S. L. J. Tea Sci. 73(1). 11 - 28. 2008. Printed in Sri Lanka Appraisal of the Weed Seedbank in Low-grown Tea (Camellia sinensis [(L) O. Kuntze]) Soil under different Weed Management Techniques K. G Prematilake1, R J Fraud-Williams2 and P B Ekanayake3 ('Agronomy Division, Tea Research Institute of Sri Lanka, Talawakelle department of Agricultural Botany, University of Reading, 2 Earley Gate, Berks, ReadingRG6 6AU,UK *47, Richmond Hill Road, Hantana, Kandy, Sri Lanka) ABSTRACT A field experiment was conducted at the Low country Regional Centre, Tea Research Institute, Sri Lanka during the period of 1994 - 1995 to investigate the density of weed seedbank in soil in relation to different weed management methods during first year of planting of tea. Manual (hand) weeding at 2,6,12 and 18 week intervals and herbicides such as Glyphosate, 2,4-D, Glufosinate Ammonium, Paraquat and Oxyfluorfen and some combinations of them at varied intervals were imposed on mulched plots. One slash weeding, every 6 weeks and Paraquat + Oxyfluorfen combination were imposed on unmulched plots. Weed seedbanks were determined using both 'seed germination' and "Malone's seed extraction" methods before and 12 months after planting (MAP) of tea. There were no significant differences (p=0.05) in seed density between plots before and after planting tea. However, weed seed density at both 0-5 and 5-15 cm depths was significantly different between treatments, 12 MAP. The highest weed seed density (4168 nr2) at 0-5 cm depth from mulched plots hand weeded every 6 weeks was significantly (p=0.05) greater than that of plots hand weeded every two weeks and plots were treated with Glyphosate and the combination of Oxyfluorfen + Paraquat. The greatest seed density of 5520 nr2 at 5-15 cm depth was from unmulched plots, slash weeded every 6 weeks and this was significantly (p=0.05) greater than that of all treatments except for plots hand weeded every 18 weeks. All hand weeding except for weeding every two weeks and, slash weeding every six weeks treatments had thus significantly greater seed densities at the total depth of 0-15 cm compared to all chemical weeding treatments 12 MAP. Whereas, the seed density was not significantly (p=0.05) affected by any treatment 12 MAP when compared with before planting tea. Mean weed species number significantly increased 12 MAP (23.2±0.66) when compared with that of before imposition of manual weeding treatments (16±2.45) and herbicide treatments 12 MAP (17.8± 1.52). Whereas, there was no significant difference in species number between chemically treated plots before (14±1.26) and 12 MAP. Majority of seeds found in all treatments, particularly of plots weeded every 6 weeks was of 11 desirable herbs on tea. Mulching alone is found to be unimportant in view o f weed seed bank in the soil. A n integrated approach, where the hand weeding at intervals o f <12 weeks is practised and a herbicide particularly a combination o f Oxyfluorfen and Paraquat is applied at tender phase of weed growth was found to be effective in the mitigation o f weed seed bank in tea soil. Key words: Chemical weeding, Manual weeding, Mulching, Weed management in tea, Weed seedbank INTRODUCTION Weed management in tea plantations is a critically important operation, particularly during early period o f tea establishment (Somaratne, 1988) as weeds interfere with tea and interrupt the major field operations such as plucking, fertilizer application and pruning. Weed competition becomes more adverse i f weeding was delayed for more than three months (Prematilake et al, 1999). Among the possible causes for profuse weed growth in tea fields, presence o f a 'weed seed stock' which is termed 'weed seedbank' in soil has a major attribution because, it could continuously release viable seeds.for germination followed by weed growth. Weed seedbank has been referred to as reserves o f seed present in the soil and on its surface (Roberts, 1981). Seedbanks in cultivated soils are derived from seeds produced in situ and those that have been introduced from elsewhere (Froud-Williams et al, 1983). Quantitative appraisal o f weed seeds in soil under different agronomic conditions is particularly important in planning and providing a basis for advice on control measures (Wilson and Cussans, 1975). Furthermore, investigations on species composition provide a basis for estimating the future weed infestation (Ball and Miller, 1989) and helps in planning o f control strategies (Roberts, 1981). Although it is more costly and laborious, the manual weeding is still