Document Type : Original Article
Authors
1 Rice Breeding Platform, International Rice Research Institute, Manilla-1301, Philippines.
2 Department of Agronomy, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh.
3 Department of Agriculture and Food, 75 York Road, Northam 6401, Australia.
4 Agricultural Sciences, Murdoch University, South St, WA 6150, Australia.
Abstract
The composition of weed species and the distribution of weed seeds in the soil profile vary significantly and closely correlated to the previous cropping system. Information on the effect of tillage types, crop residue mulching and crop rotation on the soil weed seedbank is a useful tool for sustainable weed management in conservation agriculture (CA). With the view to studying the trend of weed seedbank in CA, a net-house experiment was conducted at the Department of Agronomy, Bangladesh Agricultural University during January - December, 2016. Soil samplings were done at 0 - 15 cm depth from four different sites after the end of CA trials at Mymensingh and Baliaknadi during 2012 - 2015 and Durgapur and Godagari in 2010-2015, in Bangladesh. At Mymensingh, conventional tillage (CT) and strip tillage (ST), while at Durgapur and Godagari, additional bed planting (BP) were practiced. On the other hand, at Baliakandi additional Zero tillage (ZT) was included. At all sites 20% and 40 - 50% of standing mulches of previous crops were applied. A total of 290 samples replicated four times were placed in individual trays following a completely randomized design. The year-round headcount of emerged weed revealed that the smallest size of weed seedbank in terms of weed species composition and weed density was found in ST followed by CT, BP, and ZT with 40 - 50% crop mulch than 20% mulch. The ST, BP, and ZT produced a higher number of perennials weeds than annual weeds, but opposite in CT. Based on the results, it could be concluded that the continuous practice of ST with 40 - 50% residue mulch declined the size of weed seedbank with the proliferation of perennial weeds. Weed seedbank size in ST is even smaller than BP and ZT.
Keywords
Introduction
Conservation agriculture (CA) is the practice of any tillage sequence that minimizes soil disturbances with at least 30% soil cover by crop residue mulches utilizing the intensive crop rotation involving at least three different crops as defined by the United Nations’ Food and Agriculture Organization (Kassam et al. 2019). CA is being promoted globally for improving soil health; sustainably increase the overall economic productivity of mechanized agriculture (Sanyal et al. 2008). In addition to agronomic benefits, CA also has a major influence on the relative abundance of weed species (Fonteyne et al. 2020), and weed control is perceived as one of the most challenging issues due to a reduction in tillage operations. The composition and dynamics of weeds in the soil weed seedbank under minimum tillage was found to be changed compared to conventional tillage (Pittelkow et al.2015). Literatures reported that minimum tillage favors the infestation of perennial weeds like Cyperus rotundus L., Saccharum spontaneum L., Sorghum halepense L. than annual weeds, which are generallyreproduced from tubers and rhizomes and have not buried them or by failed to uproot and killed them as by conventional tillage. According to Woźniak (2018), populations of annual grass increase in no-tillage systems concurrent with a decrease in populations of dicotyledonous weeds. Again, Fonteyne et al. (2020) observed annual and perennial grasses, and perennial dicot species would increase, and annual dicot species would decrease in reduced tillage. Moreover, Moonen and Barberi (2004) recorded fivefold higher seedbank density in reduced tillage systems compared to full tillage systems. Barberi and Lo Cascio (2001) discovered Amaranthus spp. seedling density was much higher in no-till environments than tilled environments.
Despite the widespread promotion, in Bangladesh the practice of CA began in 2005 (Hossain et al. 2015) to validate its’ principles for small farm hold. But adequate information on weed species composition in the soil seedbank under CA is not available which is expected to be a problem after several years. Reduced tillage practice with the residue mulching of previous crops in different cropping patterns changes weed seed density in the soil by affecting the soil weed seedbank and the efficacy of weed control practices. Such knowledge on the soil weed seedbank might be a useful tool for sustainable weed management in CA. Hence, this study was undertaken to assess the trend the soil weed seedbank in 3-5 years long trials of CA in Bangladesh context. By necessity, it was hoped that understanding gained about weed seedbanks could be used to fill the gaps in weed seedbank information and the best managing of weeds to trigger the widespread adoption of CA.
