GRAZING MANAGEMENT EFFECTS ON SEDIMENT AND PHOSPHORUS IN SURFACE RUNOFF

Authors: HAAN, MATHEW - IOWA STATE UNIVERSITY; RUSSELL, JAMES - IOWA STATE UNIVERSITY; POWERS, WENDY - IOWA STATE UNIVERSITY; Kovar, John; BENNING, JAMIE - UNIVERSITY OF NEBRASKA

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2006
Publication Date: 9/1/2006
Citation: Haan, M.M., Russell, J.R., Powers, W.J., Kovar, J.L., Benning, J.L. 2006. Grazing management effects on sediment and phosphorus in surface runoff. Rangeland Ecology and Management. 59:607-615.

Interpretive Summary: The amounts of sediment and phosphorus (P) in surface runoff from agricultural lands are of concern because of the potential for siltation and eutrophication of waterways. At this time, there is limited information on total sediment and P losses in runoff from pastureland in the Midwest. Because aboveground biomass limits soil disruption caused by the impact of raindrops and roots hold soil particles, forages harvested at an appropriate height through suitable grazing management should promote water infiltration and minimize sediment and P loss in runoff water from pastures. In this three-year central Iowa study, we measured the amounts of sediment, total P, and soluble P in runoff water from pasturelands managed five different ways. Data were collected in June, August, and October of each year, and April of the following year. We found that forage management did not affect the amounts of sediment lost in runoff water. The amount of P lost was greatest from continuously grazed pastures. Pastures rotationally grazed until the grass was 10 cm high lost no more P than pastures that were not grazed. Of the soil and grass characteristics we measured, the percentage surface cover was the variable most related to sediment and P losses, which led us to conclude that surface runoff from pastures managed to maintain adequate residual forage cover should not contribute greater sediment or P to surface water than ungrazed grassland. The results of this research will provide useful information to cattle producers, local environment groups, and Cooperative Extension and Natural Resources Conservation Service (NRCS) personnel.

Technical Abstract: Sediment and phosphorus (P) in runoff from pastures are potential non-point source pollutants in surface waters that may be influenced by surface cover, sward height, treading damage, surface slope, soil moisture, and soil P. The objectives of the current study were to evaluate their relationships with the physical and chemical characteristics of the soil and sward. Five forage management treatments; ungrazed (U), hay harvest/fall stockpile grazing (HS), continuous stocking to a forage height of 5 cm (5C), and rotational stocking to forage heights of 5 (5R) or 10 (10R) cm, were established in 0.40-ha paddocks in three smooth bromegrass (Bromus inermis Leyss.) pastures and maintained for three years. Rainfall simulations were conducted at a rainfall intensity of 7.1 cm hr-1 for 1.5 hours over a 0.5-m2 area in three locations at two slope ranges in each paddock in June, August, and October of each year and the subsequent April. Forage management did not affect mean sediment load (7.3 + 5.0 kg ha-1 hr-1). Mean total P load was greatest from 5C treatment (0.071 + 0.011 kg ha-1 hr-1), did not differ between the U, HS, and 10R treatments (0.019 + 0.011 kg ha-1 hr-1) and was intermediate in the 5R treatment (0.053 + 0.011 kg ha-1 hr-1). Mean soluble P load was greatest from 5R and 5C treatments (0.037 + 0.004 kg ha-1 hr-1) and did not differ between U, HS, and 10R treatments (0.011 + 0.004 kg ha-1 hr-1). Of the soil and sward characteristics measured, percentage surface cover was most highly related to sediment load (R2 = 0.16) and total P load (R2 = 0.10). Surface runoff from pastures managed to maintain adequate residual forage cover did not contribute greater sediment or P to surface waters than an ungrazed grassland.