Trace element composition of tree fodder and potential nutritional use for livestock
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.