INFLUENCE OF SUMMER MANAGEMENT PRACTICES OF GRAZED WHEAT PASTURES ON RUNOFF, SEDIMENT, AND NUTRIENT LOSSES

The agricultural economy of the southern Great Plains relies on practices that incorporate grazed winter wheat and associated summer management practices. Information exists about the impact of these practices on water quality, but data related to runoff and associated nutrient and sediment movement due to high intensity, late summer storms in the southern Great Plains are limited. This study examined runoff and runoff water quality from two winter wheat management strategies: winter wheat with summer chemical fallow (WWF) and winter wheat with summer legumes (WWSL) and two grazing treatments (grazed and ungrazed) from 1998 to 2002. Four pastures were planted in conservation winter wheat and grazed from November to May. Summer legumes were direct seeded in two of the pastures in March and grazed mid-July to September. Runoff from plots (1.5 × 3 m) was quantified with a rainfall simulator, with rainfall intensities representing a late summer, short duration (15 min), high intensity (10 cm/h) summer storm. Runoff samples were analyzed for nitrate-N (NO3-N), bioavailable and water-soluble phosphorus (BAP and WSP, respectively), and sediment yield. Overall, the WWF practice had greater runoff, sediment, and nutrient losses than the WWSL strategy. Likewise, grazing produced greater runoff, sediment, and nutrient losses than ungrazed plots. The worst-case scenario was WWF pastures that incorporated winter grazing, with 71% of applied rainfall lost as runoff. The greatest losses for sediment (284 kg/ha), NO3-N (124 kg/ha), BAP (380 g/ha), and WSP (38 g/ha) were found with the grazed WWF practice. Understanding the mechanism of interaction between late summer storms and summer management practices will improve large-scale mitigation strategies to reduce erosion and enhance capture of water resources.