Effectiveness of Maize Cob Powder in Controlling Weevils in Stored Maize Grain
Abstract: The broad objective of this study is to assess the
effectiveness of maize cob powder in controlling maize weevils in stored maize
grain. A completely randomized block design, in
which twelve small bags of maize containing 0.5 kg of maize grain (SC5313 dent
variety), were used. Three concentration levels of maize cob powder 5g, 45g and
75g per 0.5 kg were compared with the control experiment containing
conventional chemical Actellic super at 5g. Findings showed that conventional chemical control was more effective
than traditional method at 5g and 45g levels of maize cob powder. However,
greater effectiveness of cob powder was observed at 75g level. In the absence
of conventional methods of control, which are unavailable due to local supply
bottlenecks, the study recommends use of maize cob powder to control weevils.
The importance of the
maize sub sector within the Southern African Development Community (SADC)
region is well researched and documented. Maize is an integral component of the
staple diet of the citizenry of this region and is also considered to be an
essential source of cash for farmers. It is also used in the
manufacture of a wide spectrum of industrial products ranging from animal feeds
to food products. Thus, the maize sub sector has important backward and forward
linkages with industry.
In recent years, the region
has witnessed dramatic changes in rainfall patterns ushered in by global
warming that culminated in more frequent droughts.
The effect of intermittent droughts has manifested itself in declining maize
output, further exacerbating the livelihoods of farmers and the general
populace. The effects of poverty largely stem from the failure of agriculture
to sustain the lives of the rural poor. It is therefore not a surprise that the
majority of the families in SADC live on less than $1 per day.
Declining food production
exposes farmers to chronic and transitory food shocks. This creates the need
for farmers to come up with mechanisms for conserving their scarce food
resource base. Maize, which is normally stored in granaries, is usually treated
with a different array of chemicals for preservation against pests such as weevils.
Indeed, weevils (stophilus zeamais) are often identified as one of the
major problems causing loss of stored grain in Africa.
It is estimated that weevil attack accounts for approximately 5-10% of maize
grain loss in Southern Africa.
two main chemicals -- Shumba and Actellic Chirindamatura Dust ---- are used to
conventionally treat stored maize grain. However, the inimical
macro-economy has resulted in high prices for these products and the need for
identifying other methods of preserving maize grain. One port of call for rural
farmers is using localized methods of preservation found within indigenous
Indigenous knowledge is unique
to a given culture or society. It
creates the basis for local level decision-making in agriculture, health care,
food preparation and preservation, education, and natural resource management.
Indigenous knowledge is an important ingredient for development but is grossly
A cocktail of methods,
extracted from local knowledge, have been used to preserve maize grain for
generations in Zimbabwe. However, one of the most eminent "indigenous
approaches" of maize preservation is using maize cob powder. After shelling
maize, the remnants are burnt and the resultant powder is sprinkled on maize
grain and this confers longevity to maize grain.
This method can be questioned
from two angles. Firstly, to what extent does this method improve the shelf
life of stored grain? Secondly, does the resultant soot from burnt cob powder
discolor the maize grain or render it unfit for human consumption? Thus, the
crux of the matter in this study is to broadly look at the value of local
knowledge in preserving stored maize grain.
Conventional approaches to
development in Africa have often required the transfer of technology from
developed to developing countries. This obliterates the importance of local
knowledge and experiences in solving local problems peculiar to rural
communities. Although, weevils account severe losses in stored maize grain in Southern Africa, the sustainability of conventional chemicals used
to preserve grain is questionable given the high level of poverty present in
the rural communities in Africa. Local prescriptions emanating from indigenous
knowledge base, such as the use of maize cob powder are grossly
under-researched. Thus, there is a paucity of information which illustrates the
value of indigenous knowledge in the preservation of agricultural and food products.
This study therefore provides a scientific inquiry to practices passed on for
many generations in Zimbabwe.
- To what extent is
maize cob powder effective in controlling stored maize grain?
- Do the maize ashes
have an effect on the quality of the stored grain?
The broad objective of this
study is to ascertain the value of indigenous knowledge through determining the
effectiveness of traditional methods of preserving maize grain.
