THE CONTROL OF CYP~RUS ROTUNDUS IN SUGAR CANE

Transcription

1 THE CONTROL OF CYP~RUS ROTUNDUS IN SUGAR CANE FIELDS AND IN FALLOW LAND G. Mclntyre, C. Barbe, J. Pitchen and M. Yerriah Mauritius Sugar Industry Research Institute ABSTRACT Cyperus rotundus is one of the most troublesome weeds growing in agricultural lands of Mauritius. Efforts are constantly being made to eradicate it by testing a number of herbicidal treatments in ratoon canes and in fallow land. In ratoon canes, the best results have been obtained with post-emergence spraying of Velpar K, (hexazinone + diuron) used either alone or in mixtures with Actril-DS (ioxynil-2,4-d) or 2,4-D mine salt. On C. rotundus subsp. tuberosus, only one application was needed to get very satisfactory control whereas on rotundus, two applications were necessary. In fallow land exceptional control was obtained with glyphosate. Very satisfactory result was also obtained from. the application of hexazinone. INTRODUCTION Cyperus rotundus is known to be one of the world's worst weeds. It is one of the most troublesome weeds in Mauritius. It occurs very commonly in all climatic regions and in different soil types. It is highly competitive and often causes complete failure of crops, particularly smallsized annual crops like carrots. In dry regions,perennial crops like sugarcane are severely affected (Mclntyrel ). In Mauritius there are two subspecies of Cyperus rotundus which can be easily differentiated by the following characters: 1. Cyperus rotundus L. subsp. rotundus Hooper, bears a reddish, darkbrown inflorescence; it is commonly found in all climatic regions and soil types. 2. Cyperus rotundus L. subsp. tuberosus (Rottb.) Kuk bears a golden brown or straw colored inflorescence. When growing under the same conditions, this subspecies is usually larger than the former.

2 It must be noted that under local conditions, C. rotundus does not seem to reproduce by seeds. Vegetative propagation by bulbs is very efficient with a tenfold increase in the population of the weed occurring in just a few months. Vaughan and Mclntyrez have described various aspects relating to the weed in their leaflet. MATERIALS AND METHODS Two trials were made in ratoon canes and another two in fallow land in order to determine which herbicidal treatment would achjeve the most satisfactory control of C. rotundus under different conditions. In all the trials, the statistical design used was a randomized block with three replicates for each treatment. In canes, the plot size was of 4 rows, 6.5 meters long; in fallow land, plot size was 50 square meters in one of the trials and 250 square meters in the other. The amount of water used to apply the herbicides was 650 liters per hectare and spraying was done by hand-pump knapsack sprayers. Trials in ratoon canes 1 In the first trial (Trial 1) both subspecies of rotundus were present but subsp. rotundus occurred only occasionally. Six post-emergence treatments were compared to an untreated control, C. rotundus subsp. tuberosus had alrdadq flowered abundantly and its percentage ground cover was estimated throughout the experiment, just before spraying, six weeks after the previous harvest. The experiment was visited regularly and a final assessment was made ten weeks after spraying. In all the other trials, only C, rotundus subsp. rotundus occurred. Consequently, no mention of the subspecies will henceforth be made. I I In the second trial (Trial II) eight herbicidal treatments were compared to an untreated control. Four of these treatments were Eradicane 6E (EPTC + R25788). Rates of 3 and 6 kg. a.e./ha. were tested. Each was either sprayed and then manually incorporated with a hoe, or sprayed without incorporation. The other treatments were also sprayed in post-emergence of the weed which had just started to flower. Eight weeks later, all the treatments, except Eradicane 6E were re-sprayed in order to further improve weed control. Trials in fallow land Seven post-emergence herbicidal treatments were tested in one of the trials (Trial Ill) where the plot size was 250 square meters. Eradicane 6E was incorporated into the soil by means of a tractor-mounted rotary hoe, immediately after spraying. The rotary hoe was also used in half the surface area of the control plots in order to determine the level of control of Eradicane 6E and to observe whether this chemical can retard or suppress germination of C. rotundus, as it is usually claimed. At the time of spraying the weed had flowered abundantly. Four post-emergence and one pre-emergence treatments were compared to an untreated control in the fourth trial (Trial IV). Three plots were hand- (

