全 文 ：Journal of Forestry Research (2010) 21(3): 379−386
DOI 10.1007/s11676-010-0085-0





Effects of improved fallow with Sesbania sesban on maize productivity
and Striga hermonthica infestation in Western Kenya

Hans Sjögren • Keith D Shepherd • Anders Karlsson




Received: 2009-09-14 Accepted: 2009-12-15
© Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2010
Abstract: Striga hermonthica is a major constraint to smallholder sub-
sistence agriculture production in the sub-Saharan African region. Low
soil fertility and overall environmental degradation has contributed to the
build-up of the parasitic weed infestation. Improved cropping systems
have to be introduced to address the interrelated problems of S. her-
monthica and soil fertility decline. Thus, the effects of improved fallow
with leguminous shrub Sesbania sesban on maize yields and levels of S.
hermonthica infestation on farm land in the bimodal highlands of western
Kenya were investigated. The experimental treatments were arranged in a
phased entry, and randomized complete block scheme were six months
Sesbania fallow, 18 months Sesbania fallow, six months natural fallow
consisting of regrowth of natural vegetation without cultivation, 18
months natural fallow, continuous maize cropping without fertilizer
application, and continuous maize cropping with P and N fertilization.
Results show that Sesbania fallows significantly (p<0.05) increase maize
yield relative to continuous unfertilized maize. S. hermonthica plant
populations decrease in continuous maize between the first season (mean
= 428 000 ± 63 000 ha-1) and second season (mean=51 000 ± 15 000 ha-1),
presumably in response to good weed management. S. hermonthica seed
populations in the soil decrease throughout the duration of the experi-
ment in the continuous maize treatments. Short-duration Sesbania fal-
lows can provide modest yield improvements relative to continuous
unfertilized maize, but short-duration weedy fallows are ineffective.
Continuous maize cultivation with good weed control may provide more
effective S. hermonthica control than fallowing.

Foundation project: This work was supported by Swedish International
Development Cooperation Agency (Sida)
The online version is available at http://www.springerlink.com
Hans Sjögren • Anders Karlsson
Department of Forest Ecology and Management, Swedish University of
Agricultural Sciences S-901 83 Umeå Sweden.
Email: Hans.Sjogren@ssko.slu.se
Keith D Shepherd
World Agroforestry Centre (ICRAF), P.O Box 30677-00100, Nairobi,
Kenya.
Responsible editor: Hu Yanbo

Keywords: agroforestry; crop yield; improved fallow; residual effect;
root parasite; soil fertility replenishment


Introduction

The obligate root parasitic weed Striga hermonthica (Del.) Benth.
constitutes one of the most important biotic constraints to the
production of food crops for small-scale farmers in sub-Saharan
Africa (Scholes and Malcolm 2008; Kiwia et al 2009). It infests
and may cause severe stunting and yield losses of 30–90% (van
Ast et al. 2005) in the staple food and industrial crops of the
region, including cereals (e.g. sorghum, pearl millet, finger millet,
maize, rice, wheat, and sugar cane), some broadleaf crops such
as cowpeas, sunflower, and soybean, and various cultivars of
beans. The incidence of S. hermonthica is known to negatively
correlate with soil fertility, particularly nitrogen (N) availability
(Pieterse and Verkleij 1991). Several studies have confirmed
reduced S. hermonthica infestation and increased crop yield
when high level of N was applied to the crop. In the East African
highlands, despite high yield potential, continuous cropping with
low levels of nutrient inputs has led to declining soil fertility and
a high prevalence of related crop pests and diseases, resulting in
low crop yields (Buresh et al. 1997). In Kenya, S. hermonthica
was estimated to infest about 75 000 ha of maize crop (Hassan et
al. 1995) and the rate of infestation, both in severity and spread,
has been increasing over the years as depletion in soil fertility in
smallholders’ farms continues.
The control of S. hermonthica is difficult to achieve because
of its high fecundity (Andrianjaka et al. 2007); also its roots
grow into the crop roots and sap out all the plants nutrients. In
addition, the seed germination is asynchronous (Worsham and
Egley 1990). Therefore management of S. hermonthica infesta-
tion needs an integrated approach including host plant resistance,
cultural practices, and chemical and biological treatment (Andri-
anjaka et al. 2007). Improved fallow systems, which involve the
use of perennial legume shrubs, are receiving increased research
attention as a promising method for resource-poor farming com-
munities (Pisanelli et al. 2008). Improved fallow requires inter-
ORIGINAL PAPER
Journal of Forestry Research (2010) 21(3): 379−386

