Applying the double knock technique to control Conyza bonariensis

Sequential applications of glyphosate followed by another postemergent herbicide, known as the “double knock” technique, were trialled for their effectiveness in controlling Conyza bonariensis. Combinations of glyphosate with and without 2,4‐D followed by paraquat plus diquat, paraquat, or 2,4‐D were tested at a range of follow‐up application times in two field and two pot experiments. The results showed that paraquat plus diquat or paraquat following glyphosate or glyphosate plus 2,4‐D provided highly effective weed control compared to glyphosate alone. The optimum timing for follow‐up applications of paraquat or paraquat plus diquat was between 5 and 7 days after the initial glyphosate application. Combined applications of glyphosate and 2,4‐D, compared to split applications, were not significantly different. However, following glyphosate application with 2,4‐D >1 day later considerably reduced the level of control. This study showed that the double knock technique is highly effective in controlling C. bonariensis and is rapidly becoming an important tool in the management of this problem weed.