Legume Proportion of Grass-Legume Mixtures Affects Greenhouse Gas Emissions from Animals Grazing Pasture

Legume Proportion of Grass-Legume Mixtures Affects Greenhouse Gas Emissions from Animals Grazing Pasture

Summary

We are comparing legume- and grass-based beef production systems in terms of greenhouse gas emissions from animal urine and dung, and potential methane emissions from enteric fermentation. Typically enteric fermentation emissions in legume-based diets are less than those of animals grazing pure grass pastures, however greater nitrogen content in diets containing legumes may increase emissions of some greenhouse gases (such as nitrous oxide) from animal excreta. The first year of field experimentation was completed in October 2015, and laboratory and data analysis are ongoing.

Objectives/Performance Targets

1. To quantify emissions of N₂O and CH₄ from animal dung and N₂O from animal urine for these two pasture types in order to obtain emission factors of N₂O separately for each excreta type (urine or dung) and for animals with contrasting diets, and to compare emission factors obtained in this study with others reported for different production systems in the literature and those used by the IPCC model;
2. To quantify enteric fermentation CH₄ emissions from the two pasture types and compare these measurements with those estimated by the IPCC model with parameters currently used for Southeast US in order to assess the prediction capability of the model;
3. To sum emissions of N₂O from dung and urine and CH₄ from dung and enteric fermentation of animals grazing the two pasture types and use global warming potential to estimate emissions in CO₂e.

Accomplishments/Milestones

The dung and urine from animals grazing fertilized bahiagrass pastures and rhizoma peanut-rich pastures were collected and applied to the experimental area. The first year of data collection is complete for nitrous oxide and methane emitted from animal dung and urine. Data from the first year are currently under chemical analysis using gas chromatography. Future work includes finishing these analyses, calculating gas fluxes, and conducting statistical analyses. For enteric methane emission potential from pastures, forage samples were collected, dried, and ground to pass a 1-mm screen. The incubation experiment to estimate enteric methane potential of pastures will be initiated in May 2016. A second year of data collection for greenhouse gases from animal excreta and enteric methane will be initiated in June.

Impacts and Contributions/Outcomes

Most research about the adoption of legume plants into grass pastures concludes that the presence of legumes increases pasture nutritive value and sustainability of the system. It has also been indicated that animals consuming diets rich in legumes have smaller enteric methane emissions than those consuming grass-based diets. However, presence of legumes may increase nitrogen content in animal dung and urine, and serve as a source of nitrous oxide emissions which is a powerful greenhouse gas. This research will provide researchers and producers with valuable information regarding sustainability of legume- and grass-based beef production systems in the Southeastern US by quantifying greenhouse gases coming from excreta and enteric fermentation of animals fed either of those diets. It will also provide regional emission factors that can be used in greenhouse gas budget calculations, reducing uncertainty related to these estimations.

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