Grains may be lost in the pre-harvest, harvest, and post-harvest stages. Pre-harvest losses occur before the process of harvesting begins, and may be due to insects, weeds, and rusts. Harvest losses occur between the beginning and completion of harvesting, and are primarily caused by losses due to shattering. Post-harvest losses occur between harvest and the moment of human consumption. They include on-farm losses, such as when grain is threshed, winnowed, and dried. Other on-farm losses include inadequate harvesting time, climatic conditions, practices applied at harvest and handling, and challenges in marketing produce. Significant losses are caused by inadequate storage conditions as well as decisions made at earlier stages of the supply chain, including transportation, storage, and processing, which predispose products to a shorter shelf life. Important in many developing countries, particularly in Africa, are on-farm losses during storage, when the grain is being stored for auto-consumption or while the farmer awaits a selling opportunity or a rise in prices.
Potential for loss
There is potential for loss throughout the grain harvesting and agricultural marketing chains. During stripping of maize grain from the cob, known as shelling, losses can occur when mechanical shelling is not followed up by hand-stripping of the grains that are missed. Certain shellers can damage the grain, making insect penetration easier. For crops other than maize, threshing losses occur as a result of spillage, incomplete removal of the grain or by damage to grain during the threshing. They can also occur after threshing due to poor separation of grain from the chaff during cleaning or winnowing. Incomplete threshing usually occurs in regions with high labour costs, particularly at harvest time, when labour is too scarce and expensive to justify hand-stripping after an initial mechanical thresh. Certain mechanical threshers are designed only for dry grain. A wet season's paddy harvest may clog the screens and grain will be lost. Cleaning is essential before milling. On the farm, cleaning is usually a combination of winnowing and removal by hand of heavier items such as stones. Losses can be low when the operation is done carefully but high with carelessness. With correct equipment, cleaning losses should be low in mills, but grain may be separated together with dirt or, alternatively, dirt may be carried forward into the milling stages. In drying, grain that is dried in yards or on roads, as is common in parts of Asia, may be partially consumed by birds and rodents. Wind, either natural or from passing vehicles in the case of road drying, can blow grain away. The main cause of loss during drying is the cracking of grain kernels that are eaten whole, such as rice. Some grains may also be lost during the drying process. However, failure to dry crops adequately can lead to much higher levels of loss than poor-quality drying, and may result in the entire harvest becoming inedible. Adequate drying by farmers is essential if grains are to be stored on-farm and poorly dried grains for the market need to be sold quickly to enable the marketing-processing chain to carry out adequate drying before the grains become spoilt. With a high moisture content, grain is susceptible to mould, heating, discoloration and a variety of chemical changes. Ideally, most grains should be dried to acceptable levels within 2–3 days of harvest. One of the problems in assessing levels of post-harvest loss is in separating weight loss caused by the very necessary drying operations from weight loss caused by other, controllable, factors.
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