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West Africa has major problems relating to the impact of slash-and-burn shifting cultivation on soil systems. In order to design more sustainable yam cropping systems, agronomy research in Benin has implemented trials in partnership with smallholders on alternative yam-based systems using shrubby (Gliricidia sepium) and herbaceous (Aeschynomene histrix) legumes. In the first phase, farmers modified these new systems within their own constraints; the systems were then further evaluated. The agronomic and economic performance of farmer-adapted alternative yam-based cropping systems and the implications for wider international application are discussed.
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AGRICULTURE Vol 41, No 3, 2012, pp 171–178 doi: 10.5367/oa.2012.0094
171
Agronomic and economic
performance of yam-
based systems with
shrubby and herbaceous
legumes adapted by
smallholders
Raphiou Maliki, Denis Cornet, Anne Floquet and
Brice Sinsin
Abstract: West Africa has major problems relating to the impact of slash-and-burn
shifting cultivation on soil systems. In order to design more sustainable yam
cropping systems, agronomy research in Benin has implemented trials in
partnership with smallholders on alternative yam-based systems using shrubby
(Gliricidia sepium) and herbaceous (Aeschynomene histrix) legumes. In the first
phase, farmers modified these new systems within their own constraints; the
systems were then further evaluated. The agronomic and economic performance of
farmer-adapted alternative yam-based cropping systems and the implications for
wider international application are discussed.
Keywords: adaptive research; yam-based systems; net present value; modelling;
Aeschynomene histrix; Gliricidia sepium
Raphiou Maliki (corresponding author) is with L’Institut National des Recherches Agricoles du Bénin
(NRAB), BP 01-884, Cotonou, Benin. E-mail: malikird@yahoo.fr. Denis Cornet is with Centre de
Coopération Internationale en Recherche Agronomique pour le Développement (CIRAD), F-34398,
Montpellier Cedex 5, France. Anne Floquet is with Centre Béninois pour l’Environnement, le
Développement Economique et Social (CEBEDES), BP 02-331, Cotonou, Benin. Brice Sinsin is with
Faculté des Sciences Agronomiques de l’Université d’Abomey-Calavi (FSA/UAC), BP 01-526,
Cotonou, Benin.
West Africa currently produces more than 40 million t
year
–1
of yam, which equates to 90% of total worldwide
production (FAOSTAT, 2010). Increases in production
have been obtained on larger yam-cultivated areas in
slash-and-burn and shifting cultivation systems
(Torquebiau, 2007), indicating that only a limited degree
of intensification was under way. Benin is the world’s
fourth ranked producer, after Nigeria, Côte d’Ivoire and
Ghana. Farmers no longer rely on long-duration fallow
between crops as yam is cultivated in one- or two-year
herbaceous fallow–yam or maize
/
sorghum–yam rotation
systems with manual incorporation of residue into the soil
(Doumbia, 2005). In order to design a more sustainable
yam cropping system, agronomy research in Benin has
focused on trials in partnership with smallholders on
alternative yam-based cropping systems using shrubby
(Gliricidia sepium) and
/
or herbaceous (Aeschynomene
histrix) legumes. The most important constraints for
agroforestry systems adoption, notably alley cropping
(Kang and Reynolds, 1986) are its pruning workload as
well as competition between shrubs and crops for nutri-
ents and light (Floquet et al, 2006; Maliki, 2006).
It is well known that smallholders adjust technologies
developed by researchers when confronted with various
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Agronomic and economic performance of yam-based systems
Figure 1. Monthly rainfall pattern during the four study years in
(a) Dassa, (b) Savè and (c) Ouessè.
constraints (for example, land, soil quality, labour, cash).
Adaptations suggested by smallholders have been inte-
grated into the experimental design. In the
agroforestry–yam-based system, at the end of the rotation,
the plot is fallow for a few years before being cleared for
yam and Gliricidia sepium (G. sepium), which usually
grows to form a medium-sized shrub. Instead of pruning
these shrubs at land-clearing time, most of the smallhold-
ers use early burning of G. sepium fallow in order to
reduce the labour demand and improve light and nutrient
access to subsequent crops (yam), a practice that fits in
with traditional slash-and-burn wooded fallow and yam
systems. This adapted design was incorporated into a
four-year experiment. The objective of this study was to
evaluate the agronomic and economic performance of this
adapted yam-based cropping system and draw out the
implications for wider international application.
