Sheep grazing on areas with soil – with or without heavy metals – added to the vegetation – an experimental study

Wild and domestic ruminants in Norway might encounter active and closed-down shooting ranges on rangeland pastures. The encounter can represent a risk to grazing animals through exposure to soil where heavy metals such as lead (Pb) and copper (Cu) have accumulated. Animals may choose to ingest soil, but they can also have an involuntary intake through soil stuck to vegetation. Livestock grazing behaviour on shooting ranges is not well studied and understood. This study has investigated how sheep chose to forage when they had access to pasture with or without added soil, and pasture added soil with high or low heavy metal concentrations. The soils used in the study were collected from two different shooting ranges. The trials were conducted on a fine spatial scale, where different foraging decisions carried low and equal costs for the animals: they always had all alternatives available within four meters. The trial where the sheep had access to pasture with and without soil showed that the sheep preferred to graze areas without soil added to the vegetation, especially in the beginning when the vegetation was plentiful. As the clean areas were grazed, the sheep shifted towards grazing the soil-contaminated areas more. The sheep avoided grazing vegetation with soil added when given an alternative with less soil. The trial where the sheep had access to pasture added four different soils showed that the sheep grazed most on the areas added one of the soils with high concentrations of Pb and Cu. This soil had a coarser particle matrix than the three other soils, and higher concentrations of several minerals (Na, Ca). Most likely, the sheep preferred grazing on the areas with the coarse soil because this soil stuck less to the plants than the other soils. The higher content of Pb and Cu in this soil may also have had an effect, though this is considered unlikely. The higher content of Na and Ca may have contributed to the preference for this soil. A highly controlled small-scale experiment with standardized soils might estimate sheep preference or avoidance of high concentrations of Pb and Cu per se. Further studies should include rangeland area setups to study animal foraging decisions on a larger spatial scale.