380
ruption of cereal production, which may not be favorably ac-
cepted by subsistence farmers. However, it could be an attractive
option as it accelerates the process of soil rehabilitation and
thereby shortens the length of the fallow period. Improved fal-
lows where nitrogen-fixing trees are planted may increase soil
fertility more quickly than natural weedy fallows (Hassan et al.
1995). Further legume shrubs can be valuable sources of scarce
commodities (fodder and fuelwood), and improve soil nutrient
status, particularly nitrogen, through biological nitrogen fixation
and nutrient recycling (Hartemink et al. 2000). Evidence sug-
gests that perennial legume species could improve soil chemical
and physical properties, creating less favorable environment for
the pest (Gallagher et al. 1999). The trees and shrubs in the
fallow also provide another important service to the farmer: they
fill the space and impede the establishment of undesirable weeds.
Many kinds of invasive and problematic weeds thrive in open,
sunny conditions on vacant land, but do not spread into areas that
are cooler and shadier. The plants that are part of the improved
fallow create conditions that are unfavorable to most problematic
weeds, making the subsequent establishment of crops easier than
if the area had to be cleared of undesirable weeds.
In eastern and southern African region, smallholder maize
farmers are increasingly using Sesbania sesban (L.) Merr. (ses-
bania) as a major source of N input to their N-deficient soils to
increase productivity (Niang et al. 1996). It is a fast growing N2-
fixing tree with important agroforestry attributes, e.g. provision
of fuelwood, fodder and high biomass for soil fertility replen-
ishment. Sesbania, grown in rotation with crops has been shown
to improve soil fertility and increase crop yields (Kwesiga and
Coe 1994). In high rainfall areas of western Kenya, Jama et al.
(1998b) found that one and a half year Sesbania fallows were
more effective than a natural fallow in increasing maize yields.
The same pattern was found in a meta-analysis of experiments
on the response of maize to fallows in sub-Saharan Africa (Sile-
shi et al. 2008). Sesbania and natural fallows compared to maize
monoculture have been shown to increase the nitrogen in light
fraction soil organic matter and available soil N (Maroko et al.
1998). A sizable proportion of N in Sesbania fallows is a net
input of N to the soil-plant system through biological N2 fixation
and capture of nitrate from below the rooting depth of crops
(Kwesiga and Coe 1994; Jama et al. 1998a). Some of the benefi-
cial effects on yields of fallows may be related to a rotational
effect. However, there is limited knowledge (Crookston et al.
1991) on how different duration fallows affect residual crop
yields to guide economic analyses and recommendations to
farmers. Thus the present study aimed at evaluating the residual
effects of six- and 18 months Sesbania and natural fallows com-
pared with continuous maize production on maize yields, the
prevalence of S. hermonthica plants and soil seed bank.


Material and methods

Site description

The study site is located in Central Bunyore, Vihiga District
(00°06’N, 34°34’E) at an altitude of 1,430 m a.s.l. in Western
Kenya. The cropping seasons were bimodal. The long rains (LR)
cropping season are from March to July and the short rains (SR)
cropping season are from September to January. Based on data
collected from in situ mini-weather station at the site, the mean
(± SE) annual rainfall during the period (1994–1997) was 814 ±
32 mm for the long rain season and 740 ± 149 mm for the short
rain season. The overall mean annual rainfall for the period was
1,552 ± 178 mm with large inter-annual and inter-month vari-
ability (Fig. 1). The most frequently encountered soils are Kan-
diudalfic Eutrudox according to FAO soils classification system.
The site was previously cropped with maize. The site was chosen
to be P-sufficient for maize, but degraded from continuous crop-
ping and with a high prevalence of S. hermonthica.


Fig. 1 Mean monthly rainfall (mm) during the study period at Ebu-
kanga experimental site, Vihiga district, western Kenya

The agricultural system in the study area is characterized by
traditional subsistence farming of mixed crop-livestock. The
major crops are maize, mostly unimproved varieties and bean
(Phaseolus vulgaris L.). Farmlands are small (mean size < 2 ha)
due to high population densities and the subdivision of farms for
inheritance. In spite of the small holding, short duration unman-
aged fallows are common at Bunyore (Swinkels et al. 1997). For
example, a survey in western Kenya showed that 52% of the
farmers periodically fallow 10–50% of their total farmland at a
time (Swinkels et al. 1997). The lengths of the fallow varied
between one season (24% of fallowers), one year (35%) and two
or more years (42%).

Experimental design

A randomized complete block design with four replicates ex-
periment (Fig. 2) was established to examine the effect of im-
proved fallow with leguminous shrub Sesbania sesban on maize
yields and levels of S. hermonthica infestation on farm land in
the bimodal highlands of western Kenya. Six treatments were
applied: six months tree growth with Sesbania fallow (S6); 18
months tree growth with Sesbania fallow (S18); six months
natural regrowth of vegetation fallow without cultivation (N6);
18 months natural regrowth of vegetation fallow without cultiva-
tion (N18); continuous maize cropping (M), and continuous
maize cropping associated with fertilizer (60 kg P2O5 + 60 kg
N/ha/season) application (MF). A phased entry design was ap-
plied to allow comparisons between crop yields after different
Journal of Forestry Research (2010) 21(3): 379−386

381
lengths of fallows within the same year (Table 1). Gross plot size
was 10 m × 10 m and the net plot was 6 m × 7 m. Borders
around the gross plots of 1.5 m were used to allow trenching.
Border trenching was done down to 1 m below the soil surface to
minimize root competition between plots. The plots were laid so
as to minimize within plot variation, on the basis of variation in
previous maize growth and assessment of soil characteristics.

