Methodology
Study sites
The study was carried out in central Benin, in the Guinea–
Sudan transition zone (7°45'–8°40' N, 2°20'–2°35' E) at
eight sites (Adjanoudoho, Akpéro, Boubou, Dani, Gbanlin,
Gomè, Magoumi and Miniffi). The climate is tropical with
a bimodal rainfall pattern (Figure 1). The soils are
Plinthosols (Gbanlin and Akpéro) and Luvisols
(Adjanoudoho, Miniffi, Gomè, Magoumi, Dani). Gomè
and Magoumi are located in lowlands and the other sites
are on plateaux. Vegetation is a degraded, woody savanna
type. Villages were selected to represent the land and
ethnic variability in the zone (soil preparation practices,
pool of yam varieties, rotations, etc) according to small-
holders’ origin and the cropping intensity.
Experimental design
Three designs adapted by smallholders integrating A.
histrix (1) with maize intercropped with early burning of
G. sepium shrubs, (2) with maize, and (3) with maize and
sorghum, were compared with their respective controls
(Table 1). In the low population density zone, these were
slash-and-burn perennial fallows (eight years); in greater
population density, they consisted of one-year grass
fallows of A. gayanus, or even continuously cropping
maize–sorghum. G. sepium was planted in 2000 (seven
years of establishment) and trials were monitored in the
2007–08 and 2009–10 cropping seasons with 9 farmers
(Design 1), 24 farmers (Design 2) and 6 farmers (Design 3)
integrating three varieties: early maturing Dioscorea
rotundata (V
1
), late maturing Dioscorea alata (V
2
) and late
maturing Dioscorea rotundata (V
3
). For each adapted yam-
based cropping system, we used a randomized block
design with four replicates. Plot size was 5 m × 5 m for
each variety (total design per farm: 600 m
2
). Smallholders
conducted the three designs in a perennial experiment for
four years, with two-year rotations (Figure 2).
Figure 2. Experimental design of smallholders’ traditional and
improved yam-based systems in the Guinea–Sudan transition
zone (central Benin).
Note: V1 – early D. rotundata; V2 – late D. alata; V3 – late D.
rotundata; T
0
– rotation perennial fallow–yam; T
MAGB
– rotation
maize + A. histrix in G. sepium–yam.
0
50
100
150
200
250
300
350
400
450
JFMAMJJASOND
Month
Ouessè_200
7
Ouessè_200
8
Ouessè_200
9
Ouessè_201
0
0
50
100
150
200
250
300
350
JFMAMJJASOND
Month
Dassa_2007
Dassa_2008
Dassa_2009
Dassa_2010
0
50
100
150
200
250
300
JFMAMJJASOND
Month
Monthly rainfall (mm)
Savè_2007
Savè_2008
Savè_2009
Savè_2010
Monthly rainfall (mm)
Monthly rainfall (mm)
(a)
(b)
(c)
v 1
v 2
v 3
v 2
v 3
v 1
v 1
v 2
v 3
Control T
0
Replication 1
Treatment T
MAGB
Replication 1
v 2
v 3
v 1
Replication 3
v 3
v 1
v 2
v 3
v 1
v 2
Replication 2
v 1
v 3
v 2
v 1
v 3
v 2
Replication 4
Block
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AGRICULTURE Vol 41, No 3
Agronomic and economic performance of yam-based systems
Table 1. Trial designs according to the land use intensity and vegetation degradation of the sites over the four years.
Designs Year 1 (2007) Year 2 (2008) Year 3 (2009) Year 4 (2010)
Design 1 T
0
Yam T
0
Yam
T
MAGB
T
MAGB
Design 2 T
1
Yam T
1
Yam
T
MA-
T
MA-
Design 3 T
2
Yam T
2
Yam
T
MAS-
T
MAS-
Design 1:
•T
0
(control), rotation of perennial fallow–yam.
•T
MAGB
, (rotation maize
/
A. histrix
/
G. sepium–yam): G.
sepium fallow had been planted in July 2000 at a
spacing of 4 m × 4 m with a density of 629 shrubs ha
–1
.
