Birds as pests of grain stores
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by Filipinas M. Caliboso*
Birds belong to the Class Abes of Phylum Chordata. They are the only animals with feathers. Among the vertebrates they are the most highly specialized. They are also the most numerous of vertebrates, with roughly 28,500 species and subspecies as compared with 15,000 for mammals and 20,000 for fishes.
As a group they have become adapted to aerial life, although there are a number of exceptions. Adaptive features include light hollow bones, loss of right ovary and oviduct in females of most species; the exceptionally well-developed eyes; the highly specialized lung and air-sac system; the modifications of the forelimbs to form wings; the presence of feathers which although light in weight offer effective resistance to air when the bird is in flight. Like the mammals, birds have a constant body temperature, independent of the environment (homiothermous.).
The feet and beaks of birds also exhibit various adaptations to different modes of life and different kinds of food. There are birds whose feet are adapted for swimming, perching, and running. Some feed on insects, others on fruits, seeds (grain). A number of grain-eating birds have become important agricultural pests in many countries.
In the Philippines, there are various species of bird pests which feed on such crops as rice and sorghum. Collectively, these birds are known locally as maya, and are also called Philippine weavers.
Many species are attracted by ripening grain crops and to drying and threshing floors at harvest time. Some have developed a close association with the more permanent sources of cereals and cereal products and have become a nuisance in food stores and warehouses.
Grain spilled by careless handling in and around stores attracts birds and regular spillage may lead to establishment of a resident population. Accumulations of grain dust and flour residues around mills and processing plants will also attract birds as well as heavy insect infestation on the surface of stocks. Once attracted to a building, birds quickly learn how to enter. Bird populations inside a store pose some major problems such as:
1. They may settle on top of stacks and peck holes in wooven sacks in order to reach the food inside. This can cause spillage and in extreme cases, collapse of the stack.
2. They will roost and nest inside large buildings unless access is completely restricted. In a warehouse, birds, may damage screens in order to gain entry.
3. Nests in guttering and downpiping can cause blockages and lead to flooding and water damage to commodities.
4. They become hosts for lice and mites which become occasional pests of man when birds nest in buildings.
5. Nesting materials will provide harbourage and breeding sites for several of the stored insect pests.
6. As a result of their activity, bird droppings, feathers and decaying bodies may contaminate the foodstuffs, packaging and handling facilities. Droppings are likely to be infected by food poisoning bacteria Salmonella) and also constitute a major source of infection for the diseases known Histoplasmosis, Cryptociccosis and Aspergillosis which are caused by fungal spores.
CLASSIFICATION AND IDENTIFICATION
The Philippine weavers belong to the Order Passeriformes (Perching birds) whose common feature is the perching foot. The major bird pests belong to the genera Lonchura (Family Estrildidae) and Passer (Family Ploceldae). Another species, Padda oryzivorahas also been observed but in limited number and limited distribution. Studies conducted by NAPHIRE-TRED showed that warehouse bird populations are predominantly Passer montanus or mayang simbahan. The weavers are relatively tiny birds (weighing 10-30 gm) which build covered nests, often beautifully woven. They are mainly nonmigratory. Species are easily distinguished by their plumage. However, there is no sexual dimorphism among these birds. A guide to the identification of the different species is summarized in Table 1.
METHODS OF COLLECTION
1. Mist nets-Fine silk or nylon net with 3/4 inch mesh, 7 ft. wide and 18 to 38 ft. long. A taut frame of stout twine crossed by horizontal braces called "shelfatrings" is used in conjunction with the mist net, the net and the shelfstring being tight, while the net is loose. The excess netting is arranged in a loose bag or pocket 3 or 4 inches deep below each shelfstring except the topmost one. A bird striking the net from either side carries the net beyond the shelfstrings and hangs in the pocket of the net. A net properly hung, with 4 shelves, is about 6 ft. high (Giles, 1971).
Mist nets are more effective when set on a calm day against a dark background such as tall grasses in the weavers roosting area, trees and buildings. Wind greatly reduces catch. The mist nets should be placed where birds tend to fly back and forth fairly low but where there is a minimum chance of people, and animals crashing into a net. It is best to remove birds from mist nets within a few minutes before they become entangled and die form exposure or injury.
