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Production Paper

Forage Legume Regeneration from the Soil Seed Bank in Western North Dakota

North Dakota State Univ., Dickinson Res. Ext. Cent., 1133 State Ave., Dickinson, ND 58601

^* Corresponding author (pcarr{at}ndsuext.nodak.edu)

Received for publication April 7, 2004.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Rotating wheat (Triticum spp.) with fallow cannot be sustained in the Great Plains. Replacing fallow with legume pasture enhanced wheat production in Australian ley farming. The legume species used in ley farming regenerated from seed produced during previous pasture periods. Our objective was to identify legumes that regenerated from the seed bank in western North Dakota. Ten legume species were established in one experiment, 30 species in a second experiment, and 29 species in a third experiment. Seedlings were counted in the spring following the year of establishment. Over 200 seedlings m^–2 germinated in balansa clover (Trifolium michelianum Savi), berseem clover (T. alexandrinum L.), birdsfoot trefoil (Lotus corniculatus L.), black medic (Medicago lupulina L.), burr medic (M. polymorpha L.), crimson clover (T. incarnatum L.), Persian clover (T. resupinatum L.), and red clover (T. pratense L.) plots in at least one experiment. Forage dry matter yield ranged from 2 to 5 Mg ha^–1 for birdsfoot trefoil and red clover depending on the experiment and was similar to forage yield by alfalfa (M. sativa L.) that persisted in the second year following establishment (P > 0.05). Crude protein, acid detergent fiber, and neutral detergent fiber concentrations suggested that forage quality was equal or superior for birdsfoot trefoil compared with alfalfa and red clover. Birdsfoot trefoil has potential as a self-seeding pasture species in the Great Plains.

Abbreviations: ADF, acid detergent fiber • CP, crude protein • DM, dry matter • NDF, neutral detergent fiber • PLS, pure live seeds

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
CROP–FALLOW was the dominant production system for wheat in the Great Plains of North America during much of the 20th century. Adverse environmental impacts (Lafond et al., 1993; Monreal et al., 1997) and economic efficiency problems (Aase and Schaefer, 1996) associated with crop–fallow partially explain why wheat can no longer be grown profitably using this system without government price supports. For example, the projected economic returns from crop–fallow were –$65 ha^–1 for hard red spring wheat (Triticum aestivum L. emend. Thell.) during the 2003 growing season in western North Dakota when loan deficiency payments were not considered (A. Swenson, personal communication, 2002). Economic reality suggests that new methods are needed for the continued production of wheat in the Great Plains.

The adoption of ley farming, where wheat and legume pasture are rotated, was a response to the environmental and economic problems created by crop–fallow in Australia. Improvements in soil structure, reduced soil erosion, and enhanced nutrient cycling occurred as crop–fallow was replaced with ley farming (Howieson et al., 2000). Crop yields increased by an average of 48%, and grain protein concentration by 20 g kg^–1, following the widespread adoption of ley farming in southern and western Australia (Puckridge and French, 1983).

The potential of ley farming to improve the economics of dryland wheat production in the northern U.S. Great Plains was recognized in the late 1960s (Sims, 1994). Grain yields were superior when wheat followed 7 of 16 legume forage treatments compared with fallow in a study located near Bozeman, MT (Koala, 1982). Protein concentration of wheat also was elevated when grown following some of the legume species. This research was discontinued during the 1990s following the retirement of the project leader (P.R. Miller, personal communication, 2003).

Annual medic (Medicago) species and subterranean clover (Trifolium subterraneum L.) typically are used to provide pasture in Australian ley farming. An important characteristic of these legume species is the ability to regenerate naturally following wheat from seed produced during previous pasture phases. Carter (1987) suggested that medic seed reserves in excess of 200 kg ha^–1 were needed for regeneration of productive pasture following the wheat phase in Australia. This amount of seed is needed to produce a minimum population of 200 medic seedlings m^–2 considered to be necessary to maintain a productive legume pasture (Walsh et al., 2001).

