The bicarbonate retention factor (BRF) describes the proportion of infused 14C that is not recovered in the breath during Indicator Amino Acid Oxidation (IAAO) studies to determine amino acid requirements of poultry. The effects of feeding, time of day, and stage of egg formation on BRF of NaH14CO3 during a 30-hr period by Ross 508 breeder hens were determined. The birds (n=7), 60 wk of age and of similar body weights, were fed to maintain breeder recommended body weights. Each bird underwent surgery to implant a jugular catheter. Six birds were placed into individual metabolic chambers for the infusion studies. The birds received an average priming dose of 3.960.10 or 2.410.11 kBq /kg BW and a constant infusion rate of 0.7700.019 or 0.6010.021 kBq /kg BW/h of NaH14CO3 for the first and second infusion studies, respectively. The isotope was infused over a 30-hr period with a 3-d recovery period between studies. One bird was replaced for the second study due to loss of catheter. Total breath 14CO2 was collected and sampled at 30-min intervals; 14CO2-expiration rate as % of infused dose was plotted based on time of day (photoperiod vs scotoperiod) and physiological state (fed vs fasted and egg-forming vs non-egg-forming) of the bird. The isotopic plateau was confirmed by linear regression. Eggs laid during, and within 24 hrs of the end of each infusion period were collected immediately after laying. Each egg was weighed intact; shell (+ membrane), albumen and yolk were weighed. Each egg sample was analyzed for 14C retained as a % of infused dose. Overall, the average breath 14CO2 recovery was 90.46 4.17%. The presence or absence of an egg in the oviduct did not affect (P>0.05) 14CO2 BRF; neither did stage of egg formation. During the photophase and fasted period, 14CO2 recovery from the birds was less (P<0.05) than during the scotophase and fed period respectively. The BRF for breeder hens during the photoperiod, in a fed state was determined to be 14.21%. This BRF will be used to correct future IAAO studies involving AA requirements and availability in hens.

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