Organochlorines are a class of chemicals historically used to control termites and pests on agricultural crops and fruit tress. OCs such as dieldrin, dichlorodiphenyltrichloroethane (DDT), chlordane and heptachlor were common in Western Australia until they were banned in the mid-1980s.
Precipitating Australia's ban on OCs was the 1987 detection of DDT, dieldrin and heptachlor in export beef consignments sent to the United States. The National Organochlorine Residue Management (NORM) program was established in response to the United States OC detections. The NORM program commits all sectors of the meat industry to ensuring animals with OC residues in excess of the maximum residue limit (MRL) do not enter the food chain.
The program outlines the agreed principles for managing cattle properties that have historically used OCs in either broadacre or point source applications. A significant element of the NORM program involves fat testing at risk animals for OC residues prior to slaughter.
People who graze cattle on OC contaminated land must adopt the Livestock Production Assurance (LPA) Quality Assurance program and abide by a residue quarantine notice and property management plan issued by the Department of Primary Industries and Regional Development, Western Australia (DPIRD).
Principles in understanding OC behaviour
- With careful management, cattle can be successfully grazed on OC contaminated land.
- Growth and the ability to fatten is the best method known to minimise OC residue levels.
- The production of store weaners and yearlings is the most profitable enterprise for those producers wishing to graze cattle on OC contaminated land.
Ingestion of soil
Cattle accumulate OC residues in fat by ingesting contaminated soil. Cattle need only graze contaminated land for as little as two hours a day to accumulate OC residues in excess of the MRL.
OCs can persist in the environment for decades because they are insoluble (i.e. they do not dilute in water) and bind to soil particles. Poorly structured soils such as loam soils are at higher risk of being ingested. These soils tend to cling to plant root structures and are more likely to disperse through dust and mud splash on pasture.
Pasture type and coverage
Pasture type, length and root structure will impact on the amount of soil ingested by cattle during grazing. In general, the risk of soil ingestion is greater when animals graze annual pastures compared to perennial pastures or plants with root ball structures such as subterranean clovers.
Seasonal influences also impact soil ingestion – poor pasture growth in late summer/autumn leads to increased incidental soil ingestion resulting in greater residue risk. Cattle should only graze paddocks that have adequate ground cover and pasture height is over 5cm.
OC accumulation in fat
OC residues accumulate in the body fat of animals. The OC residue level in an individual animal will depend on how fat that animal is and the concentration of OCs contained in the fat. Other factors such as breed, age, and sex may also impact on the OC residue level. Studies have shown that animals can ‘dilute’ OC residues by as much as 80 percent by gaining ‘clean’ fat (i.e. fat free of OCs).
OC residue decline in animals
The capacity of an animal to fatten is an important principle in managing cattle on OC-contaminated land. Young cattle are easier to manage as they have a greater capacity to grow and fatten, thus diluting OC residues. Older animals and bulls are considered high risk as they have a reduced capacity to grow and fatten and should not have access to contaminated areas.
When animals are placed onto residue free land, changes to the amount of body fat occur through:
- Weight gain – the addition of fat is the best method of reducing OC residues provided animals are given sufficient time on clean feed/land.
- Lactation – OCs are excreted in milk fat. A lactating cow can reduce OC levels to less than one tenth of her initial level provided she is moved to ‘clean’ land early in lactation and is kept on an excellent plane of nutrition. Lactation coupled with weight gain during spring pasture growth is a practical method of reducing residue levels in breeding cows.
- Genetics – late maturing cattle have a greater growth potential than early maturing cattle, allowing residues to ‘run-down’ over a longer period. Dairy cows have greater potential for OC rundown than beef cows given their lactation productivity.
- Condition score – lean animals that gain weight have better rundown potential. Residue rundown will not occur in fat animals that lose condition and are then re-fattened on clean feed.