Materials and Methods
Experimental site
The net-house experiment was conducted at the Department of Agronomy, Bangladesh Agricultural University (BAU), located geographically at 24.75° N and 90.50° E at an average altitude of 18 m above the mean sea level.
Edaphic and climatic environments
The experiment field was located at a flood-free medium-high land on the Old Brahmaputra Floodplain of predominantly dark grey non-calcareous alluvium soils underthe Sonatala series (Brammer, 1996). The pH of sandy clay loam (50% sand, 23% silt, and 27% clay) soil was 7.2. The research site was characterized by high temperature, high humidity, and heavy monsoon rainfall with occasional gusty wind during April-September and low precipitation with moderately low temperature during October - March (Figure 1). The maximum temperature varies from 32.3 - 33.5℃ during April - June while January was the coldest month. About 95% rainfall and relative humidity was received during April - September. The rest of rainfall was very unevenly distributed and mostly uncertain. Sunshine hours differed much in during the months of rainfall due the cloudy weather.
Figure 1. Monthly temperature and rainfall distribution pattern in 2016.
Sites of long-term CA trials
CA trials were conducted at four locations. At Mymensingh, trials were conducted at the Soil Science Field Laboratory of the BAU campus at Sadar sub-district of Mymensingh district. In this site, six experiments on CA trials under T. aman rice-wheat-mungbean cropping pattern was studied during 2012 - 2015. Durgapur and Godagari sub-districts situated under the Rajshahi district, geographically at 24°22′ N and 88°36′ E in 2010 - 2015. There were, 12 trials following T. aman rice-mustard-Boro rice, T. aman rice-mungbean-lentil, and lentil-jute-T. aman rice patterns at Durgapur while T. aman rice-wheat-mungbean, T. aman rice-wheat-jute, and T. aman rice-chickpea-jute cropping patterns. Baliakandi (at 23°39'45" N and 89°29'39" E) sub-district was located at the Rajbari district where, CA trials conducted during 2012 - 2015. At this site, six trials were performed under T. aman rice-wheat-jute system. The treatments imposed in these four locations have been presented in Table 1.
Table 1. Treatments details at four locations of long-term CA trial.
Weed control strategies of long-term CA trials
In CT, weeds were controlled by hand weeding in all crops. In ST, BP, and ZT weeds were controlled using different herbicides for different crops, as stated in Table 2. Similar herbicides were used at all locations.
Table 2. Weeding regimes of long-term CA trials at four locations.
Tillage practices followed in long-term CA trials
The CT was done by a two-wheel tractor (2WT) by four plowings and cross plowing followed by sun-drying for two days (in non-rice crops), finally by inundation and laddering (in rice). The ST was done by a versatile multi-crop planter (VMP) in a single pass operation. Strips were prepared for four rows, each of six cm wide and five cm deep made at a time. In BP, raised beds (15 cm high and 90 cm wide with 60 cm tops and 30 cm furrows) were made with a bed planting machine. In ZT, the land remained untilled. Initially, at Mymensingh, the effect of ST was tested at limited areas university field laboratory.
With the view to achieving more justified results, additionally, BP was included in the farmer’s field of Durgapur and Godagari. In contrast, ZT was included along with these three tillage types at Baliakandi. That is why tillage types were not uniform across the experimental sites.
Crop residue mulching in the long-term CA trials
Two levels of crop residue mulching (height basis) were used across the experimental sites. There were 20% mulch in all locations, while 40% mulch in Mymensingh and 50% mulch in other areas. In ST operation, tines of VMP were clogged by standing 50% mulch. To avoid this interruption, this was reduced to 40% at Mymensingh.
Soil sampling procedure
The soil was collected from the field of all locations from 0-15 cm soil depth. Five samples from each plot; hence 290 samples were collected using a stainless-steel pipe of five cm diameter following the “W” shape pattern described by Chancellor (1966). After sampling, pieces were tagged and appropriately bagged for transportation to the net-house.
Experimental set-up
Sub-samples from each plot were combined, and approximately one-kilogram soil was placed immediately in an individual round-shaped plastic tray of 33 cm in diameter. Total 290 trays were arranged following a completely randomized design. The study period was January 02-December 29, 2016.