Crop and soil management

The experimental area was uniformly cropped with unfertilized
maize during the growing season (short rain in 1993) preceding
the start of the experiment to facilitate subdivision into blocks.
At the establishment of the experiment pre-trial soil physical and
chemical properties were assessed. Soil sampling was done after
tillage, when soil had settled after early rain, by taking a compos-
ite of nine samples per plot at depths 0–15, 15–30, 30–50, 50–
100, 100–150 and 150–200 cm. The 0–15 cm samples were
taken with a core sampler of internal diameter 2.5 cm and the
samples from below 15 cm were taken using a 5.28-cm diameter
auger. The soil analyses were conducted by the ICRAF soil
laboratory using standard methods, as reported by Shepherd and
Walsh (2002). Some of the physical and chemical properties of
the composite sample of the soil are presented in Table 2. Fertil-
izer test strips, outside the plots, (1 m2) were laid out at the site
prior to the experiment to determine maize response to fertilizer.
Land preparation prior to the experiment consisted of slashing of
the maize stover and weeds from the previous crop and removal
of all organic residues except on the plots where natural weed
fallow treatment was involved. Tillage was done using human
labour to limit biomass and S. hermonthica seed carryover be-
tween plots.

Table 1. Summary of the experimental treatments at each growing
season.
Treatments 1994 1995 1996 1997 1998
LR SR LR SR LR SR LR SR LR
S6 M M MS6 S6 M M M M M
S18 MS18 S18 S18 S18 M M M M M
N6 M M M N6 M M M M M
N18 M N18 N18 N18 M M M M M
M M M M M M M M M M
MF MF MF MF MF MF MF MF MF MF
LR=long rains period (March-July), SR=short rains period (September-
January), S6=six months tree growth with Sesbania fallow, S18=18 months
tree growth with Sesbania, N6=six months natural re-growth of vegetation
fallow without cultivation, N18=18 months natural re-growth of vegetation
fallow without cultivation, M=continuous maize cropping, and
MF=continuous maize cropping with fertilizer (60 kg P2O5 + 60 kg N/ha)
application, MS=Maize and Sesbania intercropped.

Table 2. Soil properties at the start of the experiment in 1994 at Ebukanga experimental site, Vihiga district, western Kenya.
Soil depth
(cm)
pH in water Exchangeable Ca
cmolc ·kg-1
Exchangeable Mg
cmolc ·kg-1
Exchangeable K
cmolc ·kg-1
Extractable P
mg·kg-1
Total organic C
%
Sand
%
Silt
%
Clay
%
0–15 6.2 8.0 1.8 0.65 13 1.81 35 30 35
15–30 6.3 7.8 1.7 0.45 18 1.43 34 21 45
30–50 6.4 6.4 1.5 0.37 10 0.96 30 16 54
50–100 6.6 4.7 1.4 0.49 6 0.64 28 18 54
100–150 6.4 3.6 0.9 0.57 3 0.41 34 18 48

Three maize seeds (Kenya Seed Company, hybrid 511 Zea
mays L.) were sown and thinned to one plant per hole after
emergence. The maize stand density for all seasons was 53 000
plants·ha-1 (0.75 m × 0.25 m spacing) which is the agricultural
extension service recommendation. The above-ground maize
biomass and weeds were removed from the plots at each harvest.
The plots were kept weed-free throughout the growing seasons
by hoe and two hand weeding operations. Weeding of S. her-
monthica was carried out as part of the regular weeding practiced
by farmer. Predominant weeds were Tephrosia holstii Taub.
Vernonia lasiopus O. Hoffm., and Digitaria velutina P. Beauv.
The tree material used was the leguminous N2-fixing species
Sesbania Kisii, Maseno, Kenyan provenance from an ICRAF
tree screening trial in Malawa, Kakamega district. The Sesbania
fallows were established through direct sowing together with
maize in the long rains of 1994 and 1995 to allow establishment
and survival of the trees. After soaking seeds overnight, Sesbania
was sown at a rate of five seeds per hole at 0.75 m × 0.25 m
spacing in the middle of the maize inter-rows. The Sesbania was
thinned to three plants per hole after emergence and to one plant
per hole when 15 cm high. The Sesbania was inoculated at the
time of sowing using compatible Rhizobium inoculants from
Nairobi University. Maize and Sesbania were growing together
for four months before the maize was harvested and the actual
fallow period started. In the 6- and 18-month natural fallows we
let the natural weed vegetation grow without interference. All the
Fig. 2 Layout of the
experimental design –
a randomised complete
block design with four
replicates (blocks)
Journal of Forestry Research (2010) 21(3): 379−386

382
Sesbania and natural weed fallows were harvested in January
1996 and then five successive maize crops were grown.

Plant measurements

At maturity, maize was harvested and the fresh weight of stover
and cobs were recorded in the entire plot. Cobs were separated
into core and grain. Subsamples of cobs and stover were taken
from each plot and air-dried. At the end, maize grain yields were
expressed on 15% water content. During each cropping season,
S. hermonthica assessment was done once. Aboveground S.
hermonthica biomass was collected within each plot from five
randomly selected spots (0.75 m × 1.33 m) at the end of each
cropping season. The location of these spots was chosen system-
atically to avoid selecting the same location in consecutive years.
Weeds were harvested manually by cutting at the base and mixed
together. Fresh weight of the weed mixture was taken and mois-
ture content was determined on a subsample of 300 g oven-dried
at 70°C until constant weight. Within each net plot of 6 m × 7 m,
we also recorded the number of individuals of S. hermonthica a
few weeks before the harvest. S. hermonthica plants were not
uprooted but left to seed until maize was harvested. Further,
quantification of seed bank dynamics of S. hermonthica was
undertaken in all plots. Core samples were collected from nine
spots at the centre of each plot, bulked and then placed in bags.
These samples were air-dried and grounded to pass through a 2-
mm sieve. From the sieved a sample of 500 g was separated to
determine the S. hermonthica seed bank using the elutriation and
column separation method (Ndung’u et al. 1993).

