PPAC) in 2019 soybean injury 4 weeks after planting (WAP) was 28%, soybean population was reduced by 39%, and yield was reduced by 27% when trifludimoxazin plus saflufenacil was applied at 25 + 50 g ha-1. The experimental conditions that corresponded to this elevated soybean injury were coarse-texture soil, low temperatures, and high precipitation at the time of soybean emergence. Lower rates of this herbicide combination resulted in less injury, and soybean response was minimized (≤ 8%) when applications were made at least 14 DBP. The addition of Group 15 herbicides to applications of trifludimoxazin plus saflufenacil at planting did not impact soybean response, except for at PPAC in 2019, where the addition of acetochlor (51%) or pyroxasulfone (46%) to 25 + 50 g ha-1 was greater than without these Group 15 herbicides at 4 WAP (22%). Field research indicated soybean response to combinations of trifludimoxazin plus saflufenacil differed by cultivar in some instances, and greenhouse experiments determined the response was attributable to differential soybean cultivar sensitivity to the saflufenacil component of the mixture.