Mist nets require great care. They are easily tangled or torn by contact with vegetation. A person operating the nets should wear clothing with the fewest buttons, exposed pencils and other things to avoid being entangled to the net in the process of removing the birds. To expedite cleaning of nets, caught birds should be placed in a holding. Between uses, nets are best preserved by removing them from the poles and storing them in plastic or cloth bag. Wet nets should be dried out to avoid fungal attack.
2. "Korag" - This is manually operated net trap composed of two rectangular nets (1" mesh, light net) each measuring 1 1/2 m x 2 m. The widths are framed by light poles and the lengths by nylon stings. The nets are set flat on the ground, parallel to each other 3 meters apart such that when activated, they close in perfectly. The nets are hung at the ends and are flipped over the birds by a jerk on the pullcord by the operator.
The operator must be hidden from the birds by a blind. Decoy birds are needed to attract the birds. The "Korag" can be used where mist nets are ineffective such as in newly harvested rice fields and open grasslands. The trap area should be level and cleared of vegetation and other debris.
3. Modified Australian crow trap-this trap is a large cage made of mesh wire with wooden frames measuring 1 1/2 m wide x 2 m long x 2 m high. The midsection of the truncated V-shaped cage top is provided with holes or slots through which birds can enter. This trap is self-operating and so must contain food, water and some of the captured birds as a decoy.
4. "Kaliked" - this is a funnel-shaped bird trap made of bamboo sticks or coconut midribs used to catch brooding birds. The trap is fitted into the mouth of the nest and another entrance is opened for the opposite end of the nest. The bird is trapped as it seeks its way out after incubating the eggs. A trap could only catch one bird.
BIOLOGY AND BEHAVIOUR
Not much is known about the Philippine weavers. Field observations reveal that the main diet of these birds consists of rice and Echinchloa seeds. They are also observed to feed on corn tassel, sorghum and certain algae. Feeding begins just after dawn until about 10:00 A.M. and at about 3:00 P.M. to dusk.
The sparrows are primarily seed-eaters. In cage and field experiments, the birds were observed to consume 30% of their body weight per day. (3 grams for Lonchura spp. and 6 grams for Passer montanus). The same study also revealed that birds infesting private stores are 15% heavier than those found at NFA (National Food Authority, National grain marketing agency in the Philippines) warehouses. Apparently, the "openess" or loose construction of private warehouses provide easy access by birds to feed inside. On the other hand, NFA-constructed warehouses partially excluded populations through its better design and provision for bird proofing such as screens. Another reason for the observed discrepancy in weight is the presence of hog and poultry pens around the warehouse. Feeds are also available in these pens and birds obviously feed on them. Results further show that grain composed 91-97% of the gizzard contents of birds collected from private and NFA stores. Other components are weed seeds, grasses and stones.
Birds are abundant in the ricefields especially around harvest, in towns and villages. They roost on pill, starapple, ipil-ipil and bamboo trees as well as houses including warehouses. Their populations follow a definite cropping pattern in an area, that is, it increases when the grains are formed and ripened. Aside from cereals, they also subsist on weed seeds, especially Echnochloa spp. and some algae. Philippine weavers are gregarious. Different species are often together is feeding, roosting and even in nesting activities.
The breeding season of these birds may start as early as February with peak of nest building and egg laying in April and May. The breeding season may extend until October. The male and female birds work together building nests in stands of tall grasses (cogon, talahib, marker grass), palms and citrus. Mating was also observed to occur in the process. In a warehouse, they build nests on walls, between window slots, crevices of walls, beneath the roofs, along gutters and on the sliding mechanism of doors.
An average of 6 small eggs (5-8) are laid in the nest at one day interval. The eggs are oval measuring 16 mm (greatest length) by 11 mm (greatest diameter) and white or pale ochraceous-salmon in color. Eggs are hatched after a 10-day incubation period.
Newly hatched birds are naked with eyes closed. On the 7th day after hatching, chirping becomes audible while on the 8 th day, eyes start to open and primary feathers appear. The body and feathers develop further and the birds are ready to fly when they are 17-19, days old.
Present information about the different species are summarized in Table 2.