practised as a major operation in young tea at present, because any herbicide-based system cannot be adhered to throughout as young tea is more vulnerable for herbicide toxicity. Hence, it is more appropriate to use an integrated weed management system, where manual and cultural methods are practised together with a suitable herbicide. Understanding o f the size and behavior o f weed seed bank in soil w i l l help in planning economical control stratergies. The objectives o f the present study are therefore to assess the density of weed seed bank in the soil and its composition in relation to different weed management techniques practised in newly planted tea at low elevation in Sri Lanka. 12 MATERIALS AND METHODS A field experiment was conducted at the Low country Regional Centre of the Tea Research Institute, Ratnapura, Sri Lanka to estimate the weed seed bank of soils of newly planted tea under different weed management techniques, during the period of June 1994 to June 1995. The elevation of the experimental site is about 60 m amsl (the latitude: 6° 41'N and longitude 80° 24'E) and the soil type is an Ultisol sandy loam. The annual rainfall is 2500-3000mm and the mean ambient temperature is 28 °C. An old tea block with some vacancies, which were infested with weeds was selected for the investigation. Following uprooting of old tea a thorough land preparation was done in order to remove all roots and boulders, and also to keep the field weed free. Drains were also cut. Field was then planted with Mana grass (Cymbopogon confertiflorus) for soil rehabilitation at spacing of 0.6 - 0.9 m x 0.15 m, for a period of 24 months. Wetamara (Gliricidia septum) was planted at a spacing of 2.4 m x 3.0 m as shade trees. Mana grass was cut every six months and thatched in situ, and finally it was cut from the base and thatched in May 1994. Planting holes (45 cm deep) were dug along the contours at a spacing of 1.2 m x 0.6 m and 8-month old poly bagged tea plants (variety KEN 16/3) were planted in holes on 15th June 1994. Forty plots each consisting of 30 tea plants were thus demarcated, leaving two rows of tea plants in the periphery of each plot as guard rows. All plots were properly hand weeded prior to allocation of treatments. Treatment combinations There were two component studies on weed management during early establishment of tea in Low country. In the first investigation, all present weed management methods were evaluated and findings were published elsewhere (Prematilake et al., 2004). The second investigation was undertaken on weed seedbank in soil before and after imposition of above weed management methods. Therefore, treatment combinations were same in both investigations. Thus, four hand pulling treatments to mulched plots at different intervals (TI -T4) and one slash weeding treatment (T5) to unmulched plots at 6-week intervals were imposed as manual weeding practices. Another four herbicide combinations to mulched (T6- T9) plots and one herbicide combination to unmulched plots were also imposed (T10) as given below. Treatment combinations TI Mulching + Hand weeding at 2 weeks interval T2 Mulching + Hand weeding at 6 weeks interval T3 Mulching + Hand weeding at 12 weeks interval 13 T4 Mulching + Hand weeding at 18 weeks interval T5 No mulching + Slash weeding at 6 weeks interval T6 Mulching + Glyphosate (36%)@ 0.99 kg a.i h a 1 + kaolin@ 3.42 kg h a 1 T7 Mulching + 2 ,4 - D (73%) @ 0.73 kg a. i. ha-' + Paraquat (20%) @ 0.15-0.22 kg a.i. ha'1 T8 Mulching + Glufosinate Ammonium (15%) @ 0.2 kg a.i.ha'1 T9 Mulching + Oxyfluorfen (24%) @ 0.29 kg a.i. ha'1 + Paraquat @ 0.15-0.22 kg a.i. ha - 1 T I 0 No mulching + Oxyfluorfen (24%) @ 0.29 kg a.i. ha"1 + Paraquat @ 0.15-0.22 kg a.i. ha - 1 The experimental design was RCBD with four replications. Tea inter-row spaces o f plots for T1-T4 and T6-T9 were laid wi th fresh Mana (Cymbopogon confertiflorus) grass @ 37 tonnes ha - 1 soon after planting tea and repeated twice in October and March 1995 (Table 1). In unmulched plots (T5), the weeds found within tea inter-row spaces were retained as a live ground cover, slashed and removed at six week interval. For chemically treated plots, alt herbicide solutions were applied at 550 L ha'1 along the tea inter-rows using a knapsack sprayer fixed with a Poli-Jet nozzle (Orifice size 042) and a spray guard. Herbicide treatments applied to mulched plots included split applications o f Round up (glyphosate, 36%) (T6), split applications of Fernoxone (powdered formulation) (2,4- D, 73%) alone and 2 , 4-D + Gramoxone (Paraquat 