OCs and stockfeed
Producers should be aware of the risk of OCs contamination when feeding animals that are either being milked or fattened for slaughter from hay or fodder produced on OC contaminated land. Animals fed only with contaminated feed will become contaminated in a few weeks.
This important for producers as the OC residue program in WA aims to ensure that no meat or milk products offered for sale exceed 50% of the maximum residue limit (MRL). Producers should have a good understanding of how to manage stockfeeds grown on OC contaminated land.
For example, steers fed solely on fodder with 0.05 parts per million (ppm) of dieldrin can be expected to reach a level of 0.1ppm dieldrin in their fat in about 20 days (1ppm = 1 milligram/kilogram).
OC levels in plants growing in affected soils can vary considerably depending on the physical contamination rate which relates to the soil type (sandy soils – less physical pick up on root ball, clay loam soils – root clumping), the seasonal conditions (dry - dust, wet- dirt wash onto plants), pasture or plant type and relative stage of plant growth (livestock grazing ground level plants pick up more soil). If you want to produce stockfeeds from contaminated land then you will need to consider these factors as well as the animals you expect to feed the fodder or grain.
Maximum residue limits
- The MRL for OCs in meat is 0.2ppm.
- The MRL for milk from sheep, goats or cows for human consumption is 0.15ppm for dieldrin and 1.25ppm for DDT.
- The MRL of OCs allowed in hay and other animal feedstuffs is 0.05ppm.
Why is the MRL for stockfeed so low?
Cattle need to eat about 3-4% of their body weight per day (measured as feed dry matter - DM) for growth. A 400kg steer needs about 16kg of dry matter (or about 17kg of hay as a sole diet) per day to grow. If the feed level were 0.02ppm dieldrin, and assuming 100% uptake, the steer would eat about 0.32 milligrams of dieldrin per day from the plant/soil contamination (i.e. 0.02ppm in plant x 16kg DM eaten = 0.32mg/day) .
When grazing pasture, cattle also eat about 0.5kg of soil per day, which would increase the intake of dieldrin by 0.1mg/day. In this example the animal would be eating about 0.42mg of dieldrin per day.
The steer's body would contain about 20% fat, or 80kg. If 0.42mg of dieldrin was eaten per day, the body fat could be expected to accumulate about 0.005mg/kg per day. (0.42mg dieldrin/80kg fat = 0.005mg).
As the MRL for dieldrin is 0.2ppm, it would only take about 20 days on this feed for the steer to reach half the MRL (i.e. 0.005mg/day x 20 days = 0.1mg dieldrin).
Generally, low growing plants, such as clover, are more contaminated with OCs than grasses, such as phalaris, when grown on the same soil type. This is due to the contamination rate being higher with low growing plants as the tops of plants have less OCs than the lower sections and taller growing plants tend to shed their lower leaves, which reduces the contamination load.
Plants harvested in spring will only contain about one-quarter of their contamination level measured in autumn.
Maize and oaten hay are generally cut at about 10 centimetres from the ground and ensiled or baled before feeding out. If these crops are grown on contaminated soil, the recommended cutting height is around 15-20cm, to reduce soil contamination.
Most hays and silage are satisfactory as a supplementary feed but they should not be the sole diet for animals with low fat levels, such as bulls. Do not feed hay and silage to animals running on land affected by OC residues as the feed will be spread over the bare dirt by cattle feeding. All levels are cumulative and problems may arise in these circumstances.
Sampling of hay crops indicates that, provided the hay is free of soil contamination, it is generally safe to use but care needs to be taken when making hay or silage. If you cut or rake too close to the ground, soil contamination will occur. Well-grown, dense swards are less likely to have excessive soil contamination on leaves.
Avoid grazing land with residues while feeding these supplements and always ensure that they only form a small proportion of the diet. OC affected stockfeed should always be mixed with clean feed.
Vegetables have high water content so those grown in OC affected soil do not accumulate residues, as OCs are insoluble in water. Low growing plants should not be used for animal feed, as they will be contaminated by the soil, unless the vegetables are thoroughly washed and soil removed before feeding out. Never allow cattle or sheep to access an OC paddock that is in vegetable production, or to “clean up” the surplus vegetable crops that have been grown on OC land.
Potatoes and carrots have caused MRL violations. These have been the result of soil contamination, or higher residues in the skin of these vegetables. Grazing animals on root vegetable crops which are grown on contaminated land will result in a more rapid accumulation of OCs in the animals' fat if used for fodder.
Management of crops
Levels of soil dieldrin residues in the range of 0 to 2.5ppm result in increasing levels of pasture/crop plant contamination. However, as previously mentioned, the plant contamination residue level depends on the OC level in the soil, soil type, soil structure, plant species and plant height.
Soil contamination, type and structure
The level of plant dieldrin residues is much lower from organic, peaty soils than it is from loam soils. Heavily cultivated, poorly structured, highly erodible loam soils can result in plant material with up to 0.8ppm dieldrin from soil dieldrin levels of 1.4ppm. Growing fodder crops or pastures for conserved feed on this type of soil will result in high levels of OC contamination.
At equivalent levels of soil dieldrin residues, grasses take up the least (the taller the plant the lower the contamination rate), followed by capeweed (leaf size), with subterranean clover (low to the ground) having the higher residue risk.
Plant dieldrin contamination residues at 0-10cm height can be more than double the residues in the 10-20cm height range, in some pasture and crop species.
Avoiding soil contamination of any feed crop is very important. Crops intended for stockfeed should not be cut close to the ground as the closer to the ground, the higher the level of contamination.
OC levels in conserved fodder will not decline during storage. Storing conserved fodder on the ground or in sheds that have OC contamination will result in the feed becoming contaminated through pick-up from the soil.
Target livestock for feeding
Where fodder harvested from contaminated land is to be used, stock should have access to clean grazing or have their ration supplemented with clean feed (e.g. grain). Fodder grown on contaminated land should not be fed to cattle being decontaminated or those being fattened for slaughter unless it is mixed with clean feed.
Supplementary feed obtained from residue-affected paddocks should not be given to animals grazing affected areas.
The grazing of root vegetables, surface vegetable crops (pumpkins), maize, clover or weeds from or on contaminated paddocks is not recommended.
1. 1ppm = 1mg/kg.