POPULATION AND LOSS ESTIMATION
There are four forces affecting the size of population:
1) nasality (births)
2) mortality (deaths)
3) immigration movement
In combined form:
Ni + 1 + Ni + (B-D) + I - E
Ni + 1 = population at time ti + 1 (final)
Ni = population at time ti (initial)
B = births
D = deaths
I = immigrants
E = emigrants
The population growth of birds is characterized by a logistic growth curve:
Characteristics of curve:
1. Slow rate of increase to start with
2. Faster rate of increase at greater population size
3. Flatters off to an asymptote
Thus, a pair of birds can give rise to 1,720 individuals in a span of one year, potentially capable of consuming 927 kg and causing a variety of losses through reduced grain quality due to contamination, damage to structures, etc.
Types of population estimations:
1. relative density-abundant, common, rare
2. absolute density-number per area
1. Measurement of relative density
- indices of abundance such as droppings, nests and amount of food consumed.
Rice hulls that remain after the birds feeding on paddy may be collected. Since sweeping may still contain paddy. these should be removed and the rice hulls weighed. From this, the population and/or loss may thus be calculated:
weight of palay loss per day (kg) = wt. of rice hulls collected/day + wt. of brown rice consumed per
To get a good estimate of loss or population, the collection of sweeping should be undertaken daily for 12 weeds per month and at least 4 months in a year, that is, 2 months for the dry season and 2 months for the wet season:
Anaverage of 0.5 kg. of rice hulls are swept daily at NFA warehouse.
weight of brown rice consumed or loss per day = 0.500 / 0.23 = 2.174 kg.
weight of palay loss per day = 0.500 + 2.174 = 2.674 kg.
2. Measurements of absolute density:
1. total counts-ex census of population; visual counting of birds.
2. sampling methods-counts of a small porportion of the population to stimated the total.
3. Capture-recapture (tagging) methods allows estimation of density, birth rate and death rate; when distance between recaptures are known, movement and home range may be determined.
i) Marked and unmarked animals are caught at the same rate (equal probability of capture)
ii) Marked and unmarked animals are subject to the same mortality rate.
iii) Marks are not lost or overlooked.
Total population size (N) / Total caught in Sample (n)
No. marked in population (M) / No. marked animals in sample (m)
Mn where sampling is continued until (m)
N = m animals are recaptured
N = population size
n = sample size
M = total marked in population
m = total marked in sample (re-capture)
Removal method (sampling without replacement)
- when animals have to be sacrificed for some reason (economic or health reasons) make it inadvisable to return trapped animals to the population. Assumptions:
i) stationary population (immigration = emigration)
ii) Probability of capture constant for each trapping Hayne's method (1948)-by regression
|Night||Catch (y)||Total previous catch|
Zippin's Method (1958)-by multinomial equation
Cleaning the hulls and spillage. Doorways can be provided with curtains or thick PVC strips suspended from the roof. This deters birds from entering but permits almost unrestricted entry by man, vehicles and commodities. It is a lot easier to prevent birds from entering than to force them to leave with scarers such as explosives, horns and amplified distress cries.
As much as possible, within the limits imposed by other considerations, the exterior of grain warehouses should be free from ledges suitable for roosting. Attractive nearby roosting sites (such as large trees) should be reduced to the minimum, the remaining pruned to lessen the cover available for birds.
Utmost care should be taken to keep the store and surrounding area clear of spillage and food debris. Immediately after intake or discharge of stocks, all spilled residues should be swept up. Residues should never be thrown away near the stores and condemmed foodstuffs should not be dumped nearby. Removal of seeding grasses and other plants from areas adjacent to the store is a helpful measure to reduce bird infestation.
Egg collection and pest destruction
Regular attention to the destruction and removal of eggs and nests will help to reduce the resident bird population. This needs to be carried out regularly because birds are very reluctant to change nesting sites, even after disturbance, and they tend to return again to the same place.
A smaller portable version of the modified Australian crow trap (1 m long x .05 m wide x .75 m high mesh wire cage with a V-shaped top with slots trough which birds can enter) has been tried and was partially successful. This is a very time-consuming method because the traps need to be attended to twice daily. Cages should be baited with an attractive food material.
Foot Stickers (glue)
Slow drying plastic jellies can be applied to sedges where birds roost. These give the perching bird an insecure sensation. Grease can also be used for this purpose. The jellies are applied to horizontal surfaces only, using a caulking gun. The jelly remains effective until clogged with dust residues.
This is of limited value but may help to prevent bird roosting in certain areas.
Noise, and other forms of explosion are likewise of limited value.
