CONTROL OF ITCH GRASS [ROTTBOELLIA COCHINCHINENSIS LOUR.) CLAYTON] IN SUGARCANE WITH POST-EMERGENCE HERBICIDE TREATMENTS

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1 Agronomy CONTROL OF ITCH GRASS [ROTTBOELLIA COCHINCHINENSIS LOUR.) CLAYTON] IN SUGARCANE WITH POST-EMERGENCE HERBICIDE TREATMENTS R. W. Millhollon U.S. Sugarcane Field Laboratory, ARS, USDA Houma, Louisiana USA ABSTRACT The sodium salt of asulam, the methyl ester of diclofop, MSMA, a mixture of asulam and the sodium salt of dalapon, and a mixture of asulam and diclofop were evaluated in Louisiana sugarcane fields for postemergence control of itch grass up to about 30 cm in height. The range of kill and average percentage kill from a single application of the treatments in a series of &x field studies were as follows: asulam at 3.7 kglha - 6 to 94%, average 53%; diclofop at 1.1 kglha - 70 to 99%, average 86%, asulam at 3.4 kglha + dalapon at 5.0 kglha - 50 to 98%, average 87%; asulam at 3.4 kglha + diclofop at 0.6 kglha - 76 to 100%, average 94%; and MSMA at 3.4 kglha - 90 to loo%, average 97%. Sugarcane was relatively tolerant to these treatments. Combinations of pre-emergence and post-emergence treatments were effective for seasonal control of heavy populations of itch grass, and the resulting sugar yields were equal to those in a h'andweeded control. INTRODUCTION Itch grass is a major weed in sugarcane and many other crops in the tropics and sub-tropics worldwide. It is a tall, profusely tillering annual grass which can cause substantial losses in sugarcane yield when not controlled. Many seeds are produced during a growing season and can remain viable in soil for up to four years (Thomas and Allisong). Heavy infestations of itch grass are difficult to control because of the periodic germination of seed during the growing season (Millhollon7) and the tolerance or partial tolerance of itch grass to many herbicides, particularly the triazines. Trifluralin incorporated into soil over sugarcane, is an effective pre-emergence treatment (Millhollon6), but the difficulties associated with incorporation have limited its use. Small itch grass plants, less than about 10 cm in height, can be controlled with herbicides having contact action such as diuron, ametryn, and dalapon [2,2-dichloropropanoic acid] (Millhollon4). However, applications must be timely, and treatments usually provide only temporary control and can injure sugarcane if applied repeatedly. Keywords: Control, itch grass (Rottboellia cochinchinensis Lour.) Clayton, postemergence herbicide

2 R.W. MILLHOLLON 8 1 For larger itch grass plants, the organic arsenicals-dsma [disodium salt of MAA] and MSMA [monosodium salt of MAA] - have given effcctive control (Millhollon4). Asulam [methyl](4-aminophenyl)sulfonyl[carbamate], effectively controls large Johnson grass [Sorghum halepense (L.) Pers.] in sugarcane and is widely used for that purpose, but observations indicate that it does not control itch grass effectively. Diclofop [( )-2-[4-(2,4-dichlorophenoxy)phenoxy,] qropanoic acid], a member of the diphenyl ether class of chemicals controls itch grass seedlings (~nderson'). In a greenhouse study, it gave 98% control of barnyard grass [Echinochloa crusgalli (L.) Beauv.], and 8;7% control of itch grass, but only 54% control of large crab grass [Digitaria sanguinalis (L.) Scop.] and 10% control of Johnson grass. Other herbicides of this type, such as fluazifop, also control itch grass, but sugarcane has shown tolerance to only diclofop (Millhollon, unpublished data). The studies reported here were designed to evaluate and compare the effectiveness of several post-emergence treatments involving the use of asulam, diclofop, dalapon and MSMA for control of itch grass in sugarcane. MATERIALS AND METHODS A series of six experiments involving fifteen field studies were conducted from 1972 to 1985 near Breaux Bridge (B.BR.), Chacahoula (CHAC.) and Loreauville (LORV.), Louisiana. Field studies were located in areas having very heavy populations of itch grass. Specific methods of herbicide