The specific objectives of the
- To assess the
effectiveness of maize cob powder in controlling weevils in stored maize
- To ascertain the
effects of maize cob powder on quality of stored grain.
Justification of study
macro-economic environment in the country implies that farmers cannot
effectively access conventional technologies for preserving maize grain.
Although indigenous knowledge is one of the options for farmers, little is
known about whether the various methods used locally are effective. Therefore
this study is important in generating information for sharing experiences with local
rural communities constrained by scarce resources.
Fig 2.1 depicts the complex
environment within which the smallholder farmer operates. Outcomes for the
farming household are shaped by social, economic, institutional, and
technological factors. Economic factors determine farmers' participation in markets
and this can have a direct bearing on farm productivity through acquisition of
production inputs. The institutional environment mainly consists of information
sources such as extension workers for enhancing access to credits and
agricultural information. Edaphic conditions are shaped by the prevailing
climatic conditions and affect productivity by determining rainfall patterns
and soil fertility, among other issues. The social environment consists of the
networks through which the local community shares information. Indigenous
knowledge is also part of the social system but is normally overridden by
formal sources of information such as extension agents.
Overall, the farmer is
operating in a constrained optimization scenario and this often leads to low
productivity. Low productivity translates into poverty since agriculture fails
to sustain farmers through sale of produce. This condemns the farmer to
subsistence production, that is producing only for family needs. However, this
food stock, mainly maize, is exposed to pests and other hazards such as
weevils. This can potentially result in further food bottlenecks for the
farmer. Since the household is not effectively participating in the cash
economy, it cannot access expensive technologies for preserving this food stock.
Assuming rationality, the farming household then resorts to using cheaper
methods of preserving food reserves such as using maize cob powder. This
ensures that the household is able to overcome transitory food shocks.
Therefore indigenous knowledge can compliment already existing conventional
methods of food preservation.
Effects of maize cob powder
on maize kennels
According to the 1995 Elwell
study, ashes from plants (wood, cobs, and stalks) used in combination with
agricultural lime has the effect of preventing insects from moving from seed to
seed. Seeds soaked in cob ashes are protected against fungal and bacterial
diseases as well as protection from weevil attack. Cob ash contains certain
natural salts and these create inimical conditions for growth and habitation of
weevils. This characteristic gives the ash some insecticidal property. Other
studies have also shown that salt mixed with maize grain confers protection to
grain for sustained periods of time.
Results from a series of
experiments in some African countries such as Zambia and Mozambique have shown
that maize protected using traditional methods of preservation (e.g. cob) ash
was not significantly different in quality from maize grain protected by
However, there are gaps in the scientific literature on using cob ash in
traditional African systems.
Conventional approaches to
protecting grain against weevil attack
A number of insecticides,
usually in the form of fumigant dust are used to control weevils. These include
methyl bromide, phosphine and carbon disulphine, and Malathion dust. Other
approaches for protection against weevils include use of airtight storage
systems to suffocate weevils and weevil proof containers.
Over the years, concerns have
been raised over the use of chemicals on human health and ecosystem balance. Argues that over-use and abuse of
agro-chemicals imposes serious costs on a nation's economy while eroding the
ecological foundations and thriving agro-ecosystems. Evidence from different
African countries illustrates that improper use of chemicals is causing loss of
life and negative repercussions on human health.