Ultra-sonic disturbances and recorded bird call (warning cries and distress call)
These are not reliably effective indoors because the sounds are reflected off in internal surfaces. This causes a modified signal to which birds eventually become accustomed.
In general, physical methods are capital-and labor-intensive especially if birds stay above the piles.
In the field, bigger predatory birds and other animals feed on the weavers. Rodents and cats may also be considered as predators inside stores. Even people catch them at their roosting places at night for food or for trade.
Chemical Methods (poison baiting)
These methods should be considered as a last resort if all appropriate alternative methods have failed. Baiting involves the use of avicides which are potentially hazardous materials and these should be introduced only where safety and supervision can be kept under control. The use of bird poison of pesticidal products intended for other purpose is not recommended. It is unnecessary and dangerous. The food bait for the poison has to be very carefully chosen and it must suit the species of bird. For example, pigeons prefer peas, wheat and cracked or whole corn; sparrows prefer small grains such as palay. All these birds will take bread or cake very readily. The bait offered must, however, be, more attractive that available foods. Pre-baiting 34 days increases the chances of success. For pre-baiting, the same sort of food to be used for the poison bait (but lacking the poison) it is put out in the same place each day.
Repellents like methiocarb discourage birds from further feeding on the seeds. Alpha-chloralose is a material which at low doses has the effect of stupefying birds without killing them. Thus, it is possible to immobilize, collect and remove troublesome burds. They can be released at a safe distance from the warehouse, or destroyed it they are not protected species. As advantage of this it that if non-target species happen to consume the bait, they too will be only temporarily affected. When used at a higher dosage rate, alpha-choralose acts as acute poison and there is no recovery. The concentration of poison to be incorporated in the bait to produce either stupefaction or mortality requires careful determination under local conditions.
Neurological frightening agents such as 4aminopyridine prepared in grain baits have been used successfully. The material (4-AP) causes birds to emit distress cries and perform erratic flight displays. The bird showing these symptoms will die but it may be sufficient to kill only few birds becaused the affected birds serve to scare away the rest of the flock. Passer montanus is more susceptible to 4-AP than methiocarb. This species also gave the most audible and greatest number of distress calls in response to 4AP (Garrison, et. al., 1981). The use of 4-AP against sparrows in paddy fields and storage warehouses shows promise.
Chemosterilants are used to sterilize a pest population of birds thereby controlling its breeding. The method, however, has disadvantages. The less-toxic chemosteritants are too short-term in action whereas those with more persistent effects are much more toxic. Furthermore, since there is no immediate reduction in population, birds continue to cause a nuisance for some time and may even propagate further because additional birds are attracted to the site by the bait.
None of these agents should be placed where poultry and other animals have access to them. If affected birds are eaten by cats of dogs, they cause secondary poisoning of the predator. For all chemical control measures, a high degree of supervision is essential. This must include regular and frequent collection of dead birds (more than once a day), the burying or destruction of the dead birds, the final removal of all treated bait and the proper disposal of unused bait.
The primary objective of bird control in storage should be to prevent or reduce grain losses and not merely kill animals. Effective bird-proofing of stores is strongly recommended for use in the first instance. Sanitation, elimination of harbourage, proper stock and warehouse maintenance and removal of bird nests should be standard practices. Quick turn-over of stocks also reduces the exposure time of grain to the pests. If this is done, then the use of other control measures will, in most cases, be found unnecessary. If it becomes essential to introduce other measures, these must be planned and carried out so as to 1) avoid public concern, 2) pose no hazard to non-pest species, man or domestic animals.
Table 1. A Guide to the Identification of Philippine Weavers
Table 2. Some notes on the biology and behaviour of Philippine weavers
Post-harvest microbial infection of cereal grain
by Lina L flag*
Spoilage of grains after harvest is due to the interaction of prevailing environmental conditions and the various organisms that attack and contaminate the grains. The environment factors are temperature, moisture, light, gases, and chemicals that may be present. The organisms responsible for grain deterioration are rodents, insects, mites and microorganisms. Changes in the non-living environment provide conditions that may either stimulate or inhibit the activity of the living organisms and thus enhance or prevent spoilage as the case may be.
Our main concern here is the spilage of grains by microorganisms.