20%) combination (T7), split applications o f Basta (Glufosinate Ammonium, 15%) alone (T8). Goal-2E (Oxyfluorfen, 24%) was first applied directly to the bare soil soon after planting tea on T10 plots but on T9 plots prior to mulching. Another, two applications of oxyfluorfen in mixture with paraquat were applied in November 1994 and Apr i l 1995. Other herbicides were applied when weeds were 10-15 cm tall (Table 1). Weeds were hand pulled prior to application o f fertilizer mixture (T-200 at 1500 kg h a 1 y r 1 ) at two-month intervals in plots. W e e d seedbank determination The weed seed density in soil was assessed fol lowing the 'Germination method 1 as described by Brenchley and Warrington (1930) and Roberts (1970) and was supported by additional seed extraction (Malone, 1967). Atotal o f 15 soil samples were obtained randomly from the depth of 0-5 and 5-15 cm before planting tea (from an area o f 218 cm 2 ) within and between tea rows in each plot, using a metal pipe (4.3 cm diameter) and stored in polythene bags at a mean temperature o f 30°C until processing within a week period. Samples from the same depth in each plot were bulked to make a composite sample o f 800 g and each was placed on a tray to a depth o f 15 cm. Five trays fi l led 14 Table 1. Calendar o f herbicide application, hand pul l ing mulching and sampling on chemical treated plots during the first year o f establishment o f tea in the field (15 t h June 1994-15* June 1995) Month June-July 94 Aug 94 Sep 94 Oct 94 WAP 0 3 6 7 9 10 12 15 16 18 19 T6 M - Gly (jly w M T7 M D w P w P w M T8 M w GA GA w M T9 O/M w P P w M T10 O P P w Month Nov 94 Dec 94 Jan 94 Feb 95 WAP 22 23 25 26 28 29 31 32 34 36 T6 Gly w Gly w w T7 P D w P D+P w T8 GA GA w T9 O+P P w T10 O+P P w Month Mar 95 Apr 95 May 95 June 95 WAP 37 39 42 44 45 46 48 50 51 52 T6 M Gly w Gly w s T7 M w D w s T8 M w GA w s T9 M O+P s T10 O+P s M : Mulching O : Oxyfluorfen D : 2,4-D Gly : Glyphosate P : Paraquat GA : Glufosinate Arrrnionium w : Hand pulling s : Soil sampling for seed bank assessments with sterilized sand were used as the control. All trays were then placed in a screen house and protected from contamination by foreign air-borne weed seeds. Soil was kept moist as necessary, facilitating weed seed germination and frequently re-arranged to avoid differential light effects. Same procedure was followed for sampling and bulking of soils, 12 MAP of tea. To estimate the weed seed density, emerged seedlings of weed species were counted, identified and removed from trays weekly over a period of seven months, until further emergence ceased in both sample sets. Soil was disturbed at six week interval to facilitate germination of buried seeds. Seed and rhizome count was thus assessed in terms ofthe number of seedlings emerged. Subsequently, the balance of seeds in soil was isolated using Malone's seed extraction method in order to determine the actual number of viable seeds present. In this method seeds were recovered by 15 floating organic soil fraction on a dense liquid solution. One hundred grams o f soil sample was mixed in a solution o f 5, 10 and 25 g o f sodium bicarbonate, sodium hexametaphosphate, and magnesium sulphate, respectively and 200 ml tap water. For dispersion of soil and floatation, soil was agitated in the solution for two minutes. Floating debris was then decanted and sieved for retention o f even smallest seeds. This process was repeated three times. Debris on the sieve was washed with tap water to remove foam and small soil particles. A l l material collected in sieve were dried and removed for identification. Viability o f extracted seeds was simply checked by pressing seeds with a forcep. Statistical analysis Count data (Seed No.) were first log transformed and they were subjected to analysis o f variance (ANOVA) using SAS package. Mean separation was done using Least Significant Difference (LSD) and Standard Error (se) at p=0.05 probability level. The original values were used for interpretation o f results. Weed species (seeds) numbers present at 0-5 cm depth, were listed with the Mean value and Standard Error (se). RESULTS AND DISCUSSION Weed seed density and composition prior to imposition of treatments There was no significant difference in seed density among these plots at both depth (0- 5 and 5-15 cm) (Table 2). However, the slight variation in seed density would be due to the spread o f forty plots over a large area, 860 m 2 , where soil fertil ity level and degree of infestation o f weeds before planting grasses might have been varied. Vacancies in