Traditional remedies used
for post harvest crop preservation in Zimbabwe: Historical Overview
State-sponsored attempts to
introduce new technological innovations among smallholder farmers in colonial Zimbabwe were begun in 1926. The Rhodesian government enacted
a two tier agricultural extension policy that served the interests of
smallholder and large scale farmers through the Department of Agricultural
Development (DEVAG) and Department of Conservation and Extension (CONEX)
respectively. The extension approaches
used were prescriptive and tended to deligitimize the use of methods derived
from indigenous knowledge by farmers. Indigenous methods of crop preservation
were used by local farmers during the 19th century and early part of
the 20th century. However, the nature of crop preservation has been
dynamic from the pre-colonial period to present day Zimbabwe. A key feature of most approaches is that they are
derived from locally available natural materials such as trees, shrubs, and
sometimes parts of crops. Since there were little or no cash demands in the
acquisition of traditional methods of preservation, it meant that they were
readily available and thereby consistent with the small farmers' social,
economic, and institutional setting. An inventory of the known methods of crop
preservation used by smallholder farmers during the pre-colonial period is
presented in this section. Some of the preservation materials are native to Zimbabwe (lipia and marigold species) whilst others are
exotic (eucalyptus species). The exotics could have been brought by early
Portuguese traders during the pre-colonial period since they started spreading
from the Southern part of the country which borders with Mozambique along routes used by the Portuguese.
Eucalyptus species (Gum
The basis for using gum trees
as a method of crop preservation is predicated on the strong aromatic smell
that characterizes the leaves of the trees. In Zimbabwe, gum tree leaves have
been used to preserve crops such as beans, Irish potatoes, maize, millet and
The oil exuded by the gum leaves repels bean bruchids whilst the aroma
emanating from the leaves repels maize weevils. In all cases the gum leaves are
spread before and after the crop is stored such that there are two layers of
leaves in the granaries. The quantity of leaves used for any given crop depends
on expectations of the likely occurrence of pests as well as the farmers'
experience with the method. Use of gum tree leaves has been restricted to areas
where the tree grows well in Zimbabwe, particularly in natural regions 1 -- 3 which
receive at least 500 mm per annum and relatively warm temperatures that range
from 18 degrees Celsius.
This is a common perennial
shrub in Zimbabwe with a characteristic aromatic smell. Historically,
the leaves have been known to repel pests and it has also been used as a spray
against aphids. In grain crops such as maize, it has been used to repel a broad
range of pests, including weevils.
Marigold is one of the most
problematic weeds in Zimbabwean crop agriculture. In spite of this, the weed is
used by some farmers to kill nematodes and suppress other oil borne infections.
In grain crops, it is used by placing alternate layers of grain with the marigold
leaves. This method is more rarely used in most parts of the country since its
effectiveness hinges on the availability of the plant within the farmer's
Salt (in dehydrated form) or
in solution has a dehydrating effect on soft-bodied insects such as
caterpillars, snails and slugs. Salt is not used in the preservation of grain
crops since most of the pests attacking these types of crops have a hard outer
Tobacco contains the strong
aromatic poison nicotine that is toxic to a wide spectrum of pests and
microorganisms. In Zimbabwe, farmers use it as a method of crop preservation,
especially against the grain weevil, only at low levels in cases of lack of
access to conventional chemicals such as Target® or
Coopers Shumba. Tobacco can also be dusted on to vegetables as a remedy against
spider mites, aphids, and various caterpillars. Farmers argue that it is less
harmful in this form. However, the main drawback of this method is that food
plants treated with tobacco must not be eaten for at least three days.
Wood ash use for the
preservation of grain crops in Zimbabwe dates back at least to the late 19th century.
During this period, agricultural production by farmers was largely subsistence
oriented. Furthermore, no manufactured chemicals were known by the farmers
themselves or traded by the early Portuguese traders who came into the country
during this time. Wood ash was by-product of fires prepared for cooking
purposes by households. It was used mainly for two purposes. Firstly, it was
applied in fields as a form of fertilization and as a deterrent to slugs,
snails and cutworms. Secondly, it also found its use in grain storage where it
was often mixed with other ingredients obtained from other plants.
smallholder agriculture in Zimbabwe
In Southern Africa,
smallholder agricultural production is relatively homogenous with the majority
of farmers practicing crop agriculture on tropical soils poor in organic matter
with low inherent soil fertility )mainly lacking nitrogen, phosphorus, and
sulphur). As a result of general economic malaise characterizing this region,
productivity has been declining thus compromising the food security situation
of rural households.
In Zimbabwe, agriculture has traditionally been the prime mover
of economic development. Prior to the recent
land reallocations, large-scale farmers occupied about 11.2 million hectares
whilst small farmers occupied 16.3 million hectares with an average of 16.3
hectares per farmer. Crops
such as maize, millet, groundnuts, and sorghum dominated the cropping pattern.
