Converting mesquite thickets to savanna through foliage modification with clopyralid

Abstract

Honey mesquite (Prosopis glandulosa Torr.) is a problem plant in much of the southwestern USA because it reduces forage production for livestock, interferes with livestock handling and reduces off-site water yield. Aerial spraying a 1:1 mixture of clopyralid (3,6-dichloro-2-pyridinecarboxylic acid, mono-ethanolamine salt) and triclopyr (3,5,6-trichloro-2-pyridinyloxyacetic acid, butyoxyethyl ester) at 0.28 kg ae ha-1 + 0.28 kg ae ha-1 usually achieves high above-ground (top-kill) and whole plant (root-kill) mortality, but limits multiple-use options of livestock and wildlife production because little mesquite foliage is left to provide screening cover for wildlife. In addition, most surviving plants resprout from basal meristems and will become multi-stemmed plants. Some managers treat mesquite in strips or blocks, leaving untreated areas for screening cover, but these areas become increasingly non-productive for livestock and wildlife forage. The objective of this study was to evaluate the potential of aerial sprays of clopyralid alone at 0.28 kg ha-1 to convert thickets of mature, multi-stemmed mesquite to savannas by reducing mesquite foliage amount to an intermediate level (by 50-70%), yet preserving apical dominance and limiting basal sprouting. The clopyralid treatment was compared to an untreated control and aerial sprays of 0.28 kg ha-1 clopyralid + 0.28 kg ha-1 triclopyr on 2 sites. The clopyralid treatment reduced foliage amount tree-1, canopy area tree-1, and stand-level mesquite cover by > 57% when compared untreated areas, and 73% of surviving trees maintained apical dominance. Apical dominance was maintained in > 70% of trees not totally top-killed if at least 20% of the original canopy survived and produced foliage following the spray year. Percent root-kill in the clopyralid-only treatment differed between sites (34 and 10%). The lower root-kill on one site was attributed to rainfall that occurred 2 days before and one day after spraying. The clopyralid+triclopyr treatment reduced foliage on original canopies by > 96% and mesquite cover by 82% on both sites. Root-kill was > 52% on both sites but only 37% of surviving plants maintained apical dominance. Results suggest that clopyralid at 0.28 kg ha-1 may be effective for converting mesquite thickets to savanna and may aid in multiple-use management.