Weed seed emergence and data collection in net-house
Emerged seedlings were identified, counted, and removed at 30 days intervals using the seedling keys of Chancellor (1966). Unnamed seedlings were transferred to another pot and grown until maturity to facilitate identification. After the removal of each batch of seedlings, soils were air-dried, thoroughly mixed, and re-wetted to permit further emergence. The number of seedlings emerged converted to the numbers m-2 using the formula, Area = ᴨr2 (r = radius of the tray= 33 cm).
Results and Discussion
Effect of tillage and mulches on weed species compositions
At Mymensingh, CT with 20% mulch produced 28 weed species consisting of 18 broadleaves, five of grass and five sedges (Table 3). Among them, 24 weeds were annuals, and four perennials. There were three less broadleaf and one less grass weed in 40% mulch than 20% mulch. In ST with 20% mulch, 25 species were found consisting of 15 broadleaf and five grass and five sedges. Among them, 18 were annuals, and seven were perennials. But in ST with 40% mulch, five less broadleaf, one grass, and one sedge were found than 20% mulch.
At Durgapur, CT with 20% mulch, produced 29 species consisting of 19 broadleaf, five grass, and sedges each, of which 25 were annuals and four perennials (Table 4). Retention of 50% mulch generated 21 annuals and four perennials. ST with 20% mulch produced 23 species, including 14 broadleaf, four grass, and five sedges. Annuals were outnumbered than perennials. In ST with 50% mulch, 18 species found having ten broadleaf, four grass, and sedges each, of which 13 annuals and five perennials. BP, with 20% mulch, made 25 species consisting of 15 broadleaf, four grass, and six sedges where 20 were annuals and five perennials. The BP with 50% mulch, had 23 species with 19 annuals and four perennials. Results reflect that the lowest number of weed species was found in ST, followed by BP and CT. Retention of 50% mulch produced lower weeds than 20% mulch. At Godagari, almost all types of weeds were found with similar trend of response to that of Durgapur (Table 5).
At Baliakandi, CT with 20% mulch produced 14 species (eight broadleaf, three grasses, and sedges each) consisting of 10 annuals and four perennials (Table 6). But at 50% mulch, 12 species found having eight broadleaf, two grass, and sedge each, including nine annuals and three perennials. The ST with 20% mulch produced ten species consisting of seven broadleaf, two grasses, and one sedge having four annuals and six perennials. But nine species were having an almost similar number of all types of weed except one with 50% mulch. In BP with 20% mulch, 17 species were found, including ten broadleaf, three grass, and four sedges. There were 16 annuals and one perennial. In 50% mulch, 15 species were found with a similar amount of grass and sedge and fewer annual broadleaf. The ZT with 20% mulch produced 19 weed species belonged to 11 broadleaf and four grass and four sedges, having 15 annuals and four perennials. But in ZT with 50% mulch, 16 species were found to have less number broadleaf grass and sedges.
Table 3. Composition of weed species in different tillage types and residue mulches at Mymensingh.
M20 = 20% mulch, M40 = 40% mulch, P = Present, A = Absent
Effect of tillage types and mulch levels on density (plants m-2) of different weed types
Data presented in Table 7 reflect that at Mymensingh, ST created 777 less number of weeds than CT, and 40% mulch produced 288 less number of weed m-2 than 20% mulch (Table 7). Broadleaf weeds were most dominant over sedges and grasses in CT but dominant over grasses and sedges in ST. Annuals led over perennials in CT, but perennials led over annuals in ST (Table 8).
Table 4. Composition of weed species in different tillage types and residue levels at Durgapur.
M20 = 20% mulch, M50 = 50% mulch, P = Present, A = Absent
At Durgapur and Godagari, BP generated the highest weed density followed by CT and ST (Table 7). Compared to CT (1738 at Durgapur and 2079 at Godagari), ST had 172 and 237 fewer weeds, but BP had 717 and 776 more number of weeds at two locations, respectively. Retention of 50% mulch produced 442 and 610 fewer numbers of weeds than 20% mulch at two locations, respectively. Broadleaf weeds were the most dominant in all types of tillage at both locations, while grasses dominated over sedges in ST at Durgapur and BP at Godagari but opposite in BP at Durgapur and ST at Godagari. Perennials led over annuals both in ST and BP but reverse in CT at both locations (Table 8).
Table 5. Composition of weed species in different tillage types and residue levels at Godagari.
