TECHNICAL BULLETIN 127

HERBICIDES FOR OKRA

P. G. Americanos and N. A. Voulounis

.- '.

AGRICULTURAL RESEARCH INSTITUTE MINISTRY OF AGRICULTURE AND NATURAL RESOURCES

NICOSIA CYPRUS

FEBRUARY 1991

ISSN 0070-2315

All responsibility for the information in this publication remains with the author(s). The use of trade names does not imply endorsement of or discrimination against any product by the Agricultural Research Institute.

HERBICIDES FOR OKRA

P. G. Americanos and N. A. Vouzounis

SUMMARY

Application of pre-emergence herbicides to direct seeded okra resulted in significant reduc­tion in hand weeding time. Crop stand was not affected in field trials. In pot trials, her­bicide treatments did not improve yields over the weeded control and in two cases they actu­ally resulted in significant yield reduction. Suitable herbicides were alachlor (2 leg a.i.), chlor­thai-dimethyl (7.0 - 8.5 kg a.i.), diphenamid (3.5 - 5.0 kg a.i.), f1uorochloridone (0.5 kg a.i.), prometryn (1.0 - 1.25 kg a.i.), and terbutryn (1.0-1.25 kg a.i./ha). Alachlor at 3.0 kg and f1u­orochloridone at 0.75 kg a.i./ha were toxic in pot trials and reduced yields significantly.

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INTRODUCTION

Okra is an important summer vegetable in Cy­prus used almost exclusively for cooking, although small quantities find their way to processing facto­ries. The crop is direct seeded, either broadcast in wide beds which are flood irrigated, or more recent­ly, in rows, the irrigation being given via drippers.

Regardless of the method of grOWing the crop, weeds -.:an seriously impede the growth and subse­quent development of the okra plants. Yield loss­es due to weeds ranging from 54% to 91% have been reported by Govindra et aJ (1982) The same authors reported significant yield increases when the weed free period increased up to 60 days after so~­ing This indicates that okra, like other seeded crops, is most susceptible to weed competition in its early stages of development. Mechanical weed­ing is not practical where drip irrigation is installed and impossible in the broadcast crop. On the other hand labour for weeding, if at all available, is very expensive. Herbicides may offer a useful alterna­tive. Although world literature on the subject is very scant a few cases of herbicide use in okra have been reported (Sutton and Burgis, 1966; Mon­telaro and Maryel. 1966; Wascom and Fontenot 1967) .

The present study was undertaken to test the ef­fectiveness of recommended herbicides and to ex­tend their range to include new compounds.

MATERIALS AND METHODS

In a screening trial conducted in the greenhouse in early spring nine pre-emergence herbicides were applied to okra sown in polystyrene trays. Six of those, which did not cause any or unacceptable damage to okra were subsequently tested in the field. Oxadiazon. oxyfluorfen and pendimethalin caused severe to moderate stunting and were dis­carded. The herbicides alachlor, cWorthal-dimethyl. diphenamid,· fluorochloridone, prometryn and terbu­tryn were subsequently tested in the field.

Field trials were conducted at two locations dif­fering in soil type and climatic conditiOns, i.e. at Ipsonas in the plains, in a medium heavy clay soil. and at Louvaras in the mountain in a light sandy soil. In all trials the crop was direct seeded in ear­ly April at Ipsonas and in June at Louvaras. Two

to three days after sowing herbicides were applied overall in 500 ! water!ha with a knapsack sprayer fitted with a flat fan nozzle. Plot size varied be­tween 12 and 25 square meters. Experimental de­sign was a randomized complete block with three replications. After herbicide application the fields were lightly sprinkler irrigated the same or the fol­lOWing day; thereafter irrigation was by means of drippers. Three to five weeks after treatment plant stand records were taken by counting the number of established plants in 12 m. of row. At the same time the plants were examined for symptoms of toxicity. Seven to eight weeks after sowing weed species appearing in each treatment were recorded, the plots were hand weeded and the time required to do so was recorded. A few weeks later a sec­ond weeding was done where required and time was again recorded. Plant development was followed til! maturity in order to detect possible stunting or de­laying effects of herbicides. No yield records were collected from these trials, since the crop is harvest­ed almost daily over a long season.

To assess the effect of herbicides on okra yield !Jot trials were conducted in 1988 and 1989 Pots of 25 cm diameter were filled with a clay-loam soil and irrigated to field capacity. When the soil had drained and could be worked five okra seeds were sown in each pot and lightly irrigated with a hose fitted with a rose. Herbicides were then applied as in the field trials and lightly sprinkled in The pots were arranged outdoors in a randomized complete block design, with treatments replicated three times. The pots were irrigated as required and when ger­

mination was completed the number of germinated seeds was counted. They were then thinned to two seedlings per pot. Irrigation, fertilization and plant protection were carried out routinely. Weeds were removed periodically. Observations for symptoms of toxicity were made frequently tiO the onset of crop­ping. The plants were harvested almost daily and the yield per plant was recorded. Cropping ended when no more marketable fruit was produced.

RESULTS AND DlSCUSSlON

Field trials

Crop stand and toxicity symptoms. Crop stand records are shown in Table 1. At Louvaras where the soil was light there was no herbicide effect on okra germination and establishment. In fact some

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Table 1. Okra stand in two soil types and effect of herbicides on hand weeding time.

