Effects of different levels of competition by Cyperus esculentus L. on the growth and surcrose yield of irrigated sugarcane (Saccharum officinarum L.) in Northern Swaziland.
Manana, Shesi Vusumuzi Innocent.
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The study was undertaken to evaluate the interference of Cyperus esculentus (L.) with growth and quality of irrigated sugarcane (Saccharum officinarum (L.)) in northern Swaziland, and to quantify or characterise yield loss in relation to C. esculentus population and its control. The study comprised two field experiments on different soil types on plant, 1st, 2nd and 3rd ratoon sugarcane located at Mhlume (Swaziland) Sugar Company during the period 1988 to 1991. Three C. esculentus population levels were established at 1680 to 1833 plants m(-2) (heavy), 1110 to 1205 plants m(-2) (medium), and 550 to 582 plants m(-2) (light). Medium and light infestation treatments were instituted through thinning by hand to populations of 67% and 33% of the original populations respectively. Four weed control methods were superimposed. These comprised two controls; a) no weed control (C. esculentus was left undisturbed throughout the growth of the crop), b) complete weed control by hand weeding throughout the season; and two levels of herbicide application rates, a recommended estate level, which was a mixture of 1.6 L MCPA (a.i.) ha(-1) (2-methyl-4-chlorophenoxyacetic acid) (400 g a.i. L(-1) ) with 1.5 L ametryn (a.i.) ha(-1) (2-methylthio-4-isopropylamino-s-triazine) (500 g a.i. L(-1) ) and one-half the recommended herbicide application. A surfactant was added to the mixture at 0.5 L ha(-1). Both trials had 12 treatments arranged in a 3x4 rectangular lattice with five replications. Significant sugarcane yield responses to weed control method were obtained on plant and 3rd ratoon sugarcane. For the plant sugarcane crop, the institution of the recommended herbicide rate and complete hand weeding gave yield increases of about 14% and 24%, respectively, compared to the no control treatment. The plant crop sucrose yield was significantly affected by weed control methods with the recommended herbicide and complete hand weeding giving 15% and 26% increase in sucrose yield, respectively, compared to the no control treatment. In the 3rd ratoon crop complete hand weeding gave an increase of 26% and 28% sugarcane and sucrose yield, respectively. Indications were that 1st and 2nd ratoon sugarcane displayed the most vigorous growth, and hence was highly competitive against the C. esculentus and suffered no yield or quality loss due to weed populations. The possible reason was that the 1st ratoon sugarcane crop grows more vigorously than the plant crop and would therefore achieve canopy earlier than in the plant sugarcane crop. Even though results under the environmental conditions and time of regeneration of young ratoons used in this study indicated little, if any, benefit of weed control, this aspect would need further study at other times of the year before no weed control on young, vigorously growing ratoons could generally be recommended. As sugarcane yield and quality were not affected by C. esculentus populations, it was not possible to establish a population level for economic control of the weed. However, regeneration of C. esculentus in subsequent years was shown to be a function of previous years' populations and control method imposed on that population. Final populations in the subsequent year were lower where weed control was instigated than where there was no weed control. The conclusion reached in this study is that weed control will not only affect competitive abilities of current C. esculentus, but also reduce future population levels of the weed. The lack of sugarcane yield and quality response to different levels of C. esculentus population made it impossible to conclusively determine the economic threshold of C. esculentus which caused sugarcane yield and quality loss in irrigated agriculture on these soils.