The Control of Casha (Acacia spp) on Native Pasture

Native pastures, dominated by guineagrass (Panicum maximums and leucaena tLeucaena leucocephalai, provide the basic feed resource for the ruminant livestock industry in the Virgin Islands. However, many of these pastures are in advanced stages ofdeterioration because of overgrazing. There are currently no standard recommendations to prevent the continued ingress of malevolent plants such as ca~ha (Acacia spp.). A split plot field trial was set up to test the efficacy of mechanical and/or chemical treatments for the control of casha. Main plots, replicated three times, consisted of mechanical shredding vs. non-shredded mature plants. These treatments were applied in May, 1990. Subplots were 6 chemical foliar sprays: 2.0 and 3.0 kglha of dicamba alone; 1.28 and 2.56 kg/ha ofTrimecR Super Brush Killer (1:4:4 dicamba, 2,4-0 and 2,4-DP); 285 Llha of diesel oil; and a herbicide check. Subplot treatments were applied 12 wk after mechanical treatments to allow for sufficient regrowth on shredded plots. Chemical application was repeated 24 wk after the first application. Plant density of casha and frequency of canopy occurrence of all dominant plant species were taken initially in May, 1990, and again after 36 wk (just before second herbicide application) and 84 wk. Shreddin-e .i.lene prevented the 14% increase (P<0.05) in the initial ~ casha plant density (1.3 plants/m" ) that was observed on the nonshredded herbicide check, but did not kill any existing casha plant over the 84-wk period. However, shredding plus regrowth combined with dicarnba or Trimel,;R application synergistically induced greater (P <0.0 I) casha mortality (68 '7c) than similar herbicide-treated mature non-shredded plots (48% mortality). A significant leaf drop and canopy reduction (62 7~) followed the first application of diesel oil on shredded plus regrowth treatment but there was no long term topkill (4%) of existing cash a plants. The higher rates of both herbicides provided the best control of casha. Approximately 80% casha mortality was obtained by the initial application of 3.0 kg/ha of dicamba or by double application of 2.56 kg/ha of TrimecR to the shredded plus regrowth treatment without any deleterious effect on the associated leucaena or guineagrass. These data signify that mechanical shredding could be integrated with available herbicides for satisfactory control of casha on native pasture.