• CHAPTER 1


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    • Abstract: CHAPTER 11. GENERAL INTRODUCTIONMaize is one of the most important cereal crops, which serves as a staple foodfor many people in the Limpopo Province. The witch weed (Striga asiatica) is aparasitic weed that plagues cereal crops including maize and sorghum. This

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CHAPTER 1
1. GENERAL INTRODUCTION
Maize is one of the most important cereal crops, which serves as a staple food
for many people in the Limpopo Province. The witch weed (Striga asiatica) is a
parasitic weed that plagues cereal crops including maize and sorghum. This
weed competes for water and nutrients as a root parasite, literally sucking the life
out of the crop on which it germinates. In doing so, maize growth is stunted
(Sallah and Afribeh, 1998) and yields are generally reduced. In the semi-arid
areas of sub-Saharan Africa the problem of striga is so bad that some farmers
have been forced to abandon their arable land. Striga does most of its damage to
its host through phytotoxins before the weed emerges from the soil. Above
ground the crop withers, and grain production is reduced. Striga infestation is a
consequence of monocropping of cereals, which host the parasite, and declining
soil fertility, which weakens the host plant to striga attack. As a result of these
cropping practices, striga- infested areas have gathered very high levels of long-
lived striga seeds in the soil with some breaking dormancy each season when
stimulated by crop exudates. Kanampiu and Friesen (2004) reported that yield
losses due to striga damage range between 20-80% to complete yield loss.
Striga is considered an indicator for poor soil fertility (Kanampiu and Friesen,
2004).
Striga infests an estimated two-thirds of the 73 million hectares devoted to cereal
crops in Africa, resulting in crop losses of up to 70% among subsistence farmers
(Jesse, 2005; Kanampiu and Friesen, 2004). Striga accounts for an estimated 4.1
million tonnes in lost cereal yields each year, and is considered by many experts
to be the greatest obstacle to food production in Africa, particularly the Sahel
region (Watson et al., 1998). In Western Kenya an estimated 75 000ha of land
(80% of farmland) are infested with striga. Every year striga damage to crops
accounts for an estimated US$7 billion in yield loss in sub-Saharan Africa, and
1
affects the welfare and livelihood of over 100 million people (Kanampiu and
Friesen, 2004).
There are several methods that are used or have been tried to control Striga
infestation in maize. Crop rotation of a cereal with legumes such as soybean can
be a highly effective means of reducing the amount of striga seeds in the soil
(Berner et al., 1997) but this practice may not be viable in the smallholder sector
of South Africa where land holdings are small and farmers always require their
staple maize. Intercropping cereal with cowpea in the same row gave the highest
yield in Cameroon and in Ethiopia (Mbwaga et al., 2001). Intercropping with
legumes also improves soil fertility through fixation of atmospheric nitrogen.
Addition of nitrogen to the soil is generally considered to alleviate the effects of
striga and to lower the amount of striga supported by the host. The effectiveness
of cereal/legume intercropping to influence striga germination depends on the
effectiveness of the produced stimulants/inhibitors, root development, fertility
improvement, shading effect and its compatibility to striga species because the
response of striga to management options is specific (Mbwaga et al.,2001).
Chivinge et al. (2001) conducted an on-farm experiment in Zimbabwe between
1996 and 1998 with the objective of determining the effectiveness of cowpea
cultivars: IT93K-8-45-5-1-5, B301, IT82D-849, IT90K-76 and Kavara (local check)
in the management of striga. All the cowpea cultivars reduced striga emergence
by at least 40% with IT82D-849 exhibiting the highest percentage reduction.
Intercropping cowpea cultivars with maize resulted in maize yield increases of
650-860% during the 1996/97 season with yields of 3.8-4.8 t ha-1. These yield
increases, however, were higher than those obtained during the subsequent
1997/98 season.
Hoe weeding remains the most common method of weed control among
smallholders Limpopo. Development of striga resistant varieties from susceptible
species has been attempted in a number of crops including sorghum, maize,
2
cowpeas, rice and millet (Mloza-Banda and Kabambe, 1997). The effectiveness
of striga resistance of local maize varieties needs to be tested in places such as
Mopani and Vhembe where striga is most problematic. This may be a viable
option since there are many early maturing cultivars suitable for these marginal
rainfall areas.
