Trees For Horse Pastures
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Adding trees in and around pastures can be beneficial for a number of reasons. Tree roots can stabilize the soil and reduce erosion where pastureland is excessively sloped. Leaf litter and small fallen twigs add organic matter to improve soil fertility but can also cause problems for livestock if the organic litter comes from toxic trees. The activity of tree roots helps to recycle soil nutrients by absorbing nutrients from the deeper soil back into the trees. Absorption by tree roots of nutrients that move below the forage root zone can help reduce the amount of nutrients that enter into ground water (e.g., nitrates).
Trees provide shelter to livestock from cold winter winds, hot drying summer winds and prevent excessive exposure to the sun. The sheltering effect of trees reduces convectional heat loss from livestock in winter and reduces radiant heat off the ground during summer. Therefore, shelter from pasture trees decreases stress in livestock by providing more comfortable living conditions. Horses and cattle are often seen resting adjacent to a tree line to shelter them from the sun or wind. Shade from too many trees in a pasture can stunt the growth of the underlying forage crops. However, a reasonable percentage of mature trees will be beneficial to livestock production and the overall environment.
Planting trees just outside the fence around the pasture perimeter is usually adequate. Mature hardwoods, such as sugar maples, will grow wide-spreading branches, which will extend over the fences and into the field to provide shade. For large fields, placing a few fenced-off patches of trees within pastures may be worthwhile. For young trees, fencing is necessary to exclude the animals from trampling the trees, crushing shallow tree roots and browsing on the trunks and branches. Fencing will prevent rubbing injury to trees from livestock that like to scratch. Even mature, full-grown trees should remain fenced off from livestock to ensure survival of the trees.
It is important to select the appropriate tree species for each pasture. There are many native and several introduced species (that are not invasive) to choose from, but species differ in their site requirements and ideal soil type. Wet imperfectly drained soils are most suited to trees that can tolerate soil that remains saturated or flooded for extended periods of time during the growing season. Deep, naturally well-drained soils, that rarely become saturated with water, are ideal for tree species that can tolerate dry soils and occasional droughty seasons. In most cases dry-site tree species cannot tolerate flooding during the growing season and quickly die out.
Some tree species are toxic to livestock and should be avoided around and within livestock pastures. Agriculture Canada maintains an excellent web site at http://sis.agr.gc.ca/pls/pp/poison. This site and its links provide a list of plants and trees that are known to contain toxic compounds. Photographs of the plants and trees can be linked to from this site.
There are many tree species that are known to be safe for use in and around livestock pastures. More appropriately, there have been no reported incidences of livestock poisonings with many tree species.
Wet soil is prone to flooding during the growing season or remains very wet most of the time. This limits the selection of native trees to eastern white cedar, black spruce, tamarack (eastern larch), willows, green ash, black ash and silver maple. Speckled alder is a larger native shrub that will grow well on wet soggy soil and can provide some shade for livestock.
Moist, poorly-drained soil remains fairly moist all year because the water table is close to the surface. It is not prone to flooding during the summer and supports the growth of native species such as silver maple, Manitoba maple, black maple, green ash, black ash, willows, poplar, honey-locust (not black locust), white cedar, black spruce, and tamarack (eastern larch).
Moist well-drained soil is typically an upland soil that allows excess water to drain quickly after rain to a depth that provides a good soil root zone for trees and rarely becomes flooded during the growing season. These site conditions support the growth of the native species sugar maple, silver maple, white and green ash, elm, trembling and large-tooth aspen, sycamore, beech, honey-locust (not black locust), , balsam fir, white and red spruce, , white cedar, eastern hemlock, and hawthorn. Introduced species include northern catalpa, Osage-Orange and Norway spruce
Dry soil is well-drained, such as deep sand or gravelly soils that never flood. They are especially suited to elm, sweet chestnut* (not introduced horse chestnut) and staghorn sumac (upright, red, cone-shaped, seed head). Introduced species include northern catalpa, Osage-Orange, Norway Spruce and Colorado spruce.
* Native sweet chestnuts were decimated by chestnut blight disease across North America during the 1800's however, there are species recovery efforts being made. Chestnut blight disease remains very active in Ontario and sources of tolerant sweet chestnut trees are still unavailable. Seedlings that are available remain susceptible to infection by blight.
Some species of trees contain natural chemicals that can be poisonous to livestock. It is important that farmers who manage pastures be aware of which trees to avoid when planting trees and how to identify risky trees that should be removed. Some trees can cause livestock poisoning when parts of the trees are ingested. Other trees can cause problems where animals come into physical contact with the trees or products that are made from the trees (e.g., black walnut shavings).
Horse owners should be particularly wary of the source of shavings for bedding. Wood shavings that contain black-walnut wood (Juglans nigra) or butternut wood (Juglans cinerea) will cause laminitis (founder) within 24 hours of being placed on bedding containing as little as 20% black walnut. Horse owners are advised to check with their supplier of shavings to ensure that the bedding material does not contain wood from walnut or butternut. Black walnut contamination is recognized by the presence of black shaving strips. Fresh shavings from conifers, such as pine and spruce, are preferred.