treatments are described under each experiment, but the following practices were common to all experiments. Commercially formulated herbicides were used; rates for asulam, diclofop and MSMA are based on the active ingredient; that for dalapon on the acid equivalent. Rates are stated as a broadcast application to a hectare of land, but both pre-emergence and post-emergence treatments usually were applied to a band 91 cm wide over lines of sugarcane planted on top of ridges which were spaced about 1.7 m apart. Other types of treatment are detailed in the text. The sides of the rows were cultivated with a disk cultivator to control weeds until the last cultivation at lay-by. Herbicides were applied in water sprays at 374 L/ha and a non-ionic surfactant was added to all herbicide solutions at concentrations shown in the tables for the various studies. Herbicide sprays were applied to cover-weed foliage and were nondirected; thus, 50% to 90% of the sugarcane surface area was wet by spray, depending on the relative sizes of weeds and cane. Injury to cane in most experiments was rated on a scale of 0 = no injury to 10 = complete kill. Treatments in all studies were arranged in a randomised complete block design with four to six replications. Sugarcane in some experiments was harvested for yield in November or December with a whole-stalk mechanical harvester set to top the tallest plants at the first hard internode below the apical meristem. A 15-stalk sample from each plot was crushed once in a three-roller sample mill; the juice was analysed for brix by hydrometer and for sucrose by polarimetry using standard methods (Meade and

3 82 AGRONOMY TABLE I. ltchgrass control in sugarcane as influenced by rate of asulam, mixture of asulam with dalapon and size of itchgrass (Experiment 1). Herbicide & rate * Kill of,itchgrass of two heights when treated Sugarcane cm cm Mean iniurv ratina Asulam Asulam (Asulam dalapon - 2.5) Dalapon study in second-ratoon CP study in second-ratoon CP Asulam Asulam Asulam (Asulam dalapon - 2.5) (Asulam dalapon - 2.5) Dalapon * ** A nonionic surfactant was added at 0.25% vlv in 1973 and at 0.5% vlv in = no injury; 10 = complete kill. Chen3); and sugar content (sugar/ton of cane) was calculated by methods previously described (Legendre and Henderson2). Experiment 1, to evaluate the kill of itch grass by asulam alone and in mixtures, was conducted near Breanx Bridge in two field studies on dark clay soil - in second ratoon of variety CP61-37 during 1973 and in second ratoon of variety CP52-68 during The cane was shaved to ground level with a rotary-blade shaver in March, and itch grass plants cm and cm in height and tillering were treated on June 15, 1973 or May 20, 1975 with asulam, dalapon, or a mixture of asulam and dalapon, as shown in Table I. The cane was about 90 cm high. In 1973, 200 itch grass plants of the cm size and 400 plants of the cm size were randomly selected among the six replicates of each treatment and identified with coloured stakes. In 1975, plants of each size group were selected and staked among three replicates. Itch grass kill and cane injury were evaluated about 30 days after treatment. Experiment 2, to evaluate diclofop and other herbicide treatments for kill of itch grass, was conducted near Chacahoula in two field studies on a silt loam soil - one in 1977 and the other in Native itch grass seed was planted during winter in a single drill in 1977 and a double drill in 1978, on top of standard beds, but no cane was planted. Itch grass began to germinate in early April each year and