Statistical analysis

Yield data and S. hermonthica counts were subjected to analysis
of variance. In the analysis counts of S. hermonthica plant and
seed, and yield data showing heterogeneity of variance were
transformed to their logarithm values before analysis to improve
the normality. To account for zero values in some of the initial
data on S. hermonthica plants log (x+1) transformation was done
before doing analysis of variance. Descriptive statistics presented
are of original untransformed data. ANOVA was performed
separately for each parameter using the following model:
ijjiij eTBY +++= μ
where Yij is the response variable, μ is the overall mean, Tj is the
effect of the treatment, Bi is the block effect and eij is the error
term. Multiple comparisons were made with Tukeys test to
detect differences between treatments at 5% level of signifi-
cance. All statistical analyses were done using SPSS 16 software
package (SPSS for Windows, Release 2007 Chicago: SPSS Inc.).


Results

Maize yield during the fallow and the rotation cycle

As a result of the fallow period there was no maize production
for one season in the 6-month fallows (SR 1995) and for three
seasons in the 18-month fallows (SR 1994 and 1995 and LR
1995). The total above-ground biomass of maize (cobs + stover)
during the first cropping season (LR 1994) of the fallow did not
differ significantly (F5, 15 = 1.71, p = 0.193) with respect to the
treatments. The total above-ground biomass (LR 1994) was 2.10
± 0.66 and 2.70 ± 0.40 t⋅ha-1, respectively in the Sesbania fallow
(S18) and the natural fallow (N18); it ranged from 2.43 ± 0.49 to
3.13 ± 0.71 t⋅ha-1 in the S6 and N6 while it was 2.73 ± 0.53 and
5.00 ± 1.34 t⋅ha-1, respectively in the continuous cropping (M)
and the continuous cropping with fertilization (MF). In addition
neither grain, core, cob or stover yield differed (p > 0.05) be-
tween treatments at the end of the long rains cropping season
during the first year of the fallow (Fig. 3). During the establish-
ment season of the Sesbania fallow, when maize and Sesbania
were intercropped, there was also a loss in maize production.
During the intercropping season, the loss in grain yield relative
to continuous unfertilized maize was 16% in S6 and 27% in S18.
The cumulative loss in grain yield during the four seasons
(maize/Sesbania one season, Sesbania fallow three seasons) of
the fallow period relative to continuous unfertilized maize was:
S18, 87%; N18, 84%; S6, 35%; and N6, 23%. On the other hand,
fertilizer application increased grain yield by 56% during the
fallow period (Fig. 3A).
Productivity of the different rotation systems was established
after four seasons. Lower maize yields were recorded in the short
rainy season as compared with the long rainy season (Fig. 3).
The residual total above-ground biomass of maize (cobs + stov-
er) in the first growing season after the fallow appeared to be the
highest (F5, 15 = 33.44, p < 0.001) at the Sesbania fallows (10.87
± 0.94 t⋅ha-1 and 9.68 ± 0.99 t⋅ha-1, respectively for S18 and S6)
while it was similar for all other treatments (mean values ranged
from 3.74 ± 0.58 t⋅ha-1 and 5.14 ± 0.66 t⋅ha-1). In the first long
rain season after the fallow, grain yield increased significantly
(F5, 15 = 57.93, p < 0.001) relative to continuous unfertilized
maize plots by 239% in the 18-month Sesbania fallow and 170%
in the 6-month Sesbania fallow. Grain production was signifi-
cantly higher on Sesbania fallows than in continuous unfertilized
maize for two growing seasons (SR and LR) for the 18 months
and for one growing season (LR) for the six months Sesbania
fallow (Fig. 3A, B). In the second year after the fallow (1997),
no statistically significantly difference was observed between
treatments for maize grain yield during the long rain season. In
contrast during the short rain cropping season, grain yield was
similarly higher in the continuous maize with fertilizer and the
S18 (F5, 15 = 7.86, p = 0.001) compared with the other treatments.
In the third year after the fallow (1998) grain yield was higher
(F5, 15 = 20.57, p < 0.012) in the continuous maize with fertilizer
than other treatments. Cumulative maize grain yield for five
seasons after the Sesbania fallow was 41% higher in S6 and 87%
higher in S18 than in continuous unfertilized maize, which
yielded 7.9 t⋅ha-1. Cumulative maize grain yields in continuous
maize were 56% higher for plot with fertilizer than without. One
season of maize cropping after the fallow was required to recover
the loss in grain production for the 6-month Sesbania fallow and
two seasons for the 18-month fallow. Four seasons of continuous
Journal of Forestry Research (2010) 21(3): 379−386

383
seasons of continuous maize cropping could not recover the
maize grain lost after the natural fallows. The overall net grain
yield due to the effect of the treatments relative to continuous
unfertilized maize over the entire experiment was: N18, -5.1 t⋅ha-
1; N6, -1.2 t⋅ha-1; S6, 1.0 t⋅ha-1; S18, 1.5 t⋅ha-1; MF, 7.9 t⋅ha-1.
These calculations presume that there was no interaction with
climate variation during that period, since precipitation and
temperature did not show marked inter-season variation during
the main part of the growing seasons.




