Optimising spot weed control regimes for Pinus radiata plantations

Spot spraying, in which only the area around individual trees is treated, is an important method of herbicide application during establishment of Pinus radiata D. Don (radiata pine) in New Zealand. Minimising the spot size reduces costs and has possible environmental benefits from reduced overall herbicide use. An analysis of six experiments at five separate sites tested the growth response of P. radiata to spot weed control on sites dominated by herbaceous vegetation, usually as a result of oversowing with a mixture of grasses and legumes. The analysis showed that, on sites dominated by herbaceous weeds, the reduction in rotation length achieved by applying total weed control compared with no postplant weed control ranged from 0.4 to 1.6 years and averaged 0.9 years. A spot diameter of 1 m maintained for 1 year achieved 28% of the benefit of total weed control, whereas a spot diameter of 2 m maintained for 1 year achieved 66% of the benefit. Generally, there was little benefit in maintaining spots beyond 1 year. An economic analysis indicated that weed control of any kind was generally unprofitable when the time-shift gain from weed control was 0.4 years or less but highly profitable for sites with a time shift of 1.6 years or more.