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| Witchweed
takes
its toll
There are about 41 species of witchweed on the African
continent and parts of Asia. Africa is believed to be
their region of origin. Two main species parasitize
cereal crops and wild grasses, and a third parasitizes
broadleaved plants and crops such as cowpea and tobacco.
Yield losses from vast areas of fields planted with
cereals and parasitized by witchweed makes it a serious
plant pest. |
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| Parasitic,
partly
A “hemi-parasitic” plant such as witchweed
draws most of its water, nutrients and carbohydrates
from the host root, but it can do photosynthesis itself
when it turns green after emergence.
However, it will always get half or more of its carbohydrates
from its host. It cannot start its life cycle without
being attached to a host. A germinated seed will die
within three days if it has not attached successfully
to a host root. |
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Posted December 14, 2006: Kaata or Itikira
is the word in the Peuhl language for a mysterious plant with beautiful
pink flowers that seems to cause wilting of millet plants wherever
it appears in northern Mali.
One of the farmers dealing with its impact is Fatoumata Nouh Tamboura.
She is one of about 150 farmers in the village of Gaye in northern
Mali, a West African mostly drylands nation located north of Senegal.
The farmers of Gaye are aware of the plant (Striga hermonthica
or witchweed) and its damage, but often do not understand how it
interferes with millet plant growth or how it reproduces.
Farmers in this community grow millet, cowpea, roselle (bissap),
okra, and a bit of groundnut and sorghum on 1 to 2 hectares of land,
spread throughout the village territory. Most experience a high
level of witchweed infestation in their millet fields.
Fatoumata is a farmer-trainer in an experimental witchweed management
project carried out at the multi-village level (see details in box
below). She doesn’t let her illiteracy get in the way of learning
and teaching new techniques. She often asks a teacher to make notes
in her book for others to read. As one of the older women in her
village, she enjoys a position of respect for what she reports.
Fatoumata brought home the insight that witchweed reproduces by
seeds, giving the community a new way to understand the pest and
reduce its negative impact.
De-mystifying the witchweed mystery
To be able to solve the witchweed problem, you first need to understand
its biology and options for control. Botanically identified as Striga
hermonthica (Del.) Benth, witchweed is a noxious, hemi-parasitic
weed in semi-arid, sub-Sahara Africa. It causes substantial yield
loss in main cereal crops such as sorghum, pearl millet, maize and
upland rice.
Witchweed parasitizes the roots of its host plants. Through a specialized
organ called a haustorium, it takes in nutrients and water from
the host root. Due to its excessive transpiration (the stomata of
witchweed cannot close), it “pulls” water, nutrients,
and assimilates from the host. (See sidebar: “Parasitic, partly.”)
An estimated 26 million hectares of cereals (maize, sorghum and
millet) are infested with witchweed in Sub-Saharan Africa leading
to an estimated loss in production of about 10.7 million tons. West
Africa alone accounts for about 17 million infested hectares and
a yield loss of about 6.5 to 7 million tons—a full 3 percent
of the pledged food aid for sub-Saharan Africa for 2005-2006, according
to the world Food and Agriculture Organization (see sidebar: “Witchweed
takes its toll”).
The most severe problems with witchweed—in terms of yield
loss as well as persistence of the weed—seem to occur where
soils are degraded, agriculture is extensive, fields are continuously
cropped with a cereal host, and nutrient inputs (both organic and
inorganic) are low. Continuous cropping of a preferred host like
millet, sorghum or maize leads to a build-up of the witchweed seed
bank. With more seeds in the soil, more of the parasite plants will
attach to the host, leading to reduced growth and less grain in
a downward cycle for the crops.
It’s heartbreaking to walk in a millet field infested with
witchweed and see the crop plants wilting in areas where subsistence
farmers already have to deal with seasons of drought, infertile
soils and many other food-production constraints.
Integrated responses
There’s no simple solution to the witchweed problem, though
research has developed ample methods and techniques for witchweed
control. Only dedicated, labor-intensive manual weeding provides
complete control of emerging witchweed plants. However, a combination
of practices can lead to meaningful reductions in emerged plants
and, ultimately, a reduction of long-term seed-bank densities.
Click image to enlarge
African farmers have adopted several control practices. In regions
where agricultural management has intensified (in terms of labor,
cropping and inputs), great advances were achieved in so-called
Integrated Witchweed Management (IWM).
However, these systems are mostly corn/maize-based systems, and
the control packages often involved maize hybrids. Particularly
in the more extensive, low-input agricultural systems of the Sahel,
farmer-acceptable measures are scarce, and technology transfer has
thus not made much progress.
Farmers here reject control techniques that aim only at reducing
witchweed. Technologies that serve additional goals such as direct-income
increase through the introduction of potential cash (trap) crops
has more chance for adoption by farmers in the drier Savannah zones.
The witchweed problem in the West African Sahel needs to be solved
by interacting with farmers to find out what measures fit their
situation and needs. Organic amendments, intercrops of cereals with
non-host plants, host resistance, late weeding and rotations are
options within reach of most farmers and easy to combine for effective
witchweed control.
