Value chain

Postharvest losses occur at each stage along the postharvest value chain, from harvesting to storage and market. The PHL estimates in APHLIS can be displayed either per value chain or as the cumulative loss that occurs (remembering that once loss has occurred at harvesting, the % loss in the next stage is a % of what remained, not of the original amount).

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Available metrics

Percentage loss (%) the cumulative % loss in weight from production of ready to consume grain incurred during harvesting, drying, handling operations (i.e. threshing, shelling, winnowing), household level storage,transport and market-level storage for the selected location, crop and year.

Dry weight loss (t) this is the ‘absolute weight loss (in tonnes)’ which occurred postharvest for the selected crop, location and year. It is calculated by multiplying the APHLIS percentage loss estimate by the annual production figures for the selected crop, location and year. It is referred to as dry weight loss as it reflects the weight of losses of the dried focal cereal and legume crops.

Financial value of loss (USD) expresses the value of the postharvest weight loss in USD. It is calculated by multiplying the APHLIS percentage loss estimates by the annual production figures by the price (current USD) for the selected crop, location and year.

Value chain steps

For a full description of the value chain steps and methods for measuring losses during each of them, see Chapter 11 of APHLIS – Postharvest cereal losses in Sub-Saharan Africa, their estimation, assessment and reduction, and/or Section 2 of Training Manual for Improving Grain Postharvest Handling and Storage 

Harvesting/field drying

Harvesting by smallholders in Africa is almost always done manually. Losses at the time of harvest arise from two sources:

1. The scattering of grain (or shattering if the grain falls from the seed head) due to a combination of the method of harvest, the type and variety of crop and its maturity.
2. The grain that is not harvested, i.e. remains on the plant.

Crops harvested too late suffer much greater scattering losses, they may also suffer losses due to bird attack and this can be estimated separately by determining the weight of grains missing from panicles or heads at time of harvest. To facilitate further drying, the crops may also be stacked or ‘stooked’ in the field and further losses can occur during this period due to more scattering of grains and/or attack by pests (insects, rodents and birds).

Losses can be measured by marking out a number of plots in a field prior to harvesting, and weighing the amount of crop harvested from those plots and then carefully collecting and weighing any grains left behind in each of the plots, and by placing sheets under stacks in the field and calculating the mass of the scattered grain versus that remaining on the stacked plants stems. 

Further drying

Prior to threshing, grain may be subject to further drying in and around the homestead. The seed heads may be hung on racks, placed on mats or tarpaulins on the ground, or plastered-areas of the yard, or specially constructed platforms or in drying cribs. Following threshing or shelling, the grain may also be further dried typically by spreading it on the ground or plastered areas of the yard or on tarpaulin or plastic sheets, and turning the grains over from time to time. The duration of these further drying stages may range from a day to several months.

Losses during drying may occur due to consumption and damage of the grain by livestock, rodents, wildlife, insects or fungi. When further drying extends for periods of several months it effectively becomes a grain storage situation during which loss could be determined by the use of visual scales to estimate losses due to biodeterioration, while sheets and gleaning can be used to collect scattered/spilt grain, and before and after weighing can be used for small quantities, although changes in weight due to reductions in the moisture content of the grain as a result of drying are not classified as losses, by recording the moisture content of the grain at the beginning and end of the drying period the weights can be adjusted to standard moisture content. 

Threshing and Shelling

Threshing or shelling is the process of separating the grain from the seed heads, panicles, cobs or pods. This may be done by hand, by beating the crop with a stick, by beating the crop against a threshing platform, or using a manual or mechanical threshing or shelling machine. Losses at threshing may arise because:

1. The threshing is incomplete (i.e. some grain remains on the seed head)
2. Grains get scattered or spilled
3. Some grains become damaged in the process.

During loss assessment, sheets and/or gleaning can be used to collect scattered and/or broken grains and compare their mass to that of the threshed portion.

Winnowing

Winnowing is done to clean and separate the shelled or threshed grain from remnants of husks or pods or stones etc. It is typically done manually using a winnowing tray/basket and tossing the grain allowing the wind to carry off the lightest impurities. Grain may be lost due to scattering during the winnowing phase.

During loss assessment, sheets and/or gleaning can be used to collect scattered grains and compare their mass to that of the winnowed portion.

Transport from field and transport to market

A variety of means of transport are used to move grain from the field to homestead and from the field or homestead to market. The transport mode is typically dependent on the quantity to be transported, gender norms, the distance, and the household's socio-economic situation. For small quantities head loads are frequently used and particularly by women, labourers' may be hired to help harvest and then transport the crop using head loads. Bicycles, motorcycles, oxen or donkey drawn carts are also commonly used with payment often being in kind.

The measurement of losses during transport requires careful collection of any grain scattered during the journey, or weighing of grain bags at the two geographical ends of the transport process. Weighing at start and finish is likely to be the easier option provided accurate scales and labour are available. If transport is relatively rapid, e.g. done within a 24h period, then no adjustments for moisture content change are likely to be needed. Otherwise, weights before and after transport should be adjusted to standard moisture content.

Household-level storage

Once grain is sufficiently dry it is either put into storage for household food supplies or later sales, or sold to traders. The storage duration may range from a few weeks to 10-12 months. Many options exist for household-level storage with regards to the choice of container and grain protectant to help reduce attack by pests (insects, rodents, moulds, or birds) during the storage period. Good storage preserves both the quantity and quality of the grain while preventing damage.

Losses during storage are mainly due to biodeterioration:

1. usually caused by pests (insects, rodents, or birds in certain cases)
2. potentially also by fungi/moulds - if the grain is not well-dried before storage, or if the storage structure does not provide adequate protection against the rain or condensation, it is possible that there might be losses due to moulds.

Loss assessment at intervals during the storage periods can be done using visual scales, or by separating the damaged and undamaged grains and counting and weighing both portions to calculate the % weight loss, or where physically feasible by weighing the produce at the beginning and end of storage and adjusting the weights to a standard moisture content as needed. Whichever method is used, it is important that the grain withdrawals made by the family for use as food or for sale during the storage period are recorded, and that representative samples of the stored produce are taken for the loss assessment.

Market storage

Assessing the grain losses at sites where Farmers’ Groups and Co-operative etc. aggregate their grain, or in market stores or large-scale stores, can be challenging. The sources of loss are usually two-fold, grain discarded due to sorting/conditioning, and grain loss due to biodeterioration from insects, water leakage into the store etc. Grain sorting and conditioning is undertaken to raise grain quality to a standard at which it can be marketed; usually in order to comply with a specified grade in a formal trading standard. This can result in a considerable loss, since the grain that is removed in this process is often not fit for human consumption. Although, the damage to this grain will have accrued at earlier stages in the postharvest chain the actual weight loss is realized at this stage.