In much of the developing world cereal grains such as maize, rice, wheat, sorghum and millet are the main food staples. For critical and wide ranging reviews of grain postharvest losses (PHLs) and loss reduction, see the reviews by Greeley (1982), Grolleaud (1997), Boxall (2001), World Bank (2011), Hodges et al. (2010), Hodges et al. (2013), Hodges and Stathers 2013. The potential effects of climate change on grain postharvest losses has been considered by Stathers et al. 2013.  Watch the video below for a brief overview of cereal postharvest losses presented by NRI's Professor Rick Hodges.

Source of losses
Grain losses may occur for two main reasons. During harvesting, handling, processing and transport, grain may be scattered, dispersed or crushed. In addition, the grain may be subject to biodeterioration. PHLs due to biodeterioration may start as the crop reaches physiological maturity, i.e. when grain moisture contents reach 20-30% and the crop is close to harvest. It is at this stage, while the crop is still standing in the field, that storage pests may make their first attack and when unseasonal rains can dampen the crop resulting in some mould growth and the associated risk of mycotoxin contamination. A key issue is the weather conditions at the time of harvest. Most smallholder farmers in developing countries rely on sun drying to ensure that their crop is sufficiently dry for storage. If weather conditions are too cloudy, humid or even wet then the crop will not be dried sufficiently and losses will be high. More unstable weather conditions due to climate change, leading to damper or cloudier conditions, may therefore increase PHLs. However, successful drying alone is not a remedy against all PHLs since insects, rodents and birds may attack well dried grain in the field before harvest and/or invade drying cribs or stores after harvest. In addition, poor handling methods can lead to grain scattering, an inferior product contaminated with foreign matter that has broken grains that are susceptible to more rapid deterioration than whole grains. Finally, poor store hygiene measures can lead to the perpetuation of storage problems between seasons.  The range of typical postharvest weight losses for maize in Sub-Saharan Africa is shown in Figure 1.   

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Figure 1. Typical range of weight losses at links in the postharvest chain of maize in Sub-Saharan Africa

There is a possibility that there may be greater absolute postharvest loss when there are bumper harvests. This may happen through shortage of labour to care for the grain or lack of incentive to provide such care as larger harvests are associated with a sharper fall in market prices. This price fall may result in a slower flow in the market leading to longer storage periods for grain. In this situation there may be an increase in loss due to insect attack both by the normal pest complex and, in the case of maize grain, the Larger Grain Borer (Prostephanus truncatus); a pest introduced into Africa from Central America in the late 1970s and associated with a doubling of storage losses. However, the impact of bumper harvests on losses has not been measured and overall the effect is likely to be small compared with the losses resulting from unfavourable climate at harvest. Certainly, farmers are often supplied with sufficient storage capacity so that at least good harvests can be accommodated in fixed stores and in exceptional years they are content to store surplus grain in sacks in their houses. In some locations, subsistence farmers now prefer to use bag storage rather than traditional structures.

At least in the 1970s, there was a popular view that PHLs at farm level were high and traditional practices were the problem. But traditional practice is an unlikely culprit since farmers have survived difficult conditions over long periods by adapting their practice to prevailing circumstances (Greeley, 1982). Nevertheless, serious losses do sometimes occur and these may have resulted from agricultural developments for which the farmer is not pre-adapted. These include the introduction of high yielding varieties that are more susceptible to pest damage, additional cropping seasons that result in the need for harvesting and drying when weather is damp or cloudy or farmers producing significant surplus grain, which because it is to be marketed rather than consumed by the household, is less well tended.

Loss estimates
Loss figures for grains have been expressed in different ways and rarely do estimates include all steps in the postharvest chain. In the 1970s/80s the initial international efforts to quantify the losses of cereal grains were mostly focused on grain once it had entered farm storage. Few data were gathered on harvesting, drying or transport losses. Although these data gaps remain, there have been recent efforts to estimate the cumulative losses of cereals suffered by farmers in Sub-Saharan Africa using a model for PHLs that includes harvesting, drying, storage and transport but not processing activities such as milling. This effort has been supported by a network of local experts who contribute the data on which the model operates and validate its outputs. The results are available through the African Postharvest Losses Information System (APHLIS http://www.aphlis.net).

Losses estimated by APHLIS typically range from 15 to 30% although, as for all crops, weight losses figures exclude an indication of loss of quality which results in a loss of market value and opportunity. The average losses by year and by cereal type for the region are shown in Table 1. Rice cultivation in Africa, especially of the high yielding upland varieties such as NERICA, has increased significantly in recent years and although PHLs for rice are given in the table they are based on very few data. Rice losses and how they may be reduced by the introduction of improved equipment and methods requires detailed study.

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Table 1. % Cumulative postharvest weight loss from annual production of cereals in Sub-Saharan Africa*

*(Source – African Post harvest Loss Information System )

Losses for pulses and oils seeds are not yet available from the APHLIS but the losses reported from field studies of storage can be very variable. However, the few data that are available suggest overall PHLs may be of similar in magnitude to those of cereal grains.

