Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2003
Publication Date: 1/1/2005
Citation: Laufenberg, S.M., Sheley, R.L., Jacobs, J.S., Borkowski, J. 2005. Herbicide effects on density and biomass of Russian knapweed (acroptilon repens) and associated plant species. Weed Technology. 19(1):62-72. Interpretive Summary: In the riparian zones throughout Montana and the northwest United States, Russian knapweed displaces livestock and wildlife forage. Controlling this invasive weed is particularly difficult because of the nearness to water. We evaluated the effects of glyphosate, fosamine, and clopyralid plus 2, 4-D for controlling Russian knapweed. Our results showed that clopyralid plus 2, 4-D provided the best control, and increased non-native grass production, but not forbs or shrubs. We concluded that plant productivity and wildlife habitat would consist primarily of grasses and would, therefore, lack key plants needed to resist reinvasion by Russian knapweed.
Technical Abstract: Sustainable invasive weed management must address treatment effects on desired vegetation. Our objective was to determine the influence of clopyralid plus 2,4-D, glyphosate, and fosamine at various application rates and timing, on the density and biomass of Russian knapweed and associated desired plant groups. In a randomized complete block design with four replications, 28 treatments (3 herbicides x 3 herbicides rates x 3 herbicide application timings, and an untreated control) were applied to two sites located along the Missouri River riparian corridor in Montana. Clopyralid plus 2,4-D (3,6-dichloropicolinic acid + 2,4-dichlorophenoxyacetic acid), glyphosate [N'(phosphonomethyl) glycine], and fosamine [ethyl hydrogen (aminocarbonyl) phosphonate] were applied in June (spring rosette stage of Russian knapweed), July (bud to bloom stage of Russian knapweed), and August (flowering stage of Russian knapweed). Herbicide rates were clopyralid plus 2,4-D at 0.08 (clopyralid) + 0.42 (2,4-D), 0.13 + 0.67, and 0.18 + 0.92 kg a.i. ha -1; glyphosate at 0.6, 1.2, and 1.8 kg a.i. ha-1; fosamine at 3.6, 7.2, and 10.8 kg a.i. ha-1. Density and biomass of all species were sampled three years after treatment. Russian knapweed biomass decreased from 125 g m-2 to about 25 g m-2 using clopyralid plus 2,4-D, irrespective of rate or timing of application. Density of Russian knapweed was reduced by about one-half by that herbicide. Grass density and biomass was either maintained (non-native grass understory) or increased (native grass understory) using clopyralid at medium or high rates. Neither glyphosate nor fosamine provided substantial Russian knapweed control or increases in grasses. No herbicides increased native forbs, which are particularly important to the sustainability of the plant community. Without sufficient community structure and competition from this functional group, Russian knapweed will likely recover from suppression treatments. We believe that herbicides must be combined with revegetation in areas lacking a diverse mixture of species capable of occupying the newly opened niches.