Essay: Utilization of weeds in traditional medicine as an enhanced weed management

Abstract:
Weedy species growing in Agricultural lands and disturbed areas are considered a menace both to crops and surrounding environment. Worldwide, 13% loss of agricultural productivity is attributed to weeds. The losses amount to 25% in least developed nations due to poor management practices.  In the US, an estimated $27b is spent each year to control weeds, mostly on chemical herbicides. Although it is efficient, associated ill-effects on surrounding environment, run-off water bodies, fish, animals and human beings are grossly ignored. Most of the chemicals are not biodegradable thus getting accumulated year by year in the ecosystem.  In order to minimize ecological damage, weed management other than chemical usage is recommended. In this review, the authors highlighted the importance of finding an effective utilization of weeds as an appropriate and effective weed management doing away with current practices of harmful chemical usage. We recommend utilization of weeds in traditional medicines that can create a suitable livelihood options for local communities/tribes as well as minimizing the use of chemical herbicides. A list of 13 edible weeds, 59 medicinal weeds, 14 climbers commonly found in the cultivated estates and 5 weeds cultivated for medicinal purpose are tabulated.  We discuss on the status of few agricultural weeds, due to their ethno medical importance, have been cultivated at commercial scale on par with primary crops thus generating huge income and meeting the needs of industry feedstock.  We conclude the article by recommending that the weed utilization is the best way to management.
Keywords: Herbal, traditional medicine, weeds, weedicides, utilization, sustainable development
Introduction
Weeds are defined as plants found growing in agricultural lands, waste lands, gardens, roadsides and mainly disturbed areas where they do not depend on human intervention for their reproduction and survival1. They are considered as plant pests2 due to their tendencies to reduce biodiversity and promote habitat loss for less competitive species within the same ecosystem3. There are approximately 250,000 species of plants worldwide, of these, approximately 3%, or 8,000 spp. behave as weeds. Weed problems are likely to increase due to increased emphasis on high input agriculture. Globalization would further result in new weed species in spite of strict quarantine regulations and practices. Weeds grow in different weather conditions and can withstand unfavorable conditions for long time than domesticated or cultivated crops.
Materials and Methods
We made several field surveys to different states of our country such as Tamilnadu, Andhrapradesh, Karnataka, Madhya Pradesh, Rajasthan, Uthrakand, Sathishgadh, Jharkand, Odyssa and West Bengal. We carried out contract cultivation and arranged for collection of weeds by providing training to the collectors. Based on our observation we compiled list of weeds, their abundance and distribution pattern, etc. Herbarium specimens of the weeds are collected and deposited in the R&D Centre, Natural Remedies Private Limited, Bangalore 560100. The identity of the specimens confirmed using regional floras.
Results and discussion
It is to be noted that weeds cause more damage to agriculture crops than pathogens or insect pests. In agriculture weeds compete against regular cash crops. Some of the acknowledged issues with weeds are tabulated below4-5.
i) block major resources, compete for space, water, fertilizer, and land
ii) overgrow than regular crops and block sunlight
iii) lower quality of agricultural produce
iv) lower quality of pasture
v) increase in cost of production
vi) block irrigation
vii) harbor pests and serve as hosts for crop diseases
viii) reducing crop quality by contaminating the commodity
ix) interfering with harvest
x) limiting the choice of crop rotation
xi) producing chemical substances that can be allergens or toxins to humans, animals, or crop plants (allelopathy)
xii) producing thorns and woody stems that cause irritations and abrasions to skin, mouths, or hooves of livestock
xiii) being unsightly, dominant, aggressive, or unattractive although some weeds are used as ornamentals in gardens
xiv) obstructing visibility along roadways,
xv) interfering with delivery of public utilities (power lines, telephone wires),
xvi) obstructing the flow of water in water ways, and creating fire hazards
xvii) accelerating deterioration of recreational areas, parking lots, buildings, and equipment
xviii) invading exotic weed species that can displace native species in stabilized natural areas
Weeds deplete large quantities of mineral nutrients and other resources more efficiently than crop plants and thrive better during unfavorable conditions compared to crops. Sometimes they absorb more nutrients thus limiting the availability of the same to crop plants. Besides, the weeds inflict allelopathic effects on crop plants through root exudates. Weeds are expected to cause maximum reduction in the yields of crops than other pests and diseases. Of the total annual loss of agricultural produce, weeds account for 37%, insects for 29%, diseases for 22% and other pests for 12%6. They promote habitat loss for less competitive species within the same ecosystem3. Most successful of these plant species are those well adopted to their immediate natural environment with high fecundity7. Thus weed control is indispensable in every crop production system.