Fig. 3 Yield of maize grain (A, B), maize core (C, D), maize cob (E, F) and maize stover (G, H) during the fallow (kg⋅ha-1) and the experimental
period at Ebukanga, Vihiga district, western Kenya. Within the same year mean (±SE) with different letter are statistically different based on Tukey’s test.

In the first long rain following the fallow period core, cob and
stover productions were higher (p < 0.05) after the 18 months, 6
months Sesbania fallow than all other treatments. The residual
benefit for Sesbania fallow lasted for two seasons for stover as
Journal of Forestry Research (2010) 21(3): 379−386

384
indicated by the higher yield in the long rain of 1997 following
18 months Sesbania fallow than other land uses systems (F5, 15 =
2.62, p = 0.048) (Fig. 3G). When fertilizer was supplied (MF) a
yield increase was observed for cob and core (p < 0.001) during
the long rain of the last year of the experiment compared to the
other treatments. Yield of stover, core and cob was generally
high during the long rainy season as compared with the short
rainy season. The residual effect of the Sesbania fallow lasted for
one consecutive short rainy seasons for stover, cob and core
following S18 treatment.

Striga hermonthica plant numbers

Striga hermonthica infestation at the onset of the fallow period
was generally high (ranging from 227 000 ± 73 000 to 496 000 ±
158 000 shoot per ha in the long rainy season of 1994) although
no significant difference was observed between the treatments (p
= 0.529). After the first cropping season of the fallow period (in
the short rainy season of 1994) S. hermonthica numbers were
tremendously reduced in all treatments (Table 3). S. hermonthica
plant populations decreased dramatically in continuous maize
between the first season (mean = 428 × 103 ha-1) and second
season (mean = 61 × 103 ha-1). In the first cropping season fol-
lowing the fallow period, S. hermonthica number was signifi-
cantly (F5, 15 = 13.09, p < 0.001) higher in the N18 as compared
with the other treatments; S. hermonthica number was the lowest
at S18. Compared with the plots that were continuously maize
cropped (M), there was a temporary flush in the number of S.
hermonthica plants in the first two seasons after the fallow on the
plots that received natural weed fallow (N18) and in the second
season on the 18 months Sesbania fallow (S18). Fertilizer de-
creased S. hermonthica plant populations by 42% over all sea-
sons but the difference was less in the second four seasons (23%)
than in the first four seasons (47%) compared with continuously
maize cropping. In the long rain season of 1997, the number of
Striga plants was significantly (F5, 15 = 3.57, p = 0.025) lower on
plots receiving continuous maize and fertilizer compare with the
other treatments.

Table 3. Number of Striga plants (x 1000 ha-1) during seven seasons
(fallow period ended after SR95).
Growing season S6 S18 N6 N18 M MF
1994 LR 408±95a 496±158a 329±114a 414±63a 428±63a 227±73a
1994 SR 32±10ab 0±0a 36±14ab 0±0a 61±17b 41±18ab
1995 LR 18±8ab 0±0a 23±11ab 0±0a 39±9b 21±11ab
1995 SR 0±0a 0±0a 0±0a 0±0a 32±9b 8±2a
1996 LR 23±9a 10±4a 51±7a 139±25b 54±15a 51±10a
1996 SR 74±18a 169±76a 52±17a 123±31a 57±13a 54±11a
1997 LR 54±6b 41±12ab 30±3ab 32±4ab 39±9ab 14±5a
1997 SR 18±5a 12±2a 14±5a 10±3a 12±4a 5±3a
Data for the same year not marked with the same letter are significantly
different. For notations see Table 1. Values are mean ± Standard Error.

Striga hermonthica seed bank

S. hermonthica seed density in the soil at the onset of the fallow-
ing period was 155 ± 16 seeds per kg of soil. The majority of the
S. hermonthica seed in all treatment plots was found in the upper
0–15 cm of the soil profile. The average total seed numbers
found in all treatment plots at the two (0–15 cm and 15–30 cm)
soil depths levels were not significantly (p > 0.05) different from
each other in any of the seasons. There was a decline in seed
density with depth at the end of both short (SR) and long (LR)
rainy season for all treatments except for MF in the SR 1995 and
SR 1996 and for M in the SR of 1996. Trends in S. hermonthica
seed density at the soil depth of 15–30 cm were not significant
and the overall mean population was 49 seeds kg-1. However,
there was a decreasing trend in the top soil (0–15 cm) throughout
the duration of the experiment in the continuous maize treat-
ments and an increasing tendency in seed populations after the
fallow treatments, most notably in S18 and N18 (Table 4).

