Weed control in rice seedling nurseries

Weed control in rice seedling nurseries A. N. RAO AND K. MOODY

International Rice Research Institute, Los Ba~os, Laguna, Philippines ABSTRACT. Experiments were conducted in farmers' fields at Guimba, Nueva Ecija and at the International Rice Research Institute (IRRI), Los Bafios, Laguna to test the efficacy of different methods of controlling weeds in rice (Oryza sativa L.) seedling nurseries. Hand weeding was laborious and ineffective due to the morphological similarities between rice and the Echinochloa species and the regeneration of perennial weeds. It was also injurious to rice seedlings. Manual separation of weed seedlings from rice seedlings before transplanting was ineffective, laborious, time consuming and expensive. Thiobencarb, butachlor, pretilachlor+fenclorim, propanil and quinclorac+bensulfuron were inexpensive and effective in controlling weeds in seedling nurseries without any deleterious effect on rice.

KEYWORDS: Rice; herbicides; Philippines; rice seedlings; weed control; costs

Introduction Weeds infesting rice seedling nurseries compete with the rice seedlings (Shahi and Gill, 1979; Patel et aL, 1985) and in some instances may cause complete failure of the nursery (Shahi, Gill and Khind, 1976). Because of the morphological similarities between certain weed species such as Echinochloa spp. and rice, they are often transplanted together into the field (Kale, Hapase and Cruz, 1965; Negi, 1976; Rao and Moody, 1987). Tang and Wu (1965) reported that 7-7% of the Echinochloa crus-galli(L.) P. Beauv. growing in ricefields was introduced from seedling nurseries during transplanting. In China, where transplanting of E. crus-galli seedlings with rice seedlings is very common (Yu and Liu, 1986), yield losses of 48-71% due to the transplanting of E. crus-galli seedlings have been reported (Jiang, 1983). In the Philippines, Rao and Moody (1987) found that when 29% of the rice hills were infested with Echinochloa glabrescens Munro ex Hook.f., there was a 22% yield loss. Weed seedlings that are transplanted with rice seedlings are difficult to control by hand weeding and cannot be controlled by selective herbicides (Quadranti and Guyer, 1985). Thus, high seedling quality at transplanting is a prerequisite for high yields. Young seedlings are very sensitive to weed competition and there-fore weed control in rice seedling nurseries is necessary to obtain high-quality seedlings (Matsuo, 1961) and to avoid transplanting of weed seedlings with rice seedlings. Herbicides that have been reported to give good weed control in rice seedling nurseries include 0261-219418810310202-05 $03.00

© 1988 Butterworth & Co (Publishers)Ltd CROP PROTECTION Vol. 7 June 1988, 202-206

butachlor (Shahi, Gill and Khind, 1976; Shahi and Gill, 1979; Tang, Ye and Yu, 1984), oxadiazon (All, 1984), pendimethalin (Ali, 1984; Patel et aL, 1985), propanil (Kale, Hapase and Cruz, 1965), pretilachlor (Kim and Lee, 1985; Quadranti and Guyer, 1985), and thiobencarb (Ali, 1984). Because transplanting of weed seedlings with rice seedlings is a problem in some areas in the Philippines and since no studies have been conducted on weed control in rice seedling nurseries in the Philippines, these experiments were conducted in farmers' fields in Guimba, Nueva Ecija and at the IRRI experimental station to evaluate and compare the efficacy and economics of different weed control methods in rice seedling nurseries. Materials and m e t h o d s Land preparation consisted of one ploughing and two harrowings. The seedling nurseries were constructed so that they were about 4-5 cm above the original soil level. Individual plot size was 3 x 3m. A 0.5 m alley was maintained between plots that were separated from each other by levees. Pregerminated rice (cultivar IR62) seeds were broadcast uniformly in all plots at 100g dry seed/m2 (Xuan and Ross, 1976). The double seed rate treatment used 200 g dry seed/m2. No fertilizer was applied in the IRRI fields but 20 kg N/ha was applied to the nurseries in the farmers' fields in Guimba. The water level at IRRI was controlled so that submergence of the seedlings did not occur. However, in the farmers' fields, such precise water control was not possible.

A.

