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Potential for development of a predictive model that considers livestock and wildlife contributions to non- point source pollution from available data (Objective 2).


Study results vary on water quality impacts related to livestock densities. Low stocking levels generally yield less impacts to water quality. In the studies reviewed, stocking levels were commonly expressed in two ways, animal unit months (AUMS) or the proportion of available forage that is removed by animals. An AUM is precisely defined as the amount of forage needed to support one cow and calf unit for one month, conversely, stocking levels are expressed as light, moderate, or heavy amounts of forage removal. These categories depend on geographic location. For example, an area that supports 5 AUMs at a moderate grazing level, may be considered as a heavy grazing level in a more arid region. Describing grazing levels as light, moderate, and heavy is a more broad definition that has common meaning across geographic locations.

Several studies found that moderate to heavy stocking levels resulted in measurable non-point source pollution. In central Idaho, heavy cattle stocking (0.26-0.43 ha/AUM) lead to bank instability and reduced streamside vegetation and undercut banks (May and Somes 1982). In northeastern Oregon, Kauffman et al. (1983b) found that cattle stocking at 1.3-1.7 ha/AUM in a riparian zone caused significantly increased streambank erosion, although riparian vegetation was not severely impacted. In south-central Utah, similar stocking levels of 1.2 ha/AUM resulted in increased stream widths and sedimentation, and decreased pool quality and overhanging banks in (Starostka 1979).

Studies with light to moderate stocking densities yielded variable results in the degree of water quality or riparian zones impacts. In northeastern Oregon, after 1 year of grazing at 3.2 ha/AUM, Buckhouse et al. (1977) found that sediment production was insignificant and fecal bacteria counts in run-off were acceptable , although infiltration rates were decreased. In another northeastern Oregon study, Buckhouse et al. (1981) assessed the impacts to streambank degradation due to a moderate grazing level (3.2 ha/AUM); significant streambank loss could not be attributed to livestock grazing at this level. In southeastern Utah, Buckhouse and Gifford (1976) found fecal bacteria counts did not change significantly in fun-off at a grazing level of 2 ha/AUM compared to an ungrazed area. In southwestern Wisconsin, Sartz and Tolsted (1974) found that a moderate cattle grazing level resulted in increased run-off compared to an ungrazed pasture. Soil bulk density was higher in the grazed area.

Generally, heavy grazing levels result in damage to riparian zones and affect water quality. High grazing levels are likely to alter the hydrologic qualities of the soil including soil loss, infiltration rates, and bulk density (Dunford 1949, Rodgers 1981, Blackburn et al. 1982).

Light and moderate grazing levels results were variable. It is difficult to make comparisons of studies that use AUMs because of environmental differences among areas. Also, some studies did not indicate if the grazing level was considered to be light or moderate a review by Blackburn et al (1982) concluded that light and moderate grazing levels are not significantly different from on another in their impacts on hydrologic qualities of the soil, while Rauzi and Hanson (1966) found a linear relationship between the level of grazing and the amount of run-off produced and water intake rates. In western South Dakota, Hanson et al. (1970) found that infiltration rates were significantly higher in the lightly grazed pasture versus the moderately and heavily grazed pastures.

Because of the importance of vegetation in maintaining water quality by providing soil stability, reducing erosion, trapping sediment, and protecting soil from eroding forces, the amount of vegetation remaining after livestock grazing has been used as a management indicator of water quality. Several authors have recommended that light to moderate grazing levels (at least 70% if the plant cover must be left unutilized), is effective in minimizing soil erosion and maintaining soil infiltration rates (Packer 1953, Smeins 1975). Heavy grazing (> 35% of the vegetation removed) is significantly more detrimental by decreasing infiltration rates and increasing erosion (Dunford 1949, Rauzi and Hanson 1966).

Translating levels of unutilized vegetation into AUMs would require knowledge of the consumption of forage by livestock a particular area. The impact of any given stocking level depends on a particular area's ability to withstand grazing pressure. Amounts of available to livestock differ from place to place. This causes difficulties in predicting stocking levels that would minimize non-point source pollution.

The timing to grazing also influences the level of impacts on water quality and riparian zones. Grazing levels that maintain water quality and minimize impacts in one season may not have the same affect at another time due to seasonal trends in soil moisture (Marlow et al. 1987, Pogacnik and Marlow 1983). A particular grazing level may have less impact during the late dry season than the same level during wetter times of the year. Time of year and duration of grazing in the riparian zone may be more important than the actual number of grazing animals (Marlow and Pogacnik 1985, Marlow et al. 1989).

The proportion of riparian zone in an area also influences the grazing level that is appropriate. A grazing level in an allotment where livestock have access to a riparian zone versus an allotment where access to the riparian zone is limited may result in varying level of water quality and riparian zone degradation. Also, preference of different types of riparian areas by livestock varies, putting more pressure on certain vegetation types (Kauffman et al. 1983a).

Cattle prefer riparian zones, causing difficulties in defining stocking levels. The amount of available forage, the time of foraging, and desirability of the riparian zone influence the likelihood of cattle congregating in the riparian zone (Hayes 1978). Setting an adequate grazing level that will maintain infiltration rates and minimize erosion in the upland portion of an allotment may still result in negative impacts to water quality if livestock are permitted to spend large amounts of time in the riparian portion of an allotment (Platts and Nelson 1985d).

An alternative to recommending AUM levels for minimizing impacts to water quality is to use levels of forage removal (Gifford and Hawkins 1978). Treating riparian zones as separate or unique portions of an allotment when prescribing stocking levels that consider the amount of stream length accessible per AUM may have greater utility as management criteria for controlling non-point source pollution than the method of using AUMs (Kauffman et al. 1983b).


Information currently available on the impacts caused to water quality by wildlife is insufficient for generating predictive models of the relationship of herd levels to non-point source pollution. The small amount of literature found on the subject does not the numbers of animals present in either natural or human-induced situations.

The information on the affects of wild ungulates on water quality is too limited to make speculations on how their foraging behavior, seasonal movement, and winter feeding concentrations might be influencing water quality. Also, it is not possible to assess the relative influence of wild ungulates on water quality in different types of riparian habitats with the information available. The majority of the literature reviewed on the impacts of wild ungulates to water quality and riparian zones pertained to elk. Although quantitative data was not available, several authors have described impacts to riparian vegetation caused by elk (Weinstien 1979, Patten 1988, Skates 1988). Physical and behavioral characteristics of elk heighten the likelihood that they could cause impacts. Their gregariousness, seasonal preference for wet meadows and riparian habitats, and large size make them a species of interest when considering wild ungulate species that would be likely to cause impacts to water quality and riparian areas in Wyoming.

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