Assessment of the risks of nonpoint source pollution of pasture streams related to grazing management

Abstract

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 1 CHAPTER 1. GENERAL INTRODUCTION THESIS ORGANIZATION This thesis is organized as an introduction to the research and related literature review followed by a brief description of the hypothesis for developing this research and its objectives. Manuscripts for submission to the Journal of Animal Science and the Journal of Environmental Quality follow the literature review and introduction of research. Following the manuscripts are a general conclusion, appendices of additional information, and acknowledgements.

Grazing Management Effects on Sediment, Phosphorus, and Pathogen Loading of Streams in Cool-Season Grass Pastures

“…Th erefore, direct deposition of cattle feces into a pasture stream may add a signifi cant amount of P to the water if cattle are spending a large amount of time within the stream. However, in the current study, cattle in the CSU treatment spent 1.8% of their time in the stream, whereas cattle in the CSR and RS treatments spent 0.35 and 0.09% of their time in the stream, respectively (Schwarte et al, 2010). Although other researchers have reported that providing off -stream water reduced sediment, P, and fecal bacteria loading of pasture streams by altering the temporal and spatial distribution of grazing cattle (Sheffi eld et al, 1997;Byers et al, 2005), providing off -stream water to cows in the CSU and CSR pastures for 1 wk mo −1 did not aff ect cattle distribution in the pastures in this experiment (data not shown) (Schwarte et al, 2010).…”

“…These effects were likely caused by wet conditions observed in early spring and lower sward heights and forage mass observed early and near the end of the grazing season. Sediment and P transport in runoff were greater in year 1 than in year 2 (Year; P < 0.10), which was likely the result of the above‐average rainfall that occurred in May and June of year 1 (2008) (Schwarte et al, 2010).…”

“…Furthermore, the mean bank stability score of CSU pastures was 12 and 30% greater than CSR and RS pastures when the treatments were initiated in May 2005 (Nellesen et al, 2011), implying that the banks in the CSU pastures were more unstable than banks in the CSR and RS pastures at the initiation of treatments within these pastures. From May 2005 to September 2009, bank stability scores increased by 1.68, 1.66, and 4.03% yr −1 in CSU, CSR, and RS pastures, respectively, implying that stream bank stability in RS pastures was declining more rapidly than CSU or CSR pastures (Nellesen et al, 2011;Schwarte et al, 2010). However, trend analysis of the monthly erosion/ deposition data from 2005 through 2007 showed that RS pastures had an increasing trend (i.e., a decrease in bank erosion), whereas no trend was observed in CSU and CSR pastures (Nellesen et al, 2011).…”

“…Riparian paddocks were grazed for a maximum of 4 d or to a minimum sward height of 10 cm (Clary and Leininger, 2000) as measured by the falling plate meter. This experiment and pasture treatments and data related to the temporal and spatial distribution of the grazing cattle have been described previously (Haan et al, 2010; Schwarte et al, 2010). Data related to characteristics of the riparian area and stream bank erosion in these pastures during the first 3 yr of the study were reported by Nellesen et al (2011).…”

“…Because distribution of cattle feces is proportional to the amount of time spent within a pasture zone relative to a stream (Ballard and Krueger, 2005;Haan et al, 2010), the amounts of fecal dry matter and P excreted daily per cow were multiplied by the number of days in a month and the percentage of time cattle spent within the stream zone (0-3 m from stream center) as measured by GPS collars from Schwarte et al (2010) to calculate the total amounts of fecal dry matter and P deposited into the stream each month. Annual dry matter and P deposition in the stream per pasture were calculated as the sum of the monthly values multiplied by the stocking rate of 15 cows per pasture.…”