Table 4. Number of Striga hermonthica seeds (seeds kg-1 soil), (Stan-
dard Error) in seed numbers following different treatments
Growing
Season
Soil depth
(cm)
S6 S18 N6 N18 M MF
1993 SR 0-15 179±40a 149±41a 116±15a 150±41a 174±64a 164±48a
1994 LR 0-15 148±34a 134±48a 158±50a 181±43a 138±37a 82±31a
1994 SR 0-15 115±17a 65±5a 130±62a 106±11a 151±68a 59±10a
1995 LR 0-15 152±51a 40±7a 90±24a 76±9a 84±25a 100±35a
1995 LR 15-30 76±12a 26±11a 38±14a 64±30a 39±14a 45±18a
1995 SR 0-15 45±19a 28±6a 50±18a 38±16a 76±19a 26±9a
1995 SR 15-30 39±8a 41±4a 38±12a 36±7a 46±5a 34±11a
1996 LR 0-15 48±24a 63±16a 34±7a 146±80a 50±20a 94±69a
1996 LR 15-30 41±6a 16±4a 31±15a 58±17a 38±2a 26±6a
1996 SR 0-15 55±21a 111±59a 99±34a 125±26a 57±7a 37±10a
1996 SR 15-30 64±16a 67±29a 61±19a 119±61a 70±21a 57±26a
Data for the same year not marked with the same letter are significantly
different. For notations see Table 1.


Discussion

Continuous maize monocropping was the least productive option
in all seasons, although non-significant which is in agreement
with previous yield data from the same area (Jama et al. 1998a)
and corresponds to the low yields typical of subsistence agricul-
ture in the tropics (Stahl et al. 2002). Lower yield in the short
rainy season may be explained by the lower rainfall and higher
incidence of pests and diseases in the short rainy season (Jama et
al. 1998b). The lower yield in the short rainy season of 1997
despite the generally higher rainfall in this season can be attrib-
uted to erratic distribution of the rains which is reported to result
in severe crop stress (van Lauwe et al. 2002).
Improved fallows, as compared to continuous maize without
fertilizer or natural grass fallows, increased grain, stover, cob and
core yield of post-fallow maize. The residual benefit of fallow
rotation using Sesbania lasted for one to four cropping seasons
which is in agreement with previous results (Kwesiga et al. 1999;
Stahl et al. 2002; Ndufa et al. 2009). The beneficial residual
effect on maize yield following Sesbania can be attributed to a
rapid supply of plant available N from decomposing fallow
Journal of Forestry Research (2010) 21(3): 379−386

385
biomass. The improved fallow system may have improved water
availability to the maize crop by reducing runoff and increasing
water infiltration (Nyamadzawo 2008) in the furrow during the
fallowing period. In addition Sesbania may have contributed to
weed suppression and therefore increased nutrient availability for
the maize. Fewer weeds were observed in the maize-Sesbania
plots than in the sole maize plots (pers. observ.). Consequently,
the Sesbania system could offer an attractive option for soil
improvement, especially on the fields of resource-poor farmers
(Kwesiga et al. 1999). In addition it will provide fuelwood and
fodder benefit. The reduction of S. hermonthica emergence in
S18 in the season immediately after fallow clearance may have
contributed to the increased maize yield relative to continuous
unfertilized maize. On the other hand the increased fertility after
the fallow could have allowed maize to more effectively out-
compete S. hermonthica. The increased maize yield in most of
the post fallow seasons with the fertilization treatment (P and N)
indicates N deficiency and confirms the merit of N fertilization
of N-deficient soils similar to our experimental site.
The results indicated that at the end of the cropping season,
highest numbers of S. hermonthica seeds were found in the 0–15
cm soil profile which corresponds to the plough layer of the soil.
This finding conforms with detailed information about the spatial
seed distribution from a number of naturally infested farm fields
in Western Kenya (van Delft et al. 1997) which indicated that S.
hermonthica seeds are found to a depth of approximately 10–15
cm. Below this level few seed was found up to 30 cm in all
treatments plots which could be explained by seed migration
through soil pores and pathways caused by decomposition of
plants roots (Smith and Webb 1996). The high density of S.
hermonthica seed in the upper layer where the first roots of the
crop plants are usually located (van Delft et al. 2000) could result
in a relatively early and highly infested root system of the maize,
which in turn might explain the losses in yield observed. Since S.
hermonthica is a root parasite, the interaction between host and
parasite first occurs at the level of parasite seed germination and
attachment. S. hermonthica plant populations decreased dramati-
cally in continuous maize cropping, presumably in response to
good weed management compared with farmer management
before the experiment.
The lower number of emerged S. hermonthica shoots in maize
intercropped with legumes might be attributed to the shading
effect of the legumes or a change in humidity and temperature
conditions due to their dense canopy. Fodder legumes can reduce
S. hermonthica infestation as trap crops or due to suppression of
emerged S. hermonthica. They can form a valuable part of an
integrated S. hermonthica control strategy. In the continuous
maize treatment, a substantial proportion of the newly produced
S. hermonthica seeds may have not survived in the topsoil during
the subsequent dry season until the next cropping. This could
explain a drastic decrease in S. hermonthica seed viability or
additional factors could be responsible for reducing the seed
bank annually predominantly acting near the soil surface such as
consumption of seed by insects and earthworms, suicidal germi-
nation, stimulation by root exudates, other organic compound or
micro-organisms and infestation by fungi (van Delft et al. 2000;
Oswald and Ransom 2001). In the treatment without weeding in
N18, an increase of the seedbank occurred which is similar to an
earlier finding by (Ransom and Odhiambo 1994). The relative
increase in S. hermonthica following S18 in our study conflicts
with the pattern of reduction of S. hermonthica infection in ma-
ize following planted fallows, which is believed to occur due to
increased mineral N in the topsoil and/or depletion of S. her-
monthica seed during the fallow phase (Gacheru and Rao 2005).
This observed suppression may in fact have also been the effect
of previous years without a host and due to a persistent seed bank
found under both laboratory and field conditions (Samake et al.
2006). Fertilizer decreased S. hermonthica plant populations
which is consistent with other studies that have shown that nitro-
gen fertilizer decreases S. hermonthica populations (Odhiambo
1998).


