Post-harvest losses in wheat

AC301e10_optPAKISTAN IS the 8th largest producer of wheat after Russian, China, US, India, Canada, Australia and Turkey. Wheat is a staple food of many countries. It supplies 72 per cent calories and 14 per cent protein in an average diet with the per capita consumption at 120kg a year (highest in the world).

Wheat occupies a central position in agricultural policies because it is grown on about 41 per cent of the cropped area and accounts for 76 per cent of the total food grain production. It contributes 12.8 per cent to the value-added in agriculture and 3.2 percent to the GDP.

Post-harvest operations:

The post-harvest operations have due importance due to higher yields and increased cropping intensity. Harvesting is necessary as the crop is reaped early to make way for next crop. It sometimes coincides with environmental calamities like heavy rainfall, severe cyclone and floods. Therefore, suitable technology and time to harvest is necessary for reducing losses at the farm level. The post-harvest losses are estimated i.e. about 25 per cent.

A recent research puts the estimate that total preventable post-harvest losses of food grains are about 20 million tons a year that is nearly 10 per cent of the total produce. The principal adviser of Planning Commission stated that food grains going to be wasted during post-harvest period can feed 117 million people for a year.


Over 70 per cent of the crop is being harvested manually using sickles or with types of knives that leave 3-6 cm wheat straws above the ground level. Methods and timing of harvest is the most critical decision because these factors contribute to the total yield. If harvesting starts late, the grain becomes too dry and the rate of grain- shattering is high. The longer a mature crop is left in the field or on the threshing floor, the higher will be the losses from natural calamities including fire, birds, and rodents. If the harvest starts too early, the moisture content will be high and attract the pest attack.

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, 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. 0.35 per cent wheat is lost during harvest.


About 60-80 per cent of threshing is mechanical. Tractor-driven threshers and combine harvesters are the order of the day. The design and maintenance of the thresher contribute central to reduce the broken grain percentage. Threshing by using animals has become old fashion and time consuming. In this method animals continuously walk around a pole to crush the wheat straw and heads to separate the grains and convert the straw to bhoosa.1.5 and 0.45 per cent of total produce is lost during threshing and temporary storage respectively.


Post-harvest handling, transport and storage of grains at the farm level are done partially in bulk. Wheat is mainly transported in animal-driven carts. Large farmers use tractor-driven trolleys and trucks. Bags are used for transportation. Problems arise when old torn bags are used which spill grains, causing loss. Mostly 100kg bags are used which are cumbersome to carry. Other hazards are hooks which tear the bags, the rough surface of carts and trolleys and nails.

Transportation occurs from farm to market, market to consumer, and 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 cheap 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.


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 floors.

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. It is generally estimated that approximately 65 to 75 per cent 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 per cent of the farmers, who occupy 35 per cent 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.

The millers role in wheat storage has been limited by the government subsidies to public sector institutions, which procure about 60 per cent 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 godown can hold 5,000 tons 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 1,000 gauge) black polythene sheets and the cover is tied to the stack with the help of plastic ropes.

Wheat losses:

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 represent the range of losses.

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 minimizes mould growth, insect activity, and respiration of the seed. Further aeration provides a cooling action and equalizes 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, and 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 net weight loss which occurs during storage is equal to the difference between the conditions of grain at the end of the storage period, compared to the condition at initial storage.

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