Over 200 seedlings m^–2 were produced by three subterranean clover and four annual medic cultivars following wheat from the soil seed bank in a small-plot study in Montana (Koala, 1982). In particular, black medic was a prolific regenerator with over 1100 seedlings m^–2. However, black medic stands reportedly were poor during initial establishment, and forage production was only 0.8 Mg ha^–1 during the year when legume treatments first were established. Seed production by black medic and 16 other medic cultivars and lines was estimated to meet or exceed the 200 kg ha^–1 seed reserve minimum for regenerating legume pasture naturally over a 3-yr period in southeastern Wyoming (Walsh et al., 2001). Forage dry matter (DM) production averaged greater than 7 Mg ha^–1 over two growing seasons for the rigid medic (Medicago rigidula L.) experimental line SA-10343 in Wyoming compared with only 2.5 Mg ha^–1 for black medic. The researchers concluded that rigid medic may have the greatest potential for legume pasture if ley farming is adopted in southeastern Wyoming.

Attempts to adopt ley farming in the Great Plains and other regions have focused on screening medic and subterranean clover cultivars for adaptation to growing conditions in the region. Failure of cultivars to be adapted to local environmental conditions and seed shortages of the best-suited cultivars partially explain why attempts to adopt ley farming in West Asia and North Africa generally were unsuccessful (Springborg, 1986; Halse, 1993). Using native or naturalized legume forages already grown in a region is an alternative approach to adopt ley farming in the Great Plains.

Forage legume species are grown in the Great Plains. Alfalfa (Medicago sativa L. subsp. sativa var. sativa) and biennial sweetclover (Melilotus spp.) are distributed across the region. Alfalfa stands are maintained by the long-term survival of established plants while persistence of sweetclover stands depends on natural reseeding if extended beyond 2 yr (McGraw and Nelson, 2003). Other species grown to a limited extent in the Great Plains or in adjacent regions may be suited to ley farming. For example, birdsfoot trefoil is grown for forage and seed in Minnesota (Sheaffer et al., 2003). Although a perennial, individual plants rarely survive beyond a few years because of winter injury and disease. However, stand persistence occurs through the establishment of new plants produced from seed. Red clover is another perennial forage legume that can perpetuate stands through natural regeneration from the soil seed bank (McGraw and Nelson, 2003).

The purpose of this study was to identify legume species that could regenerate naturally from seed produced the previous year or from residual hardened seed that failed to germinate when first sown. A secondary objective was to determine relative amounts and quality of forage DM produced by the legume species included in the study.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Over 25 legume species used in this study were identified based on previous research on ley farming in Montana (Koala, 1982) and Wyoming (Walsh et al., 2001), results of unpublished research on forage legumes in North Dakota, and knowledgeable experts of forage legumes or ley farming in Minnesota (C.C. Sheaffer, personal communication, 1999), North Dakota (D.W. Meyer, personal communication, 1999), Manitoba (M.H. Entz, personal communication, 1999), and Wyoming (J.M. Krall, personal communication, 1999) with potential for pasture in the northern Great Plains (Table 1). Attempts were made to acquire seed for legume species that were identified from commercial seed suppliers when possible. Seed for a few legume species was not available commercially and was provided by crop scientists and others working with forage legumes.

Small grains were grown before establishing the three field experiments. The seedbed was lightly disked and then leveled in the spring before seeding plots in both 1999 and 2000; while in contrast, the only soil disturbance in 2001 occurred during seeding with fluted-disk openers on a low-disturbance seeder. No tillage occurred after seeding in plots in any experiment. Seeding rates for the legume species ranged from approximately 40 to 1840 pure live seeds (PLS) m^–2, depending on the legume species (Tables 2 and 3). The percentage of PLS (excluding hard seed) comprising a seed lot used in the field experiments was provided by the suppliers for all treatments except Austrian winter pea [Pisum sativum L. subsp. sativum var. arvense (L.) Poir], barrel medic (Medicago truncatula Gaertn.), berseem clover, birdsfoot trefoil, black lentil (Lens culinaris Medik.), black medic, sainfoin (Onobrychis viciifolia Scop.), and snail medic [M. scutellata (L.) Mill.] legume treatments. We used a germination chamber to determine the percentage of PLS for a subsample of nonscarified seed for these eight species. Seeding rates were provided by the suppliers for all treatments except alfalfa, black lentil, and sweetclover, and local rates (provided to the authors in kg PLS ha^–1) for these three treatments were used. Herbicides were applied, and some hand weeding occurred to minimize weed growth in plots.

