Levels also vary throughout the plant and it has been found that leaves have higher levels than seeds. The fact that contamination was found in seeds from pods shows that contamination also occurs through absorption of OCs by the plant as well as surface dirt and mud splash. The type of plant also has a bearing on the level of contamination. Taller plants such as grasses have less contamination than lower prostrate plants such as sub-clover. As annual pastures grow in height away from the contamination source they become less contaminated.
Perennial plants tend to be less contaminated than annual plants.
This is because, regardless of soil type, perennial pastures have a layer of shoorts, runners and leaves which insulate the grazing plant tops from the contaminated soil. This lessens the likelihood of plant contamination by dust or mud splash. The relationship between soil structure, pasture variety, season and animal fat contamination With an understanding of how the level of pasture contamination is determined by the level of soil contamination, soil type and structure, plant species and plant maturity, Agriculture Western Australia commenced a study in 1988 which looked at the accumulation and run down of dieldrin in wethers. It was found that on loamy soils, sheep body fat levels of dieldrin increased with increasing levels of soil contamination. A comparison of organic peat soils, however, found the level of body fat contamination was lower even where the soil contamination was higher than that of the loam soils.