N. RAo ANn K. MOODY

The experimental herbicide treatments included the following: butachlor, N-butoxymethyl-2-chloro-2',6'diethylacetanilide; thiobencarb, S-4-chlorobenzyl diethylthiocarbamate; pendimethalin, N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine; propanil, 3',4'dichloropropionanilide; pretilachlor+fenclorim, 2-chloro-2',6'-diethyl-N-(2-proppxyethyl)acetanilide+ 4,6-dichloro-2-phenylpyrimidine; bensulfuron, methyl 2-[([4,6-dimethoxypyrimidin-2-yl]aminocarbonyl)aminosulphonylmethyl]benzoate and quinclorac, 3fl-dichloro-8-quinolinecarboxylic acid. Specific details of each of the experiments follow.

Experiment 1 This experiment was conducted in May and June 1986 in four farmers' fields in Guimba, Nueva Ecija. A randomized complete block design was used. There were 10 treatments and four replicates. The treatments are listed in Table 1. TABLE 1. Effect of different weed control treatments on weed density and weed weight in rice seedling nurseries*. Guimba, Nueva Ecija, 1986 wet season

TreatmentT Unweeded check Weeds remaining in rice bundles after: Rapid separation Slow separation Double seed rate Hand weeding, 15 DAS Butachlor (0.3), 3 DBS Thiobencarb (0- 5), 6 DAS Pendimethalin (0.5), 6 DAS Propanil (1.5), 10 DAS Pretilachlor + fenclorim (0.15), 3 DAS Bensulfuron (0.02), 3 DAS

Density (no./ 0.25 m2)

Control (%)

Weight (g/ 0.25 m2)

Control (%)

46 a

--

6.2 a

--

28 3 37 29 24 26 26 34

bc d ab bc c bc bc bc

39 94 20 37 48 44 44 26

3.3 0.2 3.6 2.9 2.0 4.1 2.6 4.6

b c ab bc bc ab bc ab

47 97 42 53 68 34 58 26

24 c 27 bc

48 41

3.4 ab 3.0 bc

45 52

203

The rice and weeds were removed from three quadrats (0-5 x 0-5 m) in each plot 21 days after seeding (DAS). The weed seedlings were separated from the rice seedlings, counted, oven-dried, and their dry weights recorded. In one treatment, no weed control took place in the seedling nursery but the weed seedlings were separated from the rice seedling bundles by two methods: rapid separation involved removal of the plants that were taller or had longer roots than the majority of the rice seedlings; slow separation involved the careful separation and removal of as many weed seedlings as possible from the rice seedlings, The time taken for these operations was recorded.

Experiment 2 This experiment was conducted in two locations at the IRRI farm from October to November 1986 and in two farmers' fields at Guimba, Nueva Ecija from February to March 1987. One farmer's field was situated in the lungog (lower) and the other in the turod (upper) portion of the landscape. There were 10 treatments (see Table 4) and each treatment was replicated four times. A randomized complete block design was used. The weeds and the rice were sampled 25 DAS using the procedures discussed in Experiment 1. An economic analysis was done for all treatments (except for those herbicides that were not commercially available in the Philippines) using prices existing in Guimba during the period of experimentation. Results and discussion

E. glabrescens, Paspalum distichum L., Cyperus difformis L., C. rotundus L., Fimbristylis miliacea (L.) Vahl, Scirpus supinus L., Lindernia antipoda (L.) Alston, and Monochoria vaginalis (Burm.f.) Presl were

* Average of four nurseries. In a columns means followed by a common letter are not significantly different at the 5% level by Duncan's multiple range test t Herbicide rate in kg a.i./ha is indicated in parentheses. DAS, days after seeding; DBS, days before seeding

the major weeds observed in all the fields in Experiment 1. Using the double seed rate, a practice used by some

TABLE 2. Effect of different treatments on the number ofseedlings ofgrasses~ sedges, and broad-leaved weeds occurring in rice seediing nurseries in farmers' fields*. Guimba, Nueva Ecija, 1986 wet season Grasses

TreatmentT Unweeded check Weeds remaining after: Rapid separation Slow separation Double seed rate Hand weeding, 15 DAS Butachlor (0.3), 3 DBS Thiobencarb (0.5), 6 DAS Pendimethalin (0.5), 6 DAS Propanil (1.5), 10 DAS Pretilachlor + fenclorim (0.15), 3 DAS Bensulfuron (0.02), 3 DAS

