By Jim Russell, Animal Science Professor, Iowa State University
Poor grazing management can contribute to pollution of pasture streams with sediment, nutrients, and pathogenic microorganisms. Grazing at excessive stocking rates decreases forage height and increases bare ground, thereby creating conditions that allow soil erosion and transport of manure nutrients and pathogens in precipitation runoff.
Even at appropriate stocking rates for the total pasture, congregation of grazing cattle near pasture streams for drinking and body temperature regulation may increase bare ground, soil compaction, and manure accumulation on stream banks that will contribute to pollution of pasture streams if grazing cattle have unrestricted access to the stream.
Exclusion of grazing cattle from pasture streams with fencing will reduce pollution of pasture streams by maintaining vegetative cover and preventing soil compaction and manure accumulation in the streamside area. Such a vegetative buffer has the added advantage of providing wildlife habitat and, thereby, hunting opportunities. However, complete exclusion would require the development of an alternative water source outside the riparian buffer and decrease the amount of land available for grazing within a pasture. Furthermore, research has shown that eliminating grazing of smooth bromegrass pastures for three years reduced plant root density which may reduce its ability of prevent soil erosion over a number of years.
There are several alternatives to complete exclusion that are nearly or as effective in preventing water pollution of pasture streams. Stabilized stream access sites placed within riparian buffers allow cattle access to water. However, in research at the Iowa State University Rhodes Farm, cattle do not congregate on the 16-foot wide crossings constructed of polyethylene webbing and crushed rock. Cattle in this treatment spent less than 0.5% of their time within the stream and less than 3% of their time within 110 feet of the stream. Depending on the type of crossing, stabilized access sites may have the additional advantage of providing a crossing for vehicles.
Short-term or flash grazing of the riparian paddock will allow utilization of the forage in the paddock while maintaining adequate vegetative cover to minimize soil erosion. Grazing of this paddock to a minimum forage height of 4 in for no more than four days maintains forage growth above ground and forage root density in the soil. Furthermore, use of this paddock in a rotational grazing system allows control of the timing of grazing to avoid periods when the stream banks may be wet and most susceptible to hoof traffic damage.
Beyond management practices involving fencing, pollution of water in pasture streams may be reduced by practices that alter the distribution of cattle within the pasture. Because cattle primarily congregate near pasture streams to relieve thirst and control body temperature, providing off-stream water and/or shade may reduce bare ground and manure accumulation near pasture streams. Similarly, placing feeding sites for minerals or other nutritional supplements away from pasture streams is likely to reduce the risks of water pollution. In addition to reducing the risks of polluting pasture streams, proper location of off-stream water, shade or nutritional supplementation sites may actually improve pasture utilization by increasing the uniformity of grazing across the pasture.
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