Impacts of forage management on phosphorus cycling and sediment and phosphorus transport in surface runoff

Haan, Mathew
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Sediment and phosphorus (P) in runoff from pastures is influenced by forage and soil characteristics and are potential non-point source pollutants. Season and management affect forage productivity and nutrient content. Appropriate forage management practices will reduce sediment and P losses and provide adequate nutrition for grazing cattle. The objectives of this study were to quantify sediment, total P, and soluble P loads in runoff, to determine soil and forage characteristics relationship to sediment and P transport in runoff, and to determine how forage management impacts forage productivity and P uptake. For 3-years five forage management treatments were randomly assigned within three 2.75 ha smooth bromegrass pastures subdivided into 0.4-ha paddocks. Treatments were: ungrazed (U), summer hay harvest / fall stockpiled grazing (HS), continuous stocking to a 5 cm residual sward height (5C), and rotational stocking to a 5 (5R) or 10 cm (10R) residual sward height. Rainfall simulations we conducted four times per year at 7.1 cm-hr⁻¹ for 1.5 hours to determine sediment and P loads in surface runoff. Forage management did not affect sediment load (7.3±5.0 kg·ha⁻¹·hr⁻¹). Total P load was greatest from 5C (0.071±0.011 kg·ha⁻¹·hr⁻¹), lowest from U, HS, and 10R (0.019±0.011 kg·ha⁻¹·hr⁻¹) and intermediate from 5R (0.053±0.011 kg·ha⁻¹·hr⁻¹). No single factor was a good predictor of sediment or P transport. Forage samples were clipped monthly from April through November, forage mass and P content were determined. Forage management and sampling month had significant interactive affects on forage P content (P<0.05). In general, forage P was greatest during spring (0.21±0.016%), decreased with forage maturity, and was lowest during fall (0.13±0.016%). Forage productivity was greater (P<0.05) in the harvested treatments (6744±890 kg·ha⁻¹) than the U treatment (1872±890 kg·ha⁻¹). Phosphorus uptake followed the same trend as forage production, being greater (P<0.05) in harvested treatments (13.9±2.1 kg·ha⁻¹) than in the U treatment (3.7±2.1 kg·ha⁻¹). Surface runoff from pastures managed to maintain adequate residual forage cover did not contribute greater sediment or P to surface waters than do ungrazed grasslands. Forage management stimulated forage production and P cycling.