Post Harvest Losses in Wheat
Dr. Ali Muhammad Khushk & Aslam Memon
About 93 % of the food to feed the people of the world comes from plants, two-thirds of which is contributed by the cereals (wheat, maize, barley, sorghum, and millet). These cereals are the major source of calories and protein. About 80 % of the global cereal production comes from wheat, maize, and rice. Among the cereals, wheat is the largest. Of the two principal types of wheat, 90 % of the world's wheat is bread wheat, which accounts for 94 % of the production. Wheat is grown in 27 countries in the developing world. Pakistan is the eighth largest producer after the Russian Federation, China, U.S., India, Canada, Australia, and Turkey. Durum wheat is grown in North Africa, the Near and Middle East, Russian Federation, India, Italy, France, the Northern U.S., and some areas of Canada (Stubbs et al. 1986).
Wheat is the most important crop and it is the staple food for the majority of the population in Pakistan. Being staple food grain, wheat supplies 72 percent of the calories and protein in the average diet. The per capita consumption of wheat in the country, is 120 kg a year, is among the highest in the world. Wheat occupies a central position in agricultural policies, which is grown on about 37 percent of the cropped area and accounts for about 76 percent of the total food grain production. It contributes 13.8 percent to the value added in agriculture and 3.2 percent to GDP. Wheat was cultivated on an area of 8578.2 thousand hectares and total production was 23294.7 thousand tons during 2006-07.
Post-Harvest Operations
Post-harvest operations are assuming importance due to higher yields and increased cropping intensity. Due to introduction of modern technology, yield levels have substantially increased resulting in a marketable surplus which has to be stored till prices are favorable for sale. With increase in irrigation facilities and easy availability of fertilizers, intensive cropping is being practiced. The harvesting assumes considerable important because the crop has to be harvested as early as possible to make way for another crop. Sometimes, harvesting time may also coincide with heavy rainfall or severe cyclone and floods. In view of these situations suitable technology is, therefore, necessary for reducing the harvesting time and safe storage at farm level. The post-harvest losses are estimated to be about 25 per cent.
A recent estimate by the Ministry of Food and Civil supplies put the total preventable post-harvest losses of food grains at about 20 million tons a year, which was nearly 10 per cent of the total production. The principal adviser, planning commission stated that food grains wasted during post-harvest period could have fed up 117 million people for a year.
Although post-production operations vary from country to country and region to region throughout the world, procedures are similar among the developing countries. However, operations diversify with farm size such as small landholders, medium scale farmers and progressive growers. Post-production operations will be dissimilar between the developed and developing countries. Functions like harvesting, transportation, threshing, cleaning, drying, storage, packaging and marketing are described below.
A major proportion of the crop in Pakistan (over 70 percent) is harvested manually using sickles or with types of knives leaving 3-6 cm wheat straw above the ground level. Methods and timing of harvesting are important factors to total crop yield. The most critical decision in harvesting is not the degree of mechanization but the timing of the harvest. If the harvest starts late, the grain becomes too dry and rate of grain shattering is high. The longer a ripe crop is left in the field or on the threshing floor, the higher will be the loss from natural calamities including hailstorm, fire, birds, or rodents. The moisture content of the grain will be high, making drying difficult if the harvest start too early.
In Pakistan wheat is harvested in the dry summer months from March to May. Farmers are conscious of the fact that the harvested wheat should be dry enough for threshing and storage. Artificial drying is uncommon. The manually harvested wheat crop is tied into small bundles and stacked in bunches of 10 - 15 bundles, which are left in the field for one to three days to dry. Combine or mechanical harvesters yield a higher proportion of immature grains and pose a moisture hazard, leaving no time for the grain to dry.
Threshing
Threshing is mainly mechanical (60-80 percentage) in Pakistan. Tractor-driven threshers and at times combine harvesters are used. The design and maintenance of the thresher are central to reducing the broken grain percentage. Threshing using animals is also common in many areas of Pakistan. Several animals continuously walk around a pole to crush the wheat straw and heads to separate the grains and convert the straw to bhoosa.
Transport
Highly efficient bulk handling systems exist in developed countries to load loose wheat into trucks. Using an auger, wheat is moved to the grain-processing centre in a single trip, dumped into a receiving bin, carried by a mechanical conveyor through the cleaning and drying processes and into storage. Next, it is moved out of storage into the flour mill at the same location, where the finished flour is mechanically bagged, loaded into trucks by elevator, and taken to a commercial bakery or retail market without once being handled manually. National policy regarding the appropriate degree of mechanical wheat handling is often based on the need to maximise employment for unskilled labour.
