Feeding low protein diets to meat chickens: effects on emissions of toxic and odorous metabolites

N.K. Sharma, R.A. Swick, M.W. Dunlop, S.B. Wu And M. Choct


Meat chickens fed a high protein diet produce increased levels of putrefactive metabolites in the caeca such as ammonia, amines, phenols, indoles, skatole, cresol and branched chain fatty acids. Some of these metabolites are toxic and odorous. Feeding low protein diets formulated to provide all the required amino acids without excesses are useful to reduce putrefaction and therefore the production of toxic and odorous metabolites in the hindgut and litter.

A study was conducted to investigate the effect of a low protein diet on odorous metabolites emitted from litter. Day-old Ross 308 male chicks (n=144) were weighed and assigned to two treatments with 12 birds per floor pen and six replicate pens per treatment. Two diets were formulated to contain the same ratio of soybean, canola and meat meals but with different crude protein (CP) contents as follows: starter phase (260 g/kg vs 210 g/kg), grower phase (240 g/kg vs 195 g/kg) and finisher phase (230 g/kg vs 184 g/kg). The low CP diet was supplemented with crystalline amino acids including L-valine, L-isoleucine, L-arginine, L-lysine, D, L-methionine and L-threonine.

At day 34, two birds from each pen were euthanized to collect caecal contents for measurement of microbial metabolites using a gas chromatography-mass spectrometry (GC-MS). Odorant concentrations were measured from litter headspace at days 15, 29 and 35 using a selective ion flow tube mass spectrometer (SIFT-MS, Voice 200TM SYFT technologies, Christchurch, New Zealand) and a flux hood that had some similar design features to the U.S. EPA flux chamber (Standards Australia/Standards New Zealand, 2009) and was operated in the same manner for each sample.

Prior to each analysis, the SIFT-MS was run with standard gases, which included ethylbenzene, tetrafluorobenzene, toluene, hexafluorobenzene, ethylene, octafluorotoluene, benzene, and isobutene to ensure a properly calibrated instrument. Odorant concentrations were log transformed and analyzed as a 2×3 factorial arrangement (2 diets, 3 ages) using SAS JMP v.8 software and differences were considered significant at P<0.05.

Birds fed a low CP diet showed lower concentrations of isobutyric acid, isovaleric acid and branched chain fatty acids in the caecal contents at day 34 than those fed a high CP diet (P < 0.05). In litter, concentrations of dimethyl amine, trimethyl amine, ammonia, H2 S, phenol and benzene were lower with the low CP diet (P < 0.05). The concentration of methyl mercaptan in litter also tended to be lower with the low CP diet (P = 0.065). There was a diet × age interaction for 2,3-butanedione, 3-hydroxy-2-butanone, 3-methyl-1-butanol, 3-methyl-butanal, ethyl mercaptan, propionic acid and hexane (P < 0.05). Concentrations of these odorants were higher with the low CP diet (P < 0.01) on day 35 but not on days 15 and 29. Dietary CP content had no effect on the concentrations of dimethyl sulfide, dimethyl disulfide, dimethyl trisulfide, methyl amine, indole, skatole, cresol, acetic acid, butanoic acid, 2-butanone and butanol. These findings suggest that a low CP diet can reduce the production of odorous metabolites in litter but may increase the production of some odorants (including some ketones, aldehydes and alcohols) on day 35.

From the 27th Annual Australian Poultry Science Symposium