Stocking rate and riparian vegetation effects on physical characteristics of riparian zones of midwestern pastures

Authors: BEAR, DOUGLAS - Iowa State University; RUSSELL, JAMES - Iowa State University; TUFEKCIOGLU, MUSTAFA - Artvin Coruh University; ISENHART, THOMAS - Iowa State University; MORRICAL, DANIEL - Iowa State University; Kovar, John

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/3/2011
Publication Date: 6/9/2011
Citation: Bear, D.A., Russell, J.R., Tufekcioglu, M., Isenhart, T.M., Morrical, D.G., Kovar, J.L. 2011. Stocking rate and riparian vegetation effects on physical characteristics of riparian zones of midwestern pastures. Rangeland Ecology and Management. 65:119-128.

Interpretive Summary: Livestock grazing in pastures has been identified as a possible factor contributing to non-point source pollution of Rathbun Lake and associated water bodies in southern Iowa. Research was conducted with the objective of demonstrating reduced sediment and phosphorus (P) losses to surface water through implementation of practices that alter timing, frequency, duration, and/or intensity of cattle grazing near pasture streams in the ratbun lake watershed. Bank erosion, forage height, and the proportions of bare and manure-covered ground were measured along streams in 13 pastures with varying numbers of cattle per unit land area from 2006 through 2009. During the study period, we found that: 1) bank erosion of pasture streams is primarily affected by stream hydrology and not by grazing animals; 2) unrestricted stream access may increase sediment and P losses because of decreased vegetative cover and increased manure deposition near pasture streams; and 3) congregation of cattle near pasture streams is greatest in small and/or narrow pastures and may be reduced by restricting stream access to stabilized crossings or by rotational stocking. The results of this work will contribute useful information to cattle producers, local environmental groups, and Cooperative Extension and NRCS personnel interested in reducing the negative effects of agricultural production on water quality.

Technical Abstract: Grazing at high stocking rates may increase sediment and nutrient loading of pasture streams through transport in precipitation runoff and bank erosion. A three-year grazing study was conducted on thirteen cool-season grass pastures to quantify effects of stocking rate and botanical composition on forage sward height, proportions of bare and manure-covered ground, and bank erosion adjacent to streams. Pastures ranged from 2 to 107 ha with stream reaches of 306 to 1778 m that drained watersheds of 253 to 5660 ha. Bare and manure-covered ground were measured at a 15.2 m distance perpendicular to the stream at 30.5 m intervals at up to 30 locations on each side of the stream by the line transect method in May, July, September, and November of each year. At the midpoint of the 15.2 m line, forage sward height was measured with a falling plate meter (4.8 kg/m2) and plant species identified. In November 2006, fiberglass pins (1.6 x 76.2 cm) were driven 73.7 cm into the stream bank at 1 m intervals from the streambed to the top of the bank along 10 equidistant transect locations on each side of the stream to measure bank erosion during spring, summer, and fall of each year. Increasing pasture stocking rates increased manure-covered ground and decreased sward height, but did not affect proportions of bare ground. The greatest, intermediate, and least net soil erosion rates occurred during the winter/early spring, late spring/early summer, and late summer/fall seasons. Stocking rates between measurements, expressed as cow-days·stream m-1, were not related to bank erosion. Increasing stocking rates per unit of stream length will increase manure cover and decrease forage sward height, but not affect proportions of bare ground or bank erosion rates adjacent to pasture streams. Therefore, managing the stocking rate may reduce nutrient loading of pasture streams.