Performance of the Pre-Emergent Herbicide Indaziflam on Cheatgrass Rangelands By Charlie D. Clements and Dan Harmon Resource managers are constantly looking for effective tools to restore or rehabilitate degraded habitats. Throughout the Intermountain West, the accidental and subsequent invasion of cheatgrass has converted former big sagebrush/bunchgrass habitats into annual dominance, especially cheatgrass. Cheatgrass competition on rangelands is a major issue as cheatgrass outcompetes desirable perennial species at the seedling stage for limited resources. Mechanical and chemical treatments to reduce cheatgrass densities and associated fuels have been conducted and reported with varying degrees of success. Mechanical, disking, of cheatgrass in the spring, prior to seed maturity, can effectively reduce current years density while at the same time burying the remaining cheatgrass seed bank to depths that reduce future germination resulting in nearly 80% decrease in cheatgrass densities and associated competition with desirable perennial species. Following the disking treatment the site must be seeded the following fall with desirable species that have the inherent potential to germinate, emerge and establish in the given environment and in the face of cheatgrass completion. A major problem with mechanical treatment of cheatgrass-infested rangelands is they are very limited by topography and physical constraints of the rangeland. Chemical, herbicide, use on rangelands to reduce cheatgrass densities and associated fuels is much more popular and widely used, especially soilactive, pre-emergent herbicides. Imazapic (Plateau) is the most common pre-emergent herbicide used on cheatgrass-infested rangelands. Treated sites are recommended to be applied in late summer/early fall prior to any effective moisture that can trigger fall cheatgrass germination and emergence, that is why they are called pre-emergent herbicides. Application of pre-emergent herbicides in late fall may very well end up in a less than optimal control of cheatgrass that is necessary in the restoration/ rehabilitation seeding efforts. A more recent herbicide that has introduced itself into the conversation of cheatgrass control is Indaziflam. Indaziflam is a broad-spectrum pre-emergent herbicide that was first released in 2011 for use in several perennial cropping systems. In 2016, a supplemental label for Indaziflam was approved for release for the restoration of natural areas as well as fire rehabilitation areas, and more specifically 12 NOVEMBER/DECEMBER 2022
to target winter annual grasses such as cheatgrass. Currently, the Indaziflam product registered for use on rangelands is Rejuvra, at a recommended rate of 5 oz/acre. In 2018 we started investigating the efficacy of Indaziflam compared to the more widely used, Imazapic, at two sites in northern Nevada (TS Ranch, Boulder Valley and Bedell Flat north of Reno). In the fall of 2018, 2019 and 2020 we applied Indaziflam in replicated 1.5 acre plots at 5 oz/acre rate. We also applied Imazapic at 6 oz/acre rate in adjacent plots of the same size. Following early fall herbicide applications, the following May and June we recorded cheatgrass densities. Cheatgrass densities over this 4-year period were reduced from 36.8 – 98.6% with Indaziflam compared with Imazapic, 94.2 – 97.8% (Fig. 1). Indaziflam treated plots also resulted in significantly more broadleaf annuals, such as Russian thistle, compared to Imazapic. The best-known method at suppressing cheatgrass densities is with the establishment of perennial grasses, therefore all treated sites are fallowed for 1-year (herbicide activity of 12-15 months) and seeded to native, introduced and native/introduced perennial grass seed mixes. Indaziflam treated plots resulted in less than 1 perennial grass/ft² in both the 2018/19 and 2019/20 plots, ranging from 0.1/ft² in the native mix to 0.7/ ft² in the introduced mix, while the Imazapic treated plots ranged from 0.2/ft² in the native mix to 1.2/ft² in the introduced mix. Precipitation was a major limiting factor in both the 2018/19 and 2019/20 plots as the TS Ranch site only received 6.3” and 6.0” in each of those two years, respectfully. The Bedell Flat site received 5.7” and 7.4”, respectfully. Indaziflam is reported to have continued activity up to 3-years, which we witnessed on our plots, therefor we seeded the failed seeding plots of 2019 and 2020 at the same time we seeded the 1-year fallow 2021 plots in early October 2021 at both sites (Fig. 2). The Bedell Flat site received 11.2” of annual precipitation from October 2021 through September 2022, with 5” and 3” falling in October and December, respectfully. The TS Ranch site started out great with 2.3” falling in October, but then went dry as the site only received a total of 5.9” for the year. Even though the TS site was quite dry, the exceptional October started the The Progressive Rancher
germination process and we recorded increased seedling emergence and density in both the Indaziflam and Imazapic plots (Fig. 3a and 3b). The best performing Indaziflam plot at the TS Ranch site was the 2018 plot, 3-years post spray with 3 perennial grasses/ft² in the introduced mix plots compared to 1.2/ft² and 0.5/ft² in the native/ introduced and native mix plots, respectfully. This compared to 10.2/ft², 5.2/ft² and 1.3/ft² in the Imazapic treated plots. The best performing Indaziflam plot at Bedell Flat was also the 2018 treated plots, 3-year post-spray with 2.4/ft², 0/ft² and 1.2/ft², respectfully, compared to the Imazapic 2019/20 plot of 9.8/ft² in the introduced mix plot, 7.2/ft² native/introduced mix plot and 2.2/ ft² in the native mix plot (Fig 4). Although Indaziflam has longer lasting residue activity that appears to have negative affects on seedlings of seeded perennial grasses compared to Imazapic, we have also witnessed the benefits of this longer lasting activity on cheatgrass control and perennial grass performance. When Indaziflam was applied over exiting perennial grasses, the control of cheatgrass in the community was nearly complete, 98.6%, and the perennial grasses increased in vigor due to the increase in available moisture and nitrogen that the cheatgrass and other weeds were utilizing (Fig. 5). Also, this added vigor increased the green period of perennial grasses by as much as 5 weeks, therefor, perennial grass stands not treated with Indaziflam dried out by early July, whereas perennial grass stands treated with Indaziflam provided green plant material until mid-August. This is an important factor in wildfire prevention as the decrease in cheatgrass as a fuel source is significantly decreased and the added green period decreases the chance of ignition. At this point, we have not concluded that Indaziflam is effective at controlling cheatgrass and rehabilitating the site back to perennial grasses in the short-term, but the use of Indaziflam on exiting intact herbaceous communities or greenstrips would provide added benefits to improve stand vigor, improve maintenance of greenstrips and decrease the fine fuels associated with cheatgrass, therefor decreasing the chance, rate, spread and season of wildfires that are so destructive to Great Basin rangelands. www.progressiverancher.com
