Effect of dose and adjuvant on uptake of triclopyr and dicamba into Pinus contorta needles

Management of dense infestations of wilding Pinus contorta in New Zealand requires high doses of herbicides; 18 kg active ingredient (a.i.) ha(−1) triclopyr and 5 kg a.i. ha(−1) dicamba are used in combination with a complex mix of adjuvants (methylated seed oil, non‐ionic surfactant and ammonium sulfate) and other active ingredients. From the perspective of cost and environmental impact there is a need to reduce the complexity of this tank mix and the rates of active ingredients. Using radiolabelled herbicides, this study evaluated the effect of dose and adjuvants (crop oils, non‐ionic surfactants, and organosilicones) on needle injury and uptake of triclopyr and dicamba into P. contorta needles at 24 hr or 7 days after treatment (DAT). The uptake of triclopyr decreased significantly with increasing concentration (0.75%–6%) resulting in the highest uptake dose at the equivalent of 18 kg a.i. ha(−1) triclopyr at 7 DAT. When applied at 18 kg a.i. ha(−1), none of the adjuvants tested significantly increased the uptake of triclopyr (applied as Grazon(®)), with ~50% uptake occurring at 7 DAT. The uptake of dicamba (applied as Kamba(®) at 5 kg a.i. and 10 kg a.i. ha(−1)) was significantly affected by dose and adjuvants. The uptake of dicamba applied at 5 kg a.i. ha(−1) was low at 7 DAT with no adjuvant (31%); however, use of a methylated seed oil doubled the uptake. When triclopyr and dicamba were applied together, there was no evidence that either active ingredient negatively affected uptake of the other, with triclopyr enhancing uptake of dicamba. These results show potential to reduce the amount of herbicide used for conifer control without compromising efficacy.