The number of legume seedlings with the cotyledons open (BBCH 09; Lancashire et al., 1991) in a 0.5-m² area was recorded at two locations in each plot approximately 3 wk after seeding the experiment in 1999. Legume seedlings were counted approximately 15 and 30 d later, but only those with the cotyledons open having less than two trifolialate leaves (BBCH 10–11) to avoid recounting seedlings. The total number of legume seedlings was determined by adding together the number of seedlings on all three dates. This procedure was repeated after plots were seeded in the experiments in 2000 and 2001, except that legume seedlings were counted within two 0.25-m² areas in each plot.

Legume seedling stands were determined in the year after plots were seeded by first counting seedlings following spring warm up (mid- to late April) and then approximately 15 and 30 d later, following the procedure used when treatments were established. Total seedling counts were determined by adding together the number of seedlings on all three dates. Seedling numbers were recorded in all plots in 2001 in the field seeded in 2000 and in 2002 in the field seeded in 2001. Conversely, seedling numbers were recorded only in plots where plants that became established in the first year generally failed to persist in the second year in the experiment seeded in 1999.

Forage DM production was determined by harvesting legume plants at the early flowering stage (BBCH 60–61) within a 0.5-m² area in each plot and drying at 50°C for approximately 72 h until a constant weight was attained. The harvest period ranged from early June to late July, depending on the legume species and year. Forage was harvested from plots only once each year because DM production sometimes was limited and seed production was needed. A separate forage sample was collected by harvesting legume plants in a 3.7-m² area to a 6-cm height with a forage plot harvester at the same time (Swift Machine & Welding Ltd.,¹ Swift Current, SK). A subsample of approximately 400 g was selected randomly from the harvested material, dried, and sent to a commercial laboratory (Chemical Services Laboratory,¹ Jeffersonville, IN) where crude protein (CP) and acid detergent fiber (ADF) were determined by standard procedures (AOAC, 1990). A recommended method also was used to determine forage neutral detergent fiber (NDF) concentration (NFTA, 2002).

The three experiments were analyzed separately because legume treatment numbers and management differed across years and experiments. The ANOVA procedure from SAS (SAS Inst., 1985) was used in the analyses with legume treatments considered fixed and replicates considered random effects. The Ryan–Einot–Gariel–Welsch multiple range test was used to reduce the likelihood of making a Type II error when detecting differences between treatment means at the P < 0.05 level of significance.

Daily weather data recorded at a National Oceanographic and Atmospheric Administration weather service station within 1 km of the field experiments indicated that overwinter precipitation (September through March) was 170% of the 30-yr average of 136 mm before establishing the experiment in 1999. Conversely, overwinter precipitation was only 77% of the 30-yr average before establishing the experiment in 2000. Overwinter precipitation was 148 mm and near the 30-yr average before establishing the field experiment in 2001. Growing season precipitation was 101% of the 30-yr average of 293 mm in 1999, 87% in 2000, 130% in 2001, and 122% in 2002. Overall, greater-than-average amounts of precipitation occurred in 3 of 4 yr, with relatively dry conditions existing before establishing the field experiment and persisting during the growing season in 2000. Average air temperature during the growing season was within 1°C of the 30-yr average of 15°C in each year.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Legume Seedling Regeneration
Greater than 200 seedlings m^–2 is considered necessary to maintain productive legume pasture in ley farming, and more than this amount occurred in plots where balansa clover, berseem clover, birdsfoot trefoil, black medic, burr medic, crimson clover, Persian clover, and red clover were seeded the previous year in one or more experiments included in the study (Table 2). Black medic produced equal or greater amounts of seedlings compared with the other seven legume species in each experiment. Over 500 black medic seedlings m^–2 regenerated from the soil seed bank in the second year of each experiment. The germination percentage of seed sown in the first year suggests that most of the seedlings regenerated from new seed contributions and not from residual hard seed sown in the first year. The ability of black medic stands to regenerate from the soil seed bank is important since few plants that were observed in the first year overwintered in any experiment.

