Increasing concern about non-point source pollutants released from grazing livestock, a worldwide problem, motivated the present study on the effects of vegetative filter strips (VFS) for controlling pollutants (nutrients, micro-organisms and sediment loading) from grazed, irrigated pastures. Flood-irrigated pastures are an important source of forage for livestock during summer months in California, USA when the surrounding rangelands are dry and dormant. Significant amounts of runoff can be generated from these pastures during irrigation events.
Nine plots on an irrigated pasture were assigned randomly to one of three treatments: Control (no VFS), Treatment VFS-1 (8·3×7 m, 0·0058 ha VFS) and Treatment VFS-2 (17·1×7 m, 0·012 ha VFS). In 2000, two grazing events (in April and June/July) occurred during the irrigation season prior to the experiment; further, the experimental plots were grazed between irrigations 2 and 3. Attenuation of runoff loads by VFS treatment was measured during four irrigation events (between 1 August and 3 October 2000) for total suspended solids (TSS), ortho-phosphate (Ortho-P), inorganic phosphate (Inorg-P), total phosphate (Total-P), organic phosphate (Org-P), polyphosphate (Poly-P), total Kjeldahl nitrogen (TKN), NH3, NO3 and presumptive faecal coliforms (FC).
On average, approximately 0·43 of the applied water left the plots as runoff. Treatment effects approached significance for TSS and TKN and were significant (P<0·05) for Poly-P and NH3. Irrigation number effects were significant for all but TSS, NO3 and FC. The effects of VFS treatments were not consistent. Treatment VFS-2, although representing the largest buffer strip, did not always produce the lowest pollutant loads in runoff. Slope, relatively high runoff volumes and some channelled flow were probably responsible for the limited effectiveness of VFS in the present study. These results suggest that effectiveness of VFS for reducing sediment and nutrient transport from irrigated pastures may be questionable.