4 R.W. MILLHOLLON 8 3 treatments with asulam, dalapon, a mixture of asulam and dalapon, diclofop, or MSMA, as shown in Table 11, were applied in the middle of May. Itch grass plants of three heights cm (mean 10 cm), cm (Mean 20 cm) and cm (Mean 30 cm) - were randomly selected over three or four replicates and identified with coloured plastic stakes to determine kill. About 90 plants of each size were staked for each treatment in 1977, and 60, 130 and 200 plants, based on availability, were staked in each treatment for the 10, 20 and 30 cm sizes, respectively, in The 10 cm plants had no tillers, the 20 cm plants had one to several tillers, and the 30 cm plants had numerous tillers and were in the pre-boot stage of growth. Itch grass kill was evaluated about 30 days after treatment. TABLE II. Comparison of diclofop, asulam, dalapon and MSMA for postemergence control of itchgrass in weed nursery plots in two studies (Experiment 2). Kill bv averaae heiaht of itcharass at treatment 10 cm 20 cm 30 cm Herbicide &rate* Mean Asulam Dalapon (Asulam dalapon - 5.0) , Diclofop Diclofop MSMA * A nonionic surfactant was added at 0.1% vlv in 1977 and 0.25O/0 vlv in I Experiment 3, to compare asulam, diclofop and mixtures for kill of itch grass, was conducted at six locations -two near Breaux Bridge on a clay loam soil in 1981 and 1983, three near Chacahoula on silty clay loam in 1982, 1984 and 1985, and one near Loreauville on clay loam in Sugarcane cultivars at each location were: second ratoon CP at Breaux Bridge (198!), Chacahoula (1982) and Loreauville (1982); second-ratoon CP61-37 at Breaux Bridge (1983); plant cane CP at Chacahoula (1984); and first ratoon CP at ~hacahoula (1985). Treatments with asulam, diclofop, a mixture of asularn and diclofop, a mixture of asulam and dalapon or MSMA were applied in middle to late May. The specific treatments and range in sizes of itch grass at each location are shown in Table 111. Itch grass kill and sugarcane injury were rated about 40 days after treatment. Experiment 4 was designed to evaluate itch grass control with repeat applications of asulam, diclofop or MSMA as shown in Table IV. Two field studies were conducted, one in 1982 at Chacahoula in second ratoon CP on a silty clay loam soil and the other in 1983 at Breaux Bridge on second ratoon CP61-37 on a clay loam soil. Initial herbicide treatments were applied in the spring, on April 7 in 1982 and

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6 I R.W. MILLHOLLON TABLE IV. Control of itchgrass in sugarcane during the growing season with repeat postemergence applications of asulam, diclofop or MSMA (average of field studies in 1982 & Experiment 4). Itchgrass control and sugarcane Herbicide* & rate for: injury** following: 1st Application 2nd Application 1st Application 2nd Application (kglha) (%) Asulam Asulam (0.0) 95 (0.1) Diclofop Diclofop (0.5) 95 (0.4) MSMA MSMA (0.2) 95 (0.3) * ** A 0.5% vlv nonionic surfactant was added with the herbicides. First application was made when itchgrass was 3 to 15 cm in height; second application was made about 30 days later when itchgrass was about 15 cm in height. Average injury to CP in 1982 and CP in 1983, shown in parenthesis, is based on a 0-10 rating scale with 0 = no injury and 10 = complete kill. on May 9 (following shaving) in 1983, when itch grass was 3-15 cm in height. The second application was applied to regrowth of surviving plants and newly germinated plants up to about 15 cm in height, on May 11 in 1982 and on June 1 in Weed control and sugarcane injury were rated about 30 days after each herbicide treatment. Experiment 5, to study itch grass control with pre-emergence and post-emergence treatments, was conducted in 1972 at Breaux Bridge in a field of L60-25 plant cane on a dark clay soil. The top of the sugarcane row was shaved and herbicides for pre-emergence weed control were applied on April 12. The herbicides fenac [2,3,6- trichlorobenzeneacetic acid], terbacil [5-chloro-3-(1,l-dimethylethy1)-6-methyl-2,4 (1H,3H)-pyrimidinedione], metribuzin [4-amino-6-(1,l-dimethylethy1)-3-(methy1thio)- 1,2,4-triazin-5(4H)-one], trifluralin, tebuthiuron [N-[5-(1,l- dimethylethyl) -1,3,4- thiadiazol-2-yl]-n,n1-dimethylurea], and a mixture of TCA (trichloroacetic acid) and silvex [2-(2,4,5-trich1orophenoxy)propionic acid], were applied at rates shown in Table V. Trifluralin, the standard pre-emergence treatment, was incorporated into soil over sugarcane with two passes of a rolling cultivator. Itch grass which survived in these treatments and in an untreated