A plant is considered a weed, if it has one or more of following characteristics5:
i) abundant seed production
ii) rapid population establishment
iii) seed dormancy
iv) long-term survival of buried seed
v) adaptation for spread and quick adaption
vi) presence of vegetative reproductive structures
vii) ability to occupy sites disturbed by humans
viii) survival during stress conditions
ix) ability to survive, without human care
Illustrative list of weed species
An illustrative list of weeds (Naidu, 2012)8 that are used in traditional systems of medicine has been provided in Table 1. While the Naidu8 was consulted during preparation of the illustrative list, their habitat categorization was allotted by authors own survey of agricultural fields in different geographical locations over several years. The list includes some important weeds found both in the dry land as well as wet land in agriculture farms (Figure 1 and 2); however, fallow lands, road side lands, etc., do harbor variety of weeds. Their habitat ranges from alpine zone to arid zones.
Implications of weed control and their management
With increased awareness of environmental pollution focus is now shifting to adapt ecofriendly weed management practices. The composition and competition of weeds are dynamic which mostly depends on the type of soil, climate, cropping system and management factors. For example, competition for Andrographis paniculata, varies from agricultural land to fallow land to roadside habitat. To control weeds, in many rural agricultural fields weeding is carried out manually employing manual labor. An alternative to hand weeding is mechanical weeding and chemical weeding. In paddy fields, weeding is carried out just before application of fertilizers to avoid competition and prevent robust growth of weeds. The major constraint in hand weeding is labor shortage and cost. Thus, poor farmers employ their own family members to hand weed. However, for larger farms this is practically impossible. In some areas, deep ploughing and crop rotation is followed. Mucuna is used as a cover crop in suppressing weeds, especially spear grass9. However, this practice has its own disadvantage. Some of techniques further include frequent checking of potential weeds when they are establishing in the soil and removal before they are spread. Composting, ensiling, or feeding weeds to live-stock can destroy weeds to certain extent. Some mechanical techniques such as hand-pulling, hoeing, mowing, plowing and digging are also practices. Chemical weed control is considered one of the best methods to control weeds, but is associated with cost and environmental pollution. Uneducated or ill-informed farmers do not consider side effects on humans, animal and environment that would result from a prolonged and multiple uses of chemical weedicides. They have severe impact on animals, fish growing in nearby water bodies, persistence in soil, toxic residues in food, vegetables, fodder and even human beings. Some weeds have bioremediation activity and take up residual chemicals into their system. One typical example is presence of toxic and pesticide residues in herbal extracts of Bacopa monnieri. It is observed that in spite of best agronomy practices plant tend to accumulate toxic residues in the foliage. The explanation is simple. B. monnierie is weed in wetlands of rice fields. All the pesticide and herbicide residues present in the soil are taken up B. monnieri.
With regard to biodiversity conservation considerations weed and other invasive species are often perceived to act as plant pests3, one of two major threats to biodiversity, second to habitat loss10. This may be specially so when weed species are adapted to fires, lack natural enemies, grow faster than indigenous plant species and produce copious amounts of seeds7. This consequently results in implementing programs to eradicate sometimes indiscriminately without regard to their economic importance. Weeds do have lot of economic importance as food, feed, forage, fuel, traditional medicine and remediation. On the other hand, weed species form a component of agrobiodiversity, playing a part in the ecology of natural enemies as in harboring and supporting many beneficial species that suppresses the pest population consequently improving crop yields11.
Traditional medicine
India has a rich heritage of traditional medicine being practiced over thousands of years. Besides, China, South American nations and African nations do have their own traditional system of medicine. It is reported that in some Asian and African nations, 80% of the population depend on traditional medicine for their primary health care needs. In China, traditional herbal preparations account for 30-50% of total medicinal consumption. In Europe, North America and other industrialized regions, over 50% of the population have used complementary or alternative medicine at least once. 70% of the population in Canada has used complementary medicine at least once. In Germany, 90% of population has used natural remedies at some point in their life.