Sedges

Broad-leaved weeds

Number/ 0.25 m2

Control (e/0)

Number/ 0.25 m2

Control (07o)

Number/ 0.25 mz

Control (%)

36.1

--

9.8

--

0- I

--

22.3 2.7 30.7 24.6 21.0 21" 3 10.2 26.5 19.9 24- 8

38 93 15 32 42 41 72 27 45 31

5" 3 0.1 6.0 4.4 3.0 4.5 15.7 7.4 4.0 2.2

46 99 39 55 69 54 0 25 59 78

0.05 0.05 0- 3 --0- 2 0.1 0.1 0.1 --

50 50 0 100 100 0 0 0 0 100

* Values are averages o f four seedling nurseries "1" As Table l

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TABLE 3. Effect of different weed control treatments on the number, weight, height, and n u m b e r of leaves of rice seedlings in seedling nurseries*. Guimba, N u e v a Ecija, 1986 wet season

Treatmentt U n w e e d e d check Double seed rate H a n d weeding, 15 DAS Butachlor (0.3), 3 DBS Thiobencarb (0.5), 6 DAS P e n d i m e t h a l i n (0.5), 6 DAS Propanil (1 "5), 10 DAS Pretilachlor + fenclorim (0.15), 3 DAS Bensulfuron (0.02), 3 DAS

Number/ 0.25 m2 889 1683 870 881 869 619 915

b c b b b a b

Weight (g/ 0" 25 m 2) 59-9 97-9 62.6 61 "5 61.6 43.7 61.4

876 b 891 b

b c b b b a b

61.5 b 61-8 b

Average number of leaves/ seedling

Height (cm) 31.7 30.9 32.2 31.2 31" 5 30.0 32.6

4.9a

b b b b b a c

4.8 a 5.0 a 5-2 a 5.2 a 4.9a 5.3 a

32.3 b 32-4 b

5.4 a 5.2a

* Values are averages of four farmers' fields. In a column, means followed by a common letter are not significantlydifferent at the 5% level by Duncan's multiple range test , t As Tablel

TABLE 4. Effect of different weed control treatments on the n u m b e r of weed seedlings in a rice seedling nursery*. I R R I and Guimba, N u e v a Ecija, 1987 dry season N u m b e r of weeds/0.25 m 2 at: IRRI Guimba Field 1 Field 2

Field 1 Field2 (Turod) (Lungog) 77 a 95 a

Treatmentt U n w e e d e d check Weeds remaining after: R a p i d separation Slow separation Double seed rate Propanil (2.5), 10 DAS Thiobencarb (0.75), 6 DAS Butachlor (0.45), 3 DBS P r e t i l a c h l o r + fenclorim (0-3), 3 DAS Quinclorac (0.3), 3 DAS Bensulfuron (0.03), 3 DAS Quinclorac + bensulfuron ( 0 . 2 + 0 . 0 2 ) , 3 DAS

64 a

78 a

30 3 23 4 3 5

52 7 35 12 3 24

b c b c c c

b de bc de e cde

53 b

5e 73 a 25c

16 cd 11 de

70 b

4f 50 c 24de

21 def" 22 d e f

phytotoxicity.

herbicide

O f the 11 c 3 c 4 c

28 cd 9 de 22 cde

2 c

7 de

11 de 6 e

21 d e f 16 e f

21 c

36 cd

5e

15 ef

* In a column, means followed by a common letter are not significantly different at the 5% level by Duncan's multiple range test t As Table 1

TABLE 5.

Guimba farmers, did not reduce weed seedling number compared with the unweeded check (Table 1). Hand weeding resulted in a 37% reduction in weed density and a 53% reduction in weed weight ( Table 1 ) and the time taken was equivalent to 56" 3 h/ha. Broadleaved weeds were eliminated by hand weeding but the sedges and the grasses were only partially controlled ( Table 2). The weeders had difficulty in distinguishing between the dominant weed, E. glabrescens, and the rice seedlings. In addition, P. distichum regenerated and some of the annual weeds germinated after hand weeding. Some rice seedlings were inadvertently removed during hand weeding while others were trampled during weeding. Shahi and Gill (1979) noted that it is possible to remove some weeds in seedling nurseries through hand weeding but it is a very cumbersome, costly and labour-consuming process. Removal of the tallest seedlings and those with the longest roots from the seedling bundles, a practice used by some Guimba farmers, required the equivalent of 39 h/ha. This technique was ineffective because only 39% of the weed seedlings were removed (Table 1). Careful separation resulted in removal of 94% of the weed seedlings but it was laborious and time consuming, taking 587 h/ha on average. Wrigley (1969) noted that grasses similar in form to the rice plant may be transplanted as seedlings even by observant farmers. The herbicides used in this experiment reduced weed density by only 26-48% and weed weights by only 26--68% (Table 1) because of their inability to control the predominant grasses and sedges at the rates tested (Table 2). Low rates were used to reduce herbicides

