Poultry and swine farming are large contributors to environmental impacts, such as climate change, eutrophication, acidification, and air and water pollution. Feed production and manure management are identified as the main sources of these impacts. Reducing dietary crude protein levels is a nutritional strategy recognized to both decrease the use of high-impact feed ingredients and alter manure composition, reducing emissions of harmful components. For a successful implementation of this technique, feed-grade amino acid supplementation is crucial to maintaining animal performance. Reducing crude protein lowers nitrogen excretion, especially excess nitrogen excreted in urea or uric acid form, improving nitrogen efficiency. At the feed-gate, low–crude protein diets can reduce the carbon footprint of feed production through changes in raw material inclusion. The magnitude of this reduction mainly depends on the climate change impact of soybean meal and its land-use change on the feed-grade amino acids used. Reducing dietary crude protein also lowers the environmental impact of manure management in housing, storage, and at spreading: nitrogen emissions from manure (ammonia, nitrates, nitrous oxide) are reduced through reduction of nitrogen excretion. Moreover, synergetic effects exist with nitrogen form, water excretion, and manure pH, further reducing emissions. Volatilization of nitrogen is more reduced in poultry than in pigs, but emissions are more studied and better understood for pig slurry than poultry litter. Ammonia emissions are also more documented than other N-compounds. Low–crude protein diets supplemented with amino acids is a strategy reducing environmental impact at different stages of animal production, making life cycle assessment the best-suited tool to quantify reduction of environmental impacts. Recent studies report an efficient reduction of environmental impacts with low–crude protein diets. However, more standardization of limits and methods used is necessary to compare results. This review summarizes the current knowledge on mitigation of environmental impacts with low–crude protein diets supplemented with amino acids in poultry and swine, its quantification, and the biological mechanisms involved. A comparison between pigs and poultry is also included. It provides concrete information based on quantified research for decision making for the livestock industry and policy makers.

Functional amino acids supplementation to farm animals is considered to not only be beneficial by regulating intestinal barrier, oxidative stress, and immunity, but potentially also by impacting the gut microbiota. The impact of amino acids on a piglet-derived colonic microbiota was evaluated using a 48-h in vitro batch incubation strategy. The combination of 16S rRNA gene profiling with flow cytometry demonstrated that specific microbial taxa were involved in the fermentation of each of the amino acids resulting in the production of specific metabolites. Branched chain amino acids (leucine, isoleucine, valine) strongly increased branched-chain fatty acids (+23.0 mM) and valerate levels (+3.0 mM), coincided with a marked increase of Peptostreptococcaceae. Further, glutamine and glutamate specifically stimulated acetate (~20 mM) and butyrate (~10 mM) production, relating to a stimulation of a range of families containing known butyrate-producing species (Ruminococcaceae, Oscillospiraceae, and Christensenellaceae). Finally, while tryptophan was only fermented to a minor extent, arginine and lysine specifically increased propionate levels (~2 mM), likely produced by Muribaculaceae members. Overall, amino acids were thus shown to be selectively utilized by microbes originating from the porcine colonic microbiota, resulting in the production of health-related short-chain fatty acids, thus confirming the prebiotic potential of specific functional amino acids.

Two experiments were conducted to investigate the effect of arginine (Arg); the combination of Arg and glutamine (Gln); as well as an amino acid-based solution (MIX) containing Arg, Gln, threonine (Thr), and grape extract, on performance, intestinal permeability, and expression of selected mechanistic genes. Using 240 male Ross 308 off-sex broiler chickens, four experimental treatments were replicated six times with 10 birds per replicate. The experimental treatments included 5 g/kg Arg, 2.5 g/kg Arg and 2.5 g/kg Gln, and 1 g/kg MIX added to a basal diet as control. In the second study, the four dietary treatments were then given to 24 birds with or without a synthetic glucocorticoid, dexamethasone (DEX), as a gut dysfunction model. Feed conversion ratio was improved by all the supplemented treatments from day 7 to 35 of age (p < 0.001). DEX injections increased (p < 0.001) the intestinal permeability in all treatments, which tended to be reversed by Arg or MIX. Additional Arg, Arg-Gln, and MIX suppressed (p < 0.05) the overexpression of IL-1β generated by DEX. Feeding birds with MIX treatment increased (p < 0.05) expression of SGLT-1 and glutathione synthetase. In conclusion, tested amino acid supplements were effective in improving feed efficiency and restraining intestinal inflammation caused by DEX through IL-1β pathway.