Olupot et al. (1999) conducted series of experiments with the objective to
develop an integrated management strategy for striga under Ugandan conditions.
Treatments consisted of two levels of nitrogen and two weedings. These were
applied either singly or in various combinations on two sorghum varieties
(local/susceptible cultivar and an improved/tolerant variety- Seredo). The results
showed that a combination of tolerant variety, nitrogen applied at the rate of
80kgN/ha and the two hand weedings was superior to other treatments. The
lowest striga count and the highest sorghum grain yield were achieved in this
treatment. Ordinary hoe weeding (twice) followed by spot spraying of striga with
gramoxone every ten to fourteen days showed higher yields than ordinary hoe
weeding (twice) alone or hoe weeding coupled with hand pulling. Hand pulling for
witch weed may be much more difficult and expensive because plants are small,
less conspicuous and may be much more numerous (Mloza-Banda and
Kabambe, 1997).
There is a need to test the effectiveness of different agronomic practices in
Limpopo Province to assist farmers to control striga in their maize fields. These
practices can increase maize production. Farmers in Limpopo Province are
controlling striga by hoe weeding only. Smallholder farmers are not aware of
other methods, which can be used to control striga.
The objectives of this study were: 1) to test the effect of hand hoeing, hand
hoeing and application of inorganic fertilizer (N), and hand hoeing maize
intercropped with cowpea on striga emergence and growth, and maize yield in
3
dry land maize and 2) to evaluate the response of maize cultivar on maize striga
infestation.
4
CHAPTER 2
2. Literature review
2.1 Striga management
2.1.1 Intercropping
The roots of several legumes are known to induce suicidal germination of striga
seeds, and this feature has become incorporated into striga suppression
strategies involving cereal-legume rotation or intercropping. Silverleaf
desmodium is particularly effective in suppressing striga and has been
incorporated into a biological control system known as push and pull. In push-
pull, desmodium neutralizes striga (Woomer, 2004). Intercropping cereal with
cowpea in the same row gave the highest yield in Cameroon and in Ethiopia
(Mbwaga et al., 2001). Intercropping with legumes also improves soil fertility
through fixation of atmospheric nitrogen. Addition of nitrogen to the soil is
generally considered to alleviate the effects of striga and to lower the amount of
striga supported by the host. The effectiveness of cereal/legume intercropping to
influence striga germination depends on the effectiveness of the produced
stimulants/inhibitors, root development, fertility improvement, shading effect and
its compatibility to striga species (Mbwaga et al., 2001). Mixed cropping of
cereals and cowpea has been observed to reduce striga infestation significantly
(Khan et al., 2002). This is thought to be due to the soil cover of cowpea creating
unfavorable conditions for striga germination (Mbwaga et al., 2001; Musambasi
et al., 2002). Intercropping maize and beans in the same hole had the highest
grain yield, which was 78.6 % above the yield of pure maize stands due to the
fact that beans is able to fix nitrogen which will improve maize yield (Odhiambo
and Ariga, 2001).
5
2.1.2 Crop rotation
Crop rotation is the easiest control measure of striga control to implement
because it requires only commitment and planning (Shank, 2002). Crop rotation
or intercropping with trap or catch crops can reduce the number of witch weed
seeds in the soil. For heavily infested fields, trap crops can accelerate the
depletion of the reservoir of striga seed in the soil (Mloza-Banda and Kabambe,
1997). Crop rotation of a cereal with legumes such as soybean can be a highly
effective means of reducing the amount of striga seeds in the soil. To ensure
effectiveness of the rotation crop, the cultivars which are most effective in
stimulating striga must be included. A more desirable option is the use of
leguminous non-host crops, which stimulate striga germination, but do not
support its growth. These non-hosts can significantly deplete the soil seed bank
by inducing suicidal germination of striga (Berner et al., 1997).