On this plot, after an early fire to reduce the biomass of
Gliricidia sepium, maize (Zea mays L.) was sown at a
spacing of 80 cm × 40 cm in April (first year 2007). A.
histrix seeds (7 kg ha
–1
) were mixed with dry sand and
planted in April (first year 2007), approximately two
weeks after maize.
Design 2:
•T
1
(control), one-year fallow–yam rotation: natural
fallow of A. gayanus grass was naturally established
(first year).
•T
MA-
(rotation A. histrix
/
maize–yam rotation): similar to
T
MAGB,
but without G. sepium.
Design 3:
•T
2
(control), a one-year maize
/
sorghum–yam rotation:
sorghum was planted in June after maize. After sor-
ghum harvesting in December (first year 2007),
smallholders used their stems for staking yam vines
(following year).
•T
MAS-
(rotation A. histrix
/
maize
/
sorghum–yam): similar
to T
MA,
but with sorghum planting in June after maize
and A. histrix.
Data collection
The areal biomass of maize, sorghum, A. gayanus and A.
histrix were measured in each design. Biomass was collected
in October 2007 and 2009 in four 1 m² quadrats within each
plot. The biomass of G. sepium (leaves and branches) and
natural perennial fallow (with Daniellia oliveri dominance)
was estimated by pruning (January 2007 and 2009) in two
representative 25 m² quadrats within each plot. Grass
herbaceous (A. gayanus) biomass from the inferior stratum
of natural perennial fallow was weighted in four 1 m²
quadrats within each plot. Biomass samples collected from
different sources of organic matter were dried at 60°C until
constant weight and dry weight was then determined. In
July and December, maize and sorghum grains were har-
vested from each row on each plot and dry matter (DM)
determined. The fresh yam tuber weight was estimated on
each plot in December 2008 and 2010.
The biomass dry matter (DM) of trees was as follows:
10
–3
*E
s
*D
c
*B
f
B
c
=
——————
(1)
E
f
*N
c
where:
B
c
= biomass dry matter (DM) of trees (t ha
–1
)
E
s
= biomass DM sample (g)
E
f
= biomass fresh matter (FM) sample (g)
B
f
= biomass FM in two 25 m² quadrats (kg)
D
c
= tree density (ha
–1
)
N
c
= tree density in two 25 m² quadrats
The biomass dry matter (DM) of herbaceous residue was
as follows:
10
–3
*S*P
s
*B
fh
B
h
=
——————
(2)
S
0
*P
f
where:
B
h
= biomass dry matter (DM) of herbaceous
residue (t ha
–1
)
B
fh
= biomass FM of herbaceous residue in four 1 m²
quadrats (kg)
P
s
= biomass DM sample (g)
P
f
= biomass fresh matter (FM) sample (g)
S
0
= four 1 m² quadrats
S = 10,000 m²
Statistical analysis
Analysis of variance (ANOVA) was applied to the yam
yields using a randomized block design and a partial
nested model with six factors: year, replication, farmer,
site, variety and treatment. The logarithmic transforma-
tion was applied to yield values in order to normalize the
data and stabilize the variance of populations. The ran-
dom factors were ‘year’, ‘replication’, ‘farmer’ and ‘site’.
The fixed factors were ‘variety’ and ‘treatment’. Sites were
considered as fixed, based on criteria such as landscape
(lowland, plateau), soil type, population density and land
occupation period. The General Linear Model (GLM)
procedure (SAS, 1996) was computed to assess the interac-
tions between the factors involved. When interactions
between major factors were significant, interaction dia-
grams were established to describe the effect of each
factor. Least square means and standard error were also
computed for factor levels, and the Newman and Keuls
test was applied for differences between treatments.
Significance was observed at p 0.05.
Economic analysis
A simple financial analysis was performed to evaluate the
profitability of each yam-based cropping system. We
considered the time horizon from 2007 to 2010 (four years)
and a discount rate of 10% (World Bank standard), which
in fact is similar to bank interest rates. The choice of
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Agronomic and economic performance of yam-based systems
discount rate is always controversial among economists
(Stern, 2006). We considered discount rates ranging from
0% to 50% for sensitivity analysis. The net present value
was calculated as follows:
NPV = (TPR–TPC)
or
NPV =
i=1
n
R
(1+r )
n
n
D
n
(1+r)
n
=
n
(R
n
D
n
)
(1+r )
n
Σ
Σ
Σ
i=1 i=1
n
(3)
where:
NPV = net present value (US$)
TPR = total present revenue (US$)
TPC = total present cost (US$)
R
n
= revenue in the year n (US$)
D
n
= cost in the year n (US$)
r = discount rate (%)
The return on investment (RI%) was also calculated:
RI = 100 × (NPV
/
TPC) (4)
if RI > interest rate on capital, this implied profitability.
