Open weave polypropylene (PP) sacks are constructed from woven strands of polypropylene. After filling, the necks of the bags are sewn shut by hand or with a stitching machine. Most have PP bags have a capacity of 50kg or 100kg. The open weave allows the exchange of gas and moisture between the grain and the surrounding environment as well as entry of insect pests. They should be kept inside a house or grain store, away from walls, and placed on pallets to separate them from the floor.
Storage Period: 0-6 months
Capacity: 50kg - 100kg
Lifespan: 1 year - 2 year
Initial cost: US$ 7 - 8
Cost per tonne per year: US$ 5 - 6
All cereals, pulses and oilseeds
Materials used: Polypropylene strips
Locations made: Manufactured in most East and Southern Africa countries, often by companies associated with a milling enterprises, also specific packaging company. In Tanzania: Raffia Bag Co in Dar es Salaam; Ashraf in DSM; polypropylene (PPTL) (T) in Tanga; also several Chinese companies import to Tanzania and sell in bulk in Dar es Salaam. Kenyan users complain that these bags rip when lifting or loading.
Life span: 1 - 2 years depending initial quality and handling. Frequent handling by traders reduces lifespan, also may be holed by rodents, rough storage structures, and by pests such LGB (Prostephanus truncatus) or Rhyzopertha dominica.
Marketing and promotions: Polypropylene bags are widely marketed and used throughout most African countries, without any specific marketing campaigns. The supply chain is typically strongly developed, and in most countries they are readily available at local markets at village level during the harvest season. In Zimbabwe it is reported to be mandatory to use UV stabilised PP bags when the grain will be delivered to an strategic grain reserve or warehouse receipt system programme.
Success and nature of marketing: Most farmers use PP bags and in Zimbabwe they are mandatory for formal grain trading.
Training as part of the campaign: Most postharvest training refers and demonstrates the use of PP bags as they are so commonly used. They are included amongst the following PH training topics (in a typical 2 hr session in Tanzania): proper handling of the store; preparation before grain storage; pesticide application with proper dosage before storing in PP bags; improved traditional storage; importance of using pallets during storage; and, routine inspection of grain during storage.
Degree of adoption:
Polypropylene (PP) bag usage is high. Estimates suggest the following % of smallholder farmers use it:
- - Malawi - 90%;
- - Tanzania - 80% (but ~70% use it improperly, fewer farmers now use traditional granaries due to ease of PP bag storage);
- - Kenya - 100%;
- - Zimbabwe - 80% (in Zim the UV resistant PP is also being marketed, elsewhere it is just the normal PP bag).
A lot of larger scale traders and strategic food reserves also store produce in PP bags inside warehouses.
Reason for adoption:
- Easy to keep inside the house and thus protect from theft
- Cheap/ low initial investment price
- Multi-purpose packaging (transportation, handling and storage)
- Readily available locally, even at village level
- Convenient and portable, so that if the farmer suddenly needs to sell a sack of grain then it can put on the back of a bicycle and cycled it to town. If there is a flood or fire it can be relocated fairly easily. It can also be hidden easily when food security is a problem so that neighbours don’t know how much food you have.
- Easy to use
- Easy to monitor and listen to whether insects are chewing away on the produce inside it
- Easy to calculate amount of grain in stock
- Standard measurement to enable uniform packing, stacking, sampling, commodity accounting etc.
- When filled it becomes the trading unit for the formal market
- Wide adoption improves supply chain efficiency
- The UV resistant PP bags sold in Zimbabwe do not become brittle due to sunlight exposure
- Anti-slip weaving helps in stacking
- Weave does not break but opens when speared to draw samples
- Flexibility regards quantity and location of grain to be stored
Weaknesses of this store:
- Prone to rodent, termite, storage insect damage
- Grain stored in PP bags for more than 3 month periods needs to be treated with pesticides which increase the cost and can lead to health risks and rejection of exports
- PP bag has a relatively short lifespan
- Grain and pulses stored in central warehouses using woven PP bags require regular monitoring and fumigation
- Require use of a pallet or platform to keep them off the ground.
Barriers to adoption:
- None, hence why it is so widely adopted
- Bag is prone to damage by rodents, and insects such as LGB if grain is not treated
- Additional cost of treating grain with storage pesticides
- Occassionally supply and access to PP bags is difficult (e.g. rural areas of Mozambique) but generally they are said to be very easily available.
- Farmers are adopting them by themselves, but some extension effort still required to ensure optimum use
- Their use could be improved if training on how to ensure that only clean dry grain is loaded, that the room where the sacks are placed is clean, that they are stacked on pallets without touching the walls, are monitored regularly, and that any grain to be stored for >3 months is treated with a protectant
- Policies to regulate grain handling and make the PP bag a standard trading and handling unit
Use by institutions: Generally, used throughout all the countries in the region and by everybody who handles grain in formal channels, and by most who store grain at HH level. Institutions may store more than 1000 PP bags of grain.
In Kenya/Tanzania -
- transporting maize cobs or cassava or sweetpotato back from the field if not too badly holed,
- Flooring in house or mats to sit on outside house,
- Curtains across bathroom doors etc.
What to like:
- Low cost, so easily accessible to smallholder farmers
- The grain is kept in a presentable state, ready for transport, handling, storage and marketing,
- Bag use improves sampling and accounting
- Can easily be combined with insecticide admixture to extend storage period
What to dislike:
- Short lifespan of the bags
- Variability of product quality
- Susceptibility to rodent attack
- Need for insecticide treatment for longer storage periods