Rotating the infested maize or sorghum areas to wheat/barley, pulses, or
groundnuts are viable and effective options in Ethiopia. A season of non-host
cropping allows for a large portion of the striga seeds to deteriorate into non-
viability. Seriously infested areas should be rotated to non-host crops for two
years followed by closely supervised weeding. In Ethiopia two years of cropping
to a non-host was reported to reduce striga infestation by 50% (Shank, 2002). In
Sahel the results of a four year experiment in bush fields indicated that one
season cowpea in 1998, had a positive effect on subsequent millet grain yields,
soil organic carbon and nitrogen, and reduced striga infestation. The increase in
yields due to millet-cowpea rotation was 37% in 1999 compared to 3-5 years
continuous millet cropping (Samake, 2003).
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2.1.3 Weeding/Sanitation
Although weeding the small Striga plants is a tedious task and may not increase
the yield of already infected plants, it is necessary to prevent seed production
and reinfestation of the soil. Weeding must begin at the first sign of flowering
because pod formation and seed setting will soon follow. New shoots may sprout
out below the soil from infected plants requiring a second weeding before crop
maturity (Shank, 2002). Hoe weeding remains the most common method of weed
control in Malawi. Farmers will normally weed twice, the second time is through
the banking operation where the soil is pulled-up the ridge. Inconsistent results
have been obtained in Malawi on the effectiveness of hoe-weeding for striga
control (Mloza-Banda and Kabambe,1997). Sanitation consists of taking care to
note infested areas and to isolate them. Wind, rainwater, ploughing, and soil on
tools or root crops can spread seeds in the soil. Seedpods on striga plants
attached to maize or sorghum plants pulled for forage will infest manure and
feeding areas. It is suggested that a striga disposal pit be constructed to prevent
seed maturation of green or drying plants that are pulled (Shank, 2002). If striga
has formed flowers and matured, farmers should dig a hole about 70 cm deep,
burn the plant and bury them (Woomer, 2004).
2.1.4 Genetic Resistance
Varietal resistance to striga infestation in maize and sorghum has long been
recognized but only recently have attempts been made to utilize it. Basically, the
resistant varieties were low yielding and not desirable in other agronomic
characteristics. Recent efforts to utilize resistance in breeding and improvement
programs have met with limited success. A number of resistant lines of maize
have been identified by the Institute of Tropical Agriculture (IITA) in Nigeria and
the International Wheat and Maize Improvement Center (CIMMYT) in Tanzania
but have not been tested in Ethiopia (Shank, 2002).
7
More than 80 resistant sorghum lines have been selected by the International
Center for Research in the Semi-Arid Tropics (ICRISAT) in India. Of these, three
selections made by the Ethiopian Institute of Agriculture Research at Nazreth
have performed well in research trials and are currently being verified in large
scale tests. Seed of these varieties is available for adaptation testing in striga
infested areas. Care must be taken in distinguishing resistance from tolerance
since good yielding tolerant varieties will allow Striga growth and seed
production, thus increasing soil contamination (Shank, 2002).
Development of resistant varieties has been attempted in a number of crops
including sorghum, maize, cowpeas, rice and millet. Resistance in sorghum
materials has been confirmed for S.asiatica in Southern Africa and has been
incorporated into breeding materials. One of the major problems though is that
resistance to striga has been associated with low grain yield and poor grain
quality. At IITA, some maize lines with partial tolerance and/or resistance to
S.hermonthica and S.asiatica were identified and hybrids developed with
resistance or tolerance are commercially available in East and Southern Africa
(Mloza-Banda and Kabambe, 1997).
Development of resistant maize genotypes is further complicated by the
existence of biotypes and the presence of three different and economically
important striga species in Africa that infest maize and the potential buildup of the
parasite where tolerant maize lines are used. Additional research is needed to
confirm the role of some genotypic traits of the crop along with other host-
parasite interactions and their contribution to striga resistance (Mloza-Banda and
Kabambe, 1997). Hiriray, Higretay and Korokora are Ethiopian maize varieties
that are resistant due to their early maturing characters, which is an escape
mechanism against the infestation of striga (Kidane et al., 2004).