Labour productivity in US$ per man-day (LP) was given
by:
LP = NPV
/
L (5)
where L (man day) is the total labour requirement.
Economic yields for sorghum and maize were based on
a 15% moisture content, while that of yam was based on
fresh weight. Costs of production were divided into land
(hired land cost, US$ ha
–1
year
–1
), inputs (maize, yam and
legume seeds, fertilizer costs) and labour (farm activities,
costs for yam-based cropping systems establishment and
management). Land, inputs and labour costs were deter-
mined based on local prices. We considered the average
annual prices for food crops (maize, sorghum and yam)
based on the prevailing market price (Glazoué market in
central Benin). All values were expressed in US$
(FCFA501.8 = US$1, 1 December 2010).
Results
Biomass and yam yields
Before yam planting, improved fallows (T
MA-,
T
MAS-
and
T
MAGB
) showed significantly higher biomass (p < 0.01) than
controls (T
0
, T
1
and T
2
) during both the 2007 and 2009
Table 2. Biomass in three smallholders’ traditional rotations
compared to the three improved fallow designs during the 2007
and 2009 cropping seasons.
Biomass (t ha
–1
)
Design Treatment 2007 2009
Design 1 T
0
16.8b 2.81b
T
MAGB
18.4a 9.28a
LSD 5% 1.05 0.75
SD 2.2 1.58
Design 2 T
1
4.3b 3.91b
T
MA-
9.14a 8.29a
LSD 5% 0.3 0.27
SD 1.04 0.95
Design 3 T
2
6.53b 5.35b
T
MAS-
11.13a 9.74a
LSD 5% 0.55 0.46
SD 0.92 0.76
Note: Means denoted by the same letter within a column are not
significantly different (p < 0.05).
cropping seasons (Table 2). The average areal biomass of
A. histrix from different designs was 6 t ha
–1
year
–1
,
whereas G. sepium showed 4.2 t ha
–1
year
–1
DM, with a
drastic decrease in biomass (leaves and branches) the
subsequent year because of the early fire effect. Maize and
sorghum biomass contributions were 2.75 t and 2.5 t ha
–1
year
–1
DM respectively (results not presented). Yam-based
cropping systems with herbaceous and
/
or shrubby
legumes showed significantly higher yam yields than
traditional systems. The planted fallow integrating G.
sepium can replace traditional slash-and-burn systems of
long natural fallow: it induced a significant increase in
yields, even with an early maturing (and most demanding
in fertile soil) Dioscorea rotundata variety (Table 3).
Yam yields were significantly higher in 2008 than in the
2010 cropping seasons on all sites (Figure 3). This could be
related to the rainfall regime and
/
or to decreasing soil
fertility. Rainfall variability affected yam production. The
delayed rainy season, water stress or excess water can
influence crop growth and yam production (Degras, 1986).
Yam is demanding with regard to water, especially after
germination and between the 14
th
and 20
th
weeks of
vegetative growth (Dansi et al, 2003). The highest yam
yield in 2008 could be justified on one hand by the favour-
able weather conditions compared with 2010 (which was
Table 3. Effect of improved fallows compared to their respective controls according to yam variety (early maturing Dioscorea rotundata,
late maturing Dioscorea alata, late maturing Dioscorea rotundata) in the 2008 and 2010 cropping seasons.
Design 1 Design 2 Design 3
Yam yield (t ha
–1
) Yam yield (t ha
–1
) Yam yield (t ha
–1
)
Treatment Variety 2008 2010 Treatment Variety 2008 2010 Treatment Variety 2008 2010
T
0
V1 23.48c 20.04c T1 V1 15.58c 13.11c T2 V1 16.57c 14.9c
V2 22.54cd 19.92c V2 13.75d 11.97d V2 15.88c 14.8c
V3 17.42e 15.06e V3 10.85e 9.47e V3 12.82d 11.22d
T
MAGB
V1 32.18a 23.91a T
MA-
V1 21.53a 17.62a T
MAS-
V1 21.62a 20.02a
V2 28.97b 22.98b V2 17.95b 15b V2 19.95b 18.4b
V3 21.54d 17.67d V3 13.58d 11.76d V3 15.4c 14.11c
LSD 5% 2.04 1.24 1.3 0.97 1.57 1.44
SD 2.98 1.81 3.1 2.32 1.87 1.72
Note: Means denoted by the same letter within columns are not significantly different (p < 0.05).
