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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]

• Zinc and cobalt concentrations of willow leaves were multi-fold higher than sheep requirements. • Leaf selenium concentrations were more dependent on site than tree species. • Metabolisable energy was appropriate to support lamb growth in all species but alder was the highest. • Leaf crude protein was higher in spring but should support lamb growth in autumn to. Silvopastoral agroforestry, the integration of trees into livestock production systems, is an ancient practice with benefits to animal welfare and nutrition. Intensification of farming practices have reduced the presence of trees and hedgerows in the agricultural landscape. Environmental benefits coupled with improvements to ecological resilience and the long-term sustainability of farm productivity have led to a resurgence in interest in silvopastoral farming systems. The objective of this study was to investigate the nutritional composition and potential use of tree leaves as a supplementary fodder for ruminant livestock, with particular reference to sheep. Leaves (including petioles) were collected during spring (June) and autumn (September) from goat willow (Salix caprea), oak (Quercus spp) and alder (Alnus spp) from three sites in the UK. On the third site samples of ash (Fraxinus excelsior), beech (Fagus sylvatica), sweet chestnut (Castenea sativa) and sycamore (Acer pseudoplatanus) were also collected. Tree leaves were analysed to determine mineral content, dry matter (DM), crude protein (CP), modified acid detergent fibre (MADF) and metabolisable energy (ME) which were then compared to the nutritional requirements of grazing sheep (Ovis aries). Leaves from all tree species used in this study were able to exceed the dietary ME and CP concentration requirements (NRC) for growing lambs (40 kg lamb @ 150 g/d). Alder contained the most ME and CP of the studied species. There was no significant effect of season although CP was higher in spring than autumn for all tree species. Zinc and cobalt concentrations were found to be dependent on tree species with negligible site and season effects. All (NRC) sheep requirements of both elements were exceeded by willow, met by alder and not met by oak, willow exceeded these requirements for zinc and cobalt by approximately 3-6 and 10-15 fold respectively. Leaf selenium concentrations were site specific with site 1 almost able to meet maximal requirement, whilst all other sites (all tree species) were around the minimal requirement. To conclude, ME and CP concentrations of the tree leaves were generally within a requirement range to support adequate growth of lambs if leaves fed alone (not likely in practice). Selenium concentrations were site dependant, iodine was mainly season dependent with tree species effects for zinc and cobalt. The zinc and especially cobalt concentrations of willow leaves were sufficient to suggest that willow could be used as a bio-supplement when fed within a conventional grazing system, especially useful for growing lambs. [ABSTRACT FROM AUTHOR]