If you can t face them, replace them
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CTA. 2002. If you can?t face them, replace them. Spore 98. CTA, Wageningen, The Netherlands.
Permanent link to this item: http://hdl.handle.net/10568/46483
Internet URL: http://spore.cta.int/images/stories/pdf/old/spore98.pdf
Weeds just will not lie down and take 'No!' for an answer. After the umpteenth round of weeding, they shoot up again, ruining harvests and elbowing out crops altogether. If we understood better why weeds appear and re-appear we could work on better...
Weeds just will not lie down and take 'No!' for an answer. After the umpteenth round of weeding, they shoot up again, ruining harvests and elbowing out crops altogether. If we understood better why weeds appear and re-appear we could work on better control measures instead of simply trying to eradicate them. If we don t want them, we need to be ahead of them. Despite their pretty-pretty, esoteric names such as the devil s horse whip or prickly pigweed , weeds generally wreak more havoc than their fellow pests of insects, bacteria and viruses. They cause phenomenal losses in agriculture, especially in tropical areas where one-quarter of a crop s harvest is lost to weeds. Fighting them also accounts for huge amounts of time and energy. Weeding by hand or hoe is one of the most time-consuming tasks in agriculture, and a task performed mostly by women and children. But when exactly is a weed a weed? A couple of maize stalks in a field of tomatoes can be considered weeds but real weeds are plants that grow in places where they are really unwanted, either because they compete with crops for nutrients, water and sunlight, or because they are poisonous. Most farmers therefore make a distinction between useful and harmful weeds. The 'useful weeds' it takes some time to get that over your tongue, maybe - are the ones that are used for medicines, as fodder for animals and herbs in cooking. Or they may be the plants that keep vermin out of crops, fix nitrogen in the soil or provide protection against erosion. As long as they do not compete too much with the actual crops, they are left to grow for the purpose they serve. Alongside this approach is that of 'relaxed weeding' which means pulling out those weeds that are harmful, toxic and really have no purpose nor any use, not even as compost or fuel. In smallholder farms, weeds are regarded in a different way to on large commercial farms or plantations where any plant other than the cash crop is called a weed, whether useful or not. Weeds make haste About 8,000 plants worldwide are currently considered as real weeds and out of these, a small elite of 250 are considered as the world s major noxious, or harmful, weeds. Now you won t find them all together in your field. Obviously not. After all, one weed may be harmful or hostile to another. And, as any other plant, they all have their preferences for temperature, soils, fertility and humidity. Small wonder that weeds occur between and around crops with similar preferences. Weeds have things in common too. One classical farmer s saying explains how to tell weeds from vegetables: If you see anything growing, pull it out. If it starts to grow again, it was a weed. One trait of weeds is that they grow and propagate fast. Sweet flowers are slow and weeds make haste , as Shakespeare once wrote. Annual weeds produce large amounts of seeds which often retain their germination power for years and years. For example, Amaranthus spp produces up to 235,000 seeds per plant cycle, and is regarded as a pesky weed by many farmers, whereas it has also been a staple food for more than 4,000 years in central America. Perennial weeds propagate vegetatively through rootstocks, tubers, rhizomes (stems) or through running across the soil surface. Another thing weeds have in common is their reason to exist and why they shoot up so fast and in such large numbers on barren land. When we clear native vegetation, plough the soil and establish annual or perennial crops, we are in fact defying the ecological succession of the course of nature. We are thus holding back the process of natural plant succession, which really aims at changing the environment back into its previous state as a forest. Weeds are pioneer crops and thrive on soils with low fertility and low plant diversity. In that respect weeds are also telling us that things are none too bright in the soil. For shifting cultivators, for instance, the start of excessive weed growth has always been the signal to pack up and move on. A bit of everything For decades, it was believed that modern agriculture would achieve production increases mainly through a combination of mechanisation, mono-cropping, improved varieties, applying agro-chemicals and, indeed, fighting weeds at all cost. Applying herbicides to a weed was seen as much easier and cheaper than using labour to remove it either manually or mechanically. As time passed, however, herbicide resistance grew, as did general awareness about the environmental and health aspects of agro-chemicals. All the while, the weeds were literally gaining ground. Each year, as more and more herbicides were applied, so too did the crop losses caused by weeds. Basically, weed management revolves around five sets of measures. Using chemicals is one. Herbicides have the major disadvantage of being costly and sometimes dangerous, as well as their other constraints. Applying fertilisers is another - also costly chemical way of managing weeds, by increasing soil fertility. Biological measures present a second way of managing weeds. Applying intercropping or mulching, agroforestry or growing cover crops are examples of these (see boxes). A third way is through the genetic alteration of crops, using conventional breeding techniques or genetic engineering, to increase the crop s resistance or tolerance to weeds. The commonest way to get rid of weeds is through tillage and weeding. Other cultural techniques, such as burning vegetation and ploughing the soil can be counter-productive since they create an ideal environment or, to call a spade a spade, an ecological desert - for weeds to pop up and start their first phase of succession. And the fifth way? Well, as with many things in life, a combination of approaches can often yield the best result. If a good farmland is the reflection of a healthy natural ecosystem, where soil fertility is kept up in order and plant diversity mimics nature, then there is no need for noxious weeds to emerge in such large numbers. Where there s a will, there s a way. Where there s a will, there s no weed. [caption to illustration] The picture of proverbs. Many hands make light work, but prevention is better than cure.