A COMBINATION OF TEBUTHIURON AND DlUiRON FOR WEED CONTROL IN SOUTH AFRICAN SUGARCANE

A combination of tebuthiuron (25%) and diuron (50%) called Bimate* was evaluated for pre-emergence and postemergence weed control. An early post-emergence application was the best treatment. No phytotoxicity to either plant or ratoon cane was recorded when the recommended rates of 4,O kg of Bimate per ha were used on soil containing 8 to 30% clay, and 5,O kg of Bimate per ha on soil containing 30 to 50% clay. There was no detrimental effect of the herbicide on sucrose content of the cane and residues of tebuthiuron and diuron were less than 0,l mg/kg in the cane juice at all levels of chemicals tested. Stalk counts and height measurements showed an improvement over those obtained in the untreated control plots. The uses of this combination under various conditions are discussed. Introduction Tebuthiuron was initially developed as an industrial herbicide (Walker, Jones and Shawl, Eaton, Cebalo, Rainey, Sieck and Todd2). Development work indicated that, at rates lower than those used industrially, tebuthiuron could control weeds in sugarcane (Schwer3, Pafford5). In 1975 development trials with tebuthiuron commenced in South Africa where it was evaluated as a pre-emergence treatment. At the same time the S.A. Sugar Association Experiment Station evaluated the compound (Iggo5) at high levels and found good weed control and signs of phytotoxicity to sugarcane. Initially, field results were variable and it was decided to evaluate tebuthiuron in combination with other compounds. The combination with diuron performed well under most conditions for pre-emergence and early post-emergence weed control and was not phytotoxic to plant and ratoon cane at the recommended rates. Tne combination that is now registered for use in sugarcane in South Africa is tebuthiuron 25% and diuron 50% marketed as Bimate 75 wp, previously coded EL-6003" The chemical characteristics and toxicology of tebuthiuron were described by Schwers. Materials and Methods Replicated field trials were conducted throughout the sugarcane growing area of South Africa on soils ranging from 4 to 53% clay content and 0,31 to 7,01% organic matter content during the 1976177, 1977178 and 1978179 growing seasons. The design of the trials was randomised blocks which were usually replicated four times. Plots consisted of at least three rows which were 10 m long. Untreated control plots as well as standard treatments of metribuzinldiuron and hexazinone/diuron were included in all trials, Trials were conducted on plant and burnt ratoon cane before and after the emergence of weeds. Post-emergence trials were done when broadleaved weeds were smaller than the six leaf stage and grasses had not yet tillered. Plant cane was generally at the one to two unfurled leaf stage and ratoon cane 100 to 200rnm high. Trade mark registered by Elanco (a Division of Eli Lilly (SA) (Pty.1 Ltd.1 Cultivars on which tebuthiuronldiuron was evaluated were NCo 293, NCo 310, NCo 376, N55/805, N531216, N8 and CB36/ 14. Treatments were evaluated for weed control efficacy at 30-day intervals until the canopy had fully developed, using the Barrett Horsfall method (0 = no control, 10 = 100% control) and expressing the results as a percentage of the weeds controlled in comparison ~ i t h the untreated areas. Stalk height measurements were taken on twelve randomly selected stalks per plot and measured from ground level to the uppermost visible node. Stalk counts were made on the centre row of each plot excluding I m at both ends of the plot. Damage to cane was rated at '30 day intervals for crop stunting and chlorosis. A11 treatments were applied with a carbon dioxide precision sprayer usually with a floodjet nozz1.e spraying a 1 m swath. Volume rates varied from 180 to 470 litresl ha. Dosage rates of Bimate ranged from 2,O to 10,O kglha, that is 0,5 to 2,5 kg of tebuthiuronlha and 1,0 to 5,O kg of diuronlha. Agricura G49 or Cittowett surfactant at 0,2% was added to the spray solution for postemergence applications. Samples comprising 5 to 10 cane stalks taken at random from each treatment in each replicate were taken six to eleven months after treatment for tebuthiuron and diuron residues and sucrose determinations. The juice was expressed and the sample submitted for analysis. Residue determinations were done by the SA Bureau of Standards and the sucrose determinations by the SA Sugar Association Experiment Station. Results 1. Pre-emergence Trials (a) Broadleaf weeds: Control of the more common broadleaf weeds encountered in sugarcane, such as Acalypha eckloni, Ageratum coayzoides, Amatanthus spp., Argemone mexi~ma, Bidens spp., Chenopodium spp., Commelina bengalensis, Galinsoga parviflora, Gnaphalium pensylvanicum, Portulaca oleracea, Richardia brasilensis, Siegesbeckia orientalis, Solanum spp and Tagetes minuta was good to excellent at dosage rates of 4,O to 5,O kg Bimatelha. (b) Grass Weeds: The efficacy of Bimate against grass weeds before emergence was variable. In the majority of trials the major grass weeds encountered were well controlled. In some trials, certain grasses, notably Panicum maximum, was not adequately controlled at 3,O kg Bimatelha but good control was obtained with 4,O to 5,O kg Bimatejha. Table 1 illustrates the level of control achieved with Bimate on three common species of grass. (c) Cyprus esculentus: Bimate had little effect on C. esculentus as a pre-emergence treatment (see Table 2). Higher rates were more effective than lower rates but were not commercially acceptable. There were indications that a reasonable level of control could be achieved on light soils but this was not so in soils with a high clay content. Proceedings of The South African Sugar Technologists' Association June 1981 TABLE 1 The Efficacy of Bimate on the Percent Control of Grasses in Pre-emergence Situations TABLE 2 The Efficacy of Bimate on the percent control of C. esculentus in Pre-emergence Situations I Species Trial Number Clay % . . . . . . . . . . Days after application . . . . Species I Cyperus esculentus TABLE 3 The Efficacy of Bimate on the Percent Control of Some Grasses in Post-emergence Situations Panicum maximum Trial Number . . . . . . . . . . . . . . . . . . ---Clay "/, . . . . . . . . . . . . . . . . . . . . . . . . . Days after application . . . . . . . . . . . . . . . . . . Bimate kg/ha . . . . . . . . . . . . . . . . . . . . 3,O -4,O -5,O