Plant mortality and seedling recruitment responses to flupropanate in grassland populations of Nassella trichotoma

Flupropanate (sodium 2,2,3,3 tetrafluoropropanate), a slow-acting lipid bio-synthesis-inhibiting herbicide, was recently registered in New Zealand as Taskforce (745 g L-1 flupropanate as the sodium salt) for the selective and long-term control of Nassella trichotoma in pastures. In five dose-response experiments in permanent hill pastures in Canterbury, conducted between 2012 and 2016, we measured the efficacy of the herbicide against established plants of N. trichotoma and its residual activity against recruiting seedlings. Mortality, as an average across the five sites, was 93% 1.5 years after applying 1.49 kg flupropanate ha-1 (the label-recommended rate), and 100% at 2.98 kg ha-1. This indicates that an application rate higher than the label rate will be necessary for complete control of a N. trichotoma infestation. The presence of 1,000 and 6,250 visible seedlings ha 1 of N. trichotoma in the autumn 3.2 and 2.1 years after applying 1.49 kg flupropanate ha-1 (at a Greta Valley and Scargill site respectively) indicates that the herbicide’s soil residues had decayed within 12 months to a concentration lower than necessary to kill the germinating seedlings of N. trichotoma.