Sheep and organochlorine residues

Page last updated: Tuesday, 6 August 2019 - 8:31am

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The persistent nature of organochlorine (OC) residues in soils is an ongoing issue for livestock producers. Sheep are susceptible to the accumulation of OC residues. The presence of such residues in excess of the maximum residue limit (MRL) jeopardises overseas market access. Active management at a farm level is required to prevent sheep and sheep products with residues above the MRL from entering the food and fibre chain.

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.