M20 = 20% mulch, M50 = 50% mulch, P = Present, A = Absent.
At Baliakandi, the trend of weed density m-2 was ZT > BP > CT > ST. Compared to CT (1668), ST has 560 fewer weeds, but 386 and 2639 more weeds in BP and ZT, respectively. On the other hand, 50% of mulch produced 608 fewer weeds than 20% mulch (Table 7). In all types of tillage and mulch levels, broadleaf led over sedges and grasses. Annuals were dominant over perennials in CT, but perennials led over annuals in ST, BP, and ZT (Table 8).
In this study, the higher number of weeds composting broadleaf, grass, and sedge types was found in CT than ST. This phenomenon might be attributed to the emergence of more weed species in CT over ST. Mohler (2001) quoted dormant seeds in CT become viable to germinate by scarification, ambient CO2 concentrations, and higher nitrate concentrations, which may lead to producing higher weed emergence of new weed species in CT.
Table 6. Composition of weed species in different tillage types and residue mulches at Baliakandi.
CT= Conventional tillage, ST= Strip tillage, BP= Bed planting, ZT= Zero tillage, M20 = 20% mulch, M50 =50% mulch, P= Present, A= Absent.
Table 7. Effect of tillage types and mulch levels on the density (plants m-2) of different weed types at different locations.
CT = Conventional tillage, ST = Strip tillage, BP = Bed planting, ZT = Zero tillage, M20 = 20% mulch, M40 = 40% mulch, M50 = 50% mulch, X̄ = Mean.
Table 8. Effect of tillage types and mulch levels on the density (plants m-2) of annual and perennial weeds at different locations.
The research finding of Cardina et al. (2002) also revealed the increase of weed species composition in CT offered from the higher rate of seed viability occurred from weed seed burial in the soil profile. Such a higher rate of weed seed survivability might lead to an increase in weed composition in CT. Gallandt et al. (2004) found germination stimulus is generally higher near the soil surface and decreases with depth. In the reduced tillage system of ST, seedbanks are concentrated in the top layer of the soil; thus, a higher proportion of reduced tilled seedbanks will germinate compared with CT, which led to reduce seedbank size in ST than CT. The reduction of weed species in ST might also be due to minimizing the weed seedbank status in the soil by increasing non-viable or dormant weed seeds in the seedbank. Due to minimal soil disturbance (only 20%) at the upper soil layer in ST, most of the weed seeds remain on the soil surface. They can lose viability due to desiccation and adverse climate, as reported by Nichols et al. (2015). Losing of seed viability in ST may also be attributed to increased seed dormancy at an undisturbed deeper soil layer. Seeds remain dormant at a deeper layer suffer from suffocation for less oxygen pressure and darkness for feeble light, as weed seeds required oxygen and light for maximum germination (Oziegbe et al. 2010).
Surface accumulation of weed seeds in ST would increase predator like ants, insects, rodents, and birds (Blubaugh and Kaplan, 2015) access to weed seeds and could increase their removal rates. For example, common ground beetles or crickets can reduce weed seed emergence by 5 to 15% (White et al. 2007). Overall, the adoption of ST may encourage seed losses via predation by increasing the availability of seeds to predators, and by minimizing mortality and forced relocation of predators, therefore, represent a potentially valuable tool for reducing weed seedbank size in ST. Higher dispersal of weed seeds may also lead to an increase in the seedbank in CT over ST. Barroso et al. (2006) found the weed seeds traveled 2–3 m in the direction of full tillage, while in reduced tillage soils, the distance is negligible. Reducing tillage in ST, therefore, reduced the spread of weed seed both within and across fields and reduced seedbank size in this study. The reduced weed seedbank in ST may also have occurred from more lavish weed seed burial as strips were made in the same location over the years because the field layout and all the treatments were the same in the field study.