former old tea field were normally infested with many weeds. Furthermore, dispersal of weed seeds on plots through dissemination from surrounding fields at varying degree is quite possible, even i f the entire land block was covered by the grass for two years. As Boli and Watkinson (1993) also pointed out, the variation in seed density in soil is attributed to the in situ weed density, proximity to source, soil conditions, burial depth, cropping regime, fertilizer and organic manures etc. However, the present densities (max. 5114 n r 2 ) at the total depth of 0-15 cm was remarkably lower when compared with 13000-27000 seeds n r 2 in old tea fields as reported by Eden (1949). Such a lower figure might be attributed to the previous ground cover wi th establishment o f Mana (C. confertiflorus) for 24 month period followed by the weed free situation maintained until time o f imposition o f treatments. Seed density per unit depth at 0-5 cm in plots to be imposed with different treatments, except in one treatment, was greater than that at 5-15 cm depth (Table 2). Number o f weed species was also varied within a range o f 11-21 (15.89±3.37) in all plots except for plots to be treated with hand pull ing every 18 weeks, where only 7 species were reported (Table 4 a). 16 Table 2. Mean density o f the soil weed seed bank before planting o f tea and imposition o f treatments in 1994 § Mean seed density (No. /m 2 ) * •S Total seed No. x> | Seed No. at 0-5 cm depth Seed No. at 5-15 cm depth 0-15 depth o — • Tr ea tm en t Em er ge d Ex tra ct ed To ta l O rig in al TS $ B m Ex tra ct ed To ta l O rig in al Lo g O rig in al T I 3.03 2.00 3.05 1346 3.18 2.05 3.20 1976 3.46 3322 T2 3.17 nd 3.17 2026 3.19 2.10 3.21 1791 3.53 3817 T3 3.34 2.41 3.43 2903 3.01 2.58 3.25 2199 3.66 5102 T4 2.57 2.00 2.46 852 3.13 nd 3.13 2371 3.44 3223 T5 3.06 1.88 3.08 2223 3.43 nd 3.43 2891 3.65 5114 T6 3.10 2.68 3.27 1890 3.14 2.17 3.21 2297 3.57 4187 T7 3.10 nd 3.10 1667 3.09 2.54 3.20 1618 3.48 3285 T8 2.60 1.80 2.61 2322 3.29 2.17 3.32 2557 3.64 4879 T9 3.01 nd 3.01 1235 3.07 2.21 3.10 1816 3.36 3051 T10 2.34 2.10 2.36 1223 2.89 2.17 2.94 1235 3.30 2458 LSD at (0.05) (ns) (ns) (ns) * Mean value among 4 replicates ns: not significant at 0.05 level nd: not detected A majority o f weed seeds buried at 0-5 cm depth in all plots was o f broad-leaved species such as Stemodia verticillata, Peperomia pellucida, Molligo pentaphylla, Oldenlandia corymbosa and Lindernia cordifolia (Table 4 a). Such herbs are considered to be desirable for tea as they do cover the ground conserving soil and moisture. They are also known as 'soft herbs' (Anon, 2003) as they are tiny annual species wi th a very short life span o f 4-6 weeks, having less interference with tea (Prematilake etal, 2008). Furthermore, seeds and rhizomes o f sedges such as C. rotundus and Bulbostylis barbata were abundant in almost all treatments. Broad leaved species such as Borreria latifolia, Desmodium heterophyllum, Oxalis barrelieri and Croton hirtus were very common though occurred in moderate numbers in many plots. Seeds o f Mitrocapan villosum, Cleomi viscose, Euphobia prostate and cormes o f Caladium hortulanum were also present in majority o f plots in small numbers (Table 4a). Seeds o f grasses were infrequent. 17 Effect of various treatments on the size and composition ofthe weed seed bank, 12 MAP Effect of manual weeding treatments There was a significant difference in total seed density between treatments both at 0-5 cm and 5-15 cm depth, 12 MAP i.e. in 1995 (Table 3). The greatest number o f seeds at 0-5cm depth was present in mulched plots weeded every six weeks (4168). The seed density reported from hand weeding at two-week intervals (weed free treatment) was significantly less (p=0.05) than that o f all other manual weeding treatments, but comparable to that o f Glyphosate and Oxyfluorfen+ Paraquat treatments. At 5-15 cm depth, the highest seed density o f 5519 nv 2 was recorded in unmulched plots slash weeded every 6 weeks and this was significantly greater (p=0.05) than that o f all treatments except for hand weeding every 18 weeks. Weed seed density o f other three hand weeded and all chemically weeded treatments were thus comparable. As a Table 3. Mean weed seed density of the soil seed bank at 0-15 cm depth as affected by different treatments 12 MAP, in 1995 .