Effects of Stocking Rate, Botanical Composition, and Stream bank Erosion on the Physical Characteristics of the Streamside Zones of Pastures (A Three-Year Progress Report)

Abstract

Grazing management practices that allow congregation of cattle near pasture streams may increase sediment, phosphorus, and pathogen loading of the streams by removing the vegetation and causing manure accumulation near the streams. To assess the effects of stocking rate and pasture characteristics on the risk of nonpoint source pollution of pasture streams, forage sward height, bare and manure-covered soil, forage species, and erosion were measured along the banks of streams in 13 pastures on 12 cow-calf operations in southern Iowa. Mean sward heights, and bare and manure-covered soil were different (P<0.0001) between the 13 pastures. There were also month differences for mean sward height and manure-covered ground (P<0.0001), but not for bare soil (P>0.05). The proportions of vegetation species as tall fescue, reed canarygrass, clover, or sedge were different (P<0.05) between pastures. Proportions of tall fescue, reed canarygrass, Kentucky bluegrass, bromegrass, orchardgrass, and sedge in the vegetation species were different (P<0.05) between months. Pastures with the least proportion of tall fescue and greatest proportion of reed canarygrass had the least amount of bare soil within the streamside zones of pastures. Stepwise multiple regressions were calculated to predict using vegetative species, and sampling interval and annual stocking rates of cattle on the 13 pastures. Sward height decreased as tall fescue, bluegrass, legumes, and annual stocking of cow-days per acre increased (r² =0.56). The proportion of soil that was bare increased as the proportion of legumes increased and decreased as the proportion of reed canarygrass increased (r² =0.35). Manure-covered ground increased as the stocking rate per acre per sampling interval and the proportions of tall fescue and bluegrass in the vegetation increased, and decreased as broadleaf weeds and sedge increased (r² =0.47). Increased stocking rates will result in decreases in forage sward height and increases in manure cover in streamside zones. The presence of tall fescue may also increase cattle activity near streams reducing sward height and increasing manure-covered soil in the streamside zone. Annual stream bank erosion rates were not correlated (r² =0.001) to annual cattle stocking densities per stream foot or acre. The greatest amount of erosion occurs during the spring grazing season, indicative of the freeze-thaw cycle and hydrological effects.