Research into herbicides provides insights

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In New Zealand’s plantation forest sector, the use of herbicides is a well-established practice to allow young seedlings the chance to grow in the face of competition from the aggressive, introduced weeds and in some areas, native shrub hardwood regeneration. Although the amounts used are very low over the life of a tree crop, chemical herbicide use worldwide is a concern to many.

For the approximately 50% of the New Zealand plantation forest area that is eco-certified under the Forest Stewardship Council, the use of herbicides is even more relevant because one of the FSC principles seeks reduction of their use - elimination or substitution where possible.  In support of that objective, the Forest Stewardship Council publishes a list of chemicals classified as ‘Highly Hazardous”.

Some years back, two chemicals, terbuthylazine and hexazinone, were placed on the Forest Stewardship Council’s highly hazardous list of chemicals.  To continue to use them the industry had to apply for ‘derogations’ or specific approvals. In addition, the industry had to demonstrate an effort to find alternative herbicides or a means to reduce their usage. So, over five years ago the industry commenced a structured research programme to look for alternative herbicides and also to test other aspects related to herbicide use. For the first couple of years the eco-certified companies all contributed to a self-funded programme, undertaken by Scion in Rotorua, to start a programme of screening tests for alternative, more benign chemicals. With some potential candidates identified, the industry combined forces with the Sustainable Farming Fund (SFF), the Farm Forestry Association, Scion, two agrichemical companies and four Regional Councils to advance the research to a series of comprehensive field trials.

PF Olsen’s Environmental Manager, Kit Richards, who has been closely involved in the coordination of the research projects on behalf of the industry, says: "After three years, those trials have reached a point at which some conclusions can be drawn.   In summary, while a narrow range of alternatives proved workable, especially those that still at least had terbuthylazine present, their efficacy was not up to the level achieved by the current chemical prescriptions. A further range of potential mixes, including some that had no terbuthylazine or hexazinone, showed potential in some specific vegetation types. However, for these the results were too uncertain or inconsistent to be able to form recommendations without more research or validation."

Additional research outside of the alternative chemicals programme was undertaken to establish the leaching risk of terbuthylazine and hexazinone, and their environmental fate when applied under operational conditions. These studies indicated that in the relatively high organic content of forest soils the risk of leaching was low. Measurements of the presence of the herbicides in streams saw only brief detectable peaks immediately following application, followed by rapid decay to very low levels (well below world health specifications for drinking water).

Scion also undertook a review of the performance of a biocontrol agent (leaf feeding weevil) introduced to control Buddleia davidii, an aggressive weed in many parts of the country. The insect is spreading well throughout the North Island and is having a definite impact on the long-term growth of buddleia in many places. However, trials conducted by Scion indicated that the population of insects would not be able to build-up fast enough to provide effective control of buddleia during the establishment phase of Radiata pine. Furthermore, it was estimated that even if the weevil did provide early control of buddleia, it would only open up the vacated ecological resources to other weed species - thereby transferring the problem.

Coincidentally, following a revision of the Forest Stewardship Council’s chemicals policy and highly hazardous lists in 2014, both terbuthylazine and hexazinone were removed from the highly hazardous class.  However, at the same time, while the vertebrate toxins 1080 and sodium cyanide have always been on the highly hazardous list, all other vertebrate toxicants used by forestry companies to control pest animals or even insects, such as wasps, are now on the highly hazardous list, along with some other common herbicides and fungicides used throughout primary sectors in New Zealand.

"It is clear that ongoing research effort will be required for some time, especially in the area of understanding the fate of these chemicals in the environment", says Kit.

In the meantime, a research summary booklet is available to the public and the full information will shortly be available on the Sustainable Farming Fund website, (Project 12/038), the Forest Owners Association and Scion websites and the NZ Farm Forestry Association website along with a short video.

Guidelines to the efficacy of the alternative new chemical active ingredients tested

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