Preliminary screening of herbicide mixes for the control of five major weed species on certified Pinus radiata plantations in New Zealand.
The herbicides terbuthylazine and hexazinone are widely used by the New Zealand forest industry to control a range of weeds during first year vegetation management operations. These herbicides do not comply with certain eco-certification criteria and have been placed on a list of prohibited pesticides by the Forest Stewardship Council (FSC). Currently they cannot be used on FSC-certified land unless an approval is obtained. A pot trial was conducted to test for a combination of herbicides, not currently prohibited by FSC, that might have the potential to provide control of a wide range of weeds, including both broadleaves and grasses, with low phytotoxicity to Pinus radiata D.Don. Two groups of active ingredients were included in mixes used in the trial: (1) triclopyr, clopyralid and picloram for control of broadleaves; and (2) haloxyfop and quizalofop for control of grasses. Terbuthylazine was also included in the trial in the event that an approval is obtained for continued use there-of in New Zealand. We tested the herbicide combinations at 25%, 50%, 75% and 100% of recommended rates on six species: Pinus radiata, Cytisus scoparius L. (broom), Buddleja davidii Franchet (buddleja), Ulex europaeus L. (gorse), Holcus lanatus L. (fogg grass) and Cortaderia selloana (Schult) Asch. et Graeb. (pampas). The results indicated that potential replacements for hexazinone and terbuthylazine during year one for the control of fogg grass, pampas, broom and gorse are haloxyfop, clopyralid, triclopyr and picloram. Terbuthylazine used in combination with triclopyr and picloram was the only combination of herbicides tested that caused mortality of buddleja. The herbicide combinations and rates require further testing both in pot and field trials before robust recommendations for field application can be made.