control ranged in size from about 12 cm to 25 cm in height on May 23 when the post-emergence treatment with asulam at 3.7 kg/ha was applied. Itch grass control from pre-emergence treatments were rated on May 23; control from combinations of pre-emergence and post-emergence treatments were rated on August 1. Sugarcane was harvested for yield in early December. Experiment 6, to study the possibility of controlling itch grass for the entire growing season with a series of treatments, was conducted in 1976 at Breaux Bridge in second ratoon CP52-68 on silt loam. The top of the sugarcane row was shaved and fenac was applied as a pre-emergence treatment to a 76 cm band over sugarcane on March 2. On May 17, a mixture of asulam and dalapon was applied to the same band, in designated plots, when itch grass in fenac-treated plots ranged in size from

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8 TABLE VI. Control of itchgrass and yield of second-ratoon CP sugarcane after various combinations of preemergence, 5 postemergence, and lay-by herbicide treatments in Experiment 6 (1976). s r ltchgrass control 40 days & after lay-by treatment P 6 Within a 46 cm yield** z Herbicide & rate* band over Sugarlton Preemergence Postemergence Lay-by line of cane lnterrow Canelha of cane Sugarlha... (kglha) (%) !tons) (kg) (tons) Fenac None None b 105a 1.9a Fenac None Trifluralin b 107a 1.7a Fenac (Asulam dalapon - 2.5) None d 102 a 3.8 bc Fenac (Asulam dalapon - 2.5) Trifluralin de 102 a 4.2 bc None (Asulam dalapon -2.5) None c 104a 3.1b None (Asulam dalapon - 2.5) Trifluralin d 101 a 3.6 bc Handweeded control e 103 a 4.3 c Untreated control a 104 a 1.1 a * Preemergence treatments were applied in a 76-cm band on March 2; the postemergence treatment, (mixed with a 0.5% vlv nonionic surfactant), was applied on May 17 when itchgrass was about 19 cm in height in fenac treated plots and about 29 cm in height in other plots. Lay-by treatment was applied broadcast on June 2. Yields followed by the same letter are not significantly different at the 0.05 level of probability as determined by the Duncan's multiple range test. 03 4

9 8 8 AGRONOMY 18 to 20 cm in height and those in untreated plots 25 to 33 cm in height. At lay-by on June 2, trifluralin was applied broadcast and incorporated into soil in the interrow with a standard disk cultivator operated at about 5 m/s and adjusted to apply about 3 cm of soil to the top of the row around the base of sugarcane. The combinations of treatments are shown in Table VI. Sugarcane was harvested for yield in late November. RESULTS In Experiment 1, the average percentage kill of itch grass with asulam at 3.4 and 4.5 kg/ha was 90 and 98070, respectively, in 1973 but only 35 and 58%, respectively, in 1975 (Table I ). In both studies, asulam appeared to provide more effective kill of plants cm in height than those cm in height. Dalapon at 2.5 kg/ha stunted growth of itch grass but killed very few plants. A mixture of asulam dt3.4 kg/ha and dalapon at 2.5 kg/ha improved control of both sizes of itch grass, giving 100% control in 1973 and 76% control in In 1975, a reduction in the rate of asulam in the mixture from 3.4 to 2.5 kg/ha reduced kill by about 15%. In Experiment 2, in a weed nursery, the most effective treatments for control of itch grass ranging in height from 10 to 30 cm was MSMA (100%), diclofop at 1.7 kg/ha (83%) and a mixture of asulam at 3.4 kg/ha and dalapon at 5.0 kg/ha (75%)(Table 11). Less effective control was obtained with diclofop at 1.1 kg/ha (60%) and asulam at 3.7 kg/ha (55%). Control with dalapon at 5.0 kg/ha was only 13%. For treatments other than MSMA, control decreased as the height of itch grass increased, with diclofop at 1.7 kg/ha being the least affected by height and asulam that most affected. Mixing asulam with dalapon improved control of 20 and 30 cm tall itch grass over that with asulam alone, but control of the 30 cm plants was still only fair. In Experiment 3, the most effective treatments to kill itch grass, in terms of average kill, were MSMA (97%), and a