A survey of herbicide use and a review of environmental fate in New Zealand planted forests

Abstract

This paper examines current herbicide use in New Zealand planted forests. Compliance of key herbicides with existing Forest Stewardship Council (FSC) standards, the key environmental certification body within New Zealand, is also reviewed. Information obtained from a survey of six forest companies operating in New Zealand was used to identify major herbicides used by the New Zealand planted forest industry, estimate quantity of herbicides used on an annual basis and also determine changes in weed management practices motivated by certification. Glyphosate was the most widely used active ingredient in pre-plant weed control with terbuthylazine and hexazinone used most widely for post-plant weed control. Together these herbicides comprise 90% of the estimated 447 tonnes of active ingredient that is annually used. Average aerial application rates for these three active ingredients were estimated at 3.3 kg ha-1, 7.0 kg ha-1 and 1.8 kg ha-1, respectively. Use of terbuthylazine and hexazinone is restricted on FSC-certified forests subject to derogation. Environmental certification has resulted in a shift from broadcast application of terbuthylazine and hexazinone to greater use of spot weed control in the first year after tree planting. Spot weed control can reduce the amount of active ingredient used by up to 89%. Non-chemical weed control is not widely used by the forest industry as it is not as cost-effective as current herbicide regimes. A review of the literature indicated that, when used operationally and according to label registrations, these herbicides are unlikely to have any negative impacts on the planted forest environment. Although they have been detected in groundwater, under multiple land uses, concentrations were at levels below documented safe drinking standards. There are limited data for forest soil and no data on the effects of these herbicides on aquatic biota in New Zealand. At present time there is insufficient information to support or refute the prohibition of terbuthylazine and hexazinone in New Zealand's planted forests. This has highlighted a need to conduct field studies to determine the fate and behaviour of terbuthylazine and hexazinone in planted forests in New Zealand.