Grain handling and storage for pulses
Grain handling and storage conditions for pulses are important for insect control and grain quality. There is a range of insects that attack pulses in storage in Western Australia. Pea weevil (Bruchus pisorum) is the most serious. It completes its life cycle inside the field pea while in storage sometimes completely hollowing out the seed. The drugstore beetle (Stegobium paniceum) and the cow pea weevil (Callosobruchus spp) are other insects that have been detected in long term storage. Both of these have caused substantial damage during the storage period.
Pulses are also more sensitive to storage conditions than cereals. High temperature, high relative humidity, high seed moisture content, light exposure and an extended storage period have all been found to adversely affect quality. These factors may cause colour darkening and hard-to-cook defect in pulses. The hard to cook defect is characterised by increased energy requirements for cooking, poor palatability and reduced quality of protein. Colour darkening, especially in faba beans, decreases their market price and acceptability to the consumer.
Poor handling and storage of pulses will also adversely affect grain kept for seed. Weed seeds such as green wild radish pod can severely reduce seed germination. Similarly, poor handling can damage the seed before its end use. As an example, field peas can be damaged if a pneumatic grain handling system is used.
Grain handling for pulses
Pulses are more susceptible to impact damage than cereals. Effective handling and storage of pulse grain on farms must consider the following points:
Soft handling
Pneumatic grain conveyors should be avoided for pulses since the impact speed of grain on grain is higher than the critical 12 metres per second. Augers smaller than 125mm in diameter should also be avoided. Augers should be run full and preferably slowly to reduce damage. It is easier to reduce the speed of augers driven by petrol engines then augers driven by electric motors.
Loading and Unloading
Silos are designed to withstand uniform downward and significant uniform outward forces. Because of this they must only be loaded from the central top hatch. Loading from the topside hatch will unbalance the lateral forces on opposite sides of the silo. This may effectively distort the shell of the silo, placing extreme pressure on the side of the silo holding the high side of the stack. The same principles apply when out-loading. Only empty from the bottom central opening. Avoid using the bagging off chute unless the silo is designed to withstand off-centre loads. The off-centre pressures applied to the support frame also places uneven forces on the concrete pad. Failure to construct a pad to the design specifications may result in foundation failure, placing a sudden unbalanced distribution of forces on the silo. Failure or collapse of the silo could follow.
Rounded Grains
Rounded grain, like field peas, present a particular management problem because of their lower angle of repose and the different forces they apply. The rounded seeds exert higher than normal pressures on the walls. When transferred to the lower sections of the silo wall, these forces may cause a crimping or pleating effect. This problem has been observed in elevated and flat bottom silos. When this occurs on only one side, the silo may lean or collapse completely.
Grain storage for pulses
Store only dry and clean grain which contains no more seconds, foreign seeds, or other material than allowed by the bulk storage operators (for example, Co-operative Bulk Handling, stock feed manufacturers, maltsters).
Do not guess the moisture content of grain from the paddock; check it with moisture meters. The moisture content of standing grain can increase dramatically in the late afternoon, particularly near the coast or under cloudy conditions.
Fumigate as soon as the silo is filled to stop any insects that are present, creating moisture.
Cooling Grain
Cooler grain temperatures have several advantages:
- seed germination percentage is maintained longer
- lower temperatures allow moist grain to be stored safely for longer periods
- moisture migration is reduced
- insect breeding and hot spots are prevented from developing
- mould growth is slowed
- less gas is lost through the pressure relief valve of sealed silos
- darkening of the seed coat is slowed
Painting the silo white after installation on the pad is also a useful management tool. A white, painted silo can be 4C cooler than a weathered galvanised unit.
An aeration attachment is a valuable addition for storing pulses that should be considered when buying a new silo. Aeration allows grain to be harvested earlier and at higher moisture levels. This improves the quality of the grain.
Aerated silos are fitted with fans that push air through the grain. When aerated correctly, this cools the grain and equalises the moisture and temperature throughout the silo.
With an aeration system, a vent is installed on the top of the silo to allow the air that is forced in at the base of the silo to escape. This can be a waterproof perforated lid in place of the sealed lid or a venting tube that can be capped for fumigation.
Fans are run for extended periods during the first few days of storage. An average of 12 hours using the coolest and driest air of the day is common. For longer-term storage, aeration fans are run for a few hours when needed. The amount of cooling achieved during storage depends on the moisture content of the grain and the humidity and temperature of the incoming air. Western Australian trials have shown that correctly controlled aeration can reduce grain temperature to 20C or lower through out summer.
Before deciding to fit an aeration unit, it is important to plan the final destination of the grain. If the grain is to be fed out or used for seed on the property, aeration will keep insect damage at a low level. If the grain is to be transported off the property for sale, it must not contain live insects. Aeration alone does not guarantee insect free grain. The grain may have to be fumigated.