3 78 AGRONOMY weeded prior to spraying of the pre-emergence treatment. The percentage ground cover of C. rotundus was estimated before spraying and 20 weeks later. Trial 1 RESULTS AND DISCUSSION The treatments and the estimated percentage ground cover of C. rotundus before spraying, and ten weeks later, are given in Table 1. It can be observed that very satisfactory control of the weed was obtained with Velpar K4 used alone and in mix~ures with April-DS (ioxynil + 2,4-D) or 2,4-D amine salt. (Velpar K4 is a mixture of hexazinone and diuron in an approximate ratio of 1 :4 respectively). Good weed control was also obtained with the mixture diuron + Actrid-DS '+ sodium chlorate. Using Actril-DS or 2,4-D amine salt alone gave a fair control of the weed, with the former giving somewhat better results than the latter. It must be pointed out that 2 to 3 weeks after spraying, the weed leaves in all the treated plots have completely dessicated. Regrowth, however, started two weeks later and reached different infestation figures showing which treatment is the best. Trial I1 The percentage cover of C. rotundus was estimated prior to spraying and twenty weeks later in all the treatments. These details are given in Table 2. Eight weeks after the spraying, it was evident that practically no weed control had been obtained with Eradicane 6E irrespective of rates of incorporation. Appreciable regrowth had occurred in the other treatments and it was decided to re-spray them. Twelve weeks later, a final assessment was made. A very good control of C. rotundus with more than 90% kill, was obtained with Velpar K4 alone and in mixtures with Actril-DS or 2.4-D amine salt. Fair control was obtained with Actril-DS. The three Velpar K4 treatments &ere practically free from weeds whereas the plots treated with Actril-DS were being invaded by many grasses, particularly Digitaria adscendens ajd Setaria pallide-fusca and certain broad-leaved weeds like Agemum.2onyzoides and Cyperaceae like Kyllinga monocephala. Trials Ill Table 3 gives the treatments compared in this trial and their respective estimated percentage cover of C. rotundus prior to spraying and twelve weeks later. Spectacular weed control was obtained with Roundup (glyphosate) at 1 kg. a.e./ha. The two mixtures of Velpar K4 with Actril-DS were also satisfactory

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8 whereas fair weed control was obtained with Frenock (tetrapion) used alone or mixed with 2,4-D amine salt. No reduction in the population of C. rotundus was observed with Eradicane 6E but the germination of weed was retarded. Emergence of the weed was delayed to about 3 weeks after spraying, whereas in the control plots it had started after 10 days. In all the plots where rotary hoe had been used, the percentage weed cover had increased probably due to the dissemination of the bulbs in areas not originally infested. In fact, in the control plots, the weed cover had increased by 15%. Trial I V # The pre and post-emergence treatments and their respective percentage weed cover are given in Table 4. It must be noted that in this trial, Velpar was used as such, not as Velpar K4 which is already mixed with diuron. Very good weed control was obtained in all the post-emergence treatments. The pre-emergence treatment Destun (perfluidone) was totally ineffective. There was little difference between the results obtained from the three rates of Velpar. However, the higher the rate, the quicker the kill. The weed leaves in the Velpar plots were completely desiccated after three weeks but regrowth started approximately six weeks after spraying. However, many of these regrowths showed strong chlorosis and subsequently died. The results indicated that increase in efficacy of Velpar was very slightly influenced by increases in rates; consequently it is not necessary to use rates higher than 1 kg. a.i./ha. In fact the amount of Velpar in Velpar K4 tested in the other trials varied between 0.7 and 1.4 kg a.e./ha. CONCLUSION In fallow land, C. rotundus can be completely controlled with Roundup at 1 kg a.e./ha. Spot spraying of the regrowth should be done with the same herbicide at the same rate. This herbicide is rapidly decomposed upon contact with the soil so that any crop can be planted afterwards without any risks of phytotoxiciw. Velpar or Velpar K4 can also be used and they have the advantage of controlling other weeds in pre-emergence as well as in post-emergence. However, with a fairly long residual effect, these chemicals can cause phytotoxity, to susceptible crops. In ratoon canes, C, rotundus subsp. tuberosus is efficaciously controlled by Velpar K4 alone and in mixtures with Pctril-DS and 2,4-D amine salt. For the subsp. rotundus however, two applications are needed to get equivalent results. One application of Velpar K4 at 3 kg a.i./ha, is safe in ratoon canes, but a second application could prove detrimental to cane growth. Consequently, when twf- herbicide applications are necessary, it is advisable to have one with Velpar K4 + Actril-DS and another with Actril-DS alone. I REFERENCES 1. Mcl ntyre, G. (1970). Herbicide v/s Hand-weeding - preliminary results.

9 84 AGRONOMY Rep. Maurit. Sug. Ind. Res. Inst. 18: Vaughan, R. E. and G. Mclntyre (1979). Weeds of Mauritius. Leaflet No. 17. Cyperus rotundus Linn. Maurit. Sug. Ind. Res. Inst. CONTROL DE CYPERUS ROTUNDUS EN CANAVERALES Y TERRENO BARBECHO G. Mclntyre, C. Barbe, J. Pitchen and M. Yerriah RESUMEN Cyperus rotundus as una de las malezas mas dificultosas que crece en terrenos agricolas en Mauricio. Continuamente se realizan esfuerzos para errdicarla haciendo pruebas con numerosos herbicidas en cafias de retofio y en terreno barbecho. En retofios, se ha11 obtenido 10s mejores resultados con aspersiones post-emergentessde Velpar K4, utilizado solo o mezclas con Actril- DS o 2,4-D sal amina. Solamente era necesaria hacer una aplicacion para obtener resultados muy satisfactorios para controlar C. rotunda sub sp. tuberosus mientras que eran necesarias dos aplicaciones en el caso de la sub sp. rotundus. En terreno barbecho se obtuvo un conikol exceptional con Roundup aunque Velpar tambien fue muy satisfadtorio.