The Effects of Season and Diet Composition on the Radiocaesium Intake by Sheep Grazing on Heather Moorland

1. In north-east Scotland on a mixed heather and grassland pasture grazed by sheep, 2 x 2 m areas of Calluna vulgaris heath and Deschampsia flexuosa grassland were artificially contaminated with134Cs by soil injection. Estimates of the134Cs intake by sheep were made on the basis of measurements of dry matter intake, diet composition and134Cs concentrations in diet components, assuming that the whole pasture had been contaminated. 2. Dry matter intake by sheep was measured using a natural n-alkane of plant waxes and an orally administered n-alkane as markers. Diet composition was determined by a combination of botanical analysis of samples of ingested material and relationships between n-alkane patterns in faeces and ingested material. 3. From May to September the percentage of grasses in the diet of the sheep decreased from 74% to 10%, while the percentage of C. vulgaris increased from 1% to 77%. Over the same period dry matter intake decreased and the botanical composition of vegetation and diet became more similar. However, the similarity in botanical composition between vegetation and diet was overall relatively low. 4. The134Cs concentrations of dietary components increased in the order: Erica cinerea shoots, herbaceous dead matter, dead shoots of C. vulgaris, grasses (D. flexuosa and Festuca ovina), broad-leaved grasses (Agrostis spp. and others), Carex pilulifera, non-flowering and flowering shoots of C. vulgaris. 5. Intake of134Cs by sheep was markedly higher in July compared to May as a result of increased134Cs concentrations in all dietary components and increased consumption of C. vulgaris. Intake of134Cs did not change significantly between July and September despite a further increase in the percentage of C. vulgaris in the diet. This was predominantly due to a decline of134Cs concentrations in the green shoots of C. vulgaris. Seasonal changes in diet composition strongly influenced the relative contribution of different vegetation components to the total134Cs intake. C. vulgaris contributed 82% and 70% of the total134Cs intake in July and September respectively, while grass species contributed 82% in May. 6. It was calculated that between May and September the sheep had utilized 72% of herbage produced on the grassland and 29% of heather shoots produced on the heath. Assuming that the whole pasture had been contaminated, this was equivalent to a removal through grazing of 0.9% on the grassland and 2% on the heath of the134Cs injected into the