Pre-emergence Control of Six Invasive Winter Annual Grasses with Imazapic and Indaziflam (2016)

Managing invasive winter annual grasses on noncrop and rangeland remains a constant challenge throughout many regions of the United States. Currently, there are limited management options for controlling winter annual grasses that work consistently, provide multiple years of control, and do not injure desirable plant communities. Imazapic has been one of the most widely used herbicides for downy brome control on rangeland; however, control with imazapic has been inconsistent beyond the application year and perennial grass injury is not uncommon. Indaziflam, a new herbicide mode of action for rangeland weed management, has shown promise in providing long-term downy brome control. A greenhouse study was conducted to compare pre-emergence activity of imazapic and indaziflam on six invasive winter annual grasses: downy brome, cereal or feral rye, jointed goatgrass, Japanese brome, medusahead, and ventenata. For both herbicides, seven rates were used to develop dose-response curves for each species. Log-logistic regression was conducted to determine the herbicide dose required to reduce biomass by 50% (GR50 values). Indaziflam was significantly more active across all species compared to imazapic, with the exception of jointed goatgrass. Comparing all species, the GR50 values for imazapic were on average 12 times higher than indaziflam. Japanese brome was the most sensitive to both herbicides, whereas jointed goatgrass and feral rye were the most difficult winter annual grasses to control with indaziflam and imazapic, respectively. This research provides evidence of a potential new mode of action for land managers to control the major invasive winter annual grasses. Nomenclature: Imazapic; indaziflam; cereal or feral rye, Secale cereale L.; downy brome, Bromus tectorum L.; Japanese brome, Bromus japonicus Thunb. ex Murr.; jointed goatgrass, Aegilops cylindrica Host.; medusahead, Taeniatherum caput-medusae (L.) Nevski; ventenata, Ventenata dubia (Leers) Coss. in Dur. Management Implications: Invasive winter annual grasses pose a major threat to native plant communities in the United States. The life cycle of these species increases their invasiveness because few native species behave as winter annuals, providing a niche for invasive annual grasses to exploit moisture and nutrients when most desirable perennial plants are dormant. Although downy brome alone infests over 22 million ha of U.S. rangeland, there are five other invasive winter annual grasses that cause significant economic and ecological impacts: feral rye, Japanese brome, jointed goatgrass, medusahead, and ventenata. Currently, acetolactate synthase- (ALS) inhibiting herbicides such as imazapic and rimsulfuron are used for selective winter annual grass control, whereas nonselective herbicides like glyphosate are also recommended for dormant season applications (late fall or early spring). Unfortunately, none of these herbicides provide consistent control beyond 1 yr after treatment (YAT), resulting in rapid reinvasion of treated areas via the soil seed bank. Indaziflam (Bayer CropScience), a cellulose biosynthesis-inhibiting herbicide, is a new mode of action for invasive winter annual grass management. Previous field research demonstrated that indaziflam provided excellent downy brome and feral rye control 2 and 3 yr after treatment compared to imazapic. Two applications of indaziflam over a 5-yr period could substantially reduce or possibly eliminate the winter annual grass seed from the soil seed bank. The objective of this study was to evaluate indaziflam's potential to control other problematic invasive winter annual grasses found in the United States and compare its activity to the most commonly used herbicide, imazapic. The herbicide dose resulting in 50% reduction in dry biomass (GR50) was calculated for each invasive winter annual grass. In the greenhouse, indaziflam was significantly more active against all winter annual grasses compared to imazapic, with jointed goatgrass as an exception. Averaged across all invasive winter annual grasses, imazapic GR50 values were 12 times greater compared to indaziflam. The potential for long-term downy brome management is very encouraging; however, downy brome is only one species in a suite of winter annual grasses that threaten native ecosystems from the Great Plains to the Pacific Coast. This research indicates that indaziflam is active in controlling a range of winter annual grasses, and based on what we know about the soil seed bank of these species, indaziflam could be a key component in providing long-term management. Our findings provide evidence that indaziflam could be an alternative strategy for controlling invasive winter annual grasses, including relatively new invaders such as medusahead and ventenata. Additional field research is needed to determine if indaziflam provides the long-term control of ventenata, medusahead, jointed goatgrass, and Japanese brome that has been previously reported with downy brome and feral rye.