- Agronomic: corn, soybeans
- Animals: bovine
- Animal Production: grazing - continuous
- Crop Production: agroforestry, forestry
- Education and Training: demonstration, farmer to farmer, on-farm/ranch research, workshop
- Natural Resources/Environment: riparian buffers, riverbank protection, soil stabilization
- Soil Management: soil quality/health
Soil and phosphorus losses from stream bank erosion among different land-use practices were compared in three Iowa regions. Stream bank erosion was measured with erosion pins (estimate erosion rate), and by surveying the severe and very severe eroding banks of all treatments. Soil samples from the stream bank face were collected to estimate bulk density and soil total phosphorus. Row-cropped fields and continuous pastures had the highest soil and phosphorus losses, while riparian forest buffers, grass filters and pastures with cattle completely excluded from the stream the lowest. Erosion rates of individual banks did not differ between grazing practices or between buffers and pastures with cattle excluded from the channel. The differences in soil and phosphorus losses between practices were the result of percentages of the total bank lengths that were eroding within each practice. Intensive rotational pastures showed some indications of reducing soil and phosphorus losses from bank erosion compared to continuous pastures. Total eroding lengths varied from about 11% for the buffered and cattle excluded streams to 27% for the intensive rotational and 38% for the continuously grazed pastures. These losses translate to 4 to 7 tons km-1 yr-1 to 155 to 235 tons km-1 yr-1 for the two sets of practices.
Rotational and intensive rotational grazing are slowly replacing traditional continuous grazing in Iowa because these new practices better utilize pasture forages, and increase profitability (USDA-NRCS, 1997a; Undersander et al., 1993). Although many studies have been conducted in the Western United States on the influence of intensive rotational and rotational grazing on stream ecosystems, very few studies have been conducted in the Midwest (Lyons et al., 2000). Belsky et al. (1999) reported that many studies have shown that livestock grazing reduces stream bank stability in the Western United States. In most cases, decreased stream bank stability is the result of decreased vegetation cover that decreases the root length and mass in the soil (Kleinfelder et al., 1992; Dunaway et al., 1994). In rotational and intensive rotational grazing, in contrast to continuous grazing, the pasture is divided into smaller sections (paddocks) providing rest periods for the paddocks that allow above- and below-ground portions of the forage to recover thus increasing stream bank stability.
Stream bank erosion is a natural process and Simonson et al. (1994) suggest that high quality natural streams should have less than 20% of the total length of stream banks eroding. Major removal of the natural prairie, forest, and wetland vegetation in Iowa to promote agricultural practices has altered the hydrologic cycle causing larger and higher discharge events through streams. As a result most Iowa stream are incised with stream bank erosion contributing significant amounts of sediment and phosphorus to the stream. Stream bank erosion has been shown to contribute 30-45% of the sediment load in streams in Minnesota (Sekely et al., 2002), 45-50% in Iowa (Odggard, 1984; Schilling and Wolter, 2000), and up to 80-90% in other regions of the United States (Simon et al., 1996) and other countries (Krovang et al., 1997). Very few studies have quantified stream bank contribution to the total phosphorus load (Sekely et al., 2002). A study in Minnesota reported that only 7-10% of the total phosphorus in the stream was from stream bank erosion (Sekely et al., 2002) in contrast to a study in Illinois that found 56% (Roseboom, 1987). A study in Denmark found that stream bank erosion contributed more than 90% of the total phosphorus load to a stream (Krovang et al., 1997).
Phosphorus has been identified as the primary nutrient limiting eutrophication of many surface waters (Daniel et al., 1998) while sediment is the number one water quality problem in the United States (Simon and Darby, 1999).
Phosphorus, in most cases, is attached to sediment when transported (David and Gentry, 2000; Sharpley and Smith 1990) and therefore, should be studied together with sediment.
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The overall goal of this project was to collect and present data to convince farmers who continuously graze livestock in riparian corridors to adopt rotational and intensive rotational grazing practices because of environmental and monetary benefits. The goal would be accomplished by conducting research on the environmental benefits of rotational and intensive rotational grazing and by education/training. The specific research objective of this project was to compare the impact of different grazing practices and other agricultural practices on stream bank erosion and phosphorus movement. Bank erosion rates along continuous, rotational, and intensive rotational pastures were compared to bank erosion rates along riparian forest buffers and grass filters, pastures with cattle fenced out of the stream, and row-cropping adjacent to the streams. The hypothesized order from highest to lowest stream bank erosion was: row-cropped fields adjacent to streams, continuous pastures, rotational pastures, intensive rotational pastures, pastures with the stream excluded to cattle, grass filters, and riparian forest buffers.