NC1182: Nitrogen Cycling, Loading, and Use Efficiency in Forage-Based Livestock Production Systems (formerly NCT-196 and NC-189)
Statement of Issues and Justification
Title: Nitrogen cycling and plant-livestock use efficiency in grassland agroecosystemsSTATEMENT OF ISSUES AND JUSTIFICATION
The amount of nitrogen (N) applied annually to forage production systems of the Midwest exceeds plant uptake (Mosier 2001) and relatively little of the N consumed by grazing animals is removed from the ecosystem (Jarvis and Ledgard 2002). Significantly greater N is removed via mechanical harvesting for feed but the same problem occurs when the forage is fedthe animals consume forage N, but then excrete most of this N into the environment. Because N is usually the primary limitation to plant production in temperate ecosystems, significant manure and inorganic fertilizer N is applied to agroecosystems. Nitrogen fixation by legumes and purchased feed supplements contribute additional N to the system. Unless soil organic matter increases significantly over time, there is little potential for highly mobile N to accumulate in soils, so the fate of this surplus N is deposition in waterbodies, groundwater, the atmosphere, and adjacent terrestrial ecosystems where it can have undesirable effects (Jenkinson 2001, Janzen et al. 2003, Eickhout et al. 2006). Surplus N from agroecosystems throughout the Midwest has been identified as the primary cause of periodic hypoxia in the Gulf of Mexico (Rabalais et al. 2002, Turner and Rabalais 2003). Gaseous N emissions from soils contribute to the greenhouse effect (Robertson et al. 2000, Mosier 2001) and eutrophication of terrestrial and aquatic ecosystems (Vitousek et al. 1997, Carpenter et al. 1998, Ferm 1998). The magnitude of N loss and subsequent negative impacts on ecosystems are influenced by the frequency, intensity, and timing of management practices within the agroecosystem.
From an agronomic perspective, much is known about fertilizer type, amount, and timing of application for maximizing crop yields (Jenkinson 2001, Addiscott 2005). Likewise, forage quality and animal nutrition research is targeting ways of improving N use efficiency by plants (Singer and Moore 2003) and livestock (Scholefield et al. 1991). A previous project (NC-189 Forage Protein Characterization and Utilization for Cattle) of this committee focused on the ruminal degradation characteristics of forage proteins in commonly cultivated forages and also provided a solid understanding of the protein degradation characteristics of native prairie and range grasses found throughout the northcentral region. While mechanisms and pathways for N transformation and loss have been determined for grasslands and pastures (Ledgard 2001, Kroeze et al. 2003), major gaps exist in our knowledge of the relationships between management and harvest strategies and N pathways on managed cool-season pastures of the central US and elsewhere. For example, while empirical relationships between N applied as fertilizer and N loss from the ecosystem exist (Rotz et al. 2005), tradeoffs in N retention and carbon (C) cycling, hence forage production, are less well understood. A recent review of grazing effects on C and N cycles concluded, There is major uncertainty in the quantification of N2O emissions, in particular with regard to different livestock production systems as well as their feeding and waste management practices, which need to be underpinned by more accurate modeling of soil N2O fluxes (Steinfeld and Wassenaar 2007).
Many scientists are focused on improving N use efficiency (NUE) of the products, i.e., crop plants and livestock, in order to reduce the amount of N going to the environment and to reduce fertilizer costs to farmers. Unfortunately, tradeoffs exist between NUE and forage quality, e.g., C4 grasses have greater N use efficiency, but this translates to lower crude protein content and lower livestock production on these grasses. Current research directed at improving NUE of livestock centers on manipulating the ratio of urine to feces N because urine-derived N is more volatile and labile in the environment. Researchers are examining tannins and other secondary compounds in feed, but the same forage quality tradeoffs are in play here. On the other hand, some forages contain more degradable protein than animals require but insufficient undegradable protein. The excess degradable protein is excreted in the urine. Strategies to increase forage production (e.g., N fertilization and interseeding legumes) often result in forages that contain N far in excess of animal needs. Identification of optimal grazing management and/or supplementation strategies offers opportunities to increase N utilization. There is a voluminous literature on confinement feeding of beef and dairy cattle for maximum production. However, very little work has been directed towards developing strategies for precisely meeting animal requirements for metabolically protein and amino acids without overfeeding crude protein in grazing situations Energy supplements with low degradable protein content may be an option, including byproduct sources of highly digestible fibers. Other protein supplements that resist degradation and have amino acid patterns that are complementary to ruminal microbial protein also have promise. Mounting demand from consumers in the last decade has pushed ranchers and farmers to produce high quality foods at low prices with fewer environmental impacts while remaining profitable (Tilman et al. 2002). Hence, there is a pressing need for scientifically sound information to make decisions about how best to manage rural landscapes to simultaneously produce agricultural commodities and maintain, and develop ecosystem services such as biodiversity and soil, water, and air quality. Our project will quantify the effect of pasture management on both within-system and downstream environments, and the ability of those environments to provide ecosystem services that optimally benefit human well being. We also will determine the effect of these pasture management practices on N use efficiency in terms of animal production and economic returns. Realization of these objectives will help farmers, policymakers, and agency personnel manage for better quality of life in rural areas.