mixture of asulam and diclofop (94%) (Table 111). Treatments giving somewhat less 'control were diclofop (86%), and a mixture of asulam and dalapon (87%). Control with asulam was quite variable, ranging from 6% to 94% and averaging 53%. Sugarcane was relatively tolerant to the treatments with average injury ratings ranging from 0.0 for asulam to 0.7 for the mixture of asulam and dalapon. The highest injury rating was 1.5 for the asulam-dalapon mixture at Loreauville on CP In a two-application system of applying post-emergence herbicides, Experiment 4, asulam, diclofop and MSMA gave 79, 89 and 60% control respectively, following the first application, and 95% control at lay-by following the second application (Table IV). The poor performance of MSMA from the initial application undoubtedly I ~ resulted from the cool temperatures which normally prevail in early spring when the treatment was made. I In Experiments 2, 3 and 4, diclofop was only evaluated for control of itch grass since this was the dominant weed species, but it did not effectively control seedlings of Johnson grass, large crab grass or jungle rice (Echinochloa colonum (L.) Link). Treatments with asulam or MSMA controlled these weeds much more effectively. I In Experiment 5,'the control of itch grass with pre-emergence treatments was

10 R.W. MILLHOLLON 89 quite variable; soil-incorporated trifluralin, the standard gave 96% control, fenac 81q0, tebuthiuron 75%, and the other herbicides 21 to 40% control (Table V). Asulam applied to the surviving itch grass in these treatments either maintained control at a relatively high level or improved control cons;derably. Although asulam applied to untreated plots gave only 48% control, it significantly increased the yield of cane and sugar/ha by about 55% over that of the untreated control. A preemergence treatment followed by asulam generally produced higher yields of cane/ha but lower yields of sugar/ton of cane than asulam alone, and the resulting sugar/ha were not significantly different. The higher sugar content of cane in the untreated control or with the single asulam treatment probably reflected the stress of competition from itch grass. In Experiment 6, a single pre-emergence treatment with fenac gave only 40% control of itch grass, and the remaining infestation reduced yield of cane and sugar/ ha by over 50% as compared to the handweeded control (Table VI). Itch grass control and yields of cane and sugar/ha were improved considerably by treatmeilt of the surviving itch grass with a nlixture of asulam and dalapon, but the yields were still lower than the handweeded control. A lay-by treatment with trifluralin gave about 80% control of itch grass in the inter-row but was not effective in reducing itch grass competition and yield loss unless it was preceded by the post-emergence treatment with asulam-dalapon. The most effective itch grass control and highest yields of cane and sugar/ha were obtained with a combination of pre-emergence, postemergence and lay-by treatment~. This combination produced yields equivalent to those in the handweeded control. DISCUSSION The control of itch grass with asulam was quite variable, but asulam seemed most effective and consistent when itch grass was about 10 cm or less in height and was not tillering (Table 11). Control generally increased as the rate of asulam increased 1 from 2.2 to 4.5 kg/ha (Table I), but the 3.7 kg/ha rate that is used for Johnson grass control in Louisiana probably is an adequate rate for itch grass as well. Observations have shown that asulam, which is very slow acting, frequently will kill the primary shoot of itch grass but not a developing tiller and the plant will survive. Mixtures of asulam with dalapon or dichlofop improved the level and consistency of control over those of asulam alone, probably because these treatments work faster and thus stop development of tillers. The mixture of asulam and diclofop gave about 10% better control of itch grass than diclofop alone (Table 111), and the mixture controls a wider range of weeds, including Johnson grass. Also sugarcane tolerance probably is increased by reducing the rate of diclofop from 1.1 kg/ha when applied alone to 0.6 kg/ha in the mixture. Diclofop at 1.1 kg/ha has reduced sugar yield by about 7% in other studies (Millhollon and Koike8). The asulam-dalapon mixture was about as effective as the asulam-diclofop mixture. The rate of dalapon in the mixture varied from 2.5 to 5.0 kg/ha; although both rates gave good control, they cannot be compared directly because each was in a different study. Observations in these and other studies indicate that 5.0 kg/ha