Plants have always been associated with traditional medicine. There are numerous uses of weed species as herbal medicines to cure a host of body ailments and diseases. The popularity of dietary supplements, nutraceuticals is growing worldwide, especially their usage is prevalent in Western, developed nations. The interest in drugs of plant origin is due to several reasons, namely, conventional medicines are considered relatively safe when used, their affordability by large section of people and that the these methods are time tested. Rural population, ethnic groups, tribals who have limited or no access to modern medical care have always depended on traditional medicine. Due to their acceptance all over the world, herbal medicines are generating billions of dollars in revenue. More than 100 countries have formulated regulations for herbal medicines. Weeds are highly valued in traditional medicine systems and have been used by indigenous communities for curing different ailments for thousands of years. Stepp (2004)12 suggested weeds need to be given more attention as potential source of medicines. According to a recent study (unpublished report) weedy species growing in fallow farms, waste lands contribute to approximately 30,000 ton per annum in Indian herbal sector as a raw material. Refer to Table 2 for the list of important weeds being used in traditional medicine which are now being cultivated at commercial scale.
Benefits of Weeds
Despite the negative impacts of weeds, some plants usually thought of as weeds may actually provide some benefits, such as: stabilizing and adding organic matter to soils, providing habitat and feed for wildlife, providing nectar for bees, offering aesthetic qualities, serving as a genetic reservoir for improved crops, providing products for human consumption and medicinal use, creating employment opportunities, etc.,
Globally, agriculture extension officers, farmers and in some instances rural extension officers recommend eradicating them, it is estimated that 89% of the most widespread and aggressive weeds in the world are edible. However menace they are, studies have shown the economic benefit of weeds in crop fields11. List of edible weed species reported in different countries of Asia-Pacific region is represented in Table 3. Recent developments have revealed the importance of weeds in traditional medicine12. According to Stepp12, in the US, 36 of the 101 plants species implicated in drug discovery are weedy species.
Utilization of weeds as an effective way to enhanced weed management
In spite of all negative perception, weeds do have major uses as described in above sections. They are used in horticulture (Lantana spp, Portulaca spp.) for their beautiful and colorful canopy, as manure (compost prepared from water hyacinth), as fodder (Cyperus). Weed species most widely known for their food use and as a traditional medicine.
Livelihood options for local ethnic groups and food security
Wild/weeds provide an opportunity for cash generation13. Many weeds have significant economic value which can be derived from their collection and sale 14. If proper utilization of weeds is identified, their utilization can create livelihood options for local youth. Further, use of harmful chemical weedicides can be avoided thus preventing ecological pollution. Collection and sale of weeds to meet herbal industry’s needs can indirectly manage weed population to a desired level. In the US an estimated $27 billion is spent each year to control weeds growing in agricultural fields, mostly on herbicides sprayed on the field15. Selectively harvesting what they need, leaving behind certain portion in order to have continuing production, sustainable harvesting and cultivation of economically important weeds increases biodiversity of the areas where they grow, key to preserving ecosystem. The reader is directed to consult Bharucha & Pretty and Kim & Shin16-17 how wild plants can form a significant proportion of the global food basket for food security and cultural divergence.
Cultivation of recommended weeds of commercial importance for reclamation of degraded/ waste lands
In India and many other countries, unutilized wasteland is contributing to economic loss. Over the period, wastelands turn into unproductive due to build of salts. The potential of wasteland can be extracted well by cultivating some economically beneficial medicinal plants. In this way, one can regain wasteland utilization and on the other side provide subsistence to local poor families, thus relieving the pressure on agricultural lands.
Value of weeds and volume of usage in traditional medicine
Despite the growing recognition that weeds have several utilities ranging from food and fodder to medicine, weeds are frequently overlooked. Weed research is now focused on reducing their population by employing costly and sometimes non-ecofriendly practices leading to both waste of money and ecological damage. The run-off chemical weedicides contaminate surrounding water bodies including fish and other aquatic food. Rather one should focus on identifying new use of weeds that can effectively manage their population simultaneously providing livelihood options for local tribal families. Refer to Table 4 for annual consumption of weeds in herbal industry that have found their utility in traditional system of medicine. Heavy losses to agricultural crops may be recovered by economic utilization of weeds, both in cropped and un-cropped areas.