§ Mean seed density (No./m 2 )* j= Total seed No. | Seed No, at 0-5 cm depth Seed No. at 5-15 cm depth 0-15 depth Tr ea tm en t IS 9 B w Ex tra ct ed To ta l O rig in al Em er ge d Ex tr ac te d To ta l O rig in al Lo g O rig in al T I 2.46 3.00 3.11 1601 2.998 2.08 3.048 1481 3.40 3082 T2 3.52 2.59 3.57 4168 3.113 2.81 3.290 2163 3.76 6331 T3 3.46 2.52 3.51 3293 3.212 1.89 3.233 2033 3.70 5327 T4 3.51 2.52 3.55 3723 3.303 2.51 3.368 2653 3.80 6375 T5 3.38 1.76 3.39 2709 3.690 2.46 3.715 5519 3.90 8228 T6 2.98 2.54 3.12 1430 3.005 2.53 3.130 1506 3.44 2936 T7 3.24 2.30 3.29 2514 3.008 2.38 3.100 1314 3.54 3828 T8 3.26 2.10 3.29 2284 3.260 1.74 3.273 2276 3.59 4560 T9 2.90 2.29 3.00 1241 2.968 1.82 2.998 1026 3.34 2267 T10 3.00 2.19 3.07 1296 3.120 2.29 3.180 1621 3.44 2926 LSD at (0.05) 0.31 0.39 0.23 * Mean value among 4 replicates 18 T a b l e 4a. M e a n n u m b e r o f w e e d seedl ings emerged f r o m so i l w e e d seed b a n k at 0-5 c m depth be fore the i m p o s i t i o n o f t reatments i n June 1994 ( N o . / m 2 ) _ _ Species/Treatment TI T2 T3 T4 T5 T6 T7 T8 T9 T10 Stemodia verticillata 148±59* 321±82 432*102 25±12.5 202±80 99±49 136±46 222±88 136±25 111±29 Peperomia pellucida 136±53 210±32 nd 49±15 772±354 222±71 333±82 37±19 173±52 185±53 Molligo pentaphylla I30±28 161±42 395±135 247±63 296±37 11I±36 191±62 741±214 222±38 185±60 Oldenlandia corymbosa 11H36 49±14 99±49 nd 37±19 49±13 130±44 nd nd 62±31 Lindernia cordifolia 62±15 99±35 nd 37±19 62±31 74±30 74±24 62±31 25±12.5 12±6 Borreria latifolia 43±13 49±14 124±26 nd nd I2±06 19±8.5 25±12 37±19 62±19 B. leavis nd 247±I23 383±162 nd nd nd nd nd nd nd B. hispida nd nd nd nd nd nd nd nd nd 37±19 B. ocymoides 49±14 25±12.2 nd nd nd 99±31 nd nd 111±36 nd Desmodium heterophyllum 37± 37±12 nd nd 74±37 86±29 16]±59 37±05 nd 62±23 Oxalis barrelieri nd 111±18 nd nd 198±92 99±31 93±26 74±37 nd 25±12 Mitrocapan villosum nd nd 86±25 nd I24±62 nd 68±12 93±46 nd 74±37 Caladium bicolor 56±18 nd 136±31 nd 3 7 ± I 9 nd 37±19 nd 37±19 nd Croton hirtus nd 25±12.5 173±49 12±06 56±26 12±06 19±8.5 nd t2±06 nd Euphobia prostrata 149±66 nd 37±19 nd nd 25±12 nd nd nd nd E.thymifolia 148±74 nd 37±19 25±6 nd nd nd nd nd nd Cteomi viscosa 19±9.5 12±06 nd • nd 12±06 nd nd nd 37±19 49±14 Hedyotis auricularia nd 25±06 l l l ± 5 5 nd nd nd nd nd nd 86±43 Emilia javanica I2±6 nd 12±6 nd nd nd nd nd nd nd Theriophonum minutum 19±9.5 nd 99±25 nd I2±6 nd 25±12 nd nd nd Ocimum sanctum nd nd nd nd nd nd nd 37±19 nd nd Commelina benghelensis nd 309±76 nd nd nd nd nd nd nd nd Crassocephules crepidiodes nd 25±12 nd nd nd nd nd nd nd 37±19 Scoparia dulcis 19±9 2 5 ± 6 12±6 nd nd nd nd 19±9 161±73 25±12 Cyperus rotundus I l l ± 3 5 124±41 99±49 nd 173±42 457±92 I54±46 37±19 259±76 I2±6 Bulbostylis barbata 31±15 37±12 86±19 nd 25±12 12±06 19±85 nd nd 25±12 Digitaria sanguinalis nd nd 12±06 25±06 nd nd nd nd nd nd Paspalum conjugatum nd nd 25±13 nd nd nd nd nd nd nd Axonophus compressus nd 6 2 ± I 2 37±12 nd nd nd 19±9 nd nd nd Brachairia subauadrioora nd nd nd nd nd 37±19 25*12 nd nd nd Total spp No. IS 19 21 7 15 15 17 11 11 16 * se nd: not detected T a b l e 4 b . M e a n n u m b e r o f weed seedl ings emerged f r o m so i l w e e d seed b a n k at 0-5 c m depth 12 months af ter i m p o s i t i o n o f t reatments i n June 1995 ( N o . / m 2 ) Species T ] T2 T3 T4 T5 T6 T7 T8 T9 T10 Stemodia verticillata 57±I7* 126±30 34±5.5 57±14 138±32 99±49 138±47 115±29 138±25 57±21 Peperomia pellucida 149±54 92±25 57±5.5 92±33 69±30 80±35 23±6.5 46±16 183±72 207±72 Molligo pentaphylla 471±134 I525±557 413±62 1261±295 929±214 585±134 447±84 1032*219 161±22 172±32 Oldenlandia corymbosa 92±39 310±74 814±128 401±20 321±106 138±32 677±301 287±122 23±6.6 57±22 Lindernia cordifolia 34±5.5 149±14.5 57±14.5 34±17 138±29 69±17 80±35 23±6.5 22±11 11 ±05 Borreria latifolia nd 103±11 115±22 4 I 3 ± 6 9 11 ±05 34±9.5 126±14.5 11±5.5 23±6.5 103±14.5 B. leavis n d nd 43±9.5 195±38 69±13 11 ±05 nd 11±05 . 23±12 11 ±05 B. ocymoides 80±17 252±53 46±9.5 57±9.5 23±10 nd 80±17 92±46 23±11 35±11 Desmodium heterophyllum 34±0 23±I1 23±6.5 46±9.5 46±14 34±05 11± 80±26 11±5.5 nd Oxalis barrelieri 69±20 264±45 138±16 I72±52 103±25 11±5 252±12 46±13 149±46 16U38 Mitrocapan villosum 34±17 80±17 nd nd nd nd 46±9.5 57±11 23±12 23±12 Crotolaria juncia 23±12 nd nd 11±5.5 nd 11±5 nd nd nd nd Physalis angulata 23±12 nd 229±28 nd nd nd 11±5.5 11±5.5 nd nd Cleomi viscosa nd 57±14.5 92±16 34±11 nd nd 11±5.5 11±5.5 11±5.5 nd Hedyotis auricularia nd 11±6.5 46±8.5 ]]±5.5 nd nd nd 11±5.5 11±5.5 nd Hyptis suavaeolens 11±5.5 nd 34±5.5 23±11 11±5 nd nd nd nd nd Phylanthus neruri \l±5.5 34A5 .5 23±11 nd 23±10 23±5.5 23±6.5 nd nd 23±12 P. urinaria 45±16 nd 11±5.5 nd 23±10 nd nd nd nd nd Mikenia scandens nd 11 ±6 34±12 nd nd nd nd nd nd nd Scoparia