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• Willow leaf cobalt concentrations met the requirements for lambs; grass did not. • Lambs readily consumed willow leaves. • Willow supplemented lambs had significantly higher plasma vitamin B12 concentration. • Willow could be used by producers to improve vitamin B12 status. Cobalt is an essential trace element in sheep for the synthesis of vitamin B12 in the rumen to support growth and prevent clinical deficiencies (Pine). Willow (Salix spp.) leaves contain a high concentration of cobalt (∼6 times the requirement of lambs). The aim of this study was to determine the efficacy of supplementary willow leaves in improving plasma vitamin B12 status of sheep. Weaned lambs (n=24) on ad libitum grass were randomly allocated to one of three groups: Willow (offered up to 300 g of fresh willow leaves per lamb each day); Drench (2.8mg oral cobalt sulphate heptahydrate drench on day 0); Control (received no treatment). Plasma B12 concentration was evaluated for all lambs using blood samples taken on days 0 and 14. Values for plasma B12 concentration were log transformed prior to statistical analysis. At day 14 the mean (± s.d.) concentrations, on the log 10 scale, were 3.00 (±0.155) pmol/L, 2.61 (±0.146) pmol/L and 2.55 (±0.214) pmol/L for Willow, Drench and Control, respectively. The corresponding back transformed means were 990 pmol/L, 407 pmol/L, 351 pmol/L, respectively. The value for the Willow treatment was significantly higher than each of the other treatments, which did not differ significantly. Supplementary willow can be used by producers to improve vitamin B12 status in lambs. [ABSTRACT FROM AUTHOR]