Evaluation of hexazinone and 3,6-dichloropicolinic acid for control ofAcacia dealbataandEucalyptus spp.in youngPinus radiataplantations in Victoria

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Abstract
Three field experiments are described that evaluated the herbicides hexazinone and 3,6-dichloropicolinic acid (3,6-DP) for the control of Acacia dealbata and Eucalyptus spp. regrowth in young plantations of Pinus radiata. In Experiment 1, which compared the efficacy of the two herbicides applied in the presence and absence of a petroleum oil, it was found after 20 months that hexazinone (3 kg ha−1) plus oil (40% v/v) killed 87% and 97% of the Eucalyptus and A. dealbata populations respectively, whereas 3,6-DP (1.5 kg ha−1) plus oil caused a mortality of 20% for Eucalyptus and 80% for A. dealbata. The addition of a petroleum oil substantially improved the efficacy of 3,6-DP on A. dealbata. It was concluded that the choice of herbicide should largely be determined by the species composition of the woody weed population. In Experiment 2, 1.8 kg ha−1 of 3,6-DP in a spray volume of 51.2 L ha−1 containing 33% v/v petroleum oil was aerially applied (helicopter) to an 8.2 ha section of P. radiata plantation. Spray coverage was monitored using sensitised cards. Within 11 months of treatment, 45.4% of the A. dealbata population (average of 11 500 stems ha−1) was dead and a further 23.7% more than 75% defoliated. This result was considered to be operationally satisfactory, although the treatment was expensive ($215 ha−1). Experiment 3 examined the rate and formulation (liquid and granular) of soil-applied hexazinone for the control of Eucalyptus coppice. The quantity of herbicide applied to the soil was based on the height of coppice clumps which varied from 30–300 cm. All treatments tested caused a mortality in excess of 90% after six months, compared with 38% for a combination of stem injection with picloram/2,4,5-T and/or slashing/grubbing. The best treatment was 1 g liquid hexazinone per metre height of the clump, though caution should be exercised in extrapolating results from this experiment to other localities, as variations in edaphic and climatic factors and species composition could influence the level of control.

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