Rate Crop Stand Hand Herbicide (g a.i.Iha) (No. of plants/12 m row) weeding

Heavy Soil Light soil time, (Ipsonas) (Louvaras) (h/ha)

obtained by Brathwaite (1981). There were no sig­nificant differences among treatments, which on av­erage reduced weeding time for one hectare by 705 hours. Stipulating an 80% labour efficiency and taking £125 as the hourly wages, savings in weed­ing cost through the use of herbicides amount to £ I, 100 per hectare.

Alachlor 2,000 52.5 cd 54.0 ab 273 b

Alachlor 3,000 522 cd 57.7 a 220 b Chlorthal-dimethyl 7,000 49.3 d 62.0 a 298 b Chlorthal-dimethyl 8,500 53.0 cd 57.0 ab 335 b Diphenamid 3,500 510 cd 56.3 ab 329 b Diphenamid 5,000 53.2 cd 56.0 ab 284 b Fluorochloridone 500 55.2 c 54.0 ab 191 b Fluorocllioridone 750 61.7 b 51.3 ab 173 b Prometryn 1,000 68.7 a 543 ab 220 b Prometryn 1,250 65.3 ab 54.7 ab 193 b Terbutryn 1,000 690 a 58.3 a 178 b Terbutryn 1,250 64.0 ab 49.7 ab 233 b Untreated control 65.3 ab 44.0 b 949 a

Means separation by Duncan's Multiple Range Test

herbicide treated plots had significantly higher num­bers of plants than the contro!. At Ipsonas the pic­ture was not clear. In many treatments there were significantly fewer plants than in the control but this cannot be attributed to herbicide toxicity for several reasons. Firstly, in those herbicides the plots treated with the higher rate had more plants than those treated with the lower one; secondly growth and development of germinated plants was normal, showing neither stunting. nor yellowing or necrosis, nor any other symptoms suggesting possible herbi­cide damage; and, thirdly, the same treatments at Louvaras, where owing to the light nature of the soil toxicity would have been greater, had no ad­verse effect on crop stand and in fact a few actual­ly showed a beneficial effect A possible explanation for the results at Ipsonas could be that puddling caused by drippers at the sowing foci may have re­sulted in erratic germination. No such puddling oc­curred at Louvaras. Some initial damage in the form of slight yellowing was observed at Louvaras in plots treated with either rate of prollietryn and terbutryn, while the high rate of fluorochloridone caused some bleaching.

Effect on weeds. All herbicides significantly re­duced weeding time (Table I). Similar results were

Pot trials

Effect on germination and establishment. There was no treatment effect on the germination and establishment of okra since in all pots practical­ly all seeds germinated, and produced young plants. But as the plants developed toxicity symptoms be­gan to appear. Alachlor at the low rate slowed growth down a little while at the high rate it caused definite stunting, the plants never attaining full size Fluorochloridone also produced stunting as well as bleaching of the petiole and of the base of the leaf blade. These symptoms were more pronounced where the higher rate had been applied. Prometryn and terbutryn caused slight vein chlorosis on the co­tyledon leaves only. Chlorthal-dimethyl and diphe­namid produced no symptoms of any kind.

Effect on yield. The highest yields were ob­tained from plants grown in pots treated with 125 kg a.i.!ha of prometryn and from the untreated plants (Table 2). Three herbicide treatments, alach­lor 3.0 kg a.i., diphenamid 3.5 kg a.i. and fluoroch-

Table 2. Effect of herbicides on the yield of okra grown in pots

Rate Yield Herbicide (g a.i.jha) (g/plant)

Alachlor 2,000 283 abc Alachlor 3,000 263 bc Chlorthal-dimethyl 7,000 348 ab Chlorthal-dimethyl 8,500 360 ab Diphenamid 3,500 261 bc Diphenamid 5,000 290 abc Fluorochloridone 500 304 abc Fluorochloridone 750 247 c Prometryn 1,000 331 abc Prometryn 1.250 375 a Terbutryn 1,000 334 abc Terbutryn 1,250 319 abc Untreated control 372 a

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loridone 0.75 kg a.i./ha., resulted in significant yield '·reduction. For alachlor and fluorochloridone this re­

duction was due to the toxicity described above. Reduction from the lower diphenamid rate may be due to a larger number of weeds (d. weeding time, Table 1).

Although herbicides produced only slight or no toxicity whatever in field tIials, two of them, alach­lor at 3 kg a.i. and fluorochloridone at 0.75 kg a.i./ ha caused definite damage in pot tIials, resulting in reduced yields and, therefore cannot be recommend­ed for field use.

REFERENCES

Brathwaite, R.A.I. 1981. Weed control in produc­tion of okra in Trinidad. Agmnomy Journal 73: 690-692.

Govindra, Singh. Y.M. Bhan and 5.5. TIipathi 1982. Weed control in okra [Abelmoschus es­culentus (L) Moench] Indian Journal of Weed Science 14: 19-23.

Montelaro, 1. and M.E. Maryel. 1966. Chemical weed control for Florida vegetable crops. Circu­lar, Florida University Agricultural Experiment Station 196B, 8 p.

Sutton, P. and D.S. Burgis. 1966. Tolerance of okra to several herbicides. Pmceedings 19th Southern Weed Conference, 1966, pp. 589-592, Dover, Florida.

Wascom, B.W. and J.F. Fontenot. 1967. Effect of several herbicides on yield and weed control in cabbage and okra. Abstracts 20th Southern Weed Conference, 1967, p. 145.

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