Conclusion

The study provides evidence that on a N-deficient site infested
with S hermonthica, natural weedy fallows of 6-month and 18-
month duration had no residual effect on maize grain yield.
Improved Sesbania fallows had transient effects on residual
maize yield, the size and duration of which was approximately
proportional to the length of the fallow. There were small posi-
tive net yield effects of Sesbania fallows amounting to about 1 to
1.5 Mg⋅ha-1 over the duration of the experiment, compared with
7.7 Mg⋅ha-1 with fertilizer addition in continuous maize. Longer-
term experiments are required to test whether Sesbania and other
leguminous fallows can gradually build up larger benefits over
the long-term in addition to the short-term transient yield effects
observed in this experiment. S. hermonthica populations were
strongly reduced by the generally improved weed management
during the experiment and to fertilizer use in the continuous
maize treatment. On the other hand, weed and natural fallow
tended to increase S. hermonthica plant populations, and Sesba-
nia fallow also increased S. hermonthica seed populations in the
soil. Longer-term studies are required to confirm whether good
weed management can off-set the effects of Sesbania fallows on
increased S. hermonthica.

Acknowledgment
We thank Paul Smithson and the ICRAF soil laboratory staff in
Machakos and Maseno for soil and plant analysis services. We
thank A. Albrektson, L. Eliasson and P. Savadogo for manuscript
reading. We gratefully acknowledge financial support from the
Swedish International Development Cooperation Agency (Sida).


References

Andrianjaka Z, Bally R, Lepage M, Thioulouse J, Comte G, Kisa M, Dupon-
nois R. 2007. Biological control of striga hermonthica by cubitermes ter-
mite mound powder amendment in sorghum culture. Applied Soil Ecology,
37: 175–183.
Buresh RJ, A. SP, Calhoun F. 1997. Replenishing soil fertility in Africa.
Journal of Forestry Research (2010) 21(3): 379−386