Many birdsfoot trefoil plants produced seed in the experiment seeded in 1999 (data not presented). A majority of the plants that became established in 1999 overwintered, so seedlings that regenerated from the soil seed bank were not counted in 2000. Conversely, seedlings that regenerated from the soil seed bank in the second year were counted in the experiments seeded in 2000 and 2001 even though many plants that were established in the first year overwintered. More than 230 seedlings m^–2 occurred in both experiments (Table 2). Likewise, over 300 red clover seedlings m^–2 occurred in addition to established plants that persisted in the second year in both experiments where this species was sown. Most of the red clover and birdsfoot trefoil seedlings in the second year regenerated from new seed contributions since consideration of germination percentages of seed sown in the first year suggest that residual hardened seed sown the first year cannot account for most of the seedlings in the second year.

Balansa, crimson, and Persian clovers produced more than 200 seedlings m^–2 in plots seeded the previous year in both experiments where these legume treatments occurred (Table 2). Conversely, less than 200 seedlings m^–2 occurred in plots where berseem clover and burr medic were seeded in one experiment. The ability of burr medic and berseem clover seedlings to regenerate from the soil seed bank is important since both species are annuals.

Plants that became established during the seeding year overwintered successfully for seven perennial and both biennial legume species included in this study (data not presented). However, fewer than 200 seedlings m^–2 regenerated from the soil seed bank for these species in any experiment (Table 3). Seedlings of 14 annual legume species also failed to reseed naturally from the soil seed bank. Thus, there is limited potential for seedlings of over 20 legume species to regenerate from the soil seed bank under conditions similar to those that occurred during this study.

Forage Yield and Quality
Differences in forage DM yield were not detected between plots where alfalfa and birdsfoot trefoil were grown either during the seeding year or the year following seeding in any experiment (Table 4). Both species produced at least 2 Mg DM ha^–1 in the second year. Likewise, differences in forage DM production were not detected between alfalfa and red clover in the two experiments where both species were grown. Red clover produced almost 5 Mg DM ha^–1 in the year after seeding in the experiment seeded in 2001 but only 1.9 Mg DM ha^–1 in the experiment seeded in 2000. Red clover was not included among the legume treatments in the experiment seeded in 1999.

Differences in CP concentration were not detected between forage produced by alfalfa, birdsfoot trefoil, and red clover in the year after treatments were established in experiments where these species were sown (Table 5). The CP concentration of forage produced by other reseeding legume species generally was not determined because forage DM production was so limited that subsamples for forage quality analyses could not be collected. Forage CP concentration was less for crimson clover than birdsfoot trefoil in the single experiment where forage DM production by crimson clover was adequate, so this quality parameter could be determined. Differences were not detected between the CP concentration of forage produced by balansa clover, black medic, and birdsfoot trefoil in this experiment.

The NDF concentration of forage DM was lower for birdsfoot trefoil compared with alfalfa in the year after seeding in two of the three experiments (Table 5). Likewise, the NDF concentration of forage was lower for red clover compared with alfalfa in one experiment. Differences were not detected in the NDF concentration of forage produced by birdsfoot trefoil and red clover.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Black medic (Koala, 1982) and rigid medic (Walsh et al., 2001) have been identified as potential pasture species capable of reseeding naturally in the Great Plains. Both species were included in this investigation as were other annual legumes used for pasture in Australian ley farming, including barrel medic, snail medic, subterranean clover, and burr medic (Gramshaw et al., 1989). None of these legumes had potential as regenerating pasture species. The six species generally produced less than 1 Mg DM ha^–1 in the year after seeding, and only burr medic and black medic demonstrated an ability to regenerate from the soil seed bank.

