Aminopyralid+Triclopyr Control of Russian Olive and Saltcedar: Maintaining Grass Understory

Chemical control of saltcedar (Tamarix spp.) and Russian olive (Elaeagnus angustifolia L.) has had varying degrees of success. Often, these trees are mechanically removed but the stumps are not treated so they readily regrow. Some non-selective herbicides used to control these invasive plants cause unacceptable injury to desirable species, especially grasses in the understory, or do not control other invasive plants under the tree canopy.

Aminopyralid (Milestone®) controls many invasive herbaceous broadleaf weeds, but control of saltcedar and Russian olive had not yet been fully explored. Experiments were established in Nebraska, Colorado, and Wyoming to assess the efficacy on saltcedar and Russian olive regrowth or small trees and understory grass tolerance to aminopyralid plus triclopyr (Garlon® 3A^ or Garlon® 4 Ultra) mixtures and combinations with lower than recommended rates of the commonly used herbicide, imazapyr. Lower than recommended rates of imazapyr were used in an attempt to reduce injury to desirable understory grasses and improve saltcedar control with mixtures of aminopyralid and triclopyr. Treatments included triclopyr amine at 3.37 or 4.5 kg ae/ha (3 or 4 lb ae/A) and triclopyr ester at 2.24 or 3.37 kg ae/ha (2 or 3 lb ae/A) plus aminopyralid at 120 g ae/ha (0.l1 lbs ae/acre), Milestone® VM Plus at 9.6 L/ha [triclopyr amine at 1.12 kg ae/ha (1 lb ae/acre) plus aminopyralid 120 g ae/ha (0.11 lb ae/acre)], and combinations of imazapyr at 0.14 and 0.28 kg ae/ha (0.125 and 0.25 lb ai/acre, respectively) with some aminopyralid plus triclopyr treatments. At 326 days after application, 3.3 kg ae /ha (3 lbs ae/acre) triclopyr ester plus 120 g ae/ha aminopyralid provided excellent control (98%) of Russian olive and saltcedar (94%), similar to efficacy of imazapyr at 1.12 kg ae/ha (1 lb ae/acre) but with significantly less understory grass injury. Triclopyr plus aminopyralid treatments caused little to no grass injury (0 to 5%) compared to the imazapyr treatments (50 to 85%). Addition of imazapyr to aminopyralid plus triclopyr did not improve control of either brush species, but increased grass injury compared to aminopyralid plus triclopyr. At the Colorado site, aminopyralid plus triclopyr amine tended to cause more grass injury than aminopyralid plus triclopyr ester, but caused less injury than when imazapyr was included in treatments. Adding aminopyralid to either the triclopyr amine or triclopyr ester increased control of Russian olive and saltcedar. The combination of aminopyralid plus triclopyr is an excellent option to control Russian olive and saltcedar without injuring desirable understory grass vegetation.

by Mary Halstvedt*, Robert G. Wilson, Gustavo Sbatella, Scott Nissen, Brian Mealor, Byron B. Sleugh, Vanelle Peterson; Dow AgroSciences, Billings, MT, University of Nebraska, Scottsbluff, NE, Colorado State University, Fort Collins, CO, University of Wyoming, Laramie, WY, Dow AgroSciences, Des Moines, IA, and Dow AgroSciences, Mulino, OR.

Presented at the Western Society of Weed Science annual meeting in 2011.

^Application rate conversion for Garlon 3A® to Vastlan™ specialty herbicide