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12.4.3.2 Defoliation
Our analysis showed that defoliation by grazing or mowing reduces B. tectorum
biomass for up to 2 years and percentage cover for only 1 year (Fig. 12.3 ). This
analysis agrees with previous results indicating that defoliation treatments need to be
applied for more than 2 years because B. tectorum can regrow following defoliation
under favorable environmental conditions (Hempy-Mayer and Pyke 2008 ; Diamond
et al. 2009 ). To maximize the effect of defoliation, the intensity and timing of defo-
liation should be a primary consideration to reduce B. tectorum seed production and
minimize negative effects on perennial grass establishment (Frost and Launchbaugh
2003 ; Young and Clements 2007 ). Consequently, when designing an adaptive man-
agement plan, the prudent use of defoliation and/or grazing in mixed herbaceous
stands should include the application of time-controlled, short- duration, or high-
intensity strategies to selectively graze annual grasses when they are most palatable
and when perennial grasses are dormant and grazing tolerant (Frost and Launchbaugh
2003 ; Smith et al. 2012 ). For example, because cattle prefer to graze B. tectorum
second to bunchgrasses (except in early spring), targeted grazing to reduce annual
grass abundance should be applied cautiously to avoid undesirable impacts on peren-
nial grasses (Ralphs et al. 2007 ). Heavy grazing can also decrease perennial forb
cover and negatively impact perennial grasses in the short term, which may lead to
an increase in B. tectorum cover (Loeser et al. 2007; Davies et al. 2009 ). Achieving
longer-term defoliation impacts on B. tectorum abundance should also consider tim-
ing defoliation events when development of the annual grass is most sensitive to
defoliation stress. For example, defoliation when the seed head is enclosed within the
sheath of the fl ag leaf (i.e., during the boot stage) has greater impact on seed produc-
tion than defoliation at later phenological stages (Hempy-Mayer and Pyke 2008 ;
Diamond et al. 2009 ). However, because B. tectorum ’s annual productivity is highly
correlated with precipitation (Loeser et al. 2007), it may be more effective to plan
and implement defoliation treatments in drier years to maximize the effect on seed
production while minimizing regrowth potential—particularly in areas where defo-
liation will not be detrimental to residual native species (Hirsch-Schantz et al. 2014 ).
12.4.3.3 Herbicide Application
Of the four treatments we evaluated, only herbicide application consistently reduced
both short- and longer-term B. tectorum percentage cover and biomass (Fig. 12.3 ).
The effi cacy of herbicide application as a treatment method may stem from a rela-
tively longer research history relative to the other treatments. For example, the
development of preemergent imidazolinone and sulfonylurea herbicides that target
acetolactate synthase in plants provides high selectivity for invasive annual grasses
at low application rates (Stidham 1991 ; Monaco and Creech 2004 ). However, recent
studies indicate that seeded perennial grass density and cover can be reduced for up
to 3 years after application with the soil-active herbicide imazapic (Pyke et al. 2014 ;
Munson et al. 2015 ). For postemergent B. tectorum applications on semiarid eco-
systems, the nonselective herbicide, glyphosate, also shows promise because it can
T.A. Monaco et al.