Preventing moisture migration
Grain loaded into sealed silos must be of sufficiently low moisture content to prevent moisture migration problems. If the central store operator rejects a load because of excess moisture, do not load this grain into a sealed store where there is no escape for the moisture unless the silo is aerated and adequate ventilation is fitted. In a sealed silo, there is no free venting and therefore no escape for moisture in the headspace. Moisture can migrate and condense in the upper layers and there will be some venting of moisture to the atmosphere. This area of the grain is at high risk from insect colonisation.
Moisture in a silo comes from three sources:
- the stored grain itself and any weed seeds or impurities
- respiration of insects or mites in the grain
- And water entering through a leak
Grain is living and releases moisture as it respires. This is carried upwards in a silo by convection currents of air. These air currents are created by the temperature difference between the warm grain in the centre of the silo and cool walls or vice versa. In an aerated silo the moisture migration pattern is stopped because the entire stack is normally cooled to one temperature - usually 20C or below.
When grain is stored at less than 14 percent moisture and free of insects, the moisture increase in the upper layers of the grain will not be significant. If it is stored above 14 percent moisture content, enough moisture may be carried into the upper layers to place the grain at risk of going mouldy.
Insect activity releases moisture and heat into the spaces between the grains. Moisture builds up faster and to higher levels from insects than from grain respiration alone.
Moisture carried into the silo headspace may condense on a cold roof and fall back as free water. This can sometimes cause a ring of grain to germinate against the silo wall. When the grain, contains insects such as native budworm in large numbers, increased moisture can cause a damp mouldy layer across the top of the grain.
Water entering through structural damage will increase grain moisture content to the point where mould growth occurs. This moisture may also migrate to other areas. Insects will develop more rapidly in these high moisture zones.
Insect control using fumigation and insecticide application
Firstly maintain good hygiene to reduce the overall numbers of insects on the property. This is a fundamental requirement for protecting stored grain in all situations.
Secondly before harvest time, clean all silos then burn or bury residues. Treat the inside of the silo with a suitable insecticide (for example, Dryacide dust, malathion dust or spray, fenitrothion spray, Insectigas) to provide residual control until the grain is loaded into the store. Do not load the new crop on to old grain in the silo.
The grain temperature must be above 15 C before it can be effectively fumigated. Below 15C insects may not be controlled by the fumigation. For example, field peas may be harvested early and aerated to even out the moisture and temperature but this may also cool the grain below 15 degrees if an aeration controller is used.
When new grain is loaded it is essential to fumigate the silo to the recommended standard. To control Pea weevil a very high standard of sealed silo is needed. The fumigation needs to achieve 100 ppm of phosphine for 21 days. The three-minute half-life pressure test is the minimum standard required.
A new silo from the factory will be able to reach this standard but the silo should be tested annually to make sure it could retain its pressure. The silo hatches must be checked for rubber seal damage and the pressure relief valve filled with oil to the correct level.
Fumigation of a poorly maintained silo will not control insects effectively. Gas will be lost, and insects may survive with a degree of phosphine resistance.
Grain in a well-maintained sealed silo may be protected indefinitely by adding two phosphine-generating tablets per tonne of silo capacity. A 50 tonne silo holding only 30 tonnes of grain, still needs 100 tablets to effectively fumigate the grain. This rate will create sufficient gas to achieve complete control of Pea weevil. Place the tablets on the surface of the grain in trays. Observe the correct fumigation period related to the species of the insects present (pea weevil is the longest of the common pulse pests at 21 days).
Even though the fumigation has been carried out to instructions and the silo remains closed, check the grain at regular intervals for insect infestation.
Phosphine leaves no detectable residue when applied correctly and treated grain may be delivered at the bin without penalty but a declaration must be made. It is illegal to deliver grain with partly decomposed tablets of phosphine. Phosphine will not damage germination at the recommended rate.
Malathion is the only insecticide registered for application to grain by farmers in Western Australia. However Malathion resistance is widespread throughout the state and you should consider other insect control methods whenever possible. Where insects are still susceptible to Malathion it will protect grain for three to six months.
Fenitrothion is the only other chemical registered for treatment of grain handling machinery or empty storage's after cleaning. Never use fenitrothion or any other insecticide for protecting grain.
Bulk grain handlers reserve the use of Fenitrothion as a grain protectant; Fenitrothion leaves a residue in the grain, which is unacceptable in many foreign markets.
Further reading
Farmnote 24/2002: Aeration - for preserving grain quality
Farmnote 64/2003: Grain storage: Design and installation
Farmnote 66/2003: Grain storage: Handling and maintenance [Expired]
Farmnote 67/2003: Sealed silos make $ense
Farmnote 68/2003: It makes $ense to maintain your sealed silo [Expired]
Farmnote 69/2003: Effective fumigation needs a properly sealed silo [Expired]
Farmnote 70/2003: Underground storage of grain [Expired]
Page reviewed: 25 November 2005
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