Native forbs are an essential component in the functioning and diversity of prairie communities. Aminocyclopyrachlor (AMCP) will effectively control many noxious weeds that invade prairie communities; however, its efficacy on desirable broadleaf plants is relatively unknown. Few field studies have been reported, and locating adequate populations of native forbs for evaluation of tolerance to herbicides is difficult. The susceptibility of 10 prairie forb species to AMCP was evaluated in the greenhouse. Species were chosen to correlate with a field study of AMCP and a previous greenhouse experiment. AMCP was applied at 0, 35, 70, and 105 g ha−1 with a methylated seed oil (MSO) plus silicone-based non-ionic surfactant (NIS) blend at 0.25% v/v when plants reached the growth stage simulating a spring treatment for weed control. Blueflag iris (Iris versicolor L.) and harebell (Campanula rotundifolia L.) were relatively tolerant and would likely be unharmed following an application of AMCP in the field. Wild licorice (Glycyrrhiza lepidota Pursh), prairie wild rose (Rosa arkansana Porter), purple prairie clover (Dalea purpurea Vent.), and wild bergamot (Monarda fistulosa L.) were moderately susceptible to AMCP; however, plants might regrow in the field, since some survived at the highest AMCP application rate (105 g ha−1) evaluated. Skyblue aster [Symphyotrichum oolentangiense (Riddell) G. L. Nesom], Canada goldenrod (Solidago canadensis L.), blue cardinal-flower (Lobelia siphilitica L.), and blacksamson echinacea (Echinacea angustifolia DC.) were susceptible to AMCP even when applied at 35 g ha−1. The susceptibility of greenhouse-grown forbs to AMCP was the same or similar to species evaluated in the field and can be used to predict native forb tolerance in the field.

Canada thistle can cause greater than 50% yield loss in small grain crops, but little is known about production losses when the weed invades pasture and wildlands. Change in grass, forb, and woody species production from Canada thistle infestations was evaluated in two separate studies in North Dakota. The first measured change in production following aminopyralid applied at 120 g ha−1to control Canada thistle at two prairie sites. In general, grass, broadleaf, woody, and total plant yields were similar between treated and untreated prairie, regardless of the near-complete control of Canada thistle following aminopyralid application. Grass yield increased by 365 kg ha−1the year after treatment at one location, with no change in forb or woody species production. Plant production was also estimated at 20 ungrazed wildland preserves located within two Major Land Resource Areas (MLRAs). Similar to the prairie sites, minimal differences in production between Canada thistle–infested and noninfested sites were observed. The only exception was an increase in grass production of 425 kg ha−1at one of the MLRAs, with no change in broadleaf or woody species production between the Canada thistle–infested and noninfested sites. In contrast to cropland, pasture and wildland production of other species was not consistently reduced by Canada thistle.