Grain Legumes and Green Manures for Soil Fertility in ... - cimmyt

Baanante, 1996; Heisey and Mwangi, 1996) due to (i)
high price and poor infrastructure, (ii) risk due to
uncertainty in climate and the price of produce, and
(iii) lack of access to credit for smqll holders.
The Eastern and Central Africa Maize and Wheat
(ECAMAW) Research Network is a network of
maize and wheat scientists from the National Agricultural
Research Systems of the ten countries in
Eastern and Central Africa operating under the Sub­
Regional Organization, ASARECA. ECAMAW scientists
address priority constraints of regiunal importance
to improved maize and wheat production
and productivity and operate through a system of
small project grants overseen by a Steering Committee,
a Network Coordinator and CIMMYT project
scientists that fund the small grants program.
Due to the poor access farmers have to fertilizers,
ECAMA W scientists have focussed on green manures
and grain legumes as alternative sources of N
for maize systems. The potential for legumes to supply
N to cropping systems is well known, and the
benefits and constraints were recently reviewed by
Giller et al. (1997). Legumes in cropping systems
can be broadly classified as those that produce a
consumable seed (grain legumes) and those that are
grown solely for agronomic purposes, such as a
source of biologically fixed N (green manures),
weed control. and ground cover. While grain legumes
can fix substantial amount of N, with few exceptions
(e.g., groundnut, cowpea, pigeonpea), most
of the fixed N is harvested with the grain and little
is left to the soil and subsequent cereal crops. Green
manures provide considerable N to the soil when
grown in rotations with crops but also remove land
hom production to gain that benefit. Both grain legumes
and green manures grown as intercrops suffer
from competition from the companion crop, reducing
biomass accumulation, biological N fixation and
the potential benefits to the systems.
During 1997-2002, the ECAMA W Network supported
12 small grant projects, each spanning periods
of two or more years and often implemented
across several sites, to evaluate grain legumes and
green manures in maize systems. Most of these projects
were executed on-farm with farmer participation
at multiple sites. The objectives of this research
were to:
• Identify suitable adapted green manure and grain
legume species for the major ecologies of ECA;
• Evaluate appropriate management practices for
them in intercrops, relay crops or rotations with
maize;
& Quantify the impact of green manures and grain
legumes on maize pFOductivity;
• Determine ihe fertilizer-N equivalence of green
manures in rotations; and
• Evaluate green manures and grain legumes in systems
on-farm with farmers to ascertain farmers'
perceptions and acceptance.
This paper summarizes the results of these regional
network trials and describes on-going and future
research and dissemination activities of ECAMA W
network scientists with legumes in maize-based systems.
Methods
Evaluations of legumes for adaptation, biomass
production and N-fixation
Regional trials were established at Namulonge
(Uganda), Arusha (Tanzania), Tanga (Tanzania),
Jimma (Ethiopia) and Kakamega (Kenya) to screen
green manure and grain legume species for adaptation
to the local environment. A core set of 12 species
(Table 1) were generally evaluated at all sites;
an additional 10 species (Oolichos-Renga, Cajanus cajan,
Pueraria phaseoloides, Voandzeia subterranea, Crotalaria
brevidens, Oesmodium intortum, Lablab purpureus,
Macroptilium atropurpureum, Phaseolus vulgaris
(cv. Selian wonder) and green gram) were
evaluated at single selected sites. Species were
sown in'small plots on station at the onset of the
rains and were scored at appropriate periods for
establishment, nodulation, percent ground cover,
resistance to pests and diseases, seed and biomass
production among other criteria. N supply capacity
was estimated from the total biomass production
and N content of the biomass.
Effects of green manures and grain legumes in
maize-legume systems
Trials were conducted on station and on farm to
evaluate promising legume species (based on re­
gional screening trials) in systems with maize. Sys­
tems included the following:
• Rotations within a year (bimodal rainfall) orin alternate
years (monomodal rainfall distribution);
• Relays, including the effect of relay date on green
manure biomass production and sequenced maize
production; and
• Intercrops of green manure or grain legume species
with maize.
The effect of legume species and system on maize
productivity (yield of maize grain per hectare) was
measured. In some cases, the effect of maize on legume
biomass production was also determined, including
the N content of the aboveground biomass
where possible. All results were subjected to analyses
of variance and means were separated by the
Duncan's Multiple Range Test where appropriate.
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Grain Legumes and Green Manures for Soil Fertility in Southern Africa