Non-target woody plant responses to broadcast herbicide treatment for mesquite and pricklypear control

Aerial spraying of herbicides is an option for treating undesirable woody
species on grasslands and rangelands, but few studies have determined the
effects of these products on non-target woody plants important to
wildlife. A recently introduced herbicide containing a mixture of
clopyralid and aminopyralid (“CA”) is thought to be specific to honey
mesquite (Prosopis glandulosa) control. Our objective was to document the
effects of CA alone and mixed with other brush herbicides, including
picloram and triclopyr, on two target species, honey mesquite and
pricklypear (Opuntia spp.), and two non-target woody plants, lotebush
(Zizyphus obtusifolia) and hackberry (Celtis laevigata var reticulata).
Treatments were 1) CA, 2) CA + triclopyr (CA+Tr), 3) CA + picloram
(CA+Pc), and 4) clopyralid + triclopyr (Cp+Tr). We applied aerial spray
treatments on four, 4-ha replicated plots of mature mesquite thickets that
also contained pricklypear in each of 3 consecutive years in north-central
Texas and evaluated plots at 1 year and 2 years post-treatment (YPT). We
developed a tolerance-rating model with 5 levels (highly tolerant,
tolerant, moderately tolerant, moderately susceptible, and susceptible)
that integrated stand-level percent whole plant mortality (root-kill) and
percent canopy reduction of surviving plants. Mesquite was susceptible to
all treatments in all spray years. Pricklypear was susceptible to CA+Pc
[root-kill more than doubled (33 to 84%) from 1 to 2 YPT], but highly
tolerant of the other treatments. Lotebush was highly tolerant or tolerant
of all treatments. Hackberry was tolerant of CA and Cp+Tr but susceptible
to CA+Pc. The negative effect of CA+Pc on hackberry was greater when
hackberry was drought-stressed. We recommend inspection of drought status,
foliage condition, and abundance of non-target woody species prior to
broadcast spraying for control of targeted woody species or cacti.