8
2.1.5 Soil Fertility
It has been noted in western countries that host plant shading can restrict striga
growth when generous soil fertilizer is applied (Table 2.1.5.1). In areas of high
rainfall, factors such as high plant populations, recommended fertility levels, and
good weed control encourage lush crop growth and shading in spite of striga
parasitism. This is not feasible in moisture stressed areas since high fertilizer
applications would burn up the crop should normal soil moisture restrictions
occur. However, it appears that several small applications of fertilizer adjusted to
the level of available soil moisture could raise crop yields and shading in
favorable rainfall years (Shank, 2002).
Table 2.1.5.1. Effect of soil fertility level on striga growth and plant
characters of 4 maize hybrids in Nigeria
No of Striga No of Striga Maize plant Grain Yield
NPK % plants/m of seed height(cm) kg/plant
recommended row capsules/plant Res/Sus
0 150 12 102/53 10
30 102 54 103/65 17
50 85 33 124/75 13
100 23 6 146/119 36
(Shank, 2002)
Farina et al. (1985) conducted long-term fertilizer trials using nitrate and
ammonium N sources at 60, 120 and 180 kg/ha and found that N significantly
reduced the incidence of S. asiatica on maize in South Africa. Esilaba et al. 2000
reported that striga emergence was minimized with the application of 120kg
N/ha. Mumera and Below (1993) and Esilaba et al. (2000) reported that Striga
infestation declines with increasing N availability and the impact depends on the
severity of the infestation. Aflakpui et al. (1997) reported that to reduce the
9
population of Striga in maize, it is essential to apply a minimum of 90kg N/ha
fertilizer and it must be applied early.
2.1.6 Mulches
In Tsholotsho, Zimbabwe, mulches were applied at 2t/ha at 4 weeks after maize
planting. Mulches from Collophospermum mopane, Acacia karoo and Acacia
nilotica reduced the incidence of S.asiatica and delayed its emergence and
flowering. Mulch from C.mopane was the most effective in suppressing the weed
between the 4th and 5th weeks after its application. A. karoo increased the
number of days to emergence from 47 days to 68 days while that from C.
mopane and A.nilotica increased the number to 58 days. Mulches from A.
nilotica, A. karoo and C. mopane increased the number of days to flowering from
75 to 108, 125 and 100, respectively. It is recommended that farmers mulch their
fields at different times, depending on the mulch being used in order to suppress
weed emergence and flowering throughout the season (Chanyowedza and
Chivinge, 1999). The requirement of large amounts of mulch, however, limits the
usefulness of this approach.
2.1.7 Chemical control
A number of chemical control measures that have been practiced in the western
hemisphere are not practical or are too risky for several reasons. Soil sterilization
by means of stimulating striga seed germination with non-host plants (cotton or
soybeans) or chemical stimulants (Strigol and ethylene) is not practical in
developing countries because of cost and the resulting delay in planting the food
crop in areas where the season length is already limited by moisture (Shank,
2002). Preemergence herbicides against striga, such as oxyfluorfen and
dinitroaniline compounds, form a barrier in the top few centimetres of the soil and
kill striga as it emerges (Berner et al., 1997). Since Striga is a broadleaf plant,
preplant herbicides such as Atrazine, Goal, and Flex show some effect though
not efficient enough to be justified (Shank, 2002). Post-emergence use of 2,4-D
10
is effective when sprayed on the striga leaves. Though low in cost, this herbicide
is quite volatile and drift to nearby sensitive broadleaf crops (legumes, pepper
and tomato) and could be devastating. Also, maize and sorghum are vulnerable
to stalk twisting and lodging if 2,4-D is sprayed into the leaf whorl. Spraying
should only be done after users have been trained and cautioned to these
hazards. Experimentally, anti-transpirant type herbicides applied only to the base
of the row of sorghum-striga or maize-striga were very effective (Shank, 2002).
Herbicides such as trifluralin and pendimethalin, have been effective against S.
asiatica when incorporated shallowly in a layer above the cereal seed by
inhibiting shoot growth of the parasite (Mloza-Banda and Kabambe, 1997).