Many efforts and resources are aimed at staunching the flow of N into the atmosphere or waterbodies, including establishment of riparian buffer strips and restoration of wetlands where physical impedance or biogeochemical transformation of N can occur (Tate et al. 2000, Borin and Bigon 2002, Sabater et al. 2003). While these landscape mitigation strategies may be effective from the downstream ecosystem perspective, the farmer still loses N that otherwise might improve productivity. Further, the loss of nutrients from farm systems puts a spotlight on the farmer for the negative costs his enterprise places on society. Efforts to increase N retention within the productive agroecosystem should improve the standing of the farmer as a land steward. In our current project (NC-1021 Nitrogen Cycling, Loading, and Use Effiency in Forage-Based Livestock Production Systems), we analyzed the N efficiency of grassland ecosystems managed for beef cattle production in Wisconsin and Nebraska. Based on estimates of N inputs (e.g., fertilization, fixation, and deposition) and outputs, we calculated that 18% or less of the N entering the system was leaving as product. While these grassland ecosystems may not have been at a long-term equilibrium, our analysis indicates that these grassland/beef cattle systems were not highly efficient with respect to product. Further, unless soil organic matter was accumulating at a high rate (and our data do not suggest this), much of the N entering as fertilization, deposition, and/or fixation is likely finding its way out of the agroecosystem. These studies and others in the NC-1021 states also have demonstrated the effect of management practices on N2O fluxes. For example, management intensive rotational grazing appears to enhance N2O fluxes from grassland soils in comparison to continuously grazed, hayed, or rested grasslands. We propose to continue to conduct complementary experiments and simulation modeling to help stakeholders make informed decisions about rural landscapes. Multifunctional farming systems provide multiple ecosystem services. These services can include provisioning (i.e., meat, milk, and fiber production) as well as supporting, regulating, and cultural services. Supporting services include soil building and nutrient retention whereas carbon sequestration and water storage are regulating services, and cultural services include spiritual, aesthetic, and educational factors. Perennial grasslands vary greatly in their ability to provide these types of ecosystem services because of differing environmental and management characteristics. We will assess tradeoffs amongst these services, which should allow more informed decision-making and long-range improvements in U.S. agriculture as a result.
Our expected outcomes and predictions include ranking of management strategies in terms of N use efficiency, particularly as it relates to the capture and excretion of N in the environment, ultimately with the goal of adopting strategies/practices that ensure efficient use of N in order to positively influence environmental quality. In addition, this work will facilitate the identification of management strategies and forage systems that minimize N inputs and production costs. Minimizing expensive N inputs (e.g., fertilizers) in forage-based livestock production systems has tremendous potential to enhance their profitability. These impacts are most likely achieved through the development and implementation of a multiple state project. The members of our proposed project represent a geographically diverse set of states from the Southeast through the Midwest and Great Plains and to the Intermountain West. Our objectives of analyzing N efficiency of grassland production systems will be based on a wide range of vegetation types, environments (humid to semi-arid) and levels of management intensity (irrigated pasture to low-input pastures). The expertise, facilities and other resources required to design and conduct the proposed research in the grassland ecology and management area are not found at a single institution. The synergy coming from a multiple state effort in this area greatly enhances the likelihood of success in characterizing N use and developing appropriate management strategies for grassland agroecosystems. Furthermore, the technical feasibility of this type of research is questionable for a single university but becomes realistic when several institutions combine resources and expertise.
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