Date of Award
Master of Science
James R. Russell
Sediment, phosphorus (P), and fecal pathogens lost from grazed pastures contribute to the non-point source pollution of surface waters. Therefore, the objective of this experiment was to observe the effect of different grazing management techniques on the amount of time cattle spend in or near pasture streams and on the amount of sediment, P, and fecal pathogen loading of into the streams. During the 2008 and 2009 grazing season, a study was conducted at the Iowa State University Rhodes Research and Demonstration Farm utilizing six adjoining 12.1-ha pastures that were bisected by a 141-m reach of stream. The pastures were grouped into two blocks and assigned one of three treatments: continuous stocking with unrestricted stream access (CSU), continuous stocking with stream access restricted to 4.9-m wide stabilized crossings (CSR), or rotational stocking (RS). Pastures were stocked with 15 fall-calving black Angus cows from mid-May to mid-October for 153 days in both years. For two weeks of each month, GPS collars were placed on at least one cow per pasture. For one of the two weeks, alternative off-stream water was made available to cattle in CSU and CSR pastures to determine the effect of off-stream water on cattle distribution. Each month the cattle were stocked on the pastures, bare and fecal-covered ground was measured. Rainfall simulations were conducted in June, August, and October of 2008, April, June, August, and October of 2009, and April of 2010 at six vegetated and six bare locations on the stream banks in CSU and RS pastures and six vegetated locations on the stream banks within the riparian buffer in CSR pastures. In June and August of both years, two cows per pasture were given a bolus of Cr-mordent fiber to determine total and P fecal output. Shedding of the fecal pathogens was measured by collected fresh fecal samples from all 90 cows in June, August, and September of both years. Stream bank erosion was measured by erosion pins at 10 equidistant transects that were measured monthly from May to November. Results show that off-stream water had no effect on cattle distribution. Compared to the CSU treatment, the CSR treatment reduced the probability (P < 0.10) that cattle were within the Riparian Zone (0 to 36 m from stream center) at black globe temperature humidity index (BGTHI) of 50 to 100. Bare ground in and near the stream was generally greater in pastures with the CSU than CSR and RS treatments. Rainfall simulations resulted in greater (P < 0.10) proportions of applied precipitation and amounts of sediment and P transported in runoff from bare than vegetated sites across grazing treatments and from vegetated sites in CSU and RS pastures than vegetated sites in the CSR pastures. The proportion of applied precipitation, sediment and P loading into surface runoff was most closely related to the proportion of bare ground (R2 = 0.5217, 0.4512, 0.4082, respectively). Pathogen shedding of cattle occurred only once throughout the experiment and was never found in precipitation runoff from rainfall simulations. Bovine enterovirus, an indicator virus, was shed by an average of 24.3% of cows over the study and was collected in the runoff of 8.3 and 16.7% of the simulations on bare sites in CSU pastures in June and October of 2008, respectively, and from 8.3% of the simulations on vegetated sites in CSU pastures in April 2009. Stream bank erosion did not differ between treatments. Results of the experiment show that time spent by cattle near pasture streams can be reduced by RS or CSR treatments, thereby, decreasing risks of sediment and nutrient loading of pasture streams even during periods of increased BGTHI. Stream bank erosion via cut banks was the greatest contributor of both sediment and P loading of pasture streams; contributions of sediment and P from surface runoff and grazing animals were considerably less and were minimized by grazing management practices that reduced congregation of cattle by pasture streams.
Kirk Albert Schwarte
Schwarte, Kirk Albert, "Assessment of the risks of nonpoint source pollution of pasture streams related to grazing management" (2010). Graduate Theses and Dissertations. 11694.