dulcis nd 11±6 nd 15±6.5 241±98 nd nd 69±27 11±5.5 23±12 Commelina benghelensis nd n d nd 34±11 nd nd nd nd nd nd Theriophonum minutum nd n d nd nd nd nd 92±46 nd nd nd Begonia hirtella nd nd nd nd nd nd 23±6 nd nd nd Eleutheranthera ruderalis n d nd 23±6 11±5.5 nd nd 11±5.5 11±5.5 nd 69±27 Cyperus rotundus I95±45 298±22 115±36 344±22 218±33 126±26 126±31 69±28 149±30 I84±33 Bulbostylis barbata 34±17 172±41 69±12 80±27 34±15 nd 23±6.5 23±6.5 nd 11±5 Digitaria sanguinalis I l ± 6 298±I27 103±15 115±36 195±30 nd nd 57±5.5 nd 68±13 Perotis indica 34±17 11±5.5 23±6.5 23±6.5 11±5 11±5 nd nd 11±6.5 nd Brachairia subquadripora 11±5.5 nd 11±6 nd 23±10 nd 25±12 11±5.5 nd nd Eragrostis pilosa nd nd nd nd 23±10 nd nd nd nd nd Paspalum coniugatum nd 23±6.5 23±6.5 23±11 nd nd nd nd nd nd Total spp No. 21 25 24 23 23 13 19 22 19 16 ~se n d : n o t d e t e c t e d 1 9 9 4 - b e f o r e 1995 - 1 2 M A P I 2 3 4 5 6 7 8 9 10 Treatment combina t ions Figure 1. M e a n weed seed densi ty o f the soi l weed seed bank at 0 - 15 c m depth as affected by d i f ferent weed management t reatments consequence, final densi ty in each hand weed ing at 6 , 1 2 , 1 8 weeks intervals and slash weed ing every six mon ths t reatments, 12 M A P was s ign i f i can t l y greater (p=0 .05 ) than that o f hand weed ing every 2 weeks and al l chemica l weed ing treatments (Table 3 and F igure 1). Howeve r , none o f t h e seed densit ies under var ious t reatments recorded at the end o f 12 mon ths was s ign i f i can t l y d i f ferent f r o m that o f be fore impos i t i on o f each t reatment i.e. i n 1994 (F igure 1). A lower weed seed densi ty was thus mainta ined w i t h f requent hand weed ing every two weeks (F igure 1). The weed species compos i t i on par t i cu la r l y the 'so f t herbs ' was v i r t ua l l y s im i l a r in bo th years a l though seed number o f species such as Mollugo pentaphylla had been increased (Table 4 b ) . Increment o f seed dens i ty in the above species may p robab ly be due to the seed i n f l ux f r o m the su r round ing area. Re la t i ve ly a greater species d i ve rs i t y is also observed i n th is t reatment (18 and 21 species before and 12 M A P , respectively) may probably be due to less intense intra-specif ic compet i t ion (Table 4 ) . Such a l ower seed densi ty may be at t r ibuted to the l o w e r seed i n f l ux f r o m in situ p roduc t ion as weeds were removed du r i ng the onset o f ear ly vegetat ive g r o w t h phase p reven t i ng reproduc t i ve output . Fur thermore , seeds in the so i l m i g h t have germina ted fast since there was no barr ier or weed cover on the g round surface. T h o m p s o n (1992) had also shown that in und is turbed so i ls , the surface so i l (1-2 c m ) f r o m w h i c h most seedl ings emerge becomes rap id l y exhausted o f bu r i ed seeds. H igher seed densit ies in the treatments o f hand weed ing every 6 , 1 2 and 18 weeks at 0- 5 c m depth compared to hand weed ing every 2 weeks was m a i n l y a t t r ibuted to the 21 greater species diversity (23-25) which resulted in the presence o f a high seed count (Table 4 b). The greatest density recorded (4168 n r 2 ) in hand weeding every 6 weeks treatment has contributed about 66% to the total seed density at 0-15 cm depth (Table 3). Such a high density was mainly attributed to the early reproduction and high fecundity rate o f annual 'soft herbs' such as M. pentaphylla, O. corymbosa, S. verticillta, P. pellucida and L. cordifolia and; Borreria species, Oxalis barrelieri, M. villosum and Cleomi viscosa. Seeds o f grass species D. sanguinalis and sedges, C. rotundus and B. barbata also largely contributed at 0-5 cm depth (Table 4 b). In plots manually weeded both at 12 and 18 week intervals, about 60% o f the total seed density at 0-15 cm depth was represented by the seeds found within 0-5 cm upper layer. Also in the plots hand weeded every 12 weeks, soft herbs such as M. pentaphylla, O. corymbosa, and species o f Oxalis barrelieri, Cleomi viscosa, C. rotundus and D. sanguinalis were present largely in the upper layer. The high seed density in the treatment of hand weeding every 18 weeks was mainly attributed to the high seed count o f M. pentaphylla, O. corymbosa, Borreria spp, Oxalis barrelieri, C. rotundus and D. sanguinalis. Particularly, a massive production o f seeds o f B. latifolia was recorded. B. latifolia is the commonest weed species found in low-grown tea and it characterises a high fecundity rate within 12-20 weeks after germination (Prematilake, 1997). Higher densities in the above hand