Pasture-scale evaluation of postemergence applications of aminopyralid for controlling medusahead (Taeniatherum caput-medusae)

Authors: Rinella, Matthew - Matt; Bellows, Susan - Bartlett; DAVY, JOSH - University Of California - Cooperative Extension Service; FORERO, LARRY - University Of California - Cooperative Extension Service; HATLER, WILLIAM - Corteva Agriscience; JAMES, JEREMY - University Of California

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2021
Publication Date: 10/18/2021
Citation: Rinella, M.J., Bellows, S.E., Davy, J.S., Forero, L.C., Hatler, W.L., James, J.J. 2021. Pasture-scale evaluation of postemergence applications of aminopyralid for controlling medusahead (Taeniatherum caput-medusae). Rangeland Ecology and Management. 79(1):201-207. https://doi.org/10.1016/j.rama.2021.09.001.
DOI: https://doi.org/10.1016/j.rama.2021.09.001

Interpretive Summary: The exotic annual grass medusahead has invaded millions of hectares in the western U.S. The herbicide aminopyralid is an option for controlling medusahead. Aminopyralid reduces viability of grass seeds when applied just before seed production. In annual grasslands, desirable forage grasses begin producing viable seed before medusahead, which allows medusahead to be targeted for herbicide application somewhat selectively. In eight recent experiments, applying aminopyralid between when forage grasses and medusahead started producing seed reduced medusahead viability far more than forage grass viability. Consequently, the year after treatment, medusahead cover was near zero and forage grass cover increased. These experiments occurred in small plots, and results may differ in large pastures if the timing of seed production varies spatially. Variation could lead to less medusahead seed and more forage grass seed being susceptible to treatment. To test this possibility, we combined grazing and aminopyralid treatments in pastures and varied application timings in small plots. In pastures, aminopyralid reduced medusahead cover from about 45 to 20% one year and 60 to 20% another year. These reductions are smaller than in past research, but we do not attribute this to the larger treatment area because reductions were similar in pastures and small plots treated the same time. Despite smaller reductions in medusahead, forage grasses increased as much or more than previously, perhaps because aminopyralid more sharply reduced forage grass viability in previous experiments. Treatment responses did not vary with grazing, indicating grazing can continue during treatment since aminopyralid is not restricted for use with livestock present. Careful timing of aminopyralid application is critical: Just 12 d separated the least and most effective timings. Only a very low rate of aminopyralid is needed to reduce medusahead seed production, and the low cost of the low rate should encourage repeated applications when medusahead populations recover.

Technical Abstract: The exotic annual grass medusahead (Taeniatherum caput-medusae [L.] Nevski) has invaded millions of hectares in the western U.S. The herbicide aminopyralid is an emerging tool for controlling medusahead. Aminopyralid reduces viability of grass seeds when applied just before seed production. In annual grasslands, desirable forage grasses generally begin seed production before medusahead, and applying aminopyralid when medusahead is still mostly vegetative but forage grasses have begun producing seed allows medusahead to be targeted somewhat selectively. In eight recent experiments, this approach reduced medusahead viability and cover to near zero and increased forage grasses. These experiments occurred in small plots, and results may differ in pastures if plant stages vary due to effects of grazing or environmental heterogeneity. Moreover, the results may prove highly sensitive to application timing. To address these information gaps, we factorially combined grazing and aminopyralid treatments in pastures and varied application timings in small plots. In pastures, aminopyralid reduced medusahead cover from about 45 to 20% one year and 60 to 20% another year. These reductions are smaller than in past research, but we do not attribute this to the larger treatment area because reductions were similar in pastures and small plots treated the same time. Despite smaller reductions in medusahead cover, forage grasses increased as much or more than previously, perhaps because aminopyralid previously more sharply reduced forage grass seed viability. Seed viability responses did not vary with grazing, indicating grazing can occur during treatment, because aminopyralid does not have grazing restrictions. Careful application timing is critical: Just 12 d separated the least (jointing and boot stages) and most (boot to early flowering) effective timings for controlling medusahead. Only a very low rate of aminopyralid is needed to reduce medusahead seed production, and the low cost of the low rate should encourage repeated applications when necessary.