Grain spoilage is brought about by any of the following microorganisms:
1. Bacteria, which morphologically appear as a single cell or groups of single cells that may be spherical or rod-shaped. They reproduce by binary fission or simple cell division. They need a natural opening or wound to gain entrance to the host as they cannot penetrate the intact grain.
2. Actinomycetes, which are closely related to the bacteria but with elongated cells and form branches. These are usuall saprophytic. Many form antibiotics (such as neomyin and streptomycin) which inhibit of kill other microorganisms in a storage bin.
3. Yeasts, generally appear as single cells and mutiply by budding. They predominate in sealed silos where the oxygen supply is low and the moisture content is high. They may impart a fermentation or yeasty odor to the grains.
4. Molds, fungi that have filamentous vegetative structures called mycelium. Reproduction is complicated. They form sexual spores (after the fusion of two compatible cells) as well as a sexual spores (without previous fusion of cells). The spore is more resistant than the mycelium to adverse conditions. When a spore lands on a substrate (such as grain of rice) it germinates and sends out thread-like structures (mycelium) which grow and branch and soon colonize the entire grain, and later produce more spores to complete the cycle. Spores are continuously present in the soil and air, and are spread by air currents, insects and other agents.
Deleterious Effects of Microbial Contamination
1. Loss of Seed Viability:
The germ or embryo is a preferred site of microbial attack because it is delicate, thin-walled and it contains the nutrients needed for microbial growth. Once the scutellum is affected, the food supply for the germinating plant is destroyed.
2. Altered Nutritional Value:
The formation of free fatty acids, lowered protein digestibility and vitamin changes have been associated with deteriorative changes in grains brought about by microorganisms.
3. Health Hazards:
Grain dust contains fungal spores and bacterial cells along with a number of substances that cause a variety of symptoms and ailments such as respiratory disorders, eye irritations, skin itchiness, allergies, etc. Aspergillus fumigates can cause lung diseases in man when spores are inhaled. A. flavus produces aflatoxin which is carcinogenic and poisonous to several animals, possibly including man.
4. Spoiled grain causes a reduction in the milling yield and milling quality. Such grains are very friable.
5. Contaminated grains are often unpalatable with a musty or sour flavor.
6. Microbial infection may cause of granular products.
7. Molds may cause deterioration of packaging and sealing materials which often result in grain spillage.
8. Microorganisms cause heating of the grain. As the microorganisms respire, they emit heat and moisture which usually make the micro-environment more favorable for further microbial growth. The formation of hot spots in bulk-stored grains is mainly due to the metabolic heat from microbial respiration.
9. Certain microorganisms bring about grain discolorations such as blackening, reddening and yellowing. Yellow grain has become a major problem in the humid tropics. The percentage of yellow grains is high in wet palay that had been left unthreshed for a few days. The high moisture in the unthreshed palay allows for rapid microbial growth. This results in the release of metabolic heat from the respiring molds, causing an increase in the grain temperature in the unthreshed pile. The high temperatures, along with the various secondary metabolites released by the microorganisms, are the likely causes of grain yellowing.
Factors Affecting Microbial Growth and Development
Temperatures from 20 to 40°C favor the growth of most microorganisms. However, some may grow at as low at -90°C (psychrophilic) and at as high as 80°C (thermophilic). Temperatures below 10°C generally inhibit microbial growth but is not lethal in most cases. Thus, a return to favorable temperature for growth, after a period of cold storage, often results in resumption of luxuriant growth.
2. Moisture Content of Grains
Microorganisms are typically moisture loving and a certain amount of moisture in the substrate is required for growth. The amount of moisture determines the type of organism that is capable of growing in the grain, and the rate at which it grows. Among the various microorganisms, bacteria snd actinomycetes are the most hydrophilic followed by the yeasts, and finally the molds which are more droughtresistant. The microorganisms are inhibited at moisture contents in equilibrium with below 70% relative humidity. Fungal spores are not destroyed by dry conditions; their germination and growth are merely inhibited until the moisture in the grain rises to a favorable level.
3. Length of Time the Grain is Stored and Prevailing Storage Conditions:
The expected storage life of the grain is inversely related to temperature and moisture content. The lower the storage temperature and moisture content, the longer the storage life. Cleanliness in the storage bin and warehouse is a prime requisite in prolonging grain storage life.
4. General conditions of the Grain:
Microorganisms can easily penetrate and initiate infection in damaged or cracked grains. Grain that had been previously invaded by bacteria and fungi is more easily prone to damage than grain that has had no contamination.