The performance of legume species in western North Dakota must be interpreted within the constraints that existed during the study. Legume treatments were seeded in early spring since this is the typical practice when seeding small-seeded forage crops in the northern Great Plains. Legume treatments also were seeded in the spring or early summer in previous research on ley farming in the region (Koala, 1982; Walsh et al., 2001). However, some legume species may be better suited for self-regenerating pasture when seeded in late summer. For example, results of unpublished research in Wyoming suggest that seeding rigid medic in late summer instead of early spring is essential for proper natural conditioning of seed so that regeneration of seedlings occurs the following year (J.M. Krall, personal communication, 2003). Seeding legume forage crops in late summer may result in poor stand establishment compared with early spring seeding because of dry soil conditions that generally develop in late summer in much of the Great Plains. Research is needed to identify methods that result in consistent establishment of pasture when small-seeded legume species are sown in late summer.

Seed lots for some legume species used in this study may not have represented populations that are adapted to the environmental conditions that were encountered. For example, the seed lot of subterranean clover that was used resulted in plants that grew slowly and did not produce seed in 1999. The poor performance resulted in the dropping of this legume treatment in field experiments seeded in 2000 and 2001. Other populations may have been better adapted, but the seed lot that was used represented the only population that could be obtained in the amount needed for the study. The difficulty experienced in obtaining subterranean clover seed for this study reflects the common problem of obtaining seed of many legume species used in Australia for ley farming but not adapted elsewhere, particularly in regions with cold winters (Halse, 1993). Our research supports the alternative strategy of evaluating germplasm of regionally adapted legume species and cultivars for potential use rather than screening Australian germplasm for adaptation to growing conditions in the Great Plains.

Birdsfoot trefoil and red clover were not considered in previous research on ley farming in the region. However, our research suggests that both species have potential as pasture crops capable of regenerating from the soil seed bank in the Great Plains. Birdsfoot trefoil and red clover seedlings regenerated from the soil seed bank and produced a minimum of around 2 Mg DM ha^–1 when harvested only once in the year after seeding. One to two additional cuttings could have occurred but did not since our strategy was to maximize soil seed bank numbers, and we believed that additional cuttings could jeopardize seed production.

Birdsfoot trefoil and red clover produce high quality forage. Results of grazing studies indicate that average daily gains were equal or superior for lambs grazing birdsfoot trefoil compared with alfalfa (Marten et al., 1990). Average daily gains were similar when grazing red clover and alfalfa in the same study. However, birdsfoot trefoil pasture is nonbloating when grazed by ruminants while pure stands of red clover can cause bloat.

Forage was harvested at the early flowering growth stage in this study. The CP concentration was similar between forage produced by birdsfoot trefoil and red clover at this growth stage, but the ADF concentration was lower for forage produced by birdsfoot trefoil. Lower ADF concentration may suggest a possible enhancement in forage digestibility although the presence of condensed tannins in forage produced by birdsfoot trefoil has been implicated in reductions in DM digestibility among ruminants (Kumar and Singh, 1984). Miller and Ehlke (1996) reported that the condensed tannins in birdsfoot trefoil forage probably explain why bloat does not occur when pastures are grazed. These quality factors, along with the abilities of regenerating from the soil seed bank and producing at least 2 Mg DM ha^–1, suggest that birdsfoot trefoil offers the greatest near-term potential as a pasture species for use in ley farming within the region.

Previous research indicated that birdsfoot trefoil is not adapted to dry rainfed environments in western North Dakota (Kapusta and French, 1964). However, results of our study suggest that winter-hardy cultivars like Norcen can be grown successfully. Birdsfoot trefoil is not as tolerant to drought as are legumes commonly grown in the region like alfalfa, but birdsfoot trefoil still can survive extended dry periods, and several cultivars should be adapted (D.J. Undersander, personal communication, 2002). Adaptation studies are needed to identify which cultivars are adapted to growing conditions in the Great Plains, particularly when used as regenerating pasture in short rotations with wheat and other grain or seed crops.