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Agronomic and economic performance of yam-based systems
Figure 3. Interaction effects between year and site on yam yield
in Guinea–Sudan transition zone (central Benin) (Design 2).
characterized by an abundance of rains and floods in the
southern part of the study area) and, on the other hand,
by a delay in the onset of the rains in the central and
northern parts of the study area and a deficient rainfall
regime until June (a critical period of yam development).
Carsky et al (2001) reported a yield of approximately 10 t
ha
–1
for yam cultivated after forest or long fallow under
slash-and-burn, followed by decreasing yields in subse-
quent years. In the savanna zone of Nigeria, Watson and
Goldsworthy (1964) estimated yam yields of 11 t ha
–1
after
a three- or four-year fallow and < 6 t ha
–1
after one or two
years’ fallow.
The sites with the highest productivity are those where
the last forests are still being cleared, whilst those with
low productivity have been cultivated for more than one
generation. With regard to yam varieties, the early matur-
ing D. rotundata obtained the highest yield on all sites
(Figure 4), but this effect was significantly higher on the
sites with a high yam potential (yield > 15 t ha
–1
). On sites
with low productivity (yield < 15 t ha
–1
), the late maturing
D. alata performed best. The ANOVA partial nested model
allows these factors to be analysed simultaneously (Table
4). The ANOVA confirms that yam yields are significantly
Yam yield t ha
–1
õ
õ
õ
õ
õ
õ
õ
õ
õ
2008
2010
ü
ü
ü
ü
ü
ü
ü
ü
ü
19
18
17
16
15
14
13
12
11
10
Boubou
Adjanoudoho
Akpéro
Dani
Gbanlin
Gomè
Magoumi
Miniffi
Site
Table 4. ANOVA in a partial nested model of main factor effects of 2007–08 and 2009–10 trials on logarithmically transformed values of
yam yields in three yam-based systems.
Factors Design 1 Design 2 Design 3
Source DF F P DF F P DF F P
Farmer (site) 6 22.02 0.001 16 13.81 0.000 4 14.26 0.012
Year 1 6.52 0.085 1 22.03 0.001 1 7.28 0.233
Replication 3 2,067.53 0.000 3 1,708.7 0.000 3 694.34 0.000
Site 2 1.68 0.233 7 1.8 0.128 1 2.49 0.172
Treatment 1 14.68 0.083 1 187.56 0.001 1 1,724.88 0.732
Variety 2 13.57 0.011 2 16.28 0.000 2 14.35 0.067
Site × treatment 2 2.78 0.174 7 3.58 0.058 1 0.17 0.736
Treatment × farmer (site) 6 4.53 0.000 16 1.2 0.261 4 1.48 0.207
Year × treatment 1 9.02 0.095 1 2.88 0.133 1 1.09 0.486
Year × site 2 13.69 0.068 7 14.8 0.001 1 13.13 0.171
Treatment × variety 2 9.28 0.000 2 11.01 0.000 2 4.16 0.017
Site × variety 4 76.27 0.000 14 69.42 0.000 2 23.39 0.000
Year × variety 2 8.12 0.000 2 5.23 0.006 2 0.79 0.454
Year × site × treatment 2 6.75 0.001 7 1.41 0.199 1 0.9 0.345
Error 395 1,065 261
Adjusted R² (%) 96.76 91.56 92.68
Note: DF = degrees of freedom; F = Fisher test; P = probability.
Table 5. Estimated present annual costs of production, net present value, returns on investment, labour requirement and labour produc-
tivity of three different yam-based cropping systems: time horizon four years, discount rate 10%, 2007–08 and 2009–10 cropping seasons.
