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.