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.

航空喷施除草剂是治理草原与牧场中有害木本植物的可选手段，但目前鲜有研究明确此类药剂对野生动物至关重要的非目标木本植物的影响。近期推出的一款由绿草定（clopyralid）与氨氯吡啶酸（aminopyralid）复配而成的除草剂（下称“CA”），据信可专用于防治密花牧豆树（honey mesquite，*Prosopis glandulosa*）。本研究旨在明确单用CA，以及将其与毒莠定（picloram）、三氯吡氧乙酸（triclopyr）等其他灌丛除草剂复配后，对两种目标物种——密花牧豆树与梨果仙人掌（pricklypear，*Opuntia* spp.），以及两种非目标木本植物——钝叶枣（lotebush，*Zizyphus obtusifolia*）和网脉朴树（hackberry，*Celtis laevigata* var. *reticulata*）——的影响。试验共设置4种处理：1）单用CA；2）CA+三氯吡氧乙酸（CA+Tr）；3）CA+毒莠定（CA+Pc）；4）绿草定+三氯吡氧乙酸（Cp+Tr）。我们于德克萨斯州中北部连续3年，在均伴生梨果仙人掌的成熟牧豆树丛中设置4个重复样地（单块样地面积4公顷）开展航空喷施处理，并分别于处理后1年、2年（YPT）对样地进行调查评估。本研究构建了包含5个等级的耐性评价模型：高耐性、耐性、中等耐性、中等敏感、敏感，该模型整合了样地水平的植株全株死亡率（根死亡率）与存活植株的冠幅缩减率。密花牧豆树对所有喷施年份的所有处理均表现敏感。梨果仙人掌仅对CA+Pc处理表现敏感：处理后1年至2年，其根死亡率从33%升至84%，增幅超过一倍；而对其余处理均表现为高耐性。钝叶枣对所有处理均表现为高耐性或耐性。网脉朴树对CA与Cp+Tr处理表现为耐性，但对CA+Pc处理表现敏感。当网脉朴树处于干旱胁迫状态时，CA+Pc处理对其产生的负面影响更为显著。我们建议，在喷施除草剂以防治目标木本植物或仙人掌类植物前，应先调查评估区域内的干旱状况、叶片健康状况以及非目标木本植物的丰度。