weeding treatments recorded 12 M A P was thus, as a consequence o f subsequent replenishment o f the seedbank with in situ seed rain, which further depended upon the frequency o f weeding and the type of weeds present. The seed density in slash weeded unmulched plots every 6 weeks at 0-5 cm depth was also attributed to higher seed counts o f soft herbs such as M. pentaphylla, O. corymbosa, Lindernia cordifolia, S. verticillta, P. pellucida; broad leaf species, Oxalis barrelieri and Scoparia dulcis and; C.rotundus and D.sanguinalis. The highest total density of 8228 n r 2 recorded in this treatment, 12 MAP was thus resulted from an initial high density (5114 n r 2 ) as well as the in situ production o f seeds from weeds left as a live ground cover in tea inter-rows. The seeds of M. pentaphylla, O. corymbosa and rhizomes of C. rotundus were largely contributed to the seed bank as they could thrive in compact soil. However, many seeds were present at 5-15 cm depth and cracks formed by excessive drying o f soils during dry spell followed by washing out o f small seeds wi th rains into the cracks and coarse textured soil might explain such an increased density at deep layer. This is also in agreement with the findings o f Harper (1977) and Hopkins and Graham (1983). 22 Impact of various herbicide combinations Weed seed density in herbicide treated plots 12 MAP was not significantly different from that of before imposition of treatments, indicating the maintenance of the status quo weed control (Figure 1). Many authors have also concluded that effective weed control with herbicides has led to a decreased seedbank (Hurle, 1974; Fogelfors, 1991). There was a successful control of weeds particularly with Oxyfluorfen + Paraquat thereby weed seed production has been minimized, recording the least seed count. Initial lower seed stock together with meagre production of seeds of soft herbs and some common weeds were the constituents in seed bank. Similarly in Glyphosate treated plots, the species number was confined to 13, with the presence of seeds of soft herbs such as M. pentaphylla, 0. corymbosa, P. pellucida; Borreria species and sedge C. rotundus. These former two are known to be tolerant for Glyphosate (Table 4b). However, in 2,4-D + Paraquat and Glufosinate Ammonium treatments, a significantly greater seed density was recorded when compared to Glyphosate and Oxyfluorfen + Paraquat treatments at 0-5 cm depth (Table 3). This was caused by high species diversity (19) and presence of seeds of Oxalis barrelieri and Borreria species and rhizomes of C. rotundus in 2,4-D + Paraquat treated plots. Being a selective broad weed killer 2,4- D + Paraquat had not controlled C. rotundus and D. sanguinalis. These other broad leaved weeds were also not properly killed by both treatments. Relatively a greater seed density in Glufosinate Ammonium treated plots compared with Oxyfluorfen + Paraquat treatment was ascribed to both initial and final seed densities. Presence of seeds from higher number of species (19) and higher number of seeds of M. pentaphylla and O. corymbosa and D. heterophyllum, B. ocymoides and Scoparia dulcis were observed at the end of 12 months (Table 4b). M. pentaphylla was also initially present in higher number. Whereas, such swelling of the seed bank did not reflect remarkably in the final density i.e. 12 MAP in these two treatments (Figure 1). In all herbicide combinations, the frequent reporting of * soft herbs' may be represented mainly by the original seed stock. Furthermore, being floras with a very short life span, they had an opportunity to emerge as seedlings and produce seeds in situ during the time gap between two herbicide spraying and from uatteneded weeds by herbicides. Seed dispersal from surrounding fields is also quite possible. Rhyzomes of C. rotundus also frequently occurred before and after imposition of all treatments in varying degrees. It is known to be a tolerant species for number of herbicides. It is also capable of producing a net work of corms and underground tubers, asexually. Buried rhizomes may have thus been grown and multiplied during the course 23 DDI T I 0 T 2 • T3 • T 4 • T5 • T 6 S3 T7 S T8 Q T9 B T 1 0 7 n June - Dec. 94 Dec. 94 - June 95 June 94 - June 95 T ime Period P = ° - 0 5 Figure 2. Mean weed dry weight (t/ha) as affected by various manual and chemical weec management methods dur ing the period o f 15'1' June 94 -15 l h June 95 M a n u a l n e e d i n g ( m u l c h e d ) : T I - every two weeks ; T 2 - every six weeks ; T 3 - e v e n 12 weeks ; T 4 - every 1 8 weeks ; and ( u n m u l c h e d ) : T 5 - every six weeks (slash weed ing ) ; C h e m i c a l w e e d i n g ( m u l c h e d ) : T6 - glyphosate (0.99 kg a.i ha ) - kaolin (3.42 kg h a 1 ) ; T 7 - 2. 