Cash crops are primarily tobacco, cotton, and soybeans. Crop productivity among
smallholder farmers increased nominally by 400 percent between 1980 and 1990
from Z$148.28 million (US$ 148.28 million) in 1980 to Z$ 603 million (US$ 120
million) in 1990. This trend was attributed to the use of inputs such as
fertilizers, chemicals, and hybrid seeds. For instance, seed sales rose from 4,
500 tonnes in 1980/81 to 16,000 tonnes in 1989/1990 and fertilizer sales
increased from 24,000 tonnes in 1975 to 130,000 tonnes in 1986 and to around
200, 000 tonnes in 2003. Presently,
approximately 96% of farmers use improved seed despite evidence that open pollinated
varieties such as Salisbury White and Hickory King are surging back because of current
high seed costs. Productivity in the 1990s generally declined because of
droughts of 1991/92 and the 1993/94 seasons. The structure of agriculture
changed from the year 2000, ushering in more resource stressed smallholder
farmers. Although agricultural exports are still important, these have been
declining due to a range of problems such as rising cost of inputs and
recurring droughts from 2001 to 2004. For instance, the fertilizer price to
maize ratios from 1980 indicate that the price of fertilizer has been raising
faster than the price of maize, thus lowering the gross margin obtainable from
major crops. Therefore crop production systems relying on external inputs are
not sustainable for most smallholder producers both because of the immediate escalating
procurement costs and longer term effects on the environment.
Zimbabwe has been facing an inimical economic environment
over the past five years (2000-2005). As a result, the gross domestic product
(GDP) has been declining by an average 8% per annum. The rate of inflation has risen from 32
to 1042 percent in the years -- 1998 2006.
Foreign direct investment declined from US $ 440 million in 1998 to US $ 3.6
million in 2005 resulting in foreign currency bottlenecks.
These challenges have trickled
down to the farm level, particularly reducing the profitability of agricultural
activities and purchasing power. The population living below the poverty datum
line (US $ 1 dollar per day) was 36% between 1989-1994 and rose to above 80% in
Use of inputs by
Most smallholder farmers in
the country still use hybrid seeds which account for 90-95% of all maize seed
planted by farmers in any given season. The rising cost of maize seed has,
however, resulted in an increase in the number of farmers (20%) using retained
seeds, mainly in the
of the country. In addition, the increasing cost of
fertilizers has resulted in changes in the crop production systems. Farmers
located in southern and eastern provinces of the country are relying on cattle
manure as opposed to conventional chemical fertilizers. Farmers have also been facing post
harvest problems because of difficulties in accessing markets. Thus, there is
gravitation towards sustainable, low input production systems by Zimbabwean farmers
in the country.
Opportunities for using
traditional production methods by farmers in Zimbabwe
The unstable environment over
the last five years has negatively affected the nominal prices of fertilizers
and chemicals. Free government input supply schemes meant to bridge this gap have
been falling in nominal value and are not effective in enhancing access to
inputs by farmers. The use of inorganic fertilizers and chemicals has therefore
been declining among smallholder farmers. In Chivi District of
, for example, use of inorganic fertilizers and agro-chemicals decreased
by 78% and 71% respectively. Reliance
on expensive hybrid seeds by farmers negatively affected traditional systems of
storage and the re-use of seeds in subsequent production seasons. The need for
farmers to explore options of crop production and preservation is inevitable
given the current supply bottlenecks and also because capital markets are
increasingly becoming closed to farmers through prohibitive interest rates nearing
300%. There is gathering evidence, which
indicates that farmers in the country, particularly in the southern and eastern
districts, are resorting to traditional methods of production and crop
preservation. The main advantage of these methods is that the materials used
are locally available and within the farmers' reach. They are also relatively
easy to use and can be environmentally sustainable. Despite claims by farmers
that most of the methods based on traditional knowledge are 'effective' there
is little supporting scientific evidence within Southern Africa.