tested in this

experiment,

butachlor, pretilachlor+fenclorim, and bensulfuron gave the best weed control without damaging the crop. Pendimethalin reduced rice seedling stand, weight, and height (Table 3). However, others (Ali, 1984; Ali and Gururajan, 1985) have reported that pendimethalin controlled weeds in rice seedling nurseries without causing phytotoxicity to rice.

N u m b e r of grasses, sedges, and broad-leaved weeds in two rice seeding nurseries as affected by weed control treatments*. I R R I , 1987 dry season Grasses

Treatmentt

Number/ 0.25 m2

Control (%)

Number/ 0.25 m2

35.1

--

24.9

24- 3 4.8 18- 3 5.0 1.7 10.0 14.9 4.7 12.7

31 86 48 86 95 72 56 87 64

13.1 0" 8 6" 9 0.8 0.5 1.3 2.7 0.2 0.1

3.6

90

0"9

U n w e e d e d check Weeds r e m a i n i n g after: R a p i d separation Slow separation Double seed rate Propanil (2.5), 10 DAS Thiobencarb (0.75), 6 DAS Butachlor (0.45), 3 DBS Pretilachlor + fenclorim (0- 3), 3 DAS Quinclorac (0.3), 3 DAS Bensulfuron (0.03), 3 DAS Quinclorac + bensulfuron ( 0 - 2 + 0 - 0 2 ) , 3 DAS * Data are averages of two seedling nurseries;

t As Table 1

CROP PROTECTION Vol. 7 June 1988

Sedges

Broad-leaved weeds

Control (%)

Number/ 0" 25 m 2

Control (%)

--

11-1

--

47 97 72 97 98 95 89 99 99.6

3- 6 0.0 3- 8 2.3 0.8 3" 3 1.9 1.2 0- 2

68 100 66 79 93 70 83 89 98

96

0.05

99-5

A. N. RAo AND K. MOODY

205

TABLE 6. N u m b e r of grasses, sedges, and broad-leaved weeds as affected by weed control treatments in two rice seedling nurseries in farmers' fields ~. Guimba, N u e v a Ecija, 1987 dry season

Grasses Number/ 0.25 m2

Treatmentt U n w e e d e d check Weeds remaining after: Rapid separation Slow separation Double seed rate Propanil (2- 5), 10 DAS Thiobencarb (0.75), 6 DAS Butachlor (0.45), 3 DBS Pretilachlor + fenclorim (0" 3), 3 DAS Quinclorac (0.3), 3 DAS Bensulfuron (0.03), 3 DAS Quinclorac + bensulfuron (0- 2 ÷ 0.02), 3 DAS * As Table 5;

Sedges

Control (%)

Number/ 0.25 m2

Broad-leavedweeds

Control (%)

Number/ 0.25 m2

Control (%)

