At a recent Innovative Farmers event a farmer asked the question: which tree species should I choose to provide the best nutritional supplements for my dairy cows? Ideally, there would be a simple table of tree species with their nutritional benefits listed next to them, and this would answer her question. But nothing in farming is simple! And over-simplifying the topic of nutrition is unhelpful and misleading, as Lindsay Whistance of Organic Research Centre (ORC) has explained.
How to feed tree forage to livestock seems to be more important than what species to feed them. Using Lindsay’s advice and knowledge, this article looks at why farmers should choose a multi-species mix rather than opting for one species over another, and what management issues need to be considered if trees are to play a useful role in livestock diet.
Summary for farmers and land managers:
· A variety of species provides a variety of benefits; don’t reduce your tree choice to one species.
· Species choice should reflect the needs and conditions of the farm as a whole rather than just the nutritional benefits. For example, in coastal conditions, sycamore will tolerate both salt and wind. Nutrition and anti-nutrition levels vary in plants throughout the season and depending on age and whether fresh or dried, the same is true with minerals, vitamins, and tannins.
· Management systems are key: If trees are used for livestock browsing, ensure the quantity and access are sufficient for all livestock and take tree recovery time into consideration. Make sure that trees that represent a risk of poisoning are managed carefully if present in the landscape.
· If planning to preserve tree fodder, carefully consider your harvesting and storage methods.
· The species planted as part of the silvopasture field lab in Devon were chosen to represent the expected species that would form the dominant natural woody flora for the North Devon area on relatively heavy clays (pedunculate oak, downy birch, aspen, alder, hazel, holly, and willow). Some were chosen for specific browsing properties (willow (selenium source), wych elm (used to be cut for forage) and Scots pine (medicinal).
How are trees fed to livestock?
As with grasses, tree forage can be fed to livestock as fresh browse or as a preserved fodder, either dried as tree hay, or made into silage. Historically, providing tree browse or tree fodder was very normal on farms, though it largely died out with the introduction of winter root crops. Today these practices are still used but are not mainstream. If farmers are considering using trees for livestock feed then they need to fully consider how the trees will fit into their farm management system.
Things to consider if you want to make good use of tree browse as a livestock feed:
· Is there enough browsable material for all the animals?
· Do they all have sufficient access to trees? (For larger herds this may be in the form of a long hedgerow or strip of trees – see our article on silvopasture design for more information)
· Is the browse at an appropriate height for the whole herd?
· Are the trees old/resilient enough to withstand browsing?
· Are the trees protected enough to ensure that the animals are not going to kill them or cause damage which takes time and effort to repair? (Some farmers simply throw lopped branches into the field).
· Is sufficient recovery time factored into the rotation so trees have time to grow back?
If considering tree fodder as a livestock feed:
· How will the trees be harvested? What machinery will be used?
· Is there sufficient time for this to be carried out at the right time of year?
· Is there storage capacity on a large enough scale? Tree hay, for example, can work well for committed, smaller growers, but is harder to do with larger herds (though still possible with sufficient forethought).
· If being made into silage, has the storage process been carefully considered? Some farmers make tree silage by making grass silage with a layer of branches on top – but leaf moisture levels when ensiled must be carefully considered to prevent them from going mouldy, and of course there can be no air gaps to ensure a suitable anaerobic process.
What are the nutritional values of different tree species?
There is no wonder tree when it comes to livestock nutrition (though Mulberry may come closer than most in terms of protein content). Traditionally, livestock ate the trees which were available to them on farm, particularly elm and ash, with holly during the winter. Lime, hazel and sweet chestnut have also been historically used. But the trees were usually chosen for practical on-farm reasons rather than nutritional value, such as for use as fencing posts or for hedge laying.
For this reason the nutritional table below represents the most widely used farm trees in Scandinavia early last century, rather than being a comprehensive list of species and their nutrition levels. An online resource of wider and more up-to-date research is available here.
The nutritional content of tree leaves in terms of carbohydrates and protein is, in general, very similar to that of commonly found grasses growing in the same environment.
Although the table above is a useful indication of nutrition, it is important to recognise that nutrition and anti-nutrition levels vary in plants according to season, age of plant, and whether the leaves are fresh or dried (among other things). The same is true of the levels of minerals, vitamins and tannins. This reinforces the importance of multi-species planting so that livestock access a range of nutritional benefits. Fully thought-out planting schemes to ensure equal access to those trees is also really important (see the trial designs used for the Devon IF field lab for pros and cons of different planting schemes).
What trees have the right amount of trace elements?
Certain species of tree have high levels of minerals and trace elements, as shown in the table below. It is worth noting that these trace elements are generally higher in dried fodder compared with fresh, and that levels vary with age and season.
What trees have tannins and other compounds?
Tannins are naturally occurring compounds which are found in most plants and play a role in plant defence. Traditionally all tannins were thought of as being bad for animal health, but the reality is more nuanced. Condensed tannins (CT) are found in tree leaves and bark and at a low level can be very good for animal nutrition, since they promote rumen health.
They can also be helpful in managing internal parasites, allowing an animal to use energy more efficiently. The tannins coat the protein molecule preventing it from being digested in the rumen, resulting in higher quality protein being available to the cows as rumen bypass protein. However, too much CT, more than 4-5% of dry matter (according to Emile et al 2016) can make the animals unwell, so it’s important to get the balance right.