Traore et al. (2001) reported that use of herbicides is more cost effective than
mechanical weeding and it enhanced striga control. Use of 2,4-D cannot work in
the smallholder sector in SA where maize is often intercropped with cowpea,
cucumber and melons, and herbicide technology has largely not yet been
introduced to these farmers.
2.1.8 Biological control
Fusarium fungus that is found at low levels under normal conditions in some
African soils can be applied by coating cereal seeds first with Arabic gum and
then with dry fungal powder. It is a seed technology rather than herbicide
technology. The advantage with this approach is that fusarium can colonize the
soil and lie in wait for striga. When striga attacks the crops, it is killed by the
fusarium (Eberlee, 2000). Researchers at McGill Biopesticides Research
Laboratory discovered a fungus (Fusarium oxysporum) in the soil in Mali that can
suppress the weed’s growth (Watson et al., 1998). In a pilot study, the fungus
was grown on sorghum straw, and then spread on farmers’ fields at sowing time.
It is not toxic to humans or to cereal crops and attacks striga at an early growth
stage, resulting in dramatically increased sorghum yields. In 1994 at Samaya in
Mali, there was delayed emergence of striga in all inoculum treated plots,
achieving reduction in striga emergence from 53% to 90% in treated plots and
11
increase in sorghum yields of 100%. In 1995 at Kolokani in Mali there was
reduction in striga emergence of 75% and increase in sorghum yield of 19%. In
1996 at Cinzana in Mali there were reductions in striga emergence of 54% and
increase in sorghum yields of 23%. Lastly in Sikasso, in Mali, there was reduction
in striga emergence of 84% (Watson et al., 1998).
2.1.9 Integrated striga management
A series of experiments were conducted with the objective of developing an
integrated management strategy for striga (witch weed) under Ugandan
conditions (Olupot et al., 1999). The first of these experiments was carried out
during the first rains (March – July 1999). Treatments consisted of two levels of
nitrogen and two weedings. These were applied either singly or in various
combinations on two sorghum varieties (local/susceptible cultivar and an
improved/tolerant variety- Seredo). The results showed that a combination of
tolerant variety, nitrogen applied at the rate of 80kgN/ha and two hand weedings
was superior to other treatments. The lowest striga count and the highest
sorghum grain yield were achieved in this treatment (Olupot et al., 1999).
Ordinary hoe weeding (twice) followed by spot spraying of striga with gramoxone
every ten to fourteen days showed higher yields than ordinary hoe weeding
(twice) alone or hoe weeding coupled with hand pulling. However, the use of
herbicides poses challenges of accurate calibration of sprayers and use of
correct dosages. Hand pulling for witchweed may be much more difficult and
expensive because plants are individually small, less conspicuous and may be
much more numerous (Mloza-Banda and Kabambe, 1997). Generally, no single
method provides an acceptable level of control. An integrated striga management
strategy is required and would be strengthened by the use of natural enemies as
biological control agents (Abbasher et al., 1998).
12
CHAPTER 3
EFFECT OF WEED CONTROL METHOD ON STRIGA NUMBERS, MAIZE
GRAIN YIELD AND YIELD COMPONENTS
3.1 INTRODUCTION
The witchweed (Striga asiatica) is a parasitic weed that plagues cereal crops
including maize. This weed competes for water and nutrients as a root parasite,
literally sucking the life out of the crop on which it germinates. In doing so, crop
growth is stunted (Sallah and Afribeh, 1998) and yields are generally reduced.
Generally striga is considered an indicator for poor soil fertility (Kanampiu and
Friesen, 2004). Kanampiu and Friesen (2004) reported that yield losses due to
striga damage ranged between 20-80%.
Addition of nitrogen to the soil is generally considered to alleviate the effects of
striga and to lower the amount of striga supported by the host (Mbwaga et al.,
2001). Chivinge et al. (2001) reported that cowpea cultivars reduced striga
emergence by 40%. The effectiveness of cereal/legume intercropping to
influence striga germination depends on the effectiveness of the produced
stimulant/inhibitors, root development, fertility improvement, shading effect and
its compatibility to striga species because the response of striga to management
options is specific (Mbwaga et al.,2001).
