The use of organochlorine pesticides such as DDT, aldrin, dieldrin, heptachlor, lindane, etc., has been banned for approximately 30 years. Therefore, they should not present a risk of actionable residues in milk or meat products. Unfortunately, the latter statement is not quite true. Each year there are a few instances of unsuspected contamination, and these usually occur following periods of wet weather. By understanding the conditions that contribute to the contamination of forages, the likelihood of an expensive incident can be reduced.
Organochlorine pesticides were the first formulations to receive widespread use beginning with the production of DDT in the 1930's. Large quantities of these materials were used before they were discovered to be persistent in the environment and accumulate up the food chain. As the undesired side effects of these chemicals became known, tolerances were established to limit exposures to human foods, and ultimately in the late 1960's and early 1970's, organochlorine pesticides were banned or severely restricted in their use. So how can they still be a problem today?
The foremost characteristic of this class of chemicals is that they are extremely stable compounds and readily do not degrade. Likewise, they remain persistent in the soils where they were applied. Organochlorine pesticides are relatively insoluble in water. As a result, they do not leach downward through the soils but remain near the soil surface or within the plow zone. They may migrate with surface water, but it is usually a case where the pesticides move attached to soil particles or organic materials. These chemicals are also lipophilic, which means that they will associate with fats and oils. Thus, when consumed by animals they will accumulate in adipose tissues and will be excreted in milk fat if the animal is lactating. With the stringent tolerances that are in place for residues in milk and meat, only small amounts of these pesticides need be consumed to result in exclusion from the market.
Where dairy production is concerned, as long as these pesticides remain bound in the soil, there should not be a problem. The problem occurs when there is a route of transfer to the cattle. Fortunately, these pesticides do not translocate through the roots of plants into the forage or seed portions of plants.However, two routes of transfer have been identified. The first is fairly obvious. If animals consume sufficient quantities of contaminated soil, they will accumulate organochlorine pesticides. Whether this will become a problem depends on the amount of soil consumed and the concentration of the pesticide in that soil. Studies have been conducted that have shown that cows on pasture will consume up to 4.5 pounds of soil per day. Lesser amounts were consumed when pastures were lush, because grazing was higher on the plants, with less ground contact and with less soil material splashed on to the forage. Certainly, with a sufficient pesticide concentration in soil, consumption can result in undesirable residue concentrations in milk and meat, yet it is rare.