Effect of timing and rate of hexazinone application on weed control and early growth of Pinus radiata on two contrasting sites

Abstract

Summary The effects of timing (1–12 months after planting) and rate (1–8 kg ha −1) of hexazinone application on weed control and the growth of Pinus radiata two years after planting were studied in two field trials established on contrasting sites at Longford and Neerim in Gippsland, Victoria. Hexazinone applications were compared with a routine treatment of amitrole + atrazine in mixture (1 + 4 kg ha −1), also applied from 1 to 12 months after planting. Growth of P. radiata was closely correlated with live-weed cover. At Longford, maximum weed control and growth response occurred with 4 kg ha −1 hexazinone, whereas at Neerim, growth of P. radiata continued to increase to 8 kg ha −1 hexazinone. Maximum tree growth at Longford was achieved for hexazinone application in August-September, 1–2 months after planting. At Neerim, there appeared to be greater flexibility in time of application than at Longford, with best responses being recorded for applications in the September to December period. Amitrole + atrazine generally resulted in less weed control and growth of P. radiata than hexazinone, and provided less flexibility with respect to time of application. It is hypothesised that differences between sites in the optimum time of hexazinone application are related to differences in rainfall, soil moisture storage, and the relative intensity of competition for moisture in the first summer as determined by the rate of establishment and species of weeds.