Birdsfoot trefoil and seven other species regenerated from the soil seed bank in this study. However, considerable variation in seedling numbers occurred across plots for the eight legume treatments in each experiment (Table 2). Relatively large standard deviations were associated with mean seedling numbers, even during the seeding year. The variability in plant stands across plots may help explain the large standard deviations associated with average forage DM yields for legume treatments in the second year (Table 4). These data suggest that improvements in establishment methods are needed for birdsfoot trefoil and other small-seeded legume species in dry rainfed environments.

Reducing or eliminating tillage during seedbed preparation may offer benefits during establishment of small-seeded forage legume species since soil water is conserved and soil compaction increases as tillage is decreased (Gauer et al., 1982). Superior establishment of alfalfa occurred in untilled compared with tilled seedbeds under dry conditions in Manitoba (Allen and Entz, 1994). More plants generally were counted when legume treatments were seeded in an untilled seedbed in 2001 compared with a tilled seedbed in 2000 in our study (Tables 2 and 3). Still, considerable variation in seedling numbers existed in both tilled and untilled seedbeds, particularly in the second year of the field experiments. Methods must be developed that result in consistent establishment of legume plant stand during the initial pasture phase since these plants provide the first major contribution to the soil seed bank.

Strategies developed for improving establishment of legume seedlings from the soil seed bank were developed in Australia and may have application in the Great Plains. Shallow tillage following the wheat crop was recommended for replenishing the soil seed bank by burying medic pods when ley farming (Carter, 1983). However, many producers in the Great Plains may be hesitant to till soils even minimally because of concerns about soil water loss and also because of the growing interest in enhancing soil health by eliminating tillage. Alternatives to shallow tillage include the use of specially designed rollers and harrows that enhance seed-soil contact with minimal soil disturbance. A rotary harrow (Phoenix Rotary Equipment Ltd.,¹ Nisku, Alberta) has been used for this purpose in Wyoming with some success (J. Krall, personal communication, 2003).

The ability of birdsfoot trefoil and other legume species to regenerate from the soil seed bank following wheat in a wheat–pasture rotation was beyond the scope of this investigation, but this ability is essential for the success of ley farming. Research is needed that demonstrates legume forage species like birdsfoot trefoil can regenerate from the soil seed bank after wheat. The impact of the legume pasture phase on vegetative and reproductive growth in subsequent years must be determined in the Great Plains, particularly in dry rainfed areas where excessive water use by the pasture species could adversely affect subsequent wheat crop performance.

Methods must be developed to minimize adverse impacts on soil water content and maximize positive effects on soil nutrient and pest control provided by the legume species.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Results of this study demonstrate that birdsfoot trefoil can regenerate from the soil seed bank and produce a forage DM yield comparable to a second-year alfalfa stand. The abilities to reseed naturally and produce adequate amounts of forage DM to support grazing suggest that birdsfoot trefoil may be suited as a spring-seeded, regenerating pasture crop in short rotations with wheat in western North Dakota. Wheat production profitability increased when wheat–pasture replaced wheat–fallow systems in Australia, and similar economic advantages may result if wheat–pasture rotations are adopted in the northern U.S. Great Plains. In addition, soil conservation and quality should improve by replacing fallow with legume pasture in the region.

Wheat production systems are diversifying in the Great Plains, but few rotations incorporate forage crops, and even fewer incorporate a grazed pasture period. However, benefits in nutrient cycling and pest management can result by incorporating pasture into rotations with grain and seed crops that cannot be duplicated if grazing by livestock does not occur. This investigation identified legume species with potential as regenerating pasture crops, and these results support the development of integrated crop–livestock systems.

	ACKNOWLEDGMENTS

 
Appreciation is extended to Glenn Martin and Burt Melchior, agricultural specialists at the Dickinson Research Extension Center, for assistance in establishing the field experiments. The work reported in this manuscript was supported by the Cooperative State Research, Education and Extension Service, USDA, under Agreement no. 2001-34216-10563. All opinions, findings, conclusions, or recommendations expressed in this manuscript are those of the authors and do not necessarily reflect the view of the USDA.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
This paper is a contribution of the North Dakota State Univ. Agric. Exp. Stn.

¹ Mention of a proprietary product name is for identification purposes only and does not imply endorsement or warranty to the exclusion of other products.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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