Joint action of paraquat and 2,4-D on weeds associated with coffee in Costa Rica.
The joint action of paraquat and 2,4-D was studied both under semi-controlled (screenhouse) and field conditions in Costa Rica. Initially, a dose response bioassay was carried out with six selected weeds commonly found in coffee plantations (Bidens pilosa, Borreria latifolia, Drymaria cordata, Emilia fosbergii, Portulaca oleracea and Richardia scabra), to which each separate herbicide was applied individually and combined at different doses to two selected weeds (B. pilosa and R. scabra) to determine their combined effect based on plantlet dry weight. In addition, the effect of both herbicides (separate and mixed) was estimated in field plots of naturally-occurring and planted weeds by visual damage assessment. E. fosbergii and B. pilosa were the most sensitive to paraquat and B. latifolia was the most tolerant. Both in the screenhouse and in the field, the combined application of paraquat plus 2,4-D, especially at the highest doses, was less effective than expected, indicating antagonism in this mixture.