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

View the project final report

• 2016 annual report

Commodities

• Agronomic: Rhizoma peanut, bahiagrass
• Animals: bovine

Practices

• Animal Production: feed/forage, grazing management, Greenhouse gas emissions

Grazing systems for livestock production are an important economic activity in the southeastern US. These grass-based systems are nearly always nitrogen (N) limited. Nitrogen fertilizer can supply this need, but it has a large carbon footprint due to greenhouse gas (GHG) emissions that occur during fertilizer production, transportation, and use. In addition to GHG associated with N fertilizer, other important GHG sources in grazing systems include nitrous oxide (N2O) from animal urine and dung, and methane (CH4) from dung and enteric fermentation. Changes in emissions of these gases can significantly modify the GHG budget because they have very large global warming potential (GWP) relative to carbon dioxide. Legumes are an alternative N source to grazing systems that avoids the GHG emissions associated with N fertilizer use. Although inclusion of legumes in animal diets increases digestibility and has documented potential to decrease enteric CH4, little is known about its effect on emission of N2O and CH4 from animal excreta. Because the GWP of N2O is more than 10 times greater than that of CH4, the observed decrease in enteric CH4 may be irrelevant if including legumes in livestock diets increases N2O emissions from animal excreta. The objective of this project is to quantify the effects of including a legume in mixture with a grass on emissions of GHG from grazing systems. The studies included will quantify N2O emissions from urine and dung, and CH4 emissions from dung and enteric fermentation of animals grazing N-fertilized bahiagrass or mixed rhizoma peanut-bahiagrass pastures.

The overall objective of this project is to quantify the effects of including rhizoma peanut (RP) in mixture with bahiagrass (BG) (~75% RP, 25% BG) vs. N-fertilized BG monoculture on the emissions of greenhouse gases from grazed pasture.

Specific objectives are:

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 to 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.