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.