Design 1 Design 2 Design 3
T
0
T
MAGB
T
1
T
MA
T
2
T
MAS
Total present revenue (US$ ha
–1
) 2,373 3,324 1,235 2,108 1,661 2,272
Production cost (US$ ha
–1
)
Land 10 10 10 10 10 10
Input 697 704 697 704 702 705
Labour 171 407 219 357 334 456
Total present cost (US$ ha
–1
) 724 911 988 872 852 948
Net present value (US$ ha
–1
) 1,956 2,709 1,046 1,723 1,358 1,874
Return on investment (%) 54 59 21 39 32 39
Labour requirement (man day ha
–1
year
–1
) 51 111 63 100 98 131
Labour productivity (US$ ha
–1
year
–1
) 38.3 24.4 16.6 17.2 13.9 14.3
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Agronomic and economic performance of yam-based systems
Figure 4. Interaction effects between variety and site on yam
yield in Guinea–Sudan transition zone (central Benin) (Design 2).
Note: V1 – early maturing Dioscorea rotundata; V2 – late maturing
Dioscorea alata; V2 – late maturing Dioscorea rotundata.
different only for design 2 (p < 0.01), but these differences
become significant for all designs if the interaction be-
tween treatment and variety is taken into account. The
heterogeneity of results should be related to the small-
holders’ individual effects and practices.
Profitability of different yam-based cropping systems
Net present value (NPV) and return on investment (RI)
are higher in yam-based cropping systems with shrubby
and
/
or herbaceous legumes (T
MAGB
, T
MAS-
and T
MA-
) than in
controls (T
0
, T
1
and T
2
) (Table 5). However, the yam-based
agroforestry system with Gliricidia requires more labour
and shows a lower labour productivity than perennial
natural fallow. Figure 5 shows the NPVs of various yam-
based systems with a time horizon of four years (2007–09)
according to varying discount rates. These reflect the
alternative of the investment opportunities and diverse
farmers’ preference for investments rather than an imme-
diate income. Yam-based systems with Aeschynomene
make NPVs per surface unit higher than all the local
systems, including the perennial natural fallow (T
0
)
during the first four years. However, the systems with
Aeschynomene without agroforestry (T
MA
) exceed the
systems with degraded fallow of Andropogon (T
1
) and
continuous cropping (T
2
).
Discussion
Agronomic performances of yam-based cropping systems
Yam yields are significantly influenced by interactions in
treatment × variety, site × variety and year × variety
(Designs 1 and 2). The effects of yam-based cropping
systems with herbaceous legumes on yam yields are
significantly higher than in the degraded natural fallow of
the local yam-based cropping systems (T
0
and T
1
). In the
latter, frequent bush fires reduce the biomass restitution
from degraded A. gayanus grass fallows, and such fallows
have low nitrogen content (Adjei-Nsiah et al, 2007). Then
they produce less and lower-quality biomass than A.
histrix and maize residues (and sometimes sorghum).
These results confirm the well known importance of soil
organic matter for yam production. Diby et al (2009)
õ
õ
õ
õ
õ
õ
õ
õ
õ
ü
v1
v2
v3
ü
ü
ü
ü
ü
ü
ü
ü
Yam yield t ha
–1
26
24
22
20
18
16
14
12
10
Adjanoudoho
Site
Akpéro
Boubou
Dani
Gbanlin
Gomè
Magoumi
Miniffi
0 20 45
Discount rate (%)
Net present value in four years (US$ ha
–1
)
T
0
: Rotation long shrubby
fallow–yam (slash-and-burn)
T
MAGB
: Rotation maize +
A. histrix + G. sepium–yam
(early fire of shrubs)
T
1
: Rotation short fallow
A. gayanus (slash-and-burn)
T
MA
: Rotation maize +
A. histrix–yam
T
2
: Rotation maize +
sorghum–yam
T
MAS
: Rotation maize +
A. histrix + sorghum–yam
14,000
12,000
10,000
8,000
6,000
4,000
2,000
0
Figure 5. Profitability of yam-based cropping systems integrating shrubby and herbaceous legumes in comparison with traditional yam-
based systems (time horizon: 4 years, 2007–10 cropping seasons).
177
Outlook on
AGRICULTURE Vol 41, No 3
Agronomic and economic performance of yam-based systems
reported the high influence of soil organic matter on yam
growth and tuber production in forest (high organic
fertility soil: 14.4% C at 0–10 cm and 9.75% C at 10–20 cm)
and savanna (7.49% and 5.43% C). Organic materials
supplied contribute directly to the building of soil organic
matter (SOM), which itself improves the physical, chemi-
cal and biological composition of the soil (Sanginga and
Woomer, 2009). In general, residues from legumes decom-
pose faster than other organic residues, thus contributing
to the rapid recycling of nutrients for the subsequent crop.
