|
|
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. |
|
|
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. |
|
|
 |
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.
|
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. |
|
|