There is a need to test the effectiveness of different agronomic practices in
Limpopo Province to assist farmers to control striga in their maize fields. These
practices can have a major effect on maize production, as it is known that the
farmers in Limpopo Province are controlling striga by hoe weeding only.
Smallholder farmers are not familiar with other methods, which can be used to
control striga. These agronomic practices have been successfully tried in other
countries. The effectiveness of striga resistance of local maize varieties needs to
be tested in Districts such as Mopani and Vhembe where striga is most
13
problematic. This may be a viable option since there are many early maturing
cultivars suitable for these marginal rainfall areas. It is necessary to test the
effectiveness of selected agronomic practices on controlling striga in maize and
for farmers to evaluate them since subsistence farmers can only afford
inexpensive control measures. Smallholder farmers in South Africa almost wholly
intercrop their maize with legumes. It may, therefore, be necessary to test the
interaction of maize cultivar with a legume on control of striga, among other
treatments.
This study is aimed at improving maize productivity and household food security
by enhancing management of striga using selected agronomic practices. The
main objective of the study is to investigate the effectiveness of selected
agronomic practices on striga control in dry land maize in Limpopo Province and
the farmer’s reaction to their performance. Specific objectives of the study are: 1)
to test the effect of hand hoeing, hand hoeing and application of inorganic
fertilizer (N), and hand hoeing maize intercropped with cowpea on striga
emergence and growth, and maize yield in dry land, 2) to evaluate the effect of
maize cultivar to maize response to striga infestation and 3) to evaluate the
interaction between maize cultivar and agronomic practices in suppressing striga
and maize yield.
3.2 MATERIALS AND METHOD
3.2.1 Study area
The experiment was done at Mafarana village (Mopani District) near Tzaneen
where striga is a serious problem. The study was conducted on three fields,
belonging to Mrs. Shingwenyana, Mr. Nyathi and Mr. Mushwana, which were
selected on the criterion of having high natural striga infestation.
3.2.2 Experimental design and treatments
The experiment was carried out under a 2X3 factorial arrangement in a
Randomized Complete Block Design (RCBD) with four replications. The trial was
14
planted between the 14th and 16th December 2006. The experiment consisted of
two treatment factors; (i) two maize cultivars i.e. Zm 1421(V1) and Zm 423(V2)
which are early maturing and drought resistant open pollinated cultivars; (ii) three
agronomic practices: hand hoeing alone (M1), hand hoeing plus inorganic
fertilization using lime ammonium nitrate (LAN-28%N) at the rate of 56kg/ha (M2),
and hand hoeing plus inter-row intercropping of maize with cowpea (M3). Cowpea
cultivar Bechuana White was used for the intercropping treatment. Hand hoeing
alone was the control factor.
3.2.3 Trial management
Inter-row spacing was 90cm and in-row spacing was 50cm for maize (giving a
population of 22 220 plants per hectare). Hand weeding was done at 33 days
after planting (DAP) and 105 DAP. The cowpea was sprayed with dimethoate 40
EC for pest control as necessary.
3.2.4 Data collection
Gross plot size was 21.6m2 (4 rows X 90cm X 6m) and Net plot size was 10.8 m2
(2 central rows X 90cm X 6m). Data were collected from 10.8 m2 area leaving the
other two rows as borders. Striga counts were done at 61, 83 and 105 days after
planting (DAP). The yield of cowpea was not taken because excessive rain
caused serious flower drop and pods were insect damaged. The farmers in that
area also prefer to grow cowpeas for their leaves, which they use as vegetable.
The following data were collected:
1. Date of first emergence of striga
2. Number of striga plants in the row at 61, 83 and 105 DAP.
3. Number of maize plants per plot at physiological maturity (PM)
4. Number of ears per plot
5. Date of physiological maturity (PM) of maize
6. Unshelled weight per plot
7. Maize grain yield per plot
15
3.2.5 Data analysis
Data were analyzed using SPSS to compute analysis of variance (ANOVA)
across all locations. Treatment means were compared using the Least Significant
Difference (LSD), P


Use: 0.0215