The experiment was conducted
at Belvedere Technical Teachers College (BTTC) farm located in Mashonaland East
Province of Zimbabwe. The study site conditions are typical of agro-ecological region
II, one of the prime agricultural zones of the country. This region covers 7,343,059 ha and this
represents approximately 18.68% of the total land size in Zimbabwe. Rainfall received in a normal season ranges from
700-1050mm and is restricted to the summer season. Temperatures ranging from
20-27 degrees Celsius are experienced. Farming in this region is mixed and eminent
crops include maize, cotton, tobacco, and soybeans whilst significant beef,
dairy and pig production also take place.
The methodological framework
used a completely randomized block design with four treatments replicated three
times. Treatments were 5g, 45g, 75g of ash per bag and 5g of Actellic Chirindamatura
dust (manufacturer's recommended level). In this approach, 12 small jute bags of maize containing 0.5 kg of maize
grain (SC5313 dent variety) were used. The SC513 was chosen since it is a
commonly grown variety among smallholder farmers in semi arid areas. Harvested
maize was checked for the existence of weevils before the experiment and also
that it was free from inorganic chemicals. The nine bags were then sub-divided
into three groups. The first three bags were labeled A1, A2, and A3, the second
group B1, B2, and B3 whilst the third group was labeled C1, C2 and C3. The
three bags in each set were considered replications for the experiment. To the
first set of bags, 5g of ash were mixed with maize grain in A1, A2 and A3. For
the second set of bags, B1, B2 and B3, 45g of ash were added. On the third set
of bags, 75g of ash was added to the C1, C2 and C3. Extra three maize bags D1,
D2 and D3 were used as the control group of maize. The bags were then put under
the same conditions: which would mimic typical conditions of African granary,
i.e. dry with ambient temperatures of about 24 degrees Celsius. Equal numbers (10)
of weevils (Stophilus zeamais) were then put in each of the maize grain bags,
ensuring that the weevils did not migrate from one-grain bag to another. The
researchers then checked weekly for any changes in the containers over a period
of 2 months.
Layout of the experiment
Hessian bags were used for the experiment and were arranged in the manner depicted in Fig 3.1. The room was clean and ventilation was guaranteed to ensure that the weevils had sufficient air supply. In addition, the grain bags also allowed for air movement without allowing them to migrate out of the bags. Blocks indicate replications for each level of ash concentration.
Parameters collected from each sample unit
Two parameters were of interest in this research and these included:
- Number of live and dead weevils collected weekly at different concentration levels
- Change in the color of grain
An analysis of variance table was set to investigate whether there were statistical differences in the numbers of live and dead weevils as well as the quality of maize grain used. A one-way ANOVA table was set up to investigate whether there were differences in the grand means of weevils over time.
RESULTS AND DISCUSSIONS
Variations of live weevils at different levels of ash powder concentration
Fig 4.1 indicates the changes in the number of weevils from each bag over five weeks. The basic shapes of the functions are almost similar with the number of weevils in each bag increasing over the 5-week period. The higher the concentration of ash in the bag, the lower the incidence of weevils. This may be attributed to the toxic effect of higher levels of cob powder administered to the different bags. It can also be seen that for the control (which used Actellic Chirindamatura Dust), number of weevils were higher in the first than the 75-g ash powder concentration but this changed from week 2, with the control group increasing at a steady rate. The findings of this study are similar with those of Elwell and Booysen who illustrated that an increasing cob powder concentration results in a reduction in the incidence of weevil attack. 
Means from each sample
Mean and standard deviations from each block
Results from Table 4.2 show that the control group of bags had the lowest number of weevils throughout the 5-week period followed by the 75g cob powder concentration. The least effective rate of application was the 5g-concentration level.
Control (Acetylic Super)
The main hypothesis to be tested in this study was to determine whether there is a difference between the average numbers of weevils found per unit time across the different treatments. The results of the ANOVA table are shown below.
|| Number of Weevils
Source of variation
The hypothesis being tested by the ANOVA table is that there is no difference in the average number of live weevils per bag over the 5 weeks. Since the significant F value is less than 5%, we reject the hypothesis that the means are equal. Therefore, the effects of the conventional chemical and cob powder are statistically different at the 5% level. One can conclude that there is a difference in effectiveness of maize cob powder and acetylic Super under the given set of conditions. The results from this study are at variance with those found by Elwell, 1995. However, this could be attributed to the different conditions under which their research was carried out.
