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1326Achieving sustainability in the use of green manures

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  • Gary
    Nov 2 10:31 AM
      Achieving sustainability in the use of green manures

      By Roland Bunch

      Teaching farmers that green manure and cover crops have valuable uses
      besides maintaining soil fertility can help sustain the widespread
      use and adoption of green manuring practices.

      Today, well over 125,000 farmers are using green manure and cover
      crops in Santa Catarina, Brazil. Green manure and cover crops are
      equally popular in neighbouring Parana and Rio Grande do Sul. In
      Central America and Mexico, an estimated 200,000 farmers are using 20
      traditional systems involving some 14 different species of green
      manure and cover crops and organisations from Central Mexico to
      Nicaragua are promoting their use in at least 25 additional systems.
      Across the ocean in West Africa more than 50,000 farmers have adopted
      Mucuna spp. or Dolichos lablab as green manures in the last eight

      The present widespread use and rapid adoption of green manure and
      cover crops has taken many people by surprise. To some extent this is
      because little attention has been given to the extent to which green
      manures and cover crops have always been used in traditional systems.
      Gene Wilken, for example, in his otherwise excellent book, Good
      farmers: traditional agricultural resource management in Mexico and
      Central America stated that "cover cropping is not widespread in
      traditional Middle America," (Wilken 1987). Many scientists believed
      the technology inappropriate for village farmers. As late as 1989,
      Anthony Young in the classic Agroforestry for soil conservation
      dismissed green manuring as "a form of non-productive improved fallow
      which has rarely found favour with farmers" (Young 1989).


      For more than a decade it has been accepted that green manures and
      cover crops would only be accepted by small farmers if they could be
      grown on land that had no opportunity cost, could be intercropped
      with other produce, grown under tree crops or on fallow land and be
      cultivated in periods of expected drought or extreme cold. They would
      also be favoured if they involved no extra labour and out of pocket
      cash expense (Bunch 1995).

      Whilst these assumptions have proved correct, recent experience has
      shown that the sustainability of green manure and cover crops is more
      likely to guaranteed when they provide farmers with some other
      benefit besides fertile soil. This condition is consistent with the
      observation that village farmers generally prefer multiple use

      Experiences worldwide

      Experience from many parts of the world confirms the value
      attribute to green manures and cover crops that have multiple uses.
      In most known, traditional systems legumes are appreciated not only
      because they maintain soil fertility, but because the seeds or pods
      can also be eaten. Examples include the Vigna spp. which is
      intercropped in Southern Honduras, El Salvador and South-east Mexico
      and the high-altitude scarlet runner bean (Phaseolus coccineus),
      which is widely used from upstate New York (Seneca bean) to Mexico
      (ayocote) and from Guatemala (piloy) to Honduras (chinapopo) and
      Northern Chile.

      The velvet bean (Mucuna spp) is easily the most popular of all the
      green manures/cover crops used today and was initially used and
      spread by farmers along the southern border of the Himalayas in
      Nagaland partly because it was such a valued source of food (Young
      1989). In Central Honduras, where World Neighbours and COSECHA have
      taught farmers to intercrop velvet bean with maize, there has been a
      disappointing failure (35%+) to continue this technology except in
      those villages where it is consumed as a major component of coffee,
      hot chocolate, bread and tortillas. In fact, their value of green
      manures and cover crops as human food seems to be the strongest
      factor motivating in their sustained adoption.

      Perhaps the second most common use of green manures and cover crops
      is in weed control. In South-east Asia, a perennial species of the
      velvet bean is use to improve fallow and to control weeds. More
      modern practices include using jack bean (Canavalia ensiformis,
      tropical kudzu (Pueraria phaseoloides) and perennial peanuts (Arachis
      pintoi) under a variety of plantation crops, including coffee,
      citrus, and African oil palm. The velvet bean is also used to control
      imperata grass (Imperata spp.) and this practice is spreading rapidly
      throughout Benin, Togo, and Columbia. Velvet bean and jack bean are
      used to control paja blanca (Saccharum spp.) in Panama and to combat
      nutgrass in several other countries.

      A third practice, which is now more widespread but which is still
      under-appreciated, is the use of green manures and cover crops to
      stabilise swidden agriculture. Since decreased fertility and weed
      infestation are the two most important reasons why farmers abandon
      their fields today, and since green manures and cover crops can, to
      some extent, often solve both these problems they have proved to be
      an effective way of stabilising shifting cultivation in many

      One dramatic example can be drawn from the work of the Centro Maya in
      Guatemala's northern Peten region. In this humid forest area, farmers
      could only grow maize for one or two years and then the ground had to
      be left to regenerate. Now hundreds of farmers are growing velvet
      bean intercropped with maize on the same fields year after year.
      Those who initially adopted this system have been growing maize on
      the same land for eleven consecutive years and productivity has
      improved over time. Another interesting example is that of Central
      Ghana, where village farmers are inventing their own ways of
      stabilising their agriculture, including one system in which 30,000
      leucaena trees (Leucaena spp.) are intercropped with maize and burned
      very lightly each year. This practice has allowed maize to be planted
      on the same land for 20 years in succession.

      A fourth potential benefit that will probably acquire more
      insignificance as experience increases, is the use of green manures
      as animal feed. Most green manures and cover crop species, with the
      major exception of Melilotus albus cannot be grazed well, but many
      can be used for cutting and carrying even after months of drought,
      the most notable examples of this type being Lathyrus nigrivalvis and
      lablab bean (Dolichos lablab). Seeds also provide fodder, one good
      example being the seeds of the velvet bean which in Campeche, Mexico
      are cooked for half-hour, mixed with an equivalent amount of maize
      and then ground into pig feed. The University of Yucatan calculated
      that this velvet bean feed cost less than commercial feeds per unit
      of weight gained.

      Green manure and cover crops can be used in other ways as well. Two
      years after Alter-Vida stopped working in El Naranjito, Paraguay,
      farmers abandoned using velvet beans as a green manure, but continued
      to used to use them when they wanted to prepare their land for
      tobacco. In Southern Brazil, hundreds of thousands of farmers
      regularly use some 25 different species of green manure and cover
      crops for soil improvement partly because this allows them to
      increase the amount of organic matter in their soil to the point
      where tilling is no longer necessary. The financial as well as
      ecological advantages of zero-till systems are tremendously


      A number of conclusions can be drawn from the examples given above.
      First, the variety of sustainable green manure and crop cover systems
      already established in traditional as well as more recently
      introduced agricultural system is remarkably diverse. Green manures
      and cover crops have been adopted on a wide scale despite the
      seemingly prohibitive conditions mentioned earlier in this article.
      The fact that virtually every system we have refered to has some
      elements of these conditions confirms their predictive value. Thus,
      programmes to introduce new green manure and cover crop systems
      should teach farmers not only how these species can be used to
      improve their soil but that they have other uses as well.

      Tremendous potential still exists for the development of new green
      manure and cover crop systems. Scores of potential systems for using
      green manure and cover crops still need to be investigated, most
      notably the major possibilities of using them for animal feed; the
      potential latent in new as yet untried species, including trees and
      non-legumes, and the value to be derived from using combinations of
      green manures and cover crops rather than individual species.
      Experience leads us to believe that, with the possible exception of
      very intensive farming systems such as irrigated vegetable and rice,
      green manure and cover crops systems can probably be introduced into
      many, if not most of the world's, small-scale farming systems.

      Roland Bunch, COSECHA, Apartado 3586, Tegucigalpa, Honduras

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