69.0

--

13.5

--

3.5

48.5 4.7 50.0 18- 0 14.0 13.5 14.0 9- 5 27.5

30 93 28 74 80 80 80 86 60

10.5 0.0 7" 0 6.0 3.5 2-0 1" 5 i. 5 1 "0

22 100 48 56 74 85 89 89 93

2.5 0.0 0" 5 0- 5 1- 0 1.0 0.5 ---

29 100 86 86 71 71 86 100 100

8- 5

88

1.5

90

--

100

t As Table I

In the second experiment, E. glabrescens, P. dis-

tichum, C. difformis, F.. miliacea, Sphenoclea zeylanica Gaertn. and M. vaginalis were the major weeds in the I R R I farm, while E. glabrescens, P. distichum, C. rotundus, F. miliacea, S. supinus, and L. antipoda were predominant in Guimba. In this experiment, the use o f the double seed rate resulted in average reductions in weed density o f 60% and 26% at I R R I and in Guimba, respectively (Table4). Sedges were suppressed more than the grasses and this explains w h y greater reduction in weed density was observed at IRRI. Quick separation resulted in removal o f an average o f only 36% o f the weed seedlings from the rice seedling bundles at both sites. T h i s required 25 h/ha. T h e taller seedlings o f E . glabrescens, P. distichum and C. rotundus were removed. Slow separation resulted in 94% removal o f the weed seedlings (Table 4) and took 719 h/ha. T h e s e results support the findings recorded in E x p e r i m e n t 1. Shahi, Gill and K h i n d (1976) noted that even t h o u g h there were some features by which rice seedlings and y o u n g seedlings o f E. crus-galli can be differentiated, it is time consuming to examine every plant. At I R R I , good weed control was obtained with thiobencarb, quinclorac, quinclorac + bensulfuron, propanil, and butachlor (Table 4). In Guimba, quinc l o r a c + b e n s u l f u r o n , quinclorac, butachlor, and pretilachlor + fenclorim were most effective. Others have also reported that butachlor (Shahi and Gill, 1979; Tang, Ye and Yu, 1984), thiobencarb (Ali, 1984; Patel et al., 1985), and pretilachlor + fenclorim (Quadranti and G u y e r , 1985) were effective in controlling weeds in rice seedling nurseries. In G u i m b a , propanil did not control the sedges effectively whereas bensulfuron had only a weak effect on the grasses (Tables 5 and 6). Quinclorac caused reduction in rice seedling stand and reduction in dry weight (Table 7). Submergence o f the rice seedlings soon after spraying this herbicide resulted in some phytotoxicity. M e n c k et al. (1985) reported that rice u p

TABLE7. Effectof differentweedcontroltreatmentson the rice seedling number and weight in rice seedlingnurseries*.IRRI and Guimba~Nueva Ecija, 1987 dry season Number of rice seedlings/ 0-25 m2 Treatmentt U n w e e d e d check Double seed rate Propanil (2.5), 10 DAS Thiobencarb (0.75), 6 DAS Butachlor (0.45), 3 DAS Pretilachlor + fenclorim (0"3), 3 DAS Quinclorac (0.3), 3 DAS Bensulfuron (0.03), 3 DAS Quinclorac + bensulfuron ( 0 . 2 + 0 . 0 2 ) , 3 DAS

Dry weight of rice seedlings) (g/0-25m2)

IRRI

Guimba

I R R I ~ Guimbag

762 bc 1427a 801 b 778 bc 725 bc

806 b 1616a 815 b 808 b 773 b

775 bc 694 c 782 bc

802 b 682 b 804 b

45"9 b 43"3 b 4 4 . 9 b 32-1 c 46"8 b 41"2 b

741 bc

770 b

4 6 - 0 b 3 8 . 0 bc

44.0 b 60.2a 46"8 b 46"7 b 45.2 b

39.1 b 61.1a 39.5 b 42.6 b 42.8 b

* Data are averages of two seedling nurseries. In a column, means followed by a common letter are not significantly different at the 5% level by Duncan's multiple range test; t As Table 1; ~ Shoot weight only; g Shoot and root weight

TABLE 8. Economic analysis of different methods of weed control in rice seedling nurseries. I R R I and Guimba, N u e v a Ecija, 1986-87

Cost (US$/ha)t

Weed controlmethod* Double seed rate# Hand weeding~ Removalof weed seedlingsfrom rice seedlingbundles:~ Rapid Slow Herbicides¶ Butachlor(0-45) Thiobencarb(0-75) Pretilachlor+ fenclorim(0-3)

7.50 6"35 3"60 73-45 0.40 0.60 0"70

* Herbicide rate in kg a.i./ha is indicated in parentheses; 5" 1US$= P20; ~ Seed cost = $0.17/kg; ~ Hand weeding (data from Experiment i) and weed removal (data are averages of Experiments 1 and 2) = $0' 1125/h; ¶ Butachlor, $I 1/i; pretilachlor + fenclorim, $16; thiobencarb, $10- 50/1 (data from Experiment 2)

to the two-leaf stage had a lower tolerance to quinclorac than that at later stages. Using the double seed rate, hand weeding or removing the large seedlings from the seedling bundles resulted in less than 50% control o f weeds yet cost

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Weed control in rice seedling nurseries

5-10 times as much as the most expensive of the herbicide treatments (Table 8). Careful examination of each plant to ensure that most of the weeds were removed from the seedling bundles is impractical because it is laborious, time consuming and expensive. Higher herbicide rates controlled between 80% and 90°70 of the weeds. The cost of treating 440m2--the area recommended to plant 1 ha (Ministry of Agriculture, Philippines, 1978)--was 70 cents or less (Table 8). Herbicide treatment thus appears to be a practical method of controlling weeds in rice seedling nurseries.

