Simple bioassay for phytotoxic concentrations of the herbicide indaziflam in soil

Authors: Rinella, Matthew; Anderson, Elise; Cook, Kirsten; Bellows, Susan

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2025
Publication Date: 3/26/2025
Citation: Rinella, M.J., Anderson, E.M., Cook, K.A., Bellows, S.E. 2025. Simple bioassay for phytotoxic concentrations of the herbicide indaziflam in soil. Rangeland Ecology and Management. 100:78-82. https://doi.org/10.1016/j.rama.2025.02.007.
DOI: https://doi.org/10.1016/j.rama.2025.02.007

Interpretive Summary: Indaziflam is a recently developed herbicide that kills newly germinated plants. There is interest in using indaziflam to improve grassland restoration, but applying the herbicide around the time of seeding risks damaging seeded plants. A better strategy may be using indaziflam to deplete weed seedbanks then seeding after the herbicide dissipates. Dissipation rates vary, so some form of testing is needed to determine whether herbicide remains present. The manufacturer-recommended test involves seeding small areas then monitoring for indaziflam damage. A disadvantage here is that grassland seeding is failure-prone, so seeded species can fail to emerge whether indaziflam is present or not. Another disadvantage is that waiting to evaluate test areas can delay seeding from many months to a year, by which time the evaluation is obsolete. We sought more reliable, rapid tests. We sampled soil from nontreated and indaziflam-treated plots in two experiments treated 565 days and 204 days earlier. In these soil samples in a greenhouse, we planted seeds of native grasses and an exotic invasive grass then measured plant densities and heights. We also planted seeds in potting soil to rule out the possibility nontreated field soil was incompatible with plant growth. Densities and heights were similar across experiments and between nontreated field soil and potting soil. Indaziflam reduced native grass density 50% and height 74% and exotic grass density and height nearly 100%. Those grasses that emerged from treated soil were discolored. Restoration practitioners can easily test for indaziflam in advance of seeding. To prevent being misled by natural variability among samples, we recommend testing =5 nontreated and treated soil samples. In addition to greenhouses, other well-illuminated areas held at 16 to 24 C are sufficient for testing.

Technical Abstract: Indaziflam is a recently developed herbicide that kills newly germinated plants. There is interest in using indaziflam to improve grassland restoration, but applying the herbicide around the time of seeding risks damaging seeded plants. A better strategy may be using indaziflam to deplete weed seedbanks then seeding after the herbicide dissipates. Dissipation rates vary, so some form of testing is needed to determine whether herbicide remains present. The manufacturer-recommended test involves seeding small areas then monitoring for indaziflam damage. A disadvantage here is that grassland seeding is failure-prone, so seeded species can fail to emerge whether indaziflam is present or not. Another disadvantage is that waiting to evaluate test areas can delay seeding from many months to a year, by which time the evaluation is obsolete. We sought more reliable, rapid tests. We sampled soil from nontreated and indaziflam-treated plots in two experiments treated 565 d and 204 d earlier. In these soil 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.) then measured plant densities and heights. We also planted seeds in potting soil to rule out the possibility nontreated field soil was incompatible with plant growth. Densities and heights were similar across experiments and between nontreated field soil and potting soil. Indaziflam reduced native grass density 50±8% and height 74±6% (mean±SE) and exotic grass density and height nearly 100% (p<0.05). Those grasses that emerged from treated soil were discolored. Restoration practitioners can easily test for indaziflam in advance of seeding. To prevent being misled by natural variability among samples, we recommend testing =5 nontreated and treated soil samples. In addition to greenhouses, other well-illuminated areas held at 16 to 24 C are sufficient for testing.