More astonishing is the effect of the perennial fallow of G.
sepium (seven years), which is significantly higher than in
the degraded perennial natural fallow (eight years). The
perennial fallow of G. sepium would better rebuild the soil
organic matter stock than a spontaneous long-duration
fallow. Indeed, the traditional shifting cultivation and
slash-and-burn systems contribute to the synergistic effect
between the organic residues, the nutrients contained in
ashes directly used by the crop, staking the yam vines and
access to light, improving the photosynthetic activity of
the crop.
The agroforestry system with G. sepium + maize + A.
histrix under early burning of shrubs produces an addi-
tional effect. Incorporating a legume increases the N
stocks in the soil because of its ability to fix N
2
from the
atmosphere. In addition, legumes suppress nematodes
and contribute to soil moisture conservation, reduce soil
temperature and maintain a beneficial microbial commu-
nity in the rhizosphere. All these factors are important for
a better performance of tubers (Obiagwu, 1997). Nye and
Greenland (1960) reported on the contribution of peren-
nial fallow for soil fertility maintenance, showing why
fallow with Gliricidia sepium, which is a perennial coppice-
fallow, was more productive than the perennial natural
fallow in central Benin with some shrubs and woods on
the grass carpet – especially in year 3 when the coppice
regenerated, whereas the trees of the perennial natural
fallow were incinerated, died, and were replaced by A.
gayanus grass.
Profitability of yam-based cropping systems
The yam-based systems with G. sepium fallow under early
burning of shrubs (T
MGB
) showed the highest land and
cash productivity, with net present value levels signifi-
cantly higher than the other systems, including perennial
fallow (T
0
). These results confirm the earlier work of
Bamire and Manyong (2003) and Sodjadan et al (2005),
who reported on the profitability of intensification tech-
nologies with legumes. Nevertheless, the yam-based
system with G. sepium fallow (T
MGB
) requires much more
labour than the perennial spontaneous fallow (T
0
). Even
after smallholders have adapted this system to reduce the
additional labour demand by replacing pruning with
early burning, it still constitutes a limiting factor for
adoption. Indeed, not all smallholders will anticipate the
more than proportional improvement of output and cash.
Implications of our findings for international application
For international application, technologies including off-
forest multipurpose trees (G. sepium, Moringa oleifera,
Jatropha curcas) with an adapted density could be used
with herbaceous legumes (A. histrix, M. pruriens, S.
guianensis) or grass in a dynamic rotation with seasonal or
annual crops (maize, soybean, yam, cassava, potato, rice,
cotton). Adapted agroforestry systems with early burning
can include animal or mechanical traction. Our findings
will help farmers to save part of their financial resources
used for pruning in agroforestry systems or clearing
traditional wooded perennial fallows or forests, and will
ensure time economy with a positive impact on soil
productivity. In addition, the application will ensure that
natural resources are better preserved, chemical pollution
of the environment is reduced and agricultural production
diversified. Furthermore, this could contribute to greater
plant biodiversity, promotion of biofuel, improved access
of smallholders to land resources, as well as their capacity
to manage resources. This influence may be strengthened
by suitable organizational arrangements to promote
adapted technologies involving researchers, extension
workers and farmers.
Conclusion
Yam yields were significantly higher in rotations with
shrubby and herbaceous legumes than in traditional
rotations. Interactions of factors such as treatment, site,
variety, year-through rainfall regime and smallholders’
individual effects and practices influence yam yields
significantly. The system with the highest soil productiv-
ity combines A. histrix and agroforestry with G. sepium,
but it requires much more labour than the other systems.
Systems with A. histrix without agroforestry supersede
systems with a one-year fallow of A. gayanus as well as
continuous cropping with maize and sorghum. These
improved systems should be an alternative to farmers’
traditional slash-and-burn and shifting cultivation sys-
tems, and should then contribute to forest protection from
new field clearing. Research is needed to address further
simulations on the perennial fallow with G. sepium accord-
ing to tree density. It would be necessary to study other
ways of reducing the workload without the early burning.
