Many of the cultural practices that family forest owners use to improve forest production may be used to improve other components of their forestland. Forest owners may wish to increase production of grasses and forbs for livestock production and/ or wildlife habitat improvement. Forestland grazing presents opportunities to increase land productivity, improve cash flow, and to increase the diversity of plants and wildlife – all of which are not mutually exclusive. Most forestland grazing is found on the east side of the Cascade Mountains, as forest stocking levels tend to favor more open stand conditions. While generally discouraged by foresters due to soil impacts on water-saturated soils, limited forestland grazing is found on the west side of the Cascades, and generally on sites that are managed to promote more open stands.
Grazing can benefit forest management in several ways. For example, grazing and browsing can reduce the need for herbicides and mechanical weed control, and manure can reduce the need for fertilizer application by promoting nutrient recycling. Forest stands that include grazing as a management option are often park-like in appearance, and generally more socially acceptable than traditional plantations managed exclusively for timber. Low-intensity cattle grazing-grazing reduces competition for moisture between overstory trees and understory shrubs when the stand is very young. Studies in Oregon have shown up to 50 percent increase in forage and timber growth over 10 to 20 years with the integration of livestock into the system. Also, adding nitrogen-fixing vegetation such as legumes to the forage mix combined with recycled nutrients in dung and urine increases nitrogen uptake of trees on sites that are naturally deficient of nitrogen.
Weight gains for cattle on forested pastures may exceed those of grasslands because: (1) prolonged spring run-off provides more spring moisture to understory plants, (2) forage reaches maturity more slowly, (3) grasses are protected from sun and frost curing, and (4) forage species diversity provides a longer grazing season. Experience has shown that forests also protect cattle from weather – cutting the direct cold effect by 50 percent or more and reducing wind velocity by as much as 70 percent. Cattle protected by windbreaks gained 35 lbs. more than unprotected herds during a mild winter and lost 10.5 lbs. less during severe winters. Weight gains also improve with proximity to shade.
Livestock grazing in forests is much more common than many people realize. In a recent survey, 26 percent of Washington family forest owners reported livestock grazing on their forestland in the previous 12 months. Nationally, livestock graze about 25 percent of all forests. This forest area accounts for about 13 percent of the total land grazed in the U.S. and roughly equals the total area of improved pastures and grazed croplands, combined.
Foresters often discourage livestock grazing in timber plantations for fear that trees will be browsed, debarked, or stepped on. Once reassured that the plantations can be safely grazed without damaging trees, the next silvicultural concern is often soil compaction. Cattle, sheep, goats and other livestock can exert as much downward pressure on soil as do agricultural tractors and unloaded forestry harvest equipment. When a sustainable number of animals are managed, trampling only occurs over a limited area. In addition, soil compaction by livestock is generally confined to the top few inches of soil whereas heavy equipment can compact to depths over a foot. Extensive reviews of published literature found that grazing does compact soil—though it is unusual for livestock grazing on drained soils to sufficiently compact soils to hinder plant growth. Forested rangelands in the western U.S. are most frequently used as summer-fall range, when soils are not saturated. It is unlikely that responsible forest grazing will sufficiently compact soils to reduce tree growth unless soils are poorly drained.
Large seedlings are cost-effective in forest grazing systems. While more difficult and expensive to plant, these trees have a higher tolerance to damage from livestock and will more quickly escape the maximum height of browsing by livestock and deer. Careful management should be the norm when grazing young stands, particularly on steep slopes to see that soil is not displaced by animal hoof action. The number of trees to plant and the planting pattern vary widely with the objectives of forestland grazing. If the forest component is to be emphasized, stocking of 200 to 400 trees/ acre are common, with grazing restricted to the first decade or so after tree planting. If grazing is to be maintained over the long-term, lower tree stockings will be needed to maintain forage production with subsequent overstory tree thinning that reduces stocking to as few as 50 trees/ acre at maturity. Tree pattern becomes increasingly important as density increases. Conventional forests use rectangular grids of trees to minimize competition between trees at the expense of the understory vegetation. Square grids, single, double or triple rows, and cluster plantings have all been used in grazed forests. The grid layouts optimize the area for tree growth, while the row or cluster plantings share the site resources more evenly with the forage crop. Rows support greater understory forage and the ease of access to row plantings for agricultural operations such as fencing, fertilizing, haying, etc., make them popular with producers.
Overgrazing can lead to the removal of terminal leaders, substantial lateral branch defoliation and, more rarely, debarking. Young conifers are fairly tolerant to defoliation provided that the terminal leader is left intact. Research applicable to eastern Washington forests reported that heavy lateral branch defoliation of 4-year-old Douglas-fir did not affect tree height and reduced the current year’s growth by only 1.5 percent compared to undefoliated trees. It takes browsing of over half of the needles produced in the current year or girdling of over half of the stem to visibly reduce long-term growth. Removal of terminal leaders is a more serious matter. Loss of conifer terminal leaders not only forgoes that year’s height growth, it may also reduce diameter growth by as much as 30 percent. The risk of growth loss and tree deformation of young conifers in pastures is high enough to warrant either careful monitoring of forage availability and livestock grazing behavior, or physical protection of the trees during the first few years after plantation establishment. Given the increased competition from other trees and understory shrubbery on west side Cascade forests, the loss of a year’s height growth may eventually result in competition mortality. However, to place these concerns in perspective, studies in western Oregon forests report that native deer inflict more damage to young forests than livestock.
