Japanese scientists in cooperation with amino acid supplier Ajinomoto studied the potential reduction of greenhouse gas emission from swine manure by using a low-protein diet supplemented with synthetic amino acids.
Although effects of dietary protein reduction on nitrogen excretion are well documented, the potential of protein reduction to decrease N2O and CH4 emissions has not been widely examined.
The researchers completed nitrogen balance experiments in growing swine to assess effects of a low protein diet supplemented with amino acids on nitrogen retention. Faeces and urine obtained were used to evaluate greenhouse gas (GHG) emissions from composting and wastewater treatment processes.
Trial set up
Five crossbred barrows of 38 kg were used. The crude protein (CP) contents in the control (CONT) and low CP (LOW) diets were 171 g/kg and 145 g/kg (as fed), respectively.
The low CP diet was supplemented with lysine, threonine, methionine and tryptophan at levels sufficient to meet their requirements.
A crossover experiment was completed in which faeces and urine were collected separately and used for the subsequent experiment.
Manure from swine fed the control or low CP diet was placed in an experimental composting apparatus with forced aeration, and GHG emissions were measured during a five-week composting period.
Two bench scale activated sludge units were used for wastewater treatment. Under a hydraulic retention time of five days and biochemical oxygen demand loading rate of 0.4 kg m3/d, both sets of wastewater experiments were for six weeks.
Two five day measurements of GHG were compared between CONT and LOW.
There were no differences in daily barrow weight gain and nitrogen retention between CONT and LOW.
However, urinary nitrogen excretion of 5.5 g/kg dry matter (DM) intake and total nitrogen excretion of 10.1 g/kg DM intake were lower in the LOW group than in CONT.
The amount of nitrogen excreted in faeces and urine was reduced by 28.7% in the LOW group.
The reduction in manure GHG emissions was even higher, with a 39.1% reduction in the LOW group, although there was no change in emission factors as CH4/volatile solid or N2O/N) compared with manure from CONT.
About 0.5% of the influent N was emitted as N2O from compost and about 1.8% as N2O from wastewater.
Compared with total emissions, six times more N2O was emitted through wastewater treatment than from composting.