Research conducted at Universidad Politécnica de Madrid (UPM) and Universitat Politècnica de València (UPV) in Spain has shown that the inclusion of agroindustrial byproducts in pig feed can reduce the nitrous oxide emissions of the resulting manure slurry by as much as 65%.
The study, carried out by UPM researchers, in collaboration with the Institute for Animal Science & Technology at UPV, was to influence the ingredients of pig diet to modify the composition of slurry used as manure and to assess the possible variations in nitrous oxide emissions.
Two byproducts typical of the Mediterranean region were selected to carry out this study: orange pulp and carob. These byproducts were added to commercial swine diets in substitution of cereals, while respecting the balance these diets require to satisfy the animals' needs.
According to the results, soils amended with slurries obtained from the modified diets (with orange pulp and carob) decreased nitrous oxide emissions by 65% and 47%, respectively, compared with slurries obtained through a conventional swine diet. These results show the potential of alternative strategies of animal feeding to reduce the environmental issues associated with agriculture.
Nitrogen fertilizers -- organic or mineral -- are responsible for most of the nitrous oxide emissions from agricultural activity. This gas has 300 times greater heating potential than carbon dioxide, which is why it is essential to develop mitigation strategies. Nitrous oxide emissions are mainly caused by microbiological processes known as nitrification and denitrification.
So far, the measures to mitigate nitrous oxide emissions have focused on crop management. However, researchers from UPM and UPV focused on the beginning of the chain, where the animal byproducts are produced and then revalued as fertilizers.
It was proved that the excreted components through feces and urine (slurry) -- such as nitrogenous fractions, lignin, phenolic compounds, etc. -- varied according to diet. Slurries were used as fertilizers on agricultural soils cultivated with ryegrass, a forage plant used as food for livestock. The nitrous oxide emissions were compared to the emissions generated in soils with slurry obtained from pigs fed a traditional diet.
The slurry analysis revealed that the amount of benzoic acid and hippuric acid varied according to the type of diet. It turned out that emissions decreased in soils with the higher amount of benzoic acid provided by slurries. This is because benzoic acid reduced the denitrifying microbial capacity of the soil, which is responsible for most of the nitrous oxide emissions to the atmosphere. Taking into account that hippuric acid degrades in the soil to benzoic acid, slurries with higher amounts of these acids released less nitrous oxide than the rest of the slurries.