Simple Bioassay for Phytotoxic Concentrations of the Herbicide Indaziflam in Soil

Abstract

Indaziflam is a relatively new herbicide that kills newly germinated plants. There is interest in using indaziflam to improve rangeland restoration but applying it around the time of seeding risks damaging seeded plants. A better strategy may be using indaziflam long before seeding to deplete weed seedbanks and then seeding after it dissipates. Dissipation rates vary and are difficult to predict, so testing is needed to determine whether indaziflam remains present. The manufacturer-recommended test involves seeding small indaziflam-treated areas and then monitoring for herbicide damage. A disadvantage here is that rangeland seeding is failure-prone, so seeded species can fail to emerge whether indaziflam is present or not. Another disadvantage is that test areas cannot be reliably evaluated until many months to a year after seeding, by which time evaluations are obsolete. We sought a more reliable, rapid bioassay. We gathered soil samples from nontreated and indaziflam-treated plots in two experiments treated 565 d and 204 d earlier. In these samples in a greenhouse, we planted seeds of native grasses (Elymus lanceolatus [Scribn. & J.G. Sm.] Gould and Pascopyrum smithii [Rydb.] Á. Löve) and an exotic invasive grass (Bromus japonicus Thunb.) and then measured plant responses. Plant densities and heights were similar across experiments. Indaziflam reduced native grass density 50% ± 8%, native grass height 74% ± 6% (mean ±SE), and exotic grass density and height nearly 100% (p < 0.05). The grasses that emerged from treated soil were discolored. Plant data were gathered 18 d after seeding, which illustrates that restoration managers can rapidly test for indaziflam in advance of seeding. To prevent being misled by natural variability among samples, we recommend testing ≥ five nontreated and treated soil samples. In addition to greenhouses, other well-illuminated areas held at 16–24°C are sufficient for testing.