THE EFFECTS OF SEASON AND DIET COMPOSITION ON THE RADIOCESIUM INTAKE BY SHEEP GRAZING ON HEATHER MOORLAND

1. In north-east Scotland on a mixed heather and grassland pasture gra zed by sheep, 2 X 2 m areas of Calluna vulgaris heath and Deschampsia flexuosa grassland were artificially contaminated with Cs-134 by soil injection. Estimates of the Cs-134 intake by sheep were made on the ba sis of measurements of dry matter intake, diet composition and Cs-134 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 a s markers. Diet composition was determined by a combination of botanic al analysis of samples of ingested material and relationships between n-alkane patterns in faeces and ingested material. 3. From May to Sept ember the percentage of grasses in the diet of the sheep decreased fro m 74% to 10%, while the percentage of C. vulgaris increased from 1% to 77%. Over the same period dry matter intake decreased and the botanic al composition of vegetation and diet became more similar. However, th e similarity in botanical composition between vegetation and diet was overall relatively low. 4. The Cs-134 concentrations of dietary compon ents increased in the order: Erica cinerea shoots, herbaceous dead mat ter, dead shoots of C. vulgaris, grasses (D. flexuosa and Festuca ovin a), broad-leaved grasses (Agrostis spp. and others), Carex pilulifera, non-flowering and flowering shoots of C. vulgaris. 5. Intake of Cs-13 4 by sheep was markedly higher in July compared to May as a result of increased Cs-134 concentrations in all dietary components and increase d consumption of C. vulgaris. Intake of Cs-134 did not change signific antly between July and September despite a further increase in the per centage of C. vulgaris in the diet. This was predominantly due to a de cline of Cs-134 concentrations in the green shoots of C. vulgaris. Sea sonal changes in diet composition strongly influenced the relative con tribution of different vegetation components to the total Cs-134 intak e. C. vulgaris contributed 82% and 70% of the total Cs-134 intake in J uly and September respectively, while grass species contributed 82% in May. 6. It was calculated that between May and September the sheep ha d utilized 72% of herbage produced on the grassland and 29% of heather shoots produced on the heath. Assuming that the whole pasture had bee n contaminated, this was equivalent to a removal through grazing of 0. 9% on the grassland and 2% on the heath of the Cs-134 injected into th e soil. By taking the sheep off the pasture at the end of September ap proximately 0.08% of the injected Cs-134 would be removed from the sit e via the body tissues of the sheep.