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

<p>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 (<em>Prosopis glandulosa</em>) 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 (<em>Opuntia</em> spp.), and two non-target woody plants, lotebush (<em>Zizyphus obtusifolia</em>) and hackberry (<em>Celtis laevigata </em>var <em>reticulata</em>). 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.</p>