Submitted to: Ecological Modeling
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2024
Publication Date: 8/8/2024
Citation: Sadsad, J., Chu, M., Guzman, J., Moriasi, D.N., Fortuna, A. 2024. Transport of nitrogen in grassed watersheds accounting for the combined influence of grazing and climate. Ecological Modeling. 496. Article 110827. https://doi.org/10.1016/j.ecolmodel.2024.110827.
DOI: https://doi.org/10.1016/j.ecolmodel.2024.110827

Interpretive Summary: Grasslands account for approximately 30% of the total land area in the United States and play a major role in the country’s agricultural landscape. Nearly 35% of these grasslands are actively used for livestock production to meet the demand for livestock-derived products for a growing population. In this study, a modeling tool was used to investigate the feasibility of enhancing livestock production without increasing nitrogen loss to water bodies. The study was carried out at the eight Water Resources and Erosion controlled watersheds located in central Oklahoma. Results showed that irrespective of the stocking rate, duration, and frequency, nitrogen losses in grazed pastures occur under conducive weather conditions, primarily precipitation. However, strategically matching grazing activities with prevailing weather patterns can increase livestock production while promoting a clean environment in pasture management. For example, implementing seasonal grazing from June to September, a scheme characterized by high evapotranspiration and low soil moisture, results in a notably low potential risk of nitrogen loss. This strategic approach, implemented through targeted grazing periods, is a promising tool that farmers can use to minimize environmental risks associated with pasture nutrient losses while enhancing livestock production.

Technical Abstract: A comprehensive modeling framework utilizing MIKE-SHE to investigate the feasibility of enhancing livestock production while concurrently mitigating the impact of grassland ecosystems on surface water quality is presented. In this study, a modeling framework that simulates nitrogen transport was developed using a coupled physically based distributed model, MIKE SHE. The model was calibrated and validated using observed overland flow and water quality data from El Reno, Oklahoma’s Water Resources and Erosion Unit watersheds. Different scenarios involving variations in timing, duration, and frequency of grazing activities, stocking rates, and climate conditions were simulated using the calibrated MIKE-SHE model. The goal was to develop an intelligent grazing scheme by delineating targeted grazing windows that optimally balance heightened livestock production with environmental sustainability. The study’s findings showed that the observed variability in existing literature originates mainly from climatic differences, with precipitation being the primary driver of nutrient loss, while evapotranspiration and soil moisture conditions are secondary factors. The simulations revealed that the impact of grazing on nitrogen loss in the pasture is evident only when grazing activities, irrespective of the stocking rate, duration, and frequency, occur under weather conditions conducive to nutrient loss in the pasture. These intertwined processes suggest that the impact of grazing on nitrogen loss can only be generalized within the context of the prevailing weather conditions in the pasture. Hence, strategically matching grazing activities with prevailing weather patterns can increase livestock production while promoting environmental sustainability in pasture management.