Salicin, a main ingredient of aspirin, has antimicrobial, anti-inflammatory and antifungal properties and can be a useful dietary tool to improve animal health and aid self-medication. Since it plays a number of useful roles within the plant, salicin can be found in all plants to varying degrees but it is strongly associated with willows as their leaves and bark are high in salicin and they were originally used to make aspirin.
Are any trees poisonous to livestock?
Some trees contain chemicals which can be very useful to livestock health in limited quantities but are anti-nutritional or poisonous if too much is consumed – such as tannins. Some tree species, such as yew, certainly increase the risk of poisoning occurring though some farmers report that even yew was lightly browsed by stock without the animals showing any signs of ill health. Representation of trees will have an effect so a field otherwise devoid of trees and with a yew hedge, the yew can present a greater risk than if the yew hedge was next to a silvopastoral system. Hunger will also play a part since hungry animals are more likely to try and eat plants in greater quantities than they otherwise would have. Trees like the yew (whilst they may offer medicinal benefits in smaller quantities) should not be considered as potential food sources.
Stone fruits such as cherry or plum have high concentrations of cyanide. Interestingly, livestock can generally eat the fresh and fully dried leaves without a problem, but eating too many wilted leaves, e.g. from a fallen branch, can make them ill or even kill them. The wilted leaves have higher cyanide levels. However, hefted animals that have grown up with these trees, and have learnt feeding behaviour from their mothers, can often eat these wilted leaves with no ill effects, because they have learned not to eat too many at once and can pace themselves. If livestock are able to access stoned fruit trees for browsing, these risks should be carefully considered in the management system. The same is true for mast years when oak trees produce acorns which, when eaten in too high quantities, can be fatal for cattle and horses.
What trees are more palatable to livestock?
Some trees are generally more attractive to browsing livestock than others. The table below from Scottish Forestry ranks tree species roughly in terms of palatability (using trees which were traditionally grown on Scottish farms). Lack of palatability does not mean that the tree has no place in silvopasture design, however: alder is most unpalatable, but is nevertheless still browsed at low levels and can play other roles in the farming systems such as offering shelter and fixing nitrogen.
Palatability
| Palatability | Tree species |
| 1 | Aspen, Willow |
| 2 | Ash, Rowan |
| 3 | Hazel, Oak |
| 4 | Scots Pine, Juniper, Holly |
| 5 | Birch, Hawthorn |
| 6 | Beech |
| 7 | Alder |
Source: Scottish Forestry
Notes on the above figures:
1 = most preferred species
In lowland woodlands aspen may be in palatability class 3.
Scots pine, juniper and holly are more preferred in winter than summer because they are evergreen, however, young holly shoots, before the leaves have hardened, are also often taken.
There is some debate about where Hawthorn should be ranked and may be higher than 5.
Retaining resilience of trees when being used as feed
Of course, livestock like to browse and can cause a huge amount of damage to young trees, and over-browsing can kill trees completely. A good rule of thumb is to make sure that no more than 50% of the tree’s leaves are browsed, leaving enough for the plant to recover. Resilience depends on all sorts of variables including the rest period between browsing, and protection methods used. Very young trees will not be suitable to be grazed for several years, particularly by cattle, as they are too vulnerable.
However, some trees naturally recover more quickly than others from browsing. Some species grow faster, getting beyond browsing height more quickly. According to Scottish Forestry, conifers seem to be less resilient to browsing than broadleaved trees because they store more nutrients in their leaves, and struggle to grow quickly enough to outgrow browsing height. Birch and alder are fast growing, and in the right conditions, ash, oak and hazel can be too. Certain shade-tolerant species such as holly, oak and rowan, are likely to survive browsing better than others as they build up nutrient stores in their roots. This allows them to withstand losing leaves (Scottish Forestry).
| Resilience | Tree species |
| 1 | Eared willow, Birch, Alder |
| 2 | Holly, Juniper |
| 3 | Hazel, Oak, Rowan, Ash |
| 4 | Scots Pine |
Source: Scottish Forestry
Note on the table: 1 = most resilient.
The benefits of a multi-species approach
A multi-species approach to silvopasture is generally seen as best, because each tree offers something different:
· Ash is a traditional farm forage tree, but due to Ash dieback it is not possible to buy ash trees (though they can be transplanted within a farm).
· Elm is also a traditional farm forage tree with high levels of magnesium. It is currently rarely planted as it is subject to Dutch elm disease, though it has been found to grow well in hedgerows if maintained at a certain height.
· Willow and poplar are fast growing and therefore useful trees for regular browsing. Willow is especially popular due to its high salicin and zinc levels.
· Conifers such as Scots pine (yew is not recommended) can provide anti- insect benefits but are less resilient to browsing.
· Holly is a useful winter supplement because it is evergreen.
· Stoned fruits should be managed with care given cyanide levels in leaves.
· Alder is much less palatable than most other trees (beech is also low in palatability) but is quite resilient to browsing.
· Mulberry has a high protein content and can tolerate most soils.
The nutritional benefits provided by trees depends as much on management practice as the inherent nutritional value of the trees themselves.
Explore our website for more information on silvopasture design, handy tips and advice on silvopasture establishment, and information on our Field Lab.