While livestock can graze new plantations safely, great care should be exercised when tree terminal leaders are within the reach of livestock. Pastures can be grazed during the spring growing period with negligible defoliation of trees provided that total utilization of forage does not exceed 35 percent of current seasons forage crop.
The potential for tree damage by livestock appear to be related to several factors including season of year (spring/ early summer is when other forage plants are most palatable, but compaction may be an issue), percent utilization of forage available, age of animal, and tree heights. One study in southern Oregon comparing tree seedling growth by cattle grazing in a recently planted pine plantation versus a non-grazed plantation, experienced enhanced tree growth due to both the reduced grass and shrub vegetation competition the with the pine seedlings as well as the nitrogen inputs from manure. Damage is more likely during the first two-three years of tree growth before resinous chemical defenses are well developed. After three years, conifer foliage is not particularly palatable to cattle or elk, though sheep, goats, and deer might still be attracted. Conifer foliage is most likely to be grazed in the spring when it is newly-emerged and the anti-herbivory defense compounds have not yet fully accumulated, or any time that livestock are short of other forage, however, livestock will consume conifer foliage in low amounts even when other preferred forage is available. This very low level of tree browsing often changes quickly info substantial levels as other forage is depleted. It is not unusual for over 90 percent of tree defoliation to occur when other forage choices are limited. Livestock grazing young forests must be checked frequently and animals properly removed when forage is depleted and they begin to actively feed upon trees.
Minimizing Livestock Damage
In general, livestock breed is not a useful predictor of damaging feeding behavior as is age, sex, and past experience of animals. However, larger breeds such as Charolais, Semmental; Gelbvieh, and Limousin tend to distribute less than smaller breeds, so concentration in areas such as riparian forests can become an issue with their tendency to linger that can lead over-grazing and potential damage to soil structure. Older dry ewes do far less damage to trees than young lambs or rams. Cattle, sheep and goats that have consumed either green foliage or dry needles regularly in the past are much more likely to feed upon young trees in pastures. In every flock or herd there are individuals that seem to be predisposed to feed upon trees. Feeding behavior may be taught to others. Tree damagers should be culled as soon as they are identified. Some practitioners also report that livestock transported into grazed forests from non-forested areas will browse young trees as a “novel” food. Fencing, tubing, repellents and livestock exclusion have all been used to control browse damage by both wildlife and livestock in grazed forests. Fencing works well when trees are concentrated in closely spaced rows to maximize grazing area and minimize fencing costs. Fencing can be permanent where continuous grazing is planned or wildlife damage to trees is a concern. Portable electric fencing has been successfully used for short time periods and prescriptive grazing to reduce invasive plants. Lightweight portable fencing is erected quickly when and where needed to protect trees from livestock, so monitoring of the grazing progress is not as critical as with open grazing. Protecting individual seedling trees with plastic mesh or rigid tubes has also been used successfully, but this measure has drawbacks. Cattle trampling and tube removal by sheep and subsequent browsing of the unprotected tree is a real concern, so monitoring of the grazing is required. Attaching the tube firmly is another problem. Rigid wood stakes often break when rubbed by livestock. Resilient materials such as bamboo are more resistant to breakage.
Riparian Area Concerns
Livestock grazing becomes more complicated where riparian systems are involved. Because riparian areas remain lush and green into the summer dormancy period for upland grasses and forbs, livestock will congregate in these areas for shade, water, and forage. This situation could result in overgrazing of riparian plants critical for riparian and stream function and physical damage of stream banks. Some rules of thumb include:
- Allowing continuous, season-long grazing will damage riparian function.
- Expect that in years of good rainfall, an early growing season for grazing vegetation will encourage cattle to graze uplands, where green forage and warm temperatures are more favorable.
- Install off-stream water and salt far away from riparian areas.
- Cull animals that prefer to “camp” in riparian areas.
- Force cattle out of riparian areas with riders or substitute with herded sheep or goats.
- Exclude riparian grazing until late in the growing season, but be careful to watch for overuse of woody plants.
- Expect mixed or very site-specific results for riparian pastures in rotation systems.
- A number of successes have been observed when late winter and early growing season grazing systems were merged, but be careful to monitor compaction.
Livestock management is the key to successful forestland grazing. Important considerations for proper grazing management include where the livestock graze, season of use (timing), length of use (time), and the amount of plants grazed (intensity). Some rules of thumb include:
- Match the type of livestock to the forage base.
- Make judicious use of fencing, salting, off-riparian water, and trails to aid proper distribution and minimize damage.
- Customize rotational grazing systems to the local area and manage them intensively; time and timing will vary depending on the location, year and objectives.
- Move livestock well before browsing begins on trees.
By Andrew B. Perleberg, Regional Extension Specialist – Forestry, Washington State University and James P. Dobrowolski, Rangeland, Grassland, and Water Quantity National Program Leader, National Institute of Food and Agriculture, USDA.