Sheep management
Sheep can accumulate OC residues. However, results from Department of Primary Industries and Regional Development experiments indicate they reduce levels more rapidly than cattle when removed from the contaminated area. All owners with sheep grazing OC treated land are required to have a Quarantine Management Notice and an agreed property management plan to ensure any product sold does not have levels of residue above the MRL.
The level of OCs in an animal is related to the intake of residues, amount of fat present and the stage of production of the animal, that is, whether it is growing, lactating or losing weight. Residues can also appear in the wool. Fat levels greater than the MRL have been recorded in mature sheep but they can 'rundown' the level of OCs in their bodies quickly once placed on clean grazing land.
The maximum amount of dieldrin allowed in fat before an animal is considered unsuitable for human consumption is 0.2 ppm (or 0.2 mg per kg of fat). The MRL for DDT is 5 mg/kg. The MRL for milk for human consumption is 0.15 ppm for dieldrin and 1.25 ppm for DDT.
The level of any OC in wool has not been set, but currently any level above 3 ppm may result in withdrawal of the wool from sale and/or charges for scouring the wool. This may happen on soils contaminated with either DDT or dieldrin.
Soil testing showing levels of dieldrin over 0.1ppm indicates the paddock or area tested has a high potential for producing sheep with body fat OC levels above the allowable MRL unless managed effectively. Contaminated sheep cannot be sold for human consumption.
Sheep that have grazed on a clean area of the farm, or off-farm, for an adequate period, can be sold for slaughter for human consumption. It is strongly recommended that a sample of these animals be fat tested before sale if there is a risk that the sheep may have unacceptable OC residues.
There are a number of strategies available to producers to ensure they do not sell sheep or wool that contains OC residues above the allowable levels. This includes off-farm agistment if grazing on clean land, on farm, is not an option.
Pasture Management
Sheep can be very selective grazers; it is possible for them to rapidly accumulate OCs from pasture on contaminated land. The shorter the pasture the more soil will be eaten, particularly in winter when paddocks become muddy and OC bearing dirt is splashed onto the feed.
Good pasture management will allow more flexible management of sheep grazing contaminated land. Sheep may be grazed on risk areas, but it is the owner's responsibility to ensure they graze on OC free ground, for sufficient time, until they are below the MRL. In many cases this may only be a few weeks, but where levels are higher it may take up to several months.
At the break of season, defer grazing on OC ground for at least six weeks so as to obtain a complete pasture cover. Avoid grazing short, dry feed on OC areas. When there is less than 2,000 kg DM/ha, levels of dieldrin in body fat are likely to exceed the MRL. If the pasture is dense (soil not visible as you walk through) only graze down to 25mm. On cereal crops, or sparse (new) pasture, graze no lower than 50mm. If supplement feeding, avoid OC areas as the soil pick up rate is increased.
Alternatively use risk areas for hay production, but ensure that the hay is cut higher (50mm) and that the fingers of the rake operate clear of the ground to minimise soil contamination. Also the storage of hay should be done on clean paddocks or areas.
Managing cull wethers
The level of dieldrin in fat is seasonal and relates to pasture availability and live-weight. Dieldrin levels increase during summer and autumn when feed is limited and animals are likely to be losing weight. Sheep can be grazed on contaminated land in Western Australia when there is a flush in pasture growth, especially in winter and spring, which results in a large amount of relatively uncontaminated feed available.
Removing cull wethers from contaminated land at least three months prior to sale is usually adequate to decontaminate them. If possible wethers should be finished on good quality pasture which will hasten decontamination of OCs from the fat.
Managing cull ewes
Dry ewes should be treated in the same way as wethers. Lambing ewes will release OCs via milk and will decontaminate quickly on clean pasture. Ewes should be suitable for sale if moved off OC contaminated land at lambing and sold when lambs are weaned, provided they are not losing weight or in poor condition.
If stock are losing weight (even when lactating), they need to be decontaminated by grazing clean land and allowed to gain condition before they are sold. As a rule of thumb three months grazing on clean land with good nutrition levels should be adequate for decontamination.
Milking sheep
Ewes that have grazed contaminated land eliminate OCs in their milk, hence these chemicals can be present in milk intended for human consumption.
Managing weaner lambs
Lambs from ewes that have grazed contaminated land will get OCs via the ewe's milk. Their OC levels may be higher than their mothers so weaners should not be sold directly to slaughter. They can be sold on for wool production or decontaminated for at least three months prior to sale.
Prime lamb production
For production reasons as well as for decontamination purposes prime lambs should be finished on high quality feed. Ewes should be removed from contaminated land at lambing and given high quality feed. Their lambs will decontaminate more quickly if they are growing rapidly and gaining condition.
Producers need to be skilled at condition scoring if they are marketing lambs and managing lambing ewes. Condition scoring skills will identify sheep that are increasing their condition. These sheep decontaminate most rapidly. For more information and video on condition scoring of sheep.
Fat testing a sample of lambs will be important where a long period of decontamination is not possible prior to finishing lambs for sale. Remember that the OC levels of individual sheep may vary significantly and lots should only be sold if all sampled lambs test well below the MRL. Provided pastures are not over grazed and final fattening is on OC free pastures, lambs should meet the requirements.
Wool production
Most studies to date indicate that where sheep graze on contaminated land, even with very high levels of dieldrin contamination (i.e. over 1 ppm in soil), it is unusual for OC levels in the wool fat to exceed 3 ppm.
However, some soil types may lead to rapid increases in wool fat contamination and soil contamination of the fleece may also occur.
Autumn is the highest risk time for wool to accumulate higher levels of OC as availability of feed is low.
Legal requirements
The use of land with OC levels likely to result in agricultural produce with OC residues above the maximum residue limit (MRL) will be restricted by a quarantine notice. To use this land, the owner must seek written permission from the Department of Primary Industries and Regional Development. Approval will be given by an authorised officer if the landowner can demonstrate that produce from the area will not exceed MRL of OC residues. This will usually require an audited property management plan (PMP) that will indicate the products produced are below the MRL.