The exact cause of laminitis in horses from walnut wood shavings is not known. For most of the year, horses and ponies that stand under living walnut trees in pastures usually do not develop symptoms of laminitis. However in the springtime, pollen from the flowers of black walnut may induce laminitis. Roots of living walnut trees give off a natural chemical called juglone that acts as a natural herbicide, killing off many nearby species of plants. It is not known if juglone is the cause of laminitis in horses.
A potent killer of horses and ponies is the native red maple (Acer rubrum), also known as a soft maple. Ingestion of wilted red maple leaves causes hemolytic anemia, which can be deadly. The toxic ingredient in red maple leaves is not known but only seems to be present in wilted leaves from June to October. Older wilted leaves cause faster poisoning than wilted leaves of early summer growth. This indicates that the amount of toxin increases in leaves during the summer.
Native red maples should not be planted around pastures where horses or ponies may be kept. If red maple trees are already present, pasture owners might consider cutting them down and ensuring the stumps are completely dead to prevent sprouting of new shoots and leaves. Leaves and branches should not be placed on the manure pile where horses can reach them.
For identification, the leaf edge or leaf margin of red maple is serrated or jagged and can be used to differentiate the species from sugar maple. Leaves of sugar maple have smooth leaf edges and are not serrated. Red maple can hybridize with silver maple creating crosses of intermediate forms that should also be avoided near horse pastures. If in doubt, pull it out and plant a tree that you know is safe for livestock. Red maple is also known as a soft maple. Silver Maple is another soft maple. Its leaves are heavily indented compared to red maple.
The landscape industry sells a number of Norway maple tree types that have red and purple coloured leaves but are not the native 'red maple' tree. It is not known if these specific 'red' coloured maples will poison horses. For now, it may be best to avoid planting red coloured maples near horse paddocks until these trees are found to be safe.
Wild forest black cherry (Prunus serotina) contains prussic acid, a form of cyanide. The acid content is highest in autumn leaves and twigs. Poisoning has occurred when livestock have browsed on leaves and twigs around perimeters of a pasture. Fallen autumn leaves could also be inadvertently consumed along with forage plants.
Forest black cherry trees frequently establish and grow along treed or unmanaged fence lines where perching birds that have eaten cherry fruit deposit seed in their droppings. Where black cherry is identified, farmers who choose to cut them down should ensure the stumps are dead to prevent re-sprouting.
Livestock have also been poisoned by red chokecherry (Prunus virginiana). Leaves, branches and seeds inside the fleshy red fruit also contain quantities of cyanide that can easily kill livestock. Chokecherry often grows wild in fencerows and can be a nuisance to control.
Seeds, leaves, bark and twigs of black locust (Robinia pseudoacacia), an introduced species, contain several toxic proteins that can poison all livestock types. Children have also been poisoned by ingesting parts of black locust trees that are often found growing near older rural buildings. The native honey locust (Gleditsia triacanthos) appears to be a safe tree. It is important to distinguish between the two locust tree species. The textbook entitled 'Trees in Canada' by John Laird Farrer is available in most bookstores and will help with tree identification.
Ornamental yews (Taxus baccata and Taxus cuspidata) are often used in landscaping because they are evergreens, they are winter hardy and can be shaped to any configuration. The leaves, seeds and twigs of the yew are deadly toxic to all livestock. The ingested leaves, bark or seeds contain a toxin known as taxine. Taxine can cause collapse and sudden death. A small quantity of the plant is all that is needed to kill a horse (0.05% of body weight or 0.5 lbs. for a 1000 lb. horse). Poisonings occur commonly from the ingestion of fresh leaves as well as from accidental ingestion when horses consume shrub trimmings.
Trees that are also known to have caused livestock poisoning include white, red and black oak trees. Horses can develop a taste for oak leaves and cattle a taste for acorns. Leaves and acorns of oak contain tannins that are toxic in large enough doses. Black and red oak are more toxic than white oak.
Horse chestnuts (young leaves, flowers and nuts), Kentucky coffee tree (seeds, fruit pulp and leaves), shrubs of pin cherry (leaves) and poison or white sumac (glossy, white, berry-like fruit) are known to contain toxic compounds that have caused poisoning in livestock.
The needles of pine trees contain a variety of compounds such as resins, mycotoxins and lignols that can cause toxic reactions in livestock if ingested. However, white and red pines make excellent shelter belt trees on moist to dry soils. Their use should be limited to areas where livestock can not eat their needles.
Once you have decided what species are appropriate to your needs, there are a number of resources you can consult for advice on buying and planting trees. It can be an easy task to do - a harder one to do successfully. One resource is the 2000-2001 Native Plant Resource Guide. It includes a list of growers that can supply the stock as well as information on planning a planting project. You can contact the OMNR Information Centre 1-800-667-1940 to purchase a copy. The LandOwner Resource Centre, 1-888-571-INFO, www.lrconline.com is another source of land management information including tree planting.
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