4 D (0.73 kg a.i h a 1 ) paraquat (0.15-0.22 kg a.i h a ) ; T 8 - g lu fos inate a m m o n i u m (0.2 kg a.i ha ' ) ; T 9 - o x y f l u o r f e n (0.29 k g a.i ha" 1) / paraquat (0 .15-0.22 kg a.i ha ' ) ; and ( U n m u l c h e d ) : T 1 0 - oxy f luor fen (0.29 kg a.i ha* 1) / paraquat (0.15-0.22 kg a.i h a 1 ) . Source: Premat i lake, K G (1997) o f 12 months in the so i l . Mo re than 1100-8700 tubers and corms fm2 have been reported w o r l d w i d e (S i r iwardana and N i s h i m o t o , 1987). The pattern o f the presence o f weed seeds under d i f ferent treatments also ref lect in the amount o f weed dry matter p roduc t ion at d i f ferent phases o f the s tudy s h o w i n g a s imi la r t rend as shown in the Figure 2. Hence, there was a very good cor re la t ion between weed dry mat ter y i e l d and the seed dens i ty in so i l (F igure 3 ) . Th i s too witnesses that the h igh seed densi ty is as a consequence o f heavy weed g r o w t h and in situ seed p roduc t ion par t i cu la r l y in manua l ly weeded plots. Weed infestat ion had been increased w i t h t ime interval and type o f weed ing in manual ly weeded t reatment , where weed seed p roduc t ion had also been increased. 24 Impact of mulching There was no significant difference in seed number between unmulched plots slash weeded and mulched plots hand weeded every 6 weeks and; also between unmulched and mulched plots treated with Oxyfluorfen + Paraquat. Seed density in Oxyfluorfen + Paraquat in mulched plots would have been further reduced if the 2n d and 3rd round of Oxyfluorfen + Paraquat combination was applied to bare soil before mulching. The little swelling in seed density in unmulched plots may be ascribed to direct deposition of seeds on bare tea inter rows by immigrating from elsewhere. CONCLUSIONS Manual weeding at intervals of <12 weeks could appreciably maintain a lower weed seed density as it could prevent or minimize replenishment ofthe seedbank in soil by keeping weeds under control before they reach the reproductive phase. In contrast, delayed weeding has increased the seedbank due to in situ seed rain from many common and few uncommon weed species. Maintaining a live weed cover on tea inter-rows with regular slashing was ineffective in terms of the seedbank and it is also not practical. Chemical weeding has resulted in the least weed seed density as weed growth was arrested at early phase of growth. On this context, Oxyfluorfen + Paraquat combination or Glyphosate herbicide is found to be more promising. Weed species composition has been narrowed down in chemical treatments compared to manual weeding. Whereas, mulching of tea inter-rows has very little or no impact on the presence of the weed seed bank. 25 The seeds o f 'Soft herbs 1 have represented in the seed bank at higher numbers in the upper layer and found to be persistent in soil prior to and after imposition o f all treatments. Their presence should be promoted. Care should also be given to control C. rotundus, which is found to be persistent under all treatments. The soil weed seedbank in young tea fields could thus be kept under check wi th an integrated approach, where the weeds are removed manually at an interval o f <12 weeks and a suitable herbicide combination such as Oxyfluorfen and Paraquat is rationally used fol lowing all precautionary measures at correct time. However, the time of manual weeding has also to be adjusted keeping in mind that ground should be free o f weeds before manuring o f tea. Experiment should be repeated in other tea growing regions too as types o f weeds in these regions are different. A C K N O W L E D G E M E N T S Financial supports provided by the Agricultural Research Project (ARP), Sri Lanka and all other support o f the Tea Research Institute o f Sri Lanka are highly acknowledged. REFERENCES Anon 2003 Integrated weed management in tea. TRI Advisory Circular, No. W M 1 Serial No. 9/03, July, Tea Research Institute of Sri Lanka, Talawakelle, Sri Lanka. Ball D A and Mil ler S D1989 The comparison o f techniques for estimation o f arable soil seed banks and their relationship to weed flora. Weed Res. 29,365-373. Boli and Watkinson A R 1993 Pattern o f abundance o f weed seed bank In Brighton Crop Prot. Conf -Weeds, pp 293-298, Brighton Conference on Weeds, 1993, Brighton, UK. 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