Selective herbicides extirpate forbs more effectively than alien plants, facilitating dominance of aggressive low grazing value grasses

• Atmospheric factors cause pervasive herbicide vapour deposition. • Native forbs are highly susceptible to direct herbicide contact and drift. • Hyparrhenia hirta was highly invasive following suppression of broadleaf species. • Hyparrhenia hirta was highly invasive in wetlands with groundwater pumping. • Chemical control of dense weed infestations did not improve veld condition. Between 2005 and 2008, eight selective herbicides were applied as broadcast foliar sprays at two grassland communities, a dry site and a wetland, infested with Campuloclinium macrocephalum in Gauteng. The study found that native forbs were highly susceptible to these selective herbicides. While treatment effects for both forbs and graminoids were not significant, significant changes occurred within treatments over time, leading to a notable decline in forb richness and abundance. At the dry site, 89 of 105 species (66 % forbs) were extirpated in some plots, with 36 species (80 % forbs) locally extinct. However, one year post-treatment, 52 new species (52 % forbs) were recorded. At the mesic site, 66 % of the 49 species also faced extirpation, and only 12 new records were found in the final year. The herbicide treatments primarily affected less common forb species, while more abundant ones persisted, indicating that this was due to large initial population sizes rather than herbicide tolerance. Graminoids increased overtime at the dry site reflecting an herbicide release effect from broadleaf species competition. However, the temporary nature of the herbicide's effects on species diversity suggests that rare species losses could be lasting. Herbicide drift emerged as a key contamination source, exacerbated by atmospheric factors, and soil moisture stress further influenced plant survival. Ultimately the emergence of a highly competitive native grass with low palatability capitalizing on suppressed forb communities at both sites raised concerns about the role of herbicides in improving veld condition.