Four repletion experiments were carried out to clarify the roles of molybdenum, sulphur and iron in the inhibition of copper absorption by ingested soils. In each experiment, groups of four to six hypocupraemic Scottish Blackface ewes were repleted for 21 days with diets containing CuSO4 and/or soil Cu and the responses in plasma Cu used to assess the availability of the Cu supplied.
In Expts 1 and 2, three soils varying in Mo but high in Fe content and derived from Lower Lias Clay, Namurian Shale and Weald loam were added at 100 g/kg D.M. to diets of low (1·0) and high (4·1 g/kg D.M.) S contents: they provided 3·1, 2·8 and 0·1 mg Mo/kg D.M. respectively. When compared with groups given no soil, each soil increased plasma Cu repletion in the low S experiment and decreased it in the high S experiment. Inhibition was thus not completely dependent on soil Mo but dependent upon dietary S.
Soils varying in Fe content were compared in the manner of the above experiments in Expts 3 and 4. Two Fe-rich soils, developed from the Upper Chalk and a Barton Clay and providing 2·8 and 1·5 g Fe/kg D.M., impaired Cu repletion on the high S diet but not on the low S diet. A soil, developed on a sand from the Bagshot Beds and providing only 0·15 g Fe/kg D.M., did not impair repletion but FeSO4, providing 0·8 g Fe/kg D.M. did so, irrespective of dietary S content. Ingestion of Fe-rich soils in a high S diet caused three-fold increases in rumen sulphide concentrations 5 and 24 h after feeding. It is suggested that soil ingestion impairs Cu absorption in sheep by trapping S as heavy metal sulphides such as FeS and releasing sulphide in the acid abomasum.