Where possible, there will be additional gains in suppressing witchweed
pressure through increased use of improved varieties, increased
use of fertiliser/organic amendments, higher crop density and more
labor input per unit area.
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Farmers, and how they are learning to impact witchweed
In 2004, the Mopti region of northern Mali was plagued
by locusts that destroyed most of the food harvest.
The situation worsened in 2005 when the farmers faced
intense drought and yet another year of serious crop
losses.
These events led to a serious food security (hunger)
problem as 83 of 108 communes (towns) were officially
declared in a state of food crisis. Malian government
and several voluntary aid groups like Catholic Relief
Services (CRS) started food distribution and subsidized
grain sales. CRS intends to aid in longer-term recovery
and increase the resilience of these agro-pastoral communities
to produce food across a variety of conditions.
Providing technical assistance to start farmer field
schools was staff from the International Crops Research
Institute for the Semi-Arid Tropics (ICRISAT). This
is a non-profit, apolitical, international organization
for science-based agricultural development. An experienced
technician from the Institute d’Economie Rurale
(IER), a national agricultural research institute, helped
with the organisation and participatory planning. Teams
from CRS, ICRISAT and IER talked with farmers of the
Mopti region and learned that witchweed was one of the
top three problems limiting agricultural production.
In June 2006, project teams started farmer field schools
in six villages with about 150 farmers participating.
Each village chooses five representatives that become
farmer trainers. Thirty farmer-trainers from the six
villages come together every week for “train the
trainer” instruction. Here they learn to set up
and work an experimental field, do observations within
the experiments and discuss subjects that concern the
cultivation of millet and cowpea.
The training focused on Integrated Witchweed Management
(IWM) to teach decision-making that weighs the costs
and benefits of using multiple witchweed suppressive
techniques—including cropping changes—with
potential yields of host and non-host crops for witchweed.
This approach is designed to bring about: (1) more
yield and net returns from labor, organic and/or mineral
soil amendments and seeds; (2) reduction in the number
of emerged weeds and the witchweed seed bank; (3) increase
in soil organic matter and fertility; and (4) more sustainable
cropping systems in general.
Learning tied to crops
Class teaching content was formed around what was happening
in the crop fields at that time of the classes. Using
botanical extracts (neem tree leaves, chili pepper or
tobacco) to manage insect pests on cowpea was the topic
during the flowering phase, whereas the underground
stages of witchweed are discussed at the weed pest’s
first emergence (at the moment of millet tillering).
Experimental fields have two plots. One compares standard
farmer practice with the IWM package of strategies.
The other looks at the effects of single components
of this package.
During preparatory meetings, program staff worked with
the farmer-trainers to design the integrated management
package. It included:
- Sowing at the first big rain (as compared to local
farmer custom of dry sowing before the rains).
- Intercropping millet and cowpea in rows and at higher
plant densities than is usually done locally.
- Using organic amendments (at around 2 tons/hectare).
- Application of low doses of mineral fertiliser
(N-P-K, and urea).
- Millet plant thinning to two plants per planting
hole (as compared to allowing five or more plants).
- Pulling witchweed plants one or two weeks after
the first flowers were observed in the integrated
practices demo plot.
By harvest time in November, some things had already
become clear to the scientists and the farmers.
- Witchweed density was lower in the integrated plot,
but pulling the flowering witchweed plants manually
was still necessary to prevent unacceptably high levels
of seed production.
- Although cowpea bean production was about the same
in the two practices, cowpea fodder production was
much higher in the integrated plot.
- Millet grain production is much higher in the integrated
plot, despite a later sowing date.
From the agronomic point of view, the IWM package worked
out well. However, a final economical evaluation together
with the farmers will have to show whether the package
is also economically viable. In this evaluation, the
costs of a practice (fertilizer, manure, labor) will
be balanced against the economic yield value of the
fodder and grain, calculated by weight. Only then do
we know whether the package is also viable economically
and socially. Soil organic matter changes will also
be noted.
If the IWM package turns out to be profitable compared
to the customary farmer practices, the project leaders
will ask the farmers to evaluate whether they feel the
techniques used are practical and whether the farmers
are willing to take the risk of investment of using
them next year.
Because it’s so hard to predict whether next
year’s growing conditions will be similar to 2006,
deciding to adopt or adapt new approaches based on a
single year’s experience will be a significant
challenge.
If it turns out that the IWM package isn’t as
profitable as common farmer practice, the discussion
will be to decide if particular improved practices would
be worth using, adapting or combining.
In this overall training process, the Malian farmers
learned about managing weeds and crops, but also gained
the human capital skills of setting up and analyzing
on-farm experiments. They built upon the strong social
capital (adherence to rules, relationships of trust,
mutuality of interest) already important for them, and
this may help them to gain financial capital if they
choose to form a marketing group. The farmers hope to
continue their cooperation with CRS, ICRISAT and IER
in the season ahead. |
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