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Summary
Post-planting vegetation control in commercial Pinus radiata D.Don forests within New Zealand almost exclusively uses a mixture of the two triazine herbicides, terbuthylazine and hexazinone. However, neither of these herbicides is endorsed for use on land certified by the Forest Stewardship Council (FSC), based on criteria relating to their persistence or toxicity in the environment. This is a problem for the forest industry as these herbicides are inexpensive, are not phytotoxic to P. radiata, and their residual action enables continued control of competing vegetation for up to a year following application. Using measurements obtained from three short-term field trials where herbicides were applied as pre- and post-emergent, post-planting treatments to a wide range of competitive vegetation species, the efficacy of alternative mixes of herbicides was compared to the triazine mix (the current industry standard). As the restrictions on the use of terbuthylazine by FSC may be lifted, this herbicide was also trialled in mixes with herbicides other than hexazinone. Tree growth was quantified one year after herbicide application using a biomass index. Results from all three trials showed that the triazine mix provided the greatest level of vegetation control, with the trees in this treatment attaining the highest biomass index. Where herbicide mixes were applied as pre-emergent treatments, the best alternative herbicide, as assessed by tree growth at one year, was indaziflam applied at 300 g active ingredient (a.i.) ha–1. Compared to the triazine mix, the biomass index for this treatment was reduced by <1%. Where herbicides were applied as post-emergent treatments, the best alternatives to the triazine mix were very similar across trials and included treatments where terbuthylazine (at 7000 g a.i. ha–1) had been applied in combination with one of either mesotrione (at 150 g a.i. ha–1; 300 g a.i. ha–1), triclopyr (at 113 g a.i. ha–1), or clopyralid (at 1500 g a.i. ha–1) or where a mixture of clopyralid, triclopyr and haloxyfop (1125 g a.i. ha–1, 113 g a.i. ha–1 and 250 g a.i. ha–1) was used. Reductions in biomass index at one year in these five treatments, relative to the triazine mix, ranged between 5% and 35%, with smallest reductions being recorded for terbuthylazine used in combination with either clopyralid or mesotrione.
Acknowledgements
The authors thank Liam Wright, Tia Uaea, Catherine Banham, David Henley and Alan Leckie for the technical support required to implement and manage these trials. All forest companies and managers are also acknowledged for permitting trial installation on their property and in-kind support. This work was funded through a Sustainable Farming Fund contract with the Ministry for Primary Industries, New Zealand: Minimising the Environmental Impact of Forest Weed Control in New Zealand, Grant No. 12_038.