MENCK, B. H., ROSEBROCK,H., UNGLAUB,W. AND KIBLER, E. (1985). BAS 514.H--quinclorac. Field experience to control Echinochloa crus-galliin rice. In: Proceedingsof the 10th Asian-

Pacific Weed Science Society Conference, Chiangma~ Thailand, pp. 107-113. MINISTRY OF AGRICULTURE,PHILIPPINES(1978). Masangana 99 Rice Culture, 16 Steps, Irrigated Transplanted. Quezon City, Philippines: Ministry of Agriculture. 20 pp. NEGb N. S. (1976). Weed control in rice. PesticidesInformation 2, 94-102. PATEL, C. L., PATEL, Z. G., PATEL, R. B. AND PATEL, H. R. (1985). Herbicides for weed control in rice nurseries. International Rice Research Newsletter 10(5), 26. QUADRANTI,M. AND GUYER, R. (1985). New possibility in weed control in the nursery beds. In: Proceedingsof the lOth Asian-

Pacific Weed Science Society Conference, Chiangma~ Thailand,

Acknowledgement A. N. Rao is grateful to the Andhra Pradesh Agricultural University, India for granting him a leave of absence that enabled him to work at the International Rice Research Institute, Philippines.

References ALI, A. M. (1984). Effect of time ofherbicide application on rices of different durations. International Rice Research Newsletter 9(6),

21-22. ALl, A. M. AND GURURAJAN,B. (1985). Weed control in lowland rice nursery. Madras AgriculturalJourna172, 429-431. JIANG, S. N. (1983). Rice yield losses caused by Echinochloa crusgalli (L.) Beauv. Plant Protection 9, 40 (in Chinese). KALE, V. R., HAPASE, D. G. AND CRUZ, R. D. (1965). A new herbicide for paddy fields. PoonaAgricultural CollegeMagazine 56, 30-32. KIM, S. C. AND LEE, S. K. (1985). Factors affecting herbicidal phytotoxicity and efficacy of antidote, CGA 123,407 in rice nursery. Korean Journal of WeedScience 5, 63-72. MATSUO, T. (1961). R ice Culture inJapan. Tokyo, Japan: Yokendo Ltd.

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pp. 277-281. RAO, A. N. AND MOODY, K. (1987). Weed seedlings transplanted with rice seedlings reduce grain yields. International Rice Research Newsletter 12(3), 51. SHAHI, H. N. AND GILL, P. S. (1979). Control weeds in rice seedling nurseries with Machete. ProgressiveFarming 15(9), 5. SHAHI, H. N., GILL, P. S. AND KHIND, C. S. (1976). Chemical control of weeds in rice nurseries. ProgressiveFarming 12(9), 9. TANG, H. Y., YE, C. D. AND YU, R. J. (1984). Application of Machete for weed control in rice nursery. Acta Phytophylacica Sinica 11,204 (in Chinese, English abstract). TANG, W. T. AND WU, H. P. (1965). Testing of some herbicides in paddy fields. Journal of the AgriculturalAssociation of China 49, 15-24 (in Chinese, English abstract). WRIGLEY, G. (1969). The problem of weeds in rice. In: Rice Technical Monograph No. 1, pp. 27-31. Basle, Switzerland: Ciba Limited. XUAN, V. T. AND ROSS, E. V. (1976). Training Manual for Rice Production. Laguna, Philippines: International Rice Research Institute, College. 140 pp. Yu, R. J. AND LIU, D. J. (1986). Sampling technique and space distribution of Echinochloa crus-galli. Acta Agriculturae Shanghai 2, 57-62 (in Chinese, English abstract). Received 9 November 1987 Accepted 4 December 1987