Invasive annual grasses, such as medusahead rye (Taeniatherum caput-medusae) and downy brome (Bromus tectorum L.), impact millions of acres of rangeland in the western United States. These grasses can reduce native species richness and abundance, decrease livestock carrying capacity, alter nutrient cycling and shorten wildfire return intervals. Historical management of medusahead has concentrated on timed grazing with livestock, burning, mechanical removal of thatch, and use of herbicides.
One of the challenges with using herbicides as a management tool is achieving selective control of the invasive annual grass without causing significant injury to taxonomically similar desirable grasses or other desirable vegetation. Over the past several years, researchers and managers observed that Milestone® herbicide applied for broadleaf weed control also impacts medusahead and downy brome plants. Field and greenhouse research have recently been conducted to measure the effectiveness of Milestone at controlling medusahead. Results of these studies are discussed in this article.
Kyser GB, VF Peterson, JS Davy, JM DiTomaso. 2012. Preemergent Control of Medusahead on California Annual Rangelands with Aminopyralid. Rangeland Ecology & Management: Vol. 65, No. 4, pp. 418-425.
Scientists at University of California at Davis established replicated field research trials at three medusahead-infested sites in northern California. Herbicide treatments included Milestone at 3, 5, 7, and 14 fluid ounces per acre (fl oz/A), Matrix at 1 and 2 ounces product per acre (oz/A), and Plateau at 8 fl oz/A. Applications were made prior to medusahead germination in September and October 2009. Visual cover estimates, biomass, and seedhead samples were collected from treated and non-treated control plots in May 2010.
Results from the three field experiments indicate that medusahead reduction was greatest with Milestone at 14 fl oz/A (Figure 1). There was a significant release of desirable annual grasses including Italian ryegrass (Lolium multiflorum), soft chess (Bromus hordeaceus) and meadow barley (Hordeum brachyantherum) with Milestone at 7 fl oz/A (Figure 2). The increase in desirable annual grass production was significantly greater with Milestone® treatments compared to Matrix or Plateau, and this finding is consistent with findings from other research trials and demonstration sites where these grasses have been present in the understory.
New Research on Annual Grass Seed Viability
Rinella MJ, SE Bellows, AD Roth. 2014. Aminopyralid Constrains Seed Production of the Invasive Annual Grasses Medusahead and Ventenata. Rangeland Ecology and Management. 67:406-411.
The purpose of this greenhouse study by Rinella and others was to explore effects of growth regulator herbicides on medusahead seed production. Milestone at 4 and 7 fl oz/A was one of the herbicides applied to greenhouse-grown medusahead plants at the seedling, internode elongation and heading growth stage. A no-herbicide control was included for comparison. Seed heads were clipped from the plants when ripe and stored for four months prior to germination testing. Germinable seeds present in each pot were calculated to determine the effect of the two growth regulator herbicides on seed germination.
Results indicate that Milestone treatments reduced medusahead seed production more than 90 percent across all timings, and 96 to 100 percent when applied at the internode and heading stages (Figure 3). These results contribute to a growing body of evidence suggesting it may be possible to use growth regulator herbicides to control invasive annual grasses by depleting their short-lived seedbanks.
Invasive forbs such as yellow starthistle (Centaurea solstitialis) and other knapweeds (Centaurea sp. and Acroptilon repens) commonly grow in association with invasive annual grasses on degraded range sites. It may be possible to reduce medusahead dominance on infested sites by applying Milestone at 7 to 14 fl oz/A pre-emergence or at growth stages when broadleaf weeds and medusahead are both susceptible to Milestone.
Growth regulator herbicides pose less risk to established perennial grass plants than grass-specific herbicides. Perennial grass populations have been shown to increase when growth regulators are used to control broadleaf weeds. Combined results from field and greenhouse studies suggest a wide range of Milestone® application timings (i.e., pre-emergence, late seedling, internode, heading) may be effective in reducing or eliminating medusahead seed production.
Use Rates and Timing for Control or Suppression of Winter Annual Grasses
Milestone® applied broadcast at 7 to 14 fluid ounces per acre (fl oz/A) can suppress or control many winter annual grasses including medusahead rye (Taeniatherum caput-medusae) and downy brome (Bromus tectorum). The key to optimum results is application timing. Applications should be made in late summer prior to rains and seed germination in order to provide the best possibility of suppression or control.
Maximum Application Rate
Do not broadcast apply more than 7 fl oz/A of Milestone per year. The total amount of Milestone applied broadcast, as a re-treatment, and/or spot treatment cannot exceed 7 fl oz/A per year. Spot treatments may be applied at an equivalent broadcast rate of up to 0.22 pounds acid equivalent (14 fl oz) of Milestone per acre per annual growing season; however, not more than 50 percent of an acre may be treated at that rate. Do not apply more than a total of 0.11 pounds acid equivalent (7 fl oz) of Milestone per acre per annual growing season as a result of broadcast, spot or repeat applications.
®Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. Milestone is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Label precautions apply to forage treated with Milestone and to manure from animals that have consumed treated forage within the last three days. Consult the label for full details. Always read and follow label instructions.