11 I generally would be the more effective rate, particularly for itch grass larger than about 15 cm in height. The higher rate of dalapon in the mixture can occasionally cause significant injury to sugarcane, as in the Loreauville study (Table 111) in which CP received a 1.5 injury rating compared to a zero rating for asulam alone. MSMA was the most effective herbicide treatment for control of itch grass of all sizes under warm temperatures such as prevail in late May in Louisiana (Tables 11 and 111); it does not perform as well under the cool temperatures of April and early May (Table IV). Sugarcane was not injured markedly by either one or two applications of MSMA in these studies; however, applications later than May in Louisiana can injure sugarcane and increase arsenic content in juice and bagasse (Millhollon5). A system of early-season control involving two repeat applications of a herbicide (Table IV) appeared to be an effective way to use asulam since relatively small itch grass can be treated at each application and surviving itch grass from the first application can be controlled by the second. Various combinations of herbicides could be used in this system such as diclofop followed by asulam or MSMA followed by asulam. Disadvantages of control with repeat post-emergence treatments are: timely application of treatments may be difficult to achieve, injury to the cane may increase, and treatments may be relatively expensive. A better approach to early-season control appears to be the use of a preemergence treatment in early spring followed by a post-emergence treatment when needed (Table V). Such combination treatments have several advantages over an exclusive post-emergence tceatment approach: less risk of injury to cane, timeliness of applications is not as critical, a wider spectrum of weeds is usually controlled, and pre-emergence treatments may predispose itch grass tomore effective control by post-emergence treatments. Full season control is accomplished by including an effective pre-emergence treatment at lay-by (Table VI). These studies indicate that post-emergence herbicide treatments should be an integral part of a system of weed control for itch grass, and the treatments discussed here could be used in such a system. In the USA, registration of MSMA and diclofop by the Environmental Protection Agency will be required before these herbicides may be used on sugarcane. REFERENCES 1. Anderson, R.N. (1976). Response of monocotyledous to HOE and HOE Weed Sci. 24: Legendre, B.L. and Henderson, M.T. (1973). The history and development of sugar yield calculations. Proc. Amer. Soc. Sugar Cane Technol. 2:lO-18. \ 3. Meade, G.P. and Chen, J.C.P. (1977). Cane Sugar Handbook (10 ed). 'Yiley-Interscience Publication, John Wiley and Sons. New York, 947 p. 4. Millhollon, R. W. (1965). Growth characteristics and co.itro1 of Rottboellia exaltata L.f., a new weed of sugarcane. Sugar Bull Millhollon, R.W. (1970). MSMA for johnsongrass control in sugarcane. Weed Sci. 18: Millhollo~RrW;(1972). Soil-incorporatedEifluralin for controlling weeds in sugarcane. Proc. Amer. Soc. Sugar Cane Technol. 2(NS):

12 R.W. MILLHOLLON Millhollon, R.W. (1978). Seasonal germination pattern of Rottboellia exaltata and its control with trifluralin and terbacil. Proc. Int. Soc. Sugar Cane Technol. 16: Millhollon, R.W. and Koike, H. (1986). Combined effect of disease and herbicide treatment on yield of sugarcane (Saccharum officinarum). Weed Sci. (In press).