Lead from car exhausts persists on roadside verges, and ruminants grazing close to a busy road are likely to consume contaminated herbage. However, cattle can detect lead on herbage when it is at concentrations of 170 mg Pb/kg DM or above and have been observed to avoid grazing pasture with 304 and 462 mg Pb/kg DM (Strojan and Phillips, 1997). Herbage 7.5 m from a busy dual carriageway has been recorded with 454 mg Pb/kg DM (Motto et al., 1970).
The avoidance of lead-contaminated herbage may be the result of low digestibility of the leadcontaminated herbage, so the digestion rate of samples of herbage from the experiments of Strojan and Phillips (1997), referred to in the Introduction, was determined by the gas production technique (Menke and Steingass, 1988). Using 15 and 10 replicates/treatment from two experiments where lead acetate was sprayed onto cut (Experiment 1) and grazed (Experiment 2) herbage, 200 mg DM of each herbage sample was incubated in sheep rumen liquor in syringe pipettes.