The pyridine carboxylic acid (PCA) herbicide family can exhibit differential activity within and among plant species, despite molecular resemblances. Aminocyclopyrachlor (AMCP), a pyrimidine carboxylic acid, is a recently discovered compound with similar use patterns to those of the PCA family; however, relative activity among PCAs and AMCP is not well understood. Therefore, the objective of this study was to quantify relative activity among aminopyralid, picloram, clopyralid, triclopyr, and AMCP in canola, squash, and okra using dose-response whole-plant bioassays. Clopyralid was less active than all other herbicides in all species and did not fit dose-response models. Aminopyralid and picloram performed similarly in squash (ED₅₀ = 21.1 and 23.3 g ae ha^–1, respectively). Aminopyralid was 3.8 times and 1.7 times more active than picloram in canola (ED₅₀ = 60.3 and 227.7 g ha^–1, respectively) and okra (ED₅₀ = 10.3 and 17.3 g ha^–1, respectively). Triclopyr (ED₅₀ = 37.3 g ha^–1) was more active than AMCP (ED₅₀ = 112.9 g ha^–1) and picloram in canola. Aminocyclopyrachlor (ED₅₀ = 6.6 g ha^–1) and triclopyr (ED₅₀ = 7.8 g ha^–1) were more active in squash than aminopyralid and picloram. In okra, AMCP (ED₅₀ = 14.6 g ha^–1) and aminopyralid (ED₅₀ = 10.3 g ha^–1) performed similarly but were more active than triclopyr (ED₅₀ = 88.2 g ha^–1). Herbicidal activity among AMCP and PCAs was vastly different despite molecular similarities that could be due to variable target-site sensitivity among species.

Nomenclature: aminocyclopyrachlor; aminopyralid; clopyralid; picloram; triclopyr; canola, Brassica napus L.; squash, Cucurbita pepo L.; okra, Abelmoschus esculentus L.