Northern Prairie Wildlife Research Center
Burning grasslands has been shown to increase the nest density of prairie
chickens (Westemeier 1973) and sharp-tailed grouse (Kirsch and Kruse 1973).
Svedarsky (1979) recommended rotational spring burning of preferred prairie
chicken nest habitats in northwestern Minnesota and fall burning of willow
lowlands to create better brood habitat.
Early settlers of the Flint Hills region of Kansas discovered that cattle selected forage from burned range more readily than from unburned range and observed that steers gained weight faster by grazing on burned range (Higgins et al. 1988b). The use of fire to increase livestock production is based on a recognition that forage growing after burning becomes more palatable and is preferred by livestock. A strong positive correlation between protein content and preference by cattle and sheep was illustrated by Leigh (1961).
Burning at the SNG
Very little burning has been done for livestock management purposes in this region. A notable exception to this has been the prescribed burning that Barker and others carried out in the SNG beginning in the early 1970's. Manske (1995a) recognized prescribed burning and mowing as essential defoliation management tools needed to assist in proper ecological management of the SNG. The grassland plant communities change over a relatively short distance in the Hummocky and Deltaic Plains Habitat Associations, making management requirements quite complex. The highly productive vegetation on the foot and toe slopes grows fast and starts senescence in the early portion of the grazing season. Cattle generally do not graze these areas in mid and late season because of decreased palatability. Grazing pressure is then concentrated on the summit, shoulder and back slopes. Management practices which would encourage repeated grazing of the more productive low areas would improve both the range carrying capacity and the nutrition available to the livestock. The upland areas are more nutritious than the low and mid areas in the latter part of the grazing season, therefore, deferred grazing in these areas is desired (Erickson et al. 1978). Cool season grasses vary more in feeding values during the season as they are more affected by changes in temperature and rainfall. Upland grasses do not decline in nutritional value to the same extent as grasses in the lowland and midland sites. Manipulation of the sedge meadow community by summer mowing or prescribed spring burning returns the vegetation to an early growth stage which is very nutritious. Manske (1995a) asserted that the sedge meadow communities that are not manipulated with burning or mowing develop accumulations of standing old growth which decrease and retard growth of current years graminoid tillers and provides an environment for shrub species to grow. Cattle grazing is not effective in reducing shrub densities except when shoots are very young and have not become woody. Mowing can be used to reduce aboveground portions of young shrub plants when the stem thickness is not very great nor plants very tall. However, mowing machines can not be used effectively on old shrubs that are tall and have thick woody stems. Prescribed fall or spring burning can be used to reduce aboveground portions of shrubs when they are young or old. Repeated defoliation of the shrubs will deplete the carbohydrate stores in the root, eventually weakening and causing death of the plant. Grazing use of the forage plants of unmanipulated sedge meadow areas is generally 10% or less. Repeated spring burning not only eliminates woody species but increases livestock utilization from about 10% to 60% (Barker et al. 1988). Livestock were attracted to the burn areas for at least 2 years after the treatment, which reduced the grazing pressure on the upland and midland communities.
Shrub encroachment, largely willows, is not confined to the sedge meadow community. The mixed grass prairie and tall grass prairie communities of the SNG also benefit from burning, mowing and grazing (Manske 1995a). Many areas of potential suitable habitat on the SNG do not meet prairie chicken nesting cover requirements and are not meeting the desired future condition due to high cover values of Kentucky bluegrass and willows (Potts 1995). The Environmental Assessment calls for the use of prescribed burning as one method to reduce the amount of Kentucky bluegrass and woody vegetation, thereby increasing the cover of native warm-season grasses. The alternative of vegetation manipulation which was selected utilizes frequent late spring burns (biennial) for several years, as well as some summer burns. Such treatment has been shown effective in reducing willows, aspen, and Kentucky bluegrass while increasing native warm-season grasses (Svedarsky et al. 1986).
Jensen (1992) identified the need to intensify control of the maturation of woody vegetation to enhance prairie chicken habitat. He noted that suppression of fire has allowed willow and aspen stands to mature. Increases of sharp-tailed grouse, along with reduced nesting cover probably are the result of shrub invasion. During the 1980's mowing of lowland areas replaced prescribed burning to reduce brushlands and increase palatability of grasses. Jensen says this mowing strategy has met limited success because some conditions (e.g. wet, rough) do not allow access of machinery; or the willow clumps become too large and thick to use ordinary farm mowing equipment. Jensen recommended burning larger blocks of the SNG rather than isolating the bottoms of temporary wetlands as a means of increasing the efficiency of the prescribed burning program. Burning of upland and midland sites should take place in early spring to promote the growth of native warm season grasses. Jensen then stated that some upland and midland areas need to remain ungrazed for much or possibly all the growing season. McCarthy et al. (1995a) also believe prescribed burning should be a more prominent part of management of the SNG. They stated the focus of prescribed burns should be on habitats in close proximity to booming grounds that have undergone significant woody encroachment.
