9/1/09 We had two follow-up questions from our blog post “New strategy for attacking Canada thistle in Crellin Meadow” about repeated use of Milestone and about using Milestone in riparian areas.

On 9/2/09 Tim D’Amato replied “In my experience, broadcast spraying Canada thistle once with Milestone does an excellent job. Spot-treating some escapes the following year may be necessary. I think this is evident at the plot sites on Haystack Meadow that Phil Westra and I sprayed a year ago.  Milestone is labeled for use in wetland areas.”

9/3/09 For further details, Dow’s TechLine Publication Winter 2009 said yes: “Milestone can be applied up to the water’s edge on terrestrial sites.” I’ve excerpted parts of this article below with registration info, half life, FAQs. The article also references research on nontarget plants by Peter Rice, Division of Biological Sciences at the University of Montana.

“Aminopyralid is labeled for use on rangeland, permanent grass pastures, Conservation Reserve Program (CRP) acres, non-cropland areas such as non-irrigation ditch banks and natural areas (such as wild-life management areas, wildlife openings, wildlife habitats, recreation areas, campgrounds, trailheads and trails). Aminopyralid was registered as Milestone® herbicide in August 2005 under EPA’s Reduced Risk Initiative Program. This new pyridine carboxylic acid herbicide (growth regulator mode of action) has excellent efficacy on a considerable number of invasive weeds at application rates ¼ to 1/20 of the active ingredient rate of previously used growth regulator herbicides.

Answers to Frequently Asked Questions
What is the Environmental Fate of Milestone® herbicide?
Soil: Aerobic microbial degradation is the primary route of breakdown in soil. Average field soil half-life is 34.5 days for eight North American sites. There are no degradation metabolites of concern.
Water: Photolysis is the primary route of degradation in water. Photolysis half-life under standard conditions is 0.6 days. Groundwater contamination potential is low because of low use rates combined with moderate soil half-life.
Air: Practically non-volatile

Can Milestone be used in riparian areas?
Yes, under the following guidelines: The label restricts applications directly to water (ponds, lakes, rivers, streams and irrigation canals). Avoid applications that may result in movement of Milestone into water used to irrigate crops. The non-target plant community should be considered.”

Regarding non-target plants, the Editor’s Note says: “Milestone® herbicide has been evaluated for non-target forb effects in university trials for several years. The results of these trials are becoming available to land managers and will be presented in upcoming TechLine newsletters. The first of these studies, by Peter Rice at the University of Montana, is summarized in this issue.” This study was funded through the USDA Forest Service’s Pesticide Impact Assistance Program (PIAP). Peter Rice’s data tables rate tolerance and susceptibility of many forbs to Milestone, 1 and 2 years after spraying.