Results and Discussion

Brome Cover

In 2021, two years after treatment, brome cover was affected by both rate and timing of indaziflam application (Table 1). When rates were combined, fall application provided greater control of brome than spring application (Figure 1). This difference in brome cover could be because annual bromegrasses typically have a winter-annual growth cycle and germinate in the fall.

Compared to the control, brome cover was reduced by all three rates sprayed in the fall (Figure 2). These three means were not significantly different from each other according to pairwise comparisons. However, we can achieve at least 70.2% brome control with 95% certainty two years after treatment when spraying 2x the recommended rate in the fall, while spraying 1x the recommended rate achieves at least 55.2% control (Table 2). At the same time, with 95% certainty, perennial species cover will increase by at least 27.2% of the control under the fall 2x treatment and 18.8% under the fall 1x treatment (Table 3).

The 2021 treatment effects were noticeable even just by walking around the sites and taking a quick visual scan of the plots. For example, in the foreground of the image shown below, one can see a Fall 2x plot on the left looking greener with perennial grasses compared to the control plot on the right full of senescing bromegrass (annual bromes mature very early in the season, making them a poor forage for livestock and wildlife in the summer/fall).

Species richness and diversity

While the fall 1x and 2x rates of indaziflam may control brome, what about the impacts to the diversity of native grass and forb species? Indaziflam is a non-selective herbicide that prevents seeds from germinating, and the reason it works for invasive annual bromegrasses is they reproduce only by seed while the native grasses and forbs that make up the dry mixedgrass are largely perennial species that rely less on reproduction by seed. The idea is that by preventing seed germination for a few years, the invasive bromegrass seedbank would be depleted while existing perennial species could be sustained through their yearly regrowth until the herbicide dissipates from the soil and native seeds can germinate again.

However, the mechanism of cellulose biosynthesis inhibiting herbicides is not well understood, and even though indaziflam is not a foliar herbicide there could be non-target effects and injury to native plants growing on the field at the time of spraying. On the other hand, indaziflam may have a positive effect on native species. By releasing perennial grasses and forbs from the competition of annual bromegrass, native species may increase in cover.

To visualize treatment effect on species cover, I did an NMDS ordination. In NMDS ordination, plots with similar composition are closer together. Vectors indicate species frequencies and other variables that were not part of the ordination. NMDS axes are not defined.

NMDS ordination showed how rate separated species cover two years after fall applied treatments (Figure 4). The 1x and 2x rates had overlap and were associated with increased cover of perennial grasses. Additionally, it appeared that many 1x and 2x plots were associated with higher shannons and simpsons diversity. When annual brome is controlled, the remnant native plant community is released from its competition. Annual bromegrass is especially competitive for early spring moisture which is a valuable resource in dry grasslands.

The control was not strongly associated with any species other than the bromegrass, indicating that there were not any forbs or perennial grasses strongly negatively impacted by the higher rate herbicide treatments. Other indaziflam studies in rangeland in Colorado, US have shown no observable negative impacts to native plant species (Clark et al. 2019). However impacts to forbs from indaziflam are hard to draw, since forb cover pre treatment may already have been low due to the brome invasion. There were very few forbs at the two sites.

Additionally, effect size statistics showed that when spraying at either the fall 1x or 2x rates, there is a 99% probability of maintaining at least 90% of species richness compared to the control (Table 4)