Furthermore, the application of different herbicides might lead to having less amount of weed in ST, BP, and ZT. We used glyphosate and pendimethalin herbicide in all crops. Besides, we used ethoxysulfuron-ethyl in rice; oxadiazon in mustard; Carfentrazon-ethyl + isoproturon in wheat while fenoxaprop-p-ethyl in jute, lentil, mungbean, and chickpea. These herbicides are previously reported to reduce seed viability or induced seed dormancy in weed, which might have led to reducing weed pressure in ST than CT. It was reported that a range of herbicides could reduce seed production and germination by several folds depending on the biotypes. Glyphosate is registered to affected pollen and seed production almost 100% in Ambrosia artemisiifolia L. (Gauvrit and Chauvel, 2010) and Bromus japonicus Thunb. (Rinella et al., 2010) while and 69.8% in Conyza bonariensis L. (Wu et al. 2007). Findings of previous studies reported that herbicides could reduce the germination of weeds seeds. Tanveer et al. (2009) found pendimethalin herbicide exerted only 30.57% seed germination of Chenopodium album L., while ethoxysulfuron-ethyl killed 98-100% seeds of Echinochloa glabrescens L. (Opeña et al. 2014). Moreover, oxadiazon waived 85.81% seeds of Fimbristylis cymosa R.Br. Furthermore, carfentrazon-ethyl + isoproturon damaged 100% seeds of Emex spinosa L. (Javaid et al. 2012) and fenoxaprop-p-ethyl wrecked 96.78% seeds of Phalaris minor L. (Singh et al. 2017).
The results of these studies agree the findings of the present study demonstrated that herbicides could potentially reduce seed production and viability of weeds, thereby reducing seedbank size in ST than CT, followed by BP and ZT. On the other hand, herbicide induced seed dormancy could contribute to the altered seed dormancy found in Hordeum murinum L., Bromus diandrus Roth., and Lolium rigidum Gaud., in intensive cropping systems that relied heavily on herbicidal weed control reported by Kleemann and Gurjeet (2013) and Owen et al. (2015). The above-discussed reasons might lead to a decline in the size of the weed seedbank in ST in a trend of weed species composition following ST < CT at Mymensingh, ST < CT < BP at Durgapur and Godagari, and ST < CT < BP < ZT at Baliakandi. Bàrberi and Cascio (2001) agree with the findings of the present study as stated the higher weed density at ZT, followed by reduced tillage because of taller weeds seedlings recruitment from the topsoil in ZT. In the present study, annual weeds led over perennials in CT, but perennial weeds led over annuals in ST, BP, and ZT. Boscutti et al. (2015) agree this finding in support with Erenstein and Laxmi (2008) concluded that altering the tillage regimes changes the disturbance frequency of the field, which results in shifts in weed vegetation of that field. Many studies support our survey with the reports; CT systems favor annuals, while reduced tillage systems favor perennial weeds (Tuesca et al. 2001). Ecological succession theory (Aweto, 2013) also agrees with our research finding suggesting the dominancy of perennials weeds in less disturbed systems. Because CT kills most of the underground vegetative reproduction structures (rhizomes, tubers, bulbs, runner, and stolons) of perennials weeds, hence, reserves only annuals weeds which reproduce mostly by seeds (matured ovules). On the other hand, the vice-versa phenomenon generally occurs in tillage was minimized in ST and BP while absent in ZT, which favored perennial weeds here in the soil weed seedbank. In this study, retention of 40 or 50% crop mulch had fewer above ground weed taxa than 20% mulch. This phenomenon might be due to the drastic effect of suppressing weed seed germination caused by a physical barrier, lowering soil temperatures and allelochemicals released from decaying plant tissues, as suggested by Curran (2016). Moreover, reduced light penetration stating cooler average soil temperatures could reduce weed seed germination or causing delay germination, damage of weed seeds upon predation and decomposition by macro and microbial populations (Conklin et al., 2002) and massive moisture conservation (Manici et al. 2004); delay the emergence of etiolated plants producing lower seeds as stated earlier (Begum et al. 2006) might have reduced weed seedbank size in 40/50% mulch over 20% mulch.
Conclusion
Based on the study, it might be concluded that long-term strip tillage of 3-5 years with 40-50% crop mulch under conservation agriculture practice reduced weed seedbanks in terms of weed species composition and density, and this reduction is much higher than bed planting and zero tillage. Strip tillage, bed planting and zero tillage also increased perennial weeds in the weed seedbank while conventional tillage increased annual weeds.
Acknowledgements
This study was a part of Ph.D. research, which was funded by the Australian Centre for International Agricultural Research (ACIAR) and of Murdoch University, Australia.
Conflicts of Interest
The authors have declared no conflicts of interest.
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