5. Presence of Insects and Mites:
Insect infestation usually accompanies mold invasion. Insects and mites serve as vehicles for the spread of molds and other microorganisms. Insects are attracted to grains that harbor molds because the insects often feed on the molds as well as on the grain.
The biochemical oxidation of food materials to obtain energy for life processes (respiration) is carried out by some microorganisms only in the presence of oxygen in amounts close to those present in air at atmospheric pressure. Other microorganisms can do with less oxygen and still others grow without oxygen. The molds are strongly aerobic so they grow profusely on the surface of the commodity where athmospheric oxygen is in ample amounts. Generaly, yeasts and bacteria can do with less oxygen than the molds (microgerophilic).
The ultra-violet portion of the spectrum is lethal to microorganisms, but due to its low penetrating power, only those organisms present on the surface of the commodity are killed. Many microorganisms grow best in the. dark although some require light for sporulation.
CONTROLLING MICROBIAL SPOILAGE OF GRAINS
Since microorganisms are present everywhere, there is no practical means of totally eliminating them. However, they may be kept under control by providing conditions that will innibit growth. The following are recommended:
1. Dry harvested crops promptly to a safe moisture level (13-14% for maize and rice). This is by far the most practical means of preventing microbial spoilage. Prevention is the key control measure because once invasion occurs, with the subsequent deteriorative changes such as aflatoxin contamination, practically nothing can be done to improve the quality of the grain or to remove the toxin. After the grain has been dried to the desired moisture level, rewetting or moisture absorption should be prevented.
2. Provide aeration in bulk-stored grain to provent moisture condensation which is highly favorable for microbial growth. Aeration cools the mid-portion of the stored bulk and lowers temperature uniformly throughout the stored grain. This prevents moisture migration. Aeration should, however, not be done at night or when the relative humidity is high because the grains may take up moisture. Increased moisture content will then favor the growth of microorganisms.
3. Prevent insect and mite infestation. In addition to the direct damage they cause, these pests disseminate fungal spores and activate the growth of molds.
4. Low temperature storage (at 10°C or lower) inhibits microbial growth but this method may be expensive and impractical for bulk stored cereals.
5. Storage in air-tight structures has been studied for the control of mold growth. However, bacteria snd yeasts may develop under these conditions which often results in off-odors to the commodity. This renders the grain unfit for human consumption because the objectionable odor may be carried over to the processed products.
6. Fungicidal treatment to keep the molds in chect could be expensive and there is the added problem of toxic residues that may render the food or feed unsafe for consumption. Grains intended for seeding purposes may be chemically treated. Fungicides may also be difficult to apply and may adversely affect the processing of grain.
Mold growth can be controlled by 1-2% ammonia in high mosture corn (22-28% moisture content) for 6 months. However, bacterial growth may continue after an initial drop in their number. Calcium propionate or sodium propionate at 5000 ppm can lengthen the storage life of rough rice.
Drying, coupled with proper ventilation or aeration appars so far to be the most effective, the safest, and the most practical means of maintaining the quality of grains. Periodic sampling and testing for moisture content, temperature and presence of microorganisms will allow the detection of potential trouble before anything serious occurs.
Commodities that yield toxin-forming fungi may not necessarily contain toxin(s) because morphologically identical strains vary in their ability to form a toxin and the fungi observed may just be superficially present on the commodity. On the other hand, grains that appear free of molds may be contaminated with a toxin if the toxin-forming mold had been eliminated through desiccation, high temperature or by some other means.
Christensen, C. M. and H. H. Kauffmann. 1969. Grain Storage, the Role of Fungi in Quality Loss. Univ. of Minn. Press, Minn.
Christensen, C. M. 1974. Storage Cereal Grains and Their Products. Am. Assoc. Cereal Chem. Inc., St. Paul, Minn.
Food Storage Manual. 1970. Prepared by the Tropical Stored Products. Centre (England), World Food Program, Rome
Hall, D. W. 1970. Handling and Storage of Food Grains in Subtropical Areas. FAO, Rome.
Salunkhe (Ed.). 1985. Postharvest Biotechnology of Cereals. CRC Press, Floida.
Sinha, R. N. and W. E. Muir. 1973. Grain Storage: Part of System. Avi. Publ. Co., Inc. Conn.
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