Interactions of Iptriazopyrid with Synthetic Auxin and Residual Herbicides in Rice

Abstract

Iptriazopyrid is a novel 4-hydroxyphenylpyruvate dioxygenase inhibitor that, upon registration for rice production, will be the first azole carboxamide herbicide labeled in the United States. Because herbicides are commonly mixed to broaden the weed control spectrum and efficacy, herbicide interactions with iptriazopyrid must be evaluated. Field experiments were conducted near Colt, AR, in 2024 and 2025 to evaluate interactions between iptriazopyrid and either synthetic auxins or residual herbicides labeled for rice. The experiments were designed as a two-factor randomized complete block design, with the presence of iptriazopyrid at 50 g ai ha ^-1 as factor one and the presence of labeled synthetic auxins or residual herbicides as the second factor. Applications targeted 3- to 4-leaf barnyardgrass, and Colby’s method was utilized to determine herbicide interactions. 2,4-D was the only herbicide found to be antagonistic to iptriazopyrid throughout the experiments. Applied alone, iptriazopyrid provided 75%, 83%, and 79% barnyardgrass control at 1, 2, and 4 WAT, respectively, compared to 12%, 18%, and 8% control when mixed with 2,4-D. The iptriazopyrid plus triclopyr combination was antagonistic for barnyardgrass control at 1 and 2 weeks after treatment (WAT), with 61% and 73% control, respectively; however, the relationship was no longer significant at 4 WAT. Barnyardgrass density supported an antagonistic interaction between 2,4-D and iptriazopyrid, as combinations of the two led to higher densities than iptriazopyrid applied alone. No labeled residual herbicide antagonized iptriazopyrid for barnyardgrass control; nor did barnyardgrass densities differ from iptriazopyrid applied alone. Overall, these findings show that iptriazopyrid offers reasonable flexibility when mixed with some, but not all, synthetic auxins labeled for rice, and that there is no reduction in barnyardgrass efficacy when mixed with residual herbicides.