In Pakistan post-harvest handling, transport and storage of grains at the farm level is done partially in bulk. Wheat is mainly transported in animal driven carts or carried on camelback. Large farmers use tractor driven trolleys and trucks. In each case bags are used for transportation. Problems arise when old torn bags are used which spill grain, causing loss. Mostly 100-kg bags are used which are cumbersome to carry. Other hazards for bags are hooks which tear the bags, the rough surface of the carts and trolleys and nails, which damage sacks when they are pulled. Transportation occurs from farm to market, market to consumer, market to temporary storage, temporary storage to long term storage and long term storage to consumers. The food grain trade depends upon labour. Therefore, handling, transport and storage of marketed grains in bags is common. Availability of cheaper jute bags also encourages handling, storage and marketing of grain in bags. Large quantities of food grain have to be moved through road transport, another major factor promoting use of bags.
Storage
The wheat delivered from the farm at harvest to the village market or to a government food corporation presents different challenges. Since mills need to be able to hold sufficient grain for 30 to 60 days of milling this wheat may be kept in sheds, large steel bins, concrete silos, or in the holding bins of a flour mill. Wheat may be temporarily stored in railroad cars or in open piles in market towns where protection is little better than on a village-threshing floor.
Depending on level of the self-sufficiency of the country the marketable surplus of food grain varies by factors comprising farm and family size, productivity and other parameters. In Pakistan, it is generally estimated that approximately 65 to 75 percentage of total wheat produced is stored at the farm. Smaller farms generally keep more grain for consumption. It is estimated that the quantity of wheat entering commercial channels from farms up to maximum 4.5 ha in size is negligible. Nationally, the 4.5 ha farm is worked by about 65 percentage of the farmers, who occupy 35 percentage of the cultivated land.
The major food grains are usually stored at the farm in specially constructed mud bins, protected by a cover, inside the house or in the open courtyard. Wheat may also be stored as a heap covered by straw, mud and dung plastered, loose in a room, or in bags, metal bins, baskets and pots. These widely contrasting storage practices may explain the range of storage loss in Asian countries.
In Pakistan, the millers' role in wheat storage has been limited by Government subsidies to public sector institutions, which procure about 60% of the marketed portion of the wheat crop. Rather than procuring wheat themselves, millers found it cheaper to procure from these Government institutions which carried out most of the long-term storage. However, when the Government raised their selling price in 1989 and thereby improved the incentives for millers to store, these responded promptly by buying up more stock.
The godowns have all the facilities for fumigation, providing aeration and rat proof. Each of the godown can hold 5000 tonnes of bagged food grains. Grain is also stored in bulk using large silos. For want of required storage space in godowns food grains are also stored in the open and this method of storage is known as CAP storage. Cap stands for cover and plinth. Open spaces in warehouses and elsewhere are used for storing produce. Crates are placed on floor, mats are spread on the crates and finally bags are placed over the crates. The stacks are built in the farm of domes. As protection against rain and sun the stacks are covered with thick (600 to 1000 guage) black polythene sheets and the cover is tied to the stack with the help of plastic ropes.
Wheat losses are defined as a measurable decrease of the food quantity and quality. Losses should not be confused with superficial damage generally due to deterioration. Quantitative loss is physical and can be measured in weight or volume, while qualitative loss can only be assessed. Quantitative loss, qualitative loss, nutritional loss, seed viability loss and commercial loss may gauge this reduction.
The major factors influencing wheat loss during storage are insects, moulds, birds and rats. Biotic factors including temperature, humidity and type of storage all affect environmental conditions in storage. High temperature causes deterioration, while low temperature is good for storage. High temperature accelerates the respiration of grain, which produces carbon dioxide, heat and water, conditions favourable for spoilage. Humidity equally impacts grain storage. Increasing humidity increases spoilage, while decreasing humidity is good for storage.
The type of storage plays a fundamental role in storage efficiency. If a concrete or mud storage structure can absorb water or allow the water vapours to pass through, in the case of a jute bag, the bio-chemical changes and mould attack are minimal, but the risk of insect infestation increases. Sun drying or turning of food grain has many advantages as it provides an opportunity for inspection and precautionary measures to avoid spoilage. Aeration greatly minimises mould growth, insect activity, and respiration of the seed. Further aeration provides a cooling action and equalises the temperature throughout the mass of the grain stored. Bad odours developed by stored grains can be easily and effectively removed.
Climate conditions, grain conditions at storage (presence of infestation, moisture content, foreign matter content), the period of storage, grain and pest control practices all contribute to the rate of loss caused by insects and mould growth. As these factors interact, it is difficult to isolate them or identify one factor, which has a direct influence on loss. Average statistics for loss, whether for store types, areas, or quantities of grain stored are inconclusive. An average figure for loss for a region or a country holds no significance unless a decision regarding a new system of storage, or new pest control techniques is required. Nevertheless average loss figures are always sought. The loss figures consist of the following:
- The weight loss which occurred during storage = The difference between the condition of grain at the end of the storage period, compared to the condition at initial storage
- The weight loss, which happened before the grain was stored (Note: some of the grain under study had been stored elsewhere for an unspecified time).
