Control of Acacia flavescens with herbicides

Abstract

Acacia flavescens seedlings and suckers were effectively controlled with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). When water was used as the herbicide carrier, seedling control improved with 2,4,5-T rates up to 2.24 kg of acid equivalent (a.e.) ha-1. However, the control of root shoots (suckers) did not improve with dosages above 1.12 kg a.e. ha-1. Diesel distillate was superior to water when used as a 2,4,5-T carrier for seedling control, but there was no difference between the two for sucker control. 2,4-dichlorophenoxyacetic acid (2,4-D) was not effective on A. flavescens seedlings. At equivalent rates 4-amino-3,5,6-trichloropicolinic acid (picloram) was as effective on A. flavescens seedlings as 2,4,5-T. Picloram was more damaging to the pasture legume Stylosanthes guyanensis ssp. gayanensis than was 2,4,5-T. Bioassay did not detect picloram in the top 96 cm of soil 15 months after applying 1.12 kg a.e. ha-1 of picloram. It is possible that loss through leaching was substantial because the soil was sandy and approximately 375 cm of rain had fallen. Addition of dibutyl phthalate (DP) at 5 per cent V/V to a diesel distillate-2,4,5-T mixture had no discernible effect on the control of A. flavescens seedlings. However, DP at 20 per cent V/V impaired 2,4,5-T activity. Acacia seedling numbers were reduced in a newly sown pasture through the application of superphosphate containing 9.6 per cent P at 570 kg ha-1. On the other hand, the survival and growth of Panicum maximum (guinea grass) was appreciably improved, which suggested that increased competition from guinea grass was responsible for Acacia suppression.