Abstract
Experiments for the chemical control of Para grass [Brachiaria mutica (Forsk.)Stapf = Panicum purpurascens Raddi] were conducted at Wailua and Helemano in Hawaii, the first on a lowland alluvial soil (8 to 14 per cent organic matter, pH 4·9 to 5·5) at Wailua, and the second on a Tropeptic Haplustox (6 per cent organic matter, pH 4·5 to 5) at Helemano. The nine herbicides used were: bromacil; karbutilate; pyriclor; picloram; diuron; 3-(4 chlorophenyl)-l, 1-dimethylurea mono(trichloroacetate) (monuronTCA); prometon; simazine; and hexachloroacetone (HCA). The herbicides tested were the same at the two sites, except that picloram was included and simazine and monuronTCA were excluded at Helemano. Subjective evaluations obtained in 12 to 20 months pointed to the short residual activity of high rates of herbicides (from 11·2 to 44·8 kg/ha) under moderate to heavy rainfall. At Wailua, karbutilate, pyriclor, and bromacil [formulated as wettable powder (WP) and as a slow release 85 per cent granule (G)] gave 50 to 95 per cent control of Para grass for periods of 2 to 12 months with maximum activity at six to ten months after application. Pyriclor showed the highest initial activity (50 per cent control in two weeks) and the highest percentage control (95 per cent at eight months). Bromacil (G) had low initial activity but was the most persistent (50 per cent control at 15 months and 20 per cent at 20 months). MonuronTCA, diuron, simazine, and prometon at rates of 22·4 to 44·8 kg/ha had little effect. At Helemano, prometon was also ineffective but diuron performed better than at Wailua since it controlled 70 per cent of Para grass at eight months. Picloram + bromacil, picloram + pyriclor, and karbutilate gave over 50 per cent control for 4 to 12 months at Helemano. Treatments which provided more than 90 per cent control for a month or more included bromacil (WP), karbutilate, and picloram + pyriclor. Picloram + pyriclor was the most effective and persistent treatment (100 per cent control at six to eight months) followed by karbutilate and bromacil (WP). Bromacil + HCA was similar in effect to bromacil (WP), and was distinctly better than at Wailua. Few herbicides were effective al rates below 33·6 kg/ha and the substituted ureas and triazines were only moderately active at 44.8kg/ha.
