Second-year weed control for direct seeding of Eucalyptus porosa in a low rainfall environment

Summary

The efficacy, ‘crop’ responses, cost and ease of application of weed control treatments in the second year following direct seeding of eucalypts were compared for a shielded knockdown herbicide treatment (glyphosate at 1.08 kg/ha a.i.) and residual herbicide overspray treatments of atrazine and simazine at 1, 2 and 3 kg/ha (a.i.) and pendimethalin at 1.98 and 3.96 kg/ha (a.i.). There was a significant growth benefit from using second-year weed control, however, none of the seedlings treated with residual herbicides grew significantly more than those with the glyphosate shielded spray alone. Despite being difficult to apply, glyphosate shielded sprays are attractive because, if correctly applied, they do not have a negative effect on the seedlings and can allow follow-up germination of tree and shrub seed. Atrazine and simazine at 3 kg/ha (a.i.) and both rates of pendimethalin were too expensive. Atrazine at 2 kg/ha (a.i.) had a significantly detrimental effect on plant growth. However, according to the above criteria, simazine at 1 kg/ha (a.i.) gave the best results with a substantial safety margin, but atrazine at 1 kg/ha (a.i.) and simazine at 2 kg/ha (a.i.) were also satisfactory. It is recommended that simazine and atrazine be trialed without their inclusion with a shielded knockdown spray.