Weed to crop status (List of weed species that are being cultivated on commercial scale)
Weeds are a component of plant genetic resources that when well protected in agro systems can become useful plants to society or become crops themselves18. Thus the cultivation of weed medicinal plants is a suitable option for optimizing resource utilization, as well as decreasing over-dependence on wild habitats. Encouraging such domestication will reduce pressure on wild or forest habitat thus conserving precious biodiversity. A list of such weeds that have been commercially cultivated are provided in Table 2. Medicinal plants can provide a significant source of income for rural life in developing nations especially through the sale of collection of weed herbs or their plant parts. Due to weed-crop competition, one has never thought of commercial utilization of weeds. If utilization is realized, they can longer be a constraint, but can become a companion crop.
Weed to crop to threatened status
Agronomic operations such as fertilizer application, herbicide treatments carried out in arable lands may alter dynamics in weed populations. Sometimes, such treatments may decrease the number of animal species that depend on these weed species (for example animal larvae, snails etc.,). An emerging paradigm is the conservation and sustainability of weed species. Utilization of weeds for effective management was discussed in earlier sections. A recent study conducted on risk status of arable weeds in Turkey determined that 112 of them (76 endemic and 36 non-endemic species) were at risk according to the IUCN Red List19. Turkey is extremely diverse, with over 3000 endemic plant species out of nearly 12,000 recorded. Considering endemic weeds, they are classified in the following risk categories: critically endangered (four weed species), endangered (EN) (6), vulnerable (VU) (14), near threatened (7), least concern (41) and data deficient (DD) (4 species). Among the non-endemic weeds, three species are EN, 31 VU and two are DD. The families with the highest threatened weed species are Scrophulariaceae, Fabaceae, Asteraceae, Brassicaceae and Lamiaceae. The identification of weed species in arable habitats and their risk status is a useful tool for assessing and monitoring how the sustainability of weed populations is affected by farming practices. The above study emphasizes the necessity of adopting new environment-friendly agricultural methods to conserve the high number of endemic weed species under threat.
Conclusion
Use of wild relatives of many crop plants (such as rice and wheat) as valuable genetic resources for improvement is often endorsed by plant breeders. Some of these wild species are occurring as weeds in their native habitat. Recent study shows that even these species are threatened because of habitat loss and over exploitation20. It is imperative to maintain these weedy species in their native habitats for future crop improvement programs. Such an activity helps crops to evolve continuously and develop wide genetic diversity. Thus weeds not only serve as economic package for forest dwellers but are also useful as genetic reservoirs for crop improvement programs.
As evident by large scale cultivation of weeds for their economical usage, it not only saves money (by preventing usage of chemical weedicides), if we can encourage sustainable utilization the income generated by their utilization often surpasses income generated by actual crop plants with which they are associated. Cultivation and agronomy practices for high volume weeds are slowly emerging to bridge gap between demand and supply. Sometimes such cultivation helps conserving threatened weed species and yet meeting the needs of industry.
Household food security and nutrition issues are at the top of agenda in many developing nations21. Many weed species are identified as source of food. The wide use of weedy species is seen as imperative for their ultimate control and should form part of part of management. There is a need for integrated research and inventorize their use in traditional medicine and as rural house-hold food security. Conservation, utilization and subsequent cultivation of weeds is an effective management strategy. Without utilization, conservation has no meaning. Every plant has some utility, it is our responsibility to identify the utility and research should be focused in this direction than their eradication.
References
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Legend to figures:
Figure 1. Tribulus terrestris L. a major dry land agriculture weed found in agricultural farms with medicinal value. It is used in Indian and Chinese traditional medicine.
Figure 2. Solanum surattense Burm. f. A common weed found on waste lands and in sugar cane fields, useful in treatment of fever, rheumatism.
Figure 3. Eclipta prostrataI(L.)L. A common weed found in paddy and vegetable fields useful in treatment of hepatoprotection and skin care.
Figure 4. Boerhavia diffusa L. a well known diuretic herb, common weed perennates with its tuberous root.