Mowing at the SNG
As mentioned in preceding paragraphs, mowing is also used as a means of vegetation manipulation, especially in the lowlands. Barker et al. (1988) have shown that repeated mowing eliminated woody species but does not increase utilization by livestock as much as spring burning. They identify 1 July as the approximate best time to mow to gain increased livestock utilization and obtain high quality hay. In a summer mowing trial conducted on SNG switchgrass areas, the mean 100% visual obstruction reading (VOR) increased progressively until the 4th spring after treatment, then decreased to a height-density to what was considered a climatic equilibrium (Manske 1995a). The mean height-densities of the 100% VOR collected during the first spring after a summer mowing treatment did not meet the minimum for good concealment cover of 1.5 dm. During the second summer after mowing, sites had a mean 100% VOR of 1.5 dm. This was considered the minimum for nesting and night roosting. The mean height-densities for the 100% VOR for the third and fourth spring after mowing treatments were 2.3 dm and 2.7 dm, respectively. The mean height- densities for the 100% VOR for the fifth and sixth spring after mowing treatments were 1.8 and 1.7 dm, respectively. Manske (1995a) concluded that switchgrass areas can be enhanced as concealment habitat for prairie grouse by summer mowing if it is conducted on a 4 or 5-year cycle.
Lowlands and midlands are key
Newell (1988b) acknowledged that due to the demonstrated importance of lowlands and midlands to the prairie chicken population, modifications in the management of these 2 communities has the greatest potential for positive impact. He recognized of primary importance the adjustment of mowing patterns to provide a wider distribution of unmowed lowlands. The current practice of mowing all the lowlands in a given pasture is detrimental to nesting, brooding, and roosting habitat. While disturbance of rank vegetation is necessary to protect upland vegetation, there should be a pattern of mowing and burning that leaves some residual vegetation in each of the pastures. This is perhaps what Van Ningen (1995) had in mind when he said that increasing the intensity of burning, mowing and grazing (by attracting cattle to the lowlands) will only serve to further limit the prairie chicken nesting habitat and result in increased jeopardy for the population. Newell suggested mowing only 50% of a given pasture on a 3-year rotation. He also believed the mowing of lowlands should be delayed until 10 August to ensure that all nesting activities are complete and broods are mature enough to avoid mowers. This conflicts with the 1 July day that Barker et al. (1988) recommended as the best time to gain livestock utilization.
Comparison of defoliation methods
Grazing, mowing and burning are all ways in which foliage may be removed. However there are significant differences in the response of plants and ecosystem functions to these. Much of the response of a plant to defoliation depends upon its phenological stage. Manske (1995c:2) noted "the key to ecological management by effective defoliation is to match the timing of the defoliation to the phenological stage of growth that triggers the desired outcome." Heavy utilization of a plant during a critical period, whether by grazing, mowing or burning, can weaken it and make it more susceptible to competition. Burning at a particular time may damage actively growing plants while stimulating growth in others. This is the rationale for using fire to control woody vegetation or Kentucky bluegrass. There are factors related to the type of defoliation that are not just plant responses. Burning, for instance, returns most of the nutrients (speeds cycles) to the ecosystem in relatively available forms. Some small amounts may be volatilized and lost. Burning also changes the physical and chemical factors of the ecosystem. Grazing does not remove as much of the herbage as does burning and the defoliation is much more selective. Grazing returns some of the nutrients to the system, but many are lost when the livestock are removed. The changes in physical factors are not as great, but can be significant with trampling. Mowing removes the herbage much like grazing, although without the selectivity. It is not a natural form of ecological management as is grazing and fire, but does trigger some of the same responses. The greatest loss of nutrients occurs by mowing if the herbage is removed as is often the case. Physical and chemical changes are probably similar to grazing, perhaps less. Grazing domestic livestock and wildlife is the natural way in which herbage is most often removed. Burning and mowing are considered as methods of vegetation manipulation to enhance grazing utilization. There seems to be a clear consensus that burning and mowing are useful vegetation manipulations. Overall, burning receives the most favorable recognition, with most who are familiar with the SNG believing greater emphasis should be put on the use of fire. Personal observation by the authors leaves no doubt that woody invasion is a problem. The main question in this mix of grazing, mowing and burning is what, when, where and at what intensity. All of these need to be considered in any management option.