Effects of season, grazing intensity and diet composition on the radio caesium intake by sheep on re-seeded hill pasture.
Abstract
Herbage intake and diet composition of sheep were measured on 2 swards containing mainly Lolium perenne, Festuca rubra and Trifolium repens, on an organic soil, maintained at sward surface heights of approximately 3 or 5 cm. Small areas within the swards were artificially contaminated by injecting 134Cs into the topsoil. The 134Cs concentrations of each diet component were obtained by sampling the vegetation within the contaminated areas. The 134Cs intake by the sheep was estimated under the assumption that the complete pasture had been contaminated. The concentrations of 134Cs in the diet components increased in the order: dead matter, grasses, T. repens, Cerastium fontanum. The 134Cs intake by sheep was higher in summer than in spring and lowest in autumn. Seasonal differences in the 134Cs intake resulted from changes in the 134Cs concentrations of plant species, changes in the botanical composition of the sward and changes in herbage intake. There was a strong indication that the 134Cs intake was lower on the shorter sward as a result of lower 134Cs concentrations in the vegetation and a lower herbage intake. The botanical composition of sward and diet was compared using similarity coefficients and principal coordinate analysis. The sheep's diet closely reflected the botanical composition of the top 2 cm of the short sward and the top 3 cm of the tall sward. Diets differed slightly between seasons due to changes in sward composition. Grasses made up 70-90% of the diet and accounted for most of the 134Cs intake in spring and autumn. In summer, C. fontanum had 134Cs concentrations which were up to 9 times higher than in other species and it contributed up to 35% of the 134Cs intake, despite low proportions in the diet. The cumulative amount of 134Cs ingested by the sheep between May and Sep. through intake of herbage was equivalent to 40-70% of the 134Cs taken up into the aboveground vegetation and 2.2-2.7% of the 134Cs injected into the soil. It was estimated that most of the 134Cs ingested would be returned to the pasture via excreta and only 0.10-0.16% of the total 134Cs would be removed with the sheep if they were taken off the pasture at the end of Sep.