The yield of weeds Evaluating the costs and benefits of a strategy for weed management. A detailed case study on two small farms in South India. When trying to evaluate the costs and benefits of a strategy for weed management, we usually look at parameters such as crop yields, labour requirements, costs of purchasing herbicides and many more. There is one blind spot in nearly all such studies: the yield of weeds, the potential for positive use of weeds. A substantial portion of the food for people, animals and soil can come from weeds. The study was carried out between May and December 1994 on two small farms of six and ten acres in Bangalore and Mysore Districts, Karnataka, India. The seasonally dry tropical climate has most of its total annual rainfall of 800 mm during south-west monsoon season between June and September. The mean annual temperature is around 24°C. Soils are mostly fertile, varying from red soil to black cotton soil. As main crops farmers grow paddy, sugarcane, ragi, maize, coconut and several pulses. Livestock usually consists of cows, bullocks used for plowing, poultry, sheep and goats. More than three quarters of the food consumed is produced on the land. Both farms practise organic agriculture and the common practice for weed control is hand weeding. Species distribution and biomass production of weeds was recorded by a random sampling method. Detailed maps were drawn and soil samples taken. However, to complement this scientific approach, it was the convivial approach that filled my herbarium sheets with dried specimens and scientific names with meaning. Living on the farms, sharing the meals, working on the fields and weeding of course... I thank the families I lived with for sharing their wisdom. For every single species of "weed" encountered on the farm, the family was asked what particular advantages and disadvantages this plant meant for them. Altogether, information on more than 150 weed species was collected. Here are the details for just two plants. Encourage or burn? Solanum nigrum, of the tomato family (Solanaceae), known as "ganaki sopu" is a plant growing wild on the fields, but it really doesn't deserve to be called a weed. To the hungry man, it means food, to the hungry cow, fodder, to the sick man, cure. The spinach prepared from young shoots and leaves is really something worth trying. "The small black berries are especially good for growing children", a mother told me. Literature says they have a high content in vitamin C (Sathyavathi 1994, Rao 1990). The use as food and medicine is not strictly separated, a characteristic of ayurveda healing tradition. The plant doesn't spread easily, so farmers usually try to encourage it where they can. Parthinium hysterophorus is an example of the other extreme. The farmers on the two study farms try to pull it out before flowering. Other farmers just burn it. The plant is poisonous to cattle, spreads easily and is problematic even as compost because its seeds remain viable for so long. Various uses Total biomass of weeds was up to 2 kg dry biomass per square metre and year, depending on the crop and type of cultivation. The ways in which weeds are put to positive use range from food, fodder, medicine and pest control to compost and green manure. Food Fifteen different species were being used as food on the farm. In the case of Physalis minima and Solanum nigrum, the ripe berries are eaten raw, the berries of Solanum torvum are cooked. Amaranthus viridis, Alternanthera pungens, Oxalis corniculata, O. acetosella and Portulaca oleracea were the favourite weeds for preparing cooked greens. Wild amaranth has a much higher resistance to drought when compared to the cultivated forms. The great hardiness of some edible weeds can turn out to be a great advantage in times of scarcity. Fodder A good 50% or more of dry weight of total fodder for livestock came from weeds on the farm. Animals were either given the freshly weeded plants or taken to graze on the bunds, on newly harvested fields or such fields where there was no danger of the cow eating the crop. Nearly all grass species are relished by cattle as well as a number of dicots: Rothia indica, Euphorbia geniculata, Ageratum conyzoides and many others. A number of species, on the other hand, are not eaten by cattle at all, such as Croton sparsiflorus, Eupatorium triplinerve, Parthinium hysterophorus and Calotropis gigantea. This type of information would be very hard to come by for a merely observing scientist. The author of this article, through her participatory approach, was able to check whether a weed is really edible by having it for lunch and to confirm what the cows eat and what they don't by taking them out to graze! Medicine For 40 out of 79 species on the farm, medicinal uses can be found in literature. But the focus in this study is on the actual, not the theoretical use of weeds, so note that all numbers in Fig. 1 refer to the farmer actually using the plant for a given purpose, not to the literature on weed uses. In the case of medicinal use, at least 9 weeds are known to the family as being medicinal, but would of course be used only if illness or accident occurred. Pest control Some "weeds" such as Ocimum canum are collected and added to grains for storage to reduce storage pests. Others such as Calotropis gigantea and Argemone mexicana are used to prepare antifeedant sprays in the same way as is done with leaves from the famous neem tree (Azadirachta indica). Still others unfold their potential on the field itself, so that it is difficult to say whether the farmer is using a weed for pest control when all he does is simply leave the plant alone instead of pulling it out. Other uses A number of weeds are "holy" plants. The flowers of Gomphrena globosa, Clitoria terneata, Leucas aspera and others are used for decorating statues of Hindu godesses and gods. "Broomstick grass" (Aristida adscensionis), is used for making broomsticks, the abundant shrub Lantana camara makes excellent fencing material, other weeds are valued as snake deterrents, for making baskets or as string for tying bundles. The list of such obvious and practical uses is almost endless. Uses in the field More varied and breeding ground for controversy are the functions of weeds while still rooted in the field. Most farmers agree that weeds on field borders play a role in preventing erosion. But do they help prevent pests by providing diverse breeding ground for predators of do they house pests? Do they protect the soil from the scorching sun or do they rob the crop of what little moisture there is? Do they bring up additional nutrients from the sub-soil or do they take away nutrients from the crop? Farmers have their views on these questions and so have scientists. The answer depends on whether we use a short-term or a long-term perspective. Weeds being used as compost, mulch and green manure comes last because it is the "weed sink". All weed biomass that was not used for any of the above purposes was always used to feed the soil. Some weeds like Cassia tora or Rothia indica are considered particularly good for making compost, but even a weed left to rot on the field after weeding can be considered a green manure. 14% of weed species on one farm and 19% on the other were nitrogen-fixing plants. Yield The number of species used for a specific purpose is not a good indicator for weed yield. Some species are so rare they wouldn't even make a single meal in a year, while others are so abundant you can make a large spinach dish for the whole family every week. The study on two farms in South India showed that a large number of weed species is used for a wide variety of puposes. Biomass production of weeds is considerable and the useful species are in part very abundant. So the blind spot in conventional cost-benefit analysis turns out to be rather a whole blind eye. For the two farms studied, the application of a herbicide as an alternative measure for weed control would be a threat to the food supply for people, animals and soil. This is not to say that the loss of weed biomass as a cost might not be outweighed in some cases by the benefits of applying the herbicide, such as a decrease in labour requirement or an increase in crop yield. But there is no reason to assume that all weed biomass that is lost will be compensated for by an increase in crop yield. There are many examples in weed crop ecology where the complementary use of resources allows the combination of weed and crop to outyield the crop alone by far (Aldrich 1984). Annidation, i.e. the principle of the complementary use of resources, has been the basis for developing highly productive systems of intercropping and mixed crops. The coexistence of weeds and crops, interspersed with weeding at certain stages during crop development can even be seen as a specialised form of intercropping that makes optimal use of the limited resources available. Paula von Weizsäcker, PO Box 1301, D-53061 Bonn, FR Germany. References - Aldrich, RJ. 1984. Weed-crop ecology: principles in weed management. Breton Publishers. - Sathyavathi, GP. 1994. Some plants of pest control importance in Karnataka. UAS, GKVK, Bangalore, India. - Rao, KN & Rao, T. 1990. Medicinal plants of Tirumala. Tirumala Tirupathi Devasthanams, Tirupati, India << country/story review