386
SSSA special publication number 51. 251 pp. Soil Science Society of
America inc., Madison, USA.
Crookston RK, Kurle JE, Copeland PJ, Ford JH, Lueschen WE. 1991. Rota-
tional cropping sequence affects yield of corn and soybean. Agronomy
Journal, 83: 108–113.
Gacheru E, Rao MR. 2005. The potential of planted shrub fallows to combat
striga infestation on maize. International Journal of Pest Management, 51:
91–100.
Gallagher RS, M. FEC, McCallie EL. 1999. Weed management through short
term improved fallows in tropical agro-ecosystems. Agroforestry System,
47: 197–221.
Gomez KA, Gomez AA. 1976. Statistical procedures for agricultural research,
New York: John Whiley & Sons, 680 pp.
Hartemink AE, Buresh RJ, Van Bodegom PM, Braun AR, Jama B, Janssen
BH. 2000. Inorganic nitrogen dynamics in fallows and maize on an oxisol
and alfisol in the highlands of Kenya. Geoderma, 98: 11–33.
Hassan R, Ransom JK, Ojiem J. 1995. The spatial distribution and farmers
strategies to control striga in maize: Survey results from Kenya. In: maize
research for stress environments. Proceedings of the Eastern and Southern
Africa regional maize Conference, 4; Harare, Zimbabwe; 28 march - 1 april
1994.
Jama B, Buresh RJ, Ndufa JK, Shepherd KD. 1998a. Vertical distribution of
roots and soil nitrate: Tree species and phosphorus effects. Soil Sci Soc Am
J, 62: 280–286.
Jama B, Buresh RJ, Place FM. 1998b. Sesbania tree fallows on phosphorus-
deficient sites: Maize yield and financial benefit. Agronomy Journal, 90:
717–726.
Kiwia A, Imo M, Jama B, Okalebo JR. 2009. Coppicing improved fallows are
profitable for maize production in striga infested soils of Western Kenya.
Agroforestry Systems, 76: 455–465.
Kwesiga F, Coe R. 1994. The effect of short rotation sesbania sesban planted
fallows on maize yield. Forest Ecology and Management, 64: 199–208.
Kwesiga FR, Franzel S, Place F, Phiri D, Simwanza CP. 1999. Sesbania
sesban improved fallows in eastern zambia: Their inception, development
and farmer enthusiasm. Agroforestry Systems, 47: 49–66.
Maroko JM, Buresh RJ, Smithson PC. 1998. Soil nitrogen availability as
affected by fallow-maize systems on two soils in Kenya. Biol Fertil Soils,
26: 229–234.
Ndufa JK, Gathumbi SM, Kamiri HW, Giller KE, Cadisch G. 2009. Do
Mixed-Species Legume Fallows Provide Long-Term Maize Yield Benefit
Compared with Monoculture Legume Fallows? Agronomy Journal, 101:
1352–1362.
Ndung’u D, Odhiambo GD, Ransom JK. 1993. Methodology for quantifying
striga seed numbers in soils of East Africa. African crop science conference
proceeedings, Kampala, Uganda vol. 1 pp. 220-221.
Niang A, Gathumbi S, Amadalo B. 1996. The potential of short-duration
improved fallow for crop productivity enhancement in the highlands of
Western Kenya. East African Agricultural and Forest Journal, 62: 103–
114.
Nyamadzawo G, Nyamugafata P, Chikowo R, Giller K. 2008. Residual effects
of fallows on selected soil hydraulic properties in a kaolinitic soil subjected
to conventional tillage (CT) and no tillage (NT). Agroforestry Systems, 72:
161–168.
Odhiambo GD. 1998. Studies on witchweed [striga hermontica (del.)
benth]seed longevity and maize[zea mays l.] productivity under different
management systems in Kenya. University of Reading, School of Plant
Sciences, Department of agricultural botany. Thesis.
Oswald A, Ransom JK. 2001. Striga control and improved farm productivity
using crop rotation. Crop Protection, 20: 113–120.
Pieterse AH, Verkleij JAC. 1991. Effect of soil conditions on striga develop-
ment: A review. In: J. K. Ransom, l. J. Musselman, a. D. Worsham, and c.
Parker (eds), Proceedings of the 5th international symposium on parasitic
weeds, pp. 329-339. Cimmyt, june 1991, Nairobi, Kenya.
Pisanelli A, Poole J, Franzel S. 2008. The adoption of improved tree fallows
in Western Kenya: farmer practices, knowledge and perception. Forests,
Trees and Livelihoods, 18: 233–252
Ransom JK, Odhiambo GD. 1994. Long term effects of fertility and hand-
weeding on striga in maize. In biology and management of orobanche. Pro-
ceedings of the third international workshop on orobanche and related stri-
ga research. Eds. Pieterse AH, Verkleij JAC and Borg SJ. Pp 513-519.
Royal tropical institute, Amsterdam.
Samake O, Stomph TJ, Kropff MJ, Smaling EMA. 2006. Integrated pearl
millet management in the sahel: Effects of legume rotation and fallow man-
agement on productivity and striga hermonthica infestation. Plant and Soil,
286: 245–257.
Scholes JD, Malcolm CP. 2008. Striga infestation of cereal crops - an un-
solved problem in resource limited agriculture. Current Opinion in Plant
Biology, 11: 180–186.
Shepherd KD, Walsh MG. 2002. Development of reflectance spectral libraries
for characterization of soil properties. Soil Sci Soc Am J, 66: 988–998.
Sileshi G, Akinnifesi FK, Ajayi OC, Place F. 2008. Meta-analysis of maize
yield response to planted fallow and green manure legumes in sub-Saharan
Africa. Plant and Soil, 307: 1–19.
Smith MC, Webb M. 1996. Estimation of the seedbank of striga spp (scrophu-
lariaceae) in Malian fields and the implications for a model of biocontrol of
striga hermonthica. Weed Research, 36: 85–92.
Stahl L, Nyberg G, Hogberg P, Buresh RJ. 2002. Effects of planted tree
fallows on soil nitrogen dynamics, above-ground and root biomass, n-2-
fixation and subsequent maize crop productivity in Kenya. Plant and Soil,
243: 103–117.
Swinkels RA, Franzel S, Shepherd KD, Ohlsson E, Ndufa JK. 1997. The
economics of short rotation improved fallows: Evidence from areas of high
population density in western kenya. Agricultural Systems, 55: 99–121.
van Ast A, Bastiaans L, Katile S. 2005. Cultural control measures to diminish
sorghum yield loss and parasite success under striga hermonthica infesta-
tions. Crop Protection, 24: 1023–1034.
van Delft GJ, Graves JD, Fitter AH, Pruiksma MA. 1997. Spatial distribution
and population dynamics of striga hermonthica seeds in naturally infested
farm soils. Plant and Soil, 195: 1–15.
van Delft GJ, Graves JD, Fitter AH, Van Ast A. 2000. Striga seed avoidance
by deep planting and no-tillage in sorghum and maize. International Jour-
nal of Pest Management, 46: 251–256.
van Lauwe B, Diels J, Sanginga N, Merckx R. 2002. Integrated plant nutrient
management in sub-saharian Africa: From concept to practice. Wallingford,
CAB International, Cambridge University Press, pp. 352.
Worsham AD, Egley GH. 1990. Physiology of witchweed seed dormancy and
germination. In: Sand, P.F., Eplee, R.E., Westbrooks, R.G. (Eds.), Witch-
weed Research and Control in the United States. Weed Science Society of
America, No 5, USA, pp. 11-26.