Changes in the color of grain
At the end of 5 weeks, each of the samples was exposed to winnowing to remove the dust and ash. It was observed that for 5 and 45g ash concentration the dust could be easily removed. At 75g there was an element of discoloration, though this was not deemed significant in terms of quality.
CONCLUSIONS AND RECOMMENDATIONS
The results of the study indicate that higher levels of cob powder are associated with lower incidence of weevils in any given sample. In the experiment, two treatment levels (the 5g and 45g) were observed to be increasing functions. These two functions increased at an increasing rate in the first two weeks and increased at a decreasing rate in weeks 3 to 5. This could be attributed to the slow toxicity effect of the cob powder in the first two weeks. The subsequent shape of the functions could be explained by the attainment of a minimum toxic threshold as the weevils are further exposed to the local microenvironment within experimental bags. Lower levels of cob powder permit relatively rapid multiplication of weevils. The shape of the 75g function almost resembles that of the control, in that weevil multiplication is curtailed in the first two weeks before increasing at a decreasing rate albeit at a lower level. It is possible that levels slightly higher than the 75g could almost simulate the shape of the control function.
The results showed that there is a statistical difference in effectiveness between the conventional and traditional approaches of preserving maize. The average numbers of weevils for the experimental period were 11 for the control and 15 for the 75g treatment level. Based on these observations, the conventional chemical control is superior in conferring an environment that is toxic to the multiplication of weevils. The need to explore the effects of levels such as 80 or 85 g of cob powder cannot be over-emphasized. Higher levels of cob powder would, however, also mean greater demand for the farmers to gather cobs and fuel energy, burning the shelled cobs, and creating fine powder through removing undesired objects. This would increase the shadow economic cost of the maize cob powder.
The transaction costs of acquiring conventional chemical techniques for preservation are relatively high for most smallholder farmers who are frequently located in inaccessible production areas. These costs include searching for appropriate information on the available chemicals and markets (such products may be unavailable in formal markets due to input supply bottlenecks in the country), as well as transport costs to and from formal market outlets. These costs can act as deterrent to smallholder use of chemicals for post harvest preservation.
The farmer is faced with a dilemma: he/she wants to ensure food security from one season to the next but also needs to acquire the form of crop preservation technology at a low cost. In the absence of the required chemicals, smallholder farmers can trade efficiency of the preserving technique for convenience and affordability. Traditional techniques confer some measure of protection but this protection may only be guaranteed at high levels of ash concentration of at least 76g/0.5 kg of maize. Amounts such as 76g could be used for levels of up to 5 kg of grain. Using this upper limit, this translates into 0.76 kg per bag and 15.2 kg of ash per tonne of grain.
It is also important to note that the findings of this study were only reflecting a given set of conditions in the Middleveld of Zimbabwe and this could affect the results. It is critical to broaden the scope of the study to include different maize varieties. Although the behavior of weevils was not an explicit parameter in this study, future related study endeavors could also investigate the relationship between weevil behavior and ash.
According to study findings and experiences of some farmers, it is possible to use an application rate of at least 76 g of ash for 5 kg of maize grain. As such, a rate of around 15 kg of ash is recommended per tonne of maize grain. Since measurement challenges are encountered by farmers in quantifying the required levels of ash, a 50kg sack (by volume) full of shelled maize cobs can give 2-3 kg of ash depending on the level of burning. Thus 5*50kg sacks of maize cobs may suffice to treat 1 tonne of maize grain.
The value of indigenous knowledge cannot be doubted as illustrated by the study. Farmers may use cob powder in protecting their harvest against weevils. As a rule of thumb, effectiveness of cob powder is normally observed at high levels of concentration. Farmers will have to strike a balance between low cost and the labor time invested to remove ash when preparing for food consumption.
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