Genesis of intrinsic ammonia and its roles in broiler chickens

Ammonia (NH 3 ) is an important atmospheric air pollutant in poultry houses where it is hazardous to the health of both birds and humans alike; however, the source of atmospheric NH 3 is intrinsic NH 3 in broiler chickens. This intrinsic NH 3 is generated by bacterial fermentation along the digestive tract coupled with endogenous NH 3 derived from systemic nitrogen metabolism. Intrinsic NH 3 is metabolised to and excreted to uric acid and, critically, atmospheric NH 3 is generated by the volatilisation of excreted uric acid in litter. Thus, the focus of this review is on intrinsic NH 3 in broiler chickens. Importantly, NH 3 is inherently highly toxic, possibly more so in avian than mammalian species, but normally NH 3 is detoxified in a condensation reaction with glutamate resulting in glutamine, a reaction catalysed by glutamine synthetase primarily in the liver. Glutamine then enters the Krebs uric acid cycle in which it is converted to uric acid and excreted. As previously reported, uric acid concentrations in excreta ranged from 28.5 to 69.4 mg/g from broiler chickens offered maize-based diets with varying dietary protein content. Inadequate detoxification and elevated plasma NH 3 concentrations negatively impacts broiler growth performance, although accurate determinations of NH 3 concentrations in blood samples are not straightforward. Inadequate NH 3 detoxification is more likely to be declared in the context of reduced-crude protein (CP) diets, which may be due to amino acid imbalances, post-prandial catabolism of non-bound amino acids, and/or inadequate dietary concentrations of glutamate and glutamine. The likelihood is that L-carnitine is protective and counteracts NH 3 toxicity. Antibiotic growth promotants (AGP) have the capacity to suppress NH 3 production by gut microorganisms and this may contribute to their capacity to enhancement of growth performance. In addition, the role of NH 3 in acid-base homeostasis is pivotal because urinary excretion of ammonium ions (NH 4 + ) is a dominant vehicle for acid elimination to counteract metabolic acidosis. It is probable that NH 3 impacts on pancreatic insulin secretion and resistance to insulin, although the importance of insulin in avian species is equivocal. Similarly, probiotics may be able to depress NH 3 production by modifying the taxonomic composition of the gut microflora. The intention of this intrinsic NH 3 review is to consider its genesis; either bacterial fermentation in the gut and/or systemic metabolism, plus the above outlined topics.