Herbicide impacts on forb performance
in degraded sagebrush steppe
ecosystems
Marie-Anne de Graaff, Aislinn Johns
Boise State University
Background

Large areas of the sagebrush steppe
ecosystem have been invaded by
Bromus tectorum (cheatgrass), leading
to a change in ecosystem structure.

This threatens sagebrush dependent
species such as sage grouse, a
candidate for protection under the
Endangered Species Act.

Imazapic is used to suppress cheatgrass
invasion, but little is known about its
impact on performance of forbs
essential to sage grouse survival.
Background
• Selective herbicide for both the pre-and
post-emergent control of some annual and
perennial grasses and some broadleaf
weeds.
• Kills plants by inhibiting the production of
branched chain amino acids, which are
necessary for protein synthesis and cell
growth.
• Relatively non-toxic to terrestrial and aquatic mammals,
birds, and amphibians.
• Half-life of 120 days in soil; rapidly degraded by sunlight
in aqueous solution.
• Degraded predominantly by soil microbes.
Background

Fire can alter soil microbial activity and nutrient cycling, thus the impact of imazapic
on forbs may be different on soils that have recently burned.

We aim to assess how imazapic affects forbs in degraded unburned and recently
burned sagebrush steppe ecosystems.
Questions
Aim: assess how imazapic affects forbs in degraded unburned and recently
burned sagebrush steppe ecosystems.
1. How do different application rates (concentrations) of imazapic affect forb
performance?
2. How do recently burned soils interact with imazapic to impact forbs?
Forb Species
Sphaeralcea grossulariifolia-
Astragalus filipes-
Achillea millefolium-
gooseberry globemallow
basalt milkvetch
common yarrow
•
•
•
•
•
•
•
• Rhizomatous
• Thin seed coat
• Sage-grouse
preferred
Single crown
Hard seed coat
Sage-grouse
preferred
Legume
Single crown
Hard seed coat
Sage-grouse preferred
Greenhouse experiment
2 degraded soils (0-10 cm depth)
• Control soil from NCA site
• Recently burned site in the area
Three sage grouse preferred forbs (grown for 41
days)
• Gooseberry globemallow
• Basalt milkvetch
• Common yarrow
4 concentrations of imazapic applied 2 weeks prior
to seeding (n=10)
• 0 oz/acre
• 2 oz/acre
• 4 oz/acre
• 6 oz/acre
Impact of fire on
microbial activity
350
B
μg C/ g soil
300
250
A
200
150
100
50
0
Unburned
Burned
Imazapic application significantly suppressed biomass
production of basalt milkvetch
Biomass (g dry weight)
0.09
a
0.08
0.07
0.06
b
0.05
b
0.04
b
0.03
0.02
0.01
0.00
0
2
4
Imazapic concentration (oz/acre)
8
Unburned
Burned
Imazapic application significantly suppressed biomass
production of common yarrow
0.18
a
Biomass (g dry weight)
0.16
0.14
0.12
0.10
b
0.08
b
0.06
b
0.04
b/c
0.02
0.00
0
2
4
Imazapic concentration (oz/acre)
8
Unburned
Burned
Imazapic application significantly suppressed biomass
production of gooseberry globemallow
0.18
a
Biomass (g dry weight)
0.16
0.14
0.12
0.10
b
0.08
0.06
b
0.04
b
0.02
b
0.00
0
2
4
Imazapic concentration ( oz/acre)
8
Unburned
Burned
Shoot biomass production: basalt milkvetch was least
impacted by imazapic applications
100%
average % response
90%
b
80%
b
b
b
a
b
a
70%
60%
a
a
50%
40%
30%
20%
10%
0%
2
4
8
Imazapic concentration (oz/acre)
Basalt milkvetch
Common yarrow
Gooseberry globemallow
average % response
Root biomass production: basalt milkvetch was least
impacted by imazapic applications
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
b
b
b
b
b
b
a
a
a
2
Basalt milkvetch
4
Imazapic concentration (oz/acre)
Common yarrow
8
Gooseberry globemallow
Conclusions
• Imazapic is not specific to
cheatgrass, and its use could
impact forb performance.
Ongoing-research
B.t. S.g. A.m. A.f.
Imazapic Concentration
N=10
Plant Species
4 oz
a.i./acre
Control
Imazapic Application
2
w
8
w
4
Week Planted
16
w
Imazapic application significantly suppressed biomass
production of cheatgrass
Biomass (g dry weight)
0.025
0.02
0.015
0.01
0.005
0
0
4
Imazapic concentration (oz/acre)
Acknowledgements
Funding: Bureau of Land
Management (BLM); U.S.
Geological Survey (USGS);
National Science Foundation
EPSCoR Program under award
number EPS-0814387.
Students: Ian Duvall, Mary
Finnel, Ariane Shannon