For example, annual herbaceous legumes could be estab-
lished including a fence-based G. sepium system with trees
for yam vine staking.
Acknowledgments
The authors express their sincere appreciation and thanks
for financial support received from the Food and Agricul-
ture Organization of the United Nations (FAO), the
Cooperation Project for Academic and Scientific Research
(CORUS) and the French Embassy in Benin. Finally, our
utmost appreciation goes to smallholder farmers who
agreed to participate in the trials and made part of their
fields available for the research.
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There are needs to increase the utility of grain legumes in the Tropical Ecosystem. Methodology for selecting an ideal grain legume cover crop for various cropping systems was proposed. There were series of step-wise processes involving field observations, field experimentations, greenhouse trials, and laboratory analysis. The observed data were ranked. These ranks guide the classification or selection of crops into specific cropping systems, where they may be bio-ecologically suitable as cover crops. The ranking rated the crop productivity of the food legume crops under tests. The ranks deduced from parameter indices (PR) are multiplied by the experimentally scaled ranks (exp-R) which are relative to the yield levels achieved by crops. Thus, ‘Crop Productivity Rating’ (CPR) = PR (exp-R). Characters compatible to intercropping were also evaluated.
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Rotations are important practices for managing soil fertility on smallholder farms. Six cropping sequences (cassava, pigeonpea, mucuna– maize–mucuna, cowpea–maize–cowpea, maize–maize–maize, and speargrass fallow) were evaluated during 2003–2004 in Wenchi district of Ghana for their effects on the profitability of the different rotations and the productivity of subsequent maize. Soil chemical properties were not significantly affected by cropping sequence. On the researcher-managed and farmer-managed plots maize grain yields were significantly influenced by cropping sequence. On the researcher-managed plots maize grain yield ranged from 1.0 t ha À1 after speargrass fallow to 3.0 t ha À1 with cassava cropping when N fertiliser was not applied to maize and from 2.1 t ha À1 with continuous maize to 4.2 t ha À1 with mucuna–maize– mucuna when 60 kg N ha À1 was applied to maize. On the farmer-managed plots where N fertiliser was not applied to maize, maize grain yields ranged from 0.4 t ha À1 on speargrass fallow to 2.2 t ha À1 on plots previously cropped to pigeonpea. High maize grain yields associated with the cropping sequences involving cassava, mucuna and pigeonpea were related to the faster decomposition and N release of the biomass compared with the slower release of N by the poorer quality materials like maize stover and speargrass. Return on investment of the different rotational sequences ranged from À22% with speargrass/maize to 235% with cassava/maize when no N application was made to maize, and from 29% with continuous maize to 196% with cassava/maize when N fertiliser was applied to maize. Cassava/maize rotation was ranked by native farmers as the most preferred rotation whereas migrant farmers ranked cowpea–maize–cowpea–maize as the most preferred rotation. Among natives, male farmers ranked rotation involving cowpea as the next most preferred rotation after cassava/maize. In contrast, female farmers ranked pigeonpea/maize rotation as the second most preferred rotation, due to low labour and external input requirements of pigeonpea compared with cowpea. The choice of a particular rotational sequence is related to access to resources and the needs of the farmer. The study therefore suggests that, in a heterogeneous farming community like Wenchi, technology development should be targeted to suit the needs and resources available to each particular group of farmers. # 2007 Elsevier B.V. All rights reserved.
La culture en couloirs dans les tropiques humides et subhumides
  • B T Kang
Kang, B.T., and Reynolds, L. (1986), 'La culture en couloirs dans les tropiques humides et subhumides', Compte rendu d'un atelier international tenu à Ibadan, Nigeria, du 10 au 14 mars 1986, Ibadan, pp 115–116.
Effets des précédentes plantes de couverture sur la production de l'igname en zone de savane au Bénin et au Togo
  • Sas Institute
  • Nc Cary
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  • P Vernier
SAS (1996), SAS User's Guide: Statistics, SAS Institute, Cary, NC. Sodjadan, P.K., Toukourou, A.M., Carsky, R.J., and Vernier, P. (2005), 'Effets des précédentes plantes de couverture sur la production de l'igname en zone de savane au Bénin et au Togo', African Journal of Root and Tuber Crops, Vol 6, No 1, pp 34–40.