Loss assessment approach
The 1970s food crisis inspired an upsurge in the development of improved loss assessment techniques, first detailed in Harris and Lindblad (1978) together with documentation on the losses themselves (National Academy of Sciences, 1978 a&b). These methods were updated and reviewed by Robin Boxall (Boxall, 1986) and his publication remains an important background document on approach and methodology. In the 1990s, Julia Compton working in Ghana took her experiences of using visual scales in plant pathology to create a rapid loss assessment method for maize grain (Compton and Sherington, 1999). Although not as accurate as individual laboratory-based measures, visual scales have many advantages not least of which is their ease of application that allows a much large sample of farms. This is important as the extent of losses varies greatly between households, between geographical locations, between seasons and between years. When attempting to provide an overview of the extent of losses it is believed that the larger sample size will more than compensate for the reduced accuracy of individual measures. A manual describing the development and implementation of loss surveys using visual scales is available (Hodges, 2013) and can be downloaded from the APHLIS website.

Loss reduction opportunities
Central to any effort to reduce losses is the adoption of better postharvest practice, this is key for both smallholders who want to improve their household food security and also for those making better livelihoods from grain sales. Improvements include the diligent application of existing approaches to postharvest handling (e.g. ensuring basic hygiene, monitoring, storage practices) and the introduction of new technologies (better grain driers, shellers, stores, etc.). The assistance of knowledgeable service providers from government extension services, NGOs or the private sectors are an essential, but relatively rare, prerequisite. For farmers seeking commercial opportunities, these can be facilitated by the adoption of new marketing arrangements such as collective marketing or new financial institutions. The latter include inventory credit or warehouse receipts systems that can give access to the credit needed to enable farmers to adopt better practices and technology.

For commercial activity, the adoption by farmers of better technology (including practices) requires preconditions such as a market that offers sufficient reward for better quality grain; transport infrastructure giving reliable linkage to a market; and, the knowledge and skills to produce good quality grain in a commercial context. Farmers often find that it is not worthwhile investing in the production of higher quality grain because the financial rewards are insufficient. Critically, a suitable incentive is needed to encourage postharvest loss reduction. It is not only these farmers who would benefit from the production of good quality grain. Others working in the grain trade, the traders, transporters etc., also benefit because a successful, quality conscious grain trade offers much increased grain flows in national and regional markets, leading to better business and better nutrition for all. Currently, it is common for traders to purchase poor quality grain from farmers at a lower price. This poor quality grain may then be placed on the market directly and provided that it is consumed quickly then weight losses may not be great (Scenario 1, Figure 2) although opportunity losses are inevitably high. If traders are keen to exploit higher value markets then they may instead condition grain so that it conforms to grade requirements (Scenario 2, Figure 2). But the process of conditioning involves substantial grain losses and costs. The result is less grain on the market and at a higher price. Alternatively, farmers could use better postharvest handling techniques that retain grain quality so that subsequent conditioning is not needed (Scenario 3, Figure 2). In that case, consumers benefit as they do not have to pay the costs of the losses inherent in this process and as the losses are lower there will be more grain on the market that will result in prices rising less steeply. This highlights the need for a value-chain approach with appropriate incentives to help farmers sell better quality grain for higher prices.

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Figure 2. Three scenarios in which differing postharvest handling and storage (PHHS) procedures by smallholder farmers and by traders result in different outcomes for grain quality and grain losses

Consistent rewards to farmers for the production of better quality grain are only likely to be offered in the context of a formal market with clearly defined and enforced quality standards. In Sub-Saharan Africa most grain moves in informal markets. However, the need for formal markets is well recognised and the East African Grains Council (EAGC) has set itself the task of lobbying for this to raise standards throughout the region and pay a premium for quality. In support of this objective it has recently issued a training manual on 'Structured Grain Trading Systems in Africa'.

The principle that a quality conscious market offers the incentive needed to reduce PHLs by creating a shift from scenario 1 to 2 to scenario 3 (as shown in Figure 2 above) is demonstrated by the UN World Food Programme's Purchase for Progress programme (P4P). This programme buys grain at a clearly specified quality from local producer groups for distribution as food aid, meeting the needs of food aid beneficiaries and at the same time helping to stimulate good quality local grain markets. The Farmers' Groups are treated according to their state of development and WFP has adjusted its own procurement modalities so that these groups and co-operatives can fulfil grain tenders. The Farmers' Groups are also provided with training on business and postharvest handling and storage (PHHS). A big boost has been given to PHHS aspects of this process by the development of the new 'Training Manual for Improving Grain Postharvest Handling and Storage' (Hodges and Stathers, 2012), in both English and French, for use by WFP training staff and partners that gives clear guidelines and instructions on how to retain grain quality throughout the supply chain. The PHHS training manual is also illustrated throughout with simple, clear cartoons that express some of the many basic messages (Figure 3).

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Figure 3. A selection of cartoons used to illustrate WFP's new training manual on postharvest handling and storage (also available in French)

The manual gives recommendation on approaches to training as well as the technical details of how grain should be handled and stored to assure good quality. It is also accompanied by a CD holding a set of key presentations and facilitators' notes. The current version has been devised specifically for SSA. A loose-leaf folder style enables trainers in the different countries to customise the manual, with blank versions of the step-by-step visual training posters that can be easily converted to local languages, and to add in details of their specific standards and grain protectant recommendations.

The relatively small scale of P4P means that its own impacts on grain markets will naturally be limited. But it is a pioneering programme leading by example across a wide geographical area including Africa, Asia and Central America. If its basic message, that a quality conscious market is the key to reducing losses of both grain and livelihood opportunity, is taken up more widely then a significant step will have been taken to improve food availability and quality.

References (see a more extensive listing under 'Scientific publications')