Timing Aminopyralid to Prevent Seed Production, Control Medusahead (Taeniatherum caput-medusae), and Increase Forage Grasses

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By Matthew J. Rinella, Josh S. Davy, Guy B. Kyser, Fadzayi E. Mashiri, Susan E. Bellows, Jeremy J. James and Vanelle F. Peterson

Presented at Western Society of Weed Science Annual Meeting, 2018.

Abstract: Exotic annual grasses such as medusahead [Taeniatherum caput-medusae (L.) Nevski] and downy brome (Bromus tectorum L.) dominate millions of hectares of grasslands in the western United States. Applying picloram, aminopyralid, and other growth regulator herbicides at late growth stages reduces seed production of most exotic annual grasses. In this study, we applied aminopyralid to T. caput-medusae to determine how reducing seed production in the current growing season influenced cover in the subsequent growing season. At eight annual grassland sites, we applied aminopyralid at 55, 123, and 245 [ 1 ] g ae ha −1 spring just before T. Caput-medusae heading. The two higher rates were also applied pre-emergence (PRE) in fall to allow comparisons with this previously tested timing. When applied in spring during the roughly 10-d period between the flag leaf and inflorescence first becoming visible, just 55 g ae ha −1 of aminopyralid greatly limited seed production and subsequently reduced T. caput-medusae cover to nearly zero. Fall aminopyralid applications were less effective against T. caput-medusae, even at a rate of 245 g ae ha −1. The growing season of application, fall treatments, but not spring treatments, sometimes reduced cover of desirable winter annual forage grasses. The growing season after application, both spring and fall treatments tended to increase forage grasses, though spring treatments generally caused larger increases. Compared with other herbicide treatment options, pre-heading aminopyralid treatments are a relatively inexpensive, effective approach for controlling T. caput-medusae and increasing forage production.

[ 1 ] Aminopyralid rates of 55, 123 and 245 grams ae/ha are equivalent to Milestone® specialty herbicide at 3, 7 and 14 fluid ounces per acre respectively.