A major consideration in the conservation of water quality has been the protection of streams and impoundments from contamination by sediments, nutrients, and pesticides. Grass barriers have been employed extensively to perform this function. Numerous studies have shown the benefit of vegetative filter strips or riparian buffers in reducing the amount of sediments and other materials reaching waterways (Dillaha et al., 1989; Butler et al., 2006; Blanco-Canqui et al., 2004). Tall fescue has been the most widely used grass in planted vegetative filter strips or riparian buffers (Sleper and Buckner, 1995). Although tall fescue does an excellent job of slowing runoff and reducing nonpoint-source pollution, it has been shown that in situations where flow is concentrated by topography or other structures, it may be necessary to include a stiffer grass barrier above a filter strip to reduce runoff flow rate and prevent overflow of the filter strip. Blanco-Canqui et al. (2006) demonstrated increased effectiveness of sediment removal and runoff reduction when they included a switchgrass barrier above tall fescue filter strips.
Filtering of nutrients is another important use of vegetative filter strips to maintain water quality. Runoff of N and P can degrade water quality both through the direct impact of these nutrients and through the production of algal blooms that can be toxic or reduce water quality through the resultant increase in biological oxygen demand. Sources of nutrients in agricultural systems may be from fertilizers applied to crop production fields, fertilizers or wastes applied to cropland or pastures, or manure produced by livestock grazing pastures or held in feedlots.
Numerous studies have been conducted to evaluate the fate of applied nutrients or manure. The most common finding related to N and P export from grasslands is that nutrient loss in runoff is reduced as the time since application increases (Pierson et al., 2001; Butler et al., 2007). Regardless of the time interval since application or rainfall intensity, maintenance of vegetative cover is critical to reducing nutrient content in runoff. Butler et al. (2007) noted that total N loss from a dallisgrass (Paspalum dilatatum Poir.)-tall fescue pasture with 45 to 95% cover was reduced 85% compared to loss from bare ground. It appears that the longer the nutrients are resident on grass, the more nutrients are retained. The process of nutrient conversion and use is slower for P than for N, so retaining runoff on application sites through maintenance of ground cover is more critical for reducing P contamination of water (Pierson et al., 2001).
The use of vegetative filter strips to reduce nonpoint-source pollution from confined animal feeding operations has been promoted widely as a best management practice (Dillaha et al., 1988). Vegetative filter strips not only reduce runoff rate and filter sediments, but grass filters also make use of the dissolved nutrients contained in feedlot effluent. An excellent review of the performance and use of vegetative filter strips for treating confined animal feeding operations effluents was published by Koelsch et al. (2006).
In addition to reducing sediment and nutrient loads in waterways, vegetative filter strips also are effective in reducing nonpoint-source pollution of waters with agricultural chemicals. Tall fescue filter strips have been effective in reducing herbicide concentration in runoff from soybean and cotton (Gossypium hirsutum L.) production systems (Webster and Shaw, 1996; Rankins et al., 2001). In a comparison of four species used as vegetative filter strips in Mississippi cotton production systems, Rankins et al. (2001) found no difference among tall fescue, big bluestem, eastern gamagrass [Tripsacum dactyloides (L.) L.], and switchgrass in effectiveness at reducing runoff, sediment, or herbicide losses with all four working effectively as filters. The key issues in selecting a vegetative filter species are the adaptation and potential longevity of the species in the region and situations where it will be employed, along with its tolerance to the herbicides being used in the production system.
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