Endogenous N-losses in broilers estimated by a [15N]-isotope dilution technique: effect of dietary fat type and xylanase addition.

Male broilers were given a low protein diet (15.5% CP) spiked with [15N]H4HCO3 from day 12 to day 18 of age to label the endogenous N-constituents. Experimental diets were subsequently fed from day 19 to day 24 of age and consisted of a rye based diet (56% dietary inclusion) which contained either 10% soya oil (S) or 10% beef tallow (T), each of which was either unsupplemented (-) or supplemented (+) with a xylanase containing enzyme preparation (2700 IU/kg at pH 5.3). [15N]-atom percent excess (APE) of excreta, faeces and urine were monitored on a daily basis during both experimental periods. Furthermore, APE was measured in various tissues at the end of the experiment. The APE of urine on the last day of the experiment was between the APE of the pancreas and that of the jejunal tissue, an observation which supported the usefulness of using urinary APE as an indicator for the endogenous N-pool. Endogenous N-proportions were estimated by an isotope dilution technique at the end of the experiment by examination of the ratio of APE in faeces and urine. The endogenous N-proportion in the faeces was greatest in birds receiving the T(-) diet. The proportions were 0.321, 0.319, 0.451 and 0.289 in S(-), S(+), T(-) and T(+) fed groups, respectively. Xylanase addition reduced endogenous N-proportion, a factor which was used to correct apparent crude protein digestibility (85.6, 86.2, 84.3 and 88.5% in S(-), S(+), T(-) and T(+) fed birds, respectively) for endogenous losses resulting in almost equal true digestibilities of crude protein for all treatments (90.3, 90.6, 90.4 and 91.5%). The amounts of endogenous N in faces were estimated to be 87, 69, 244 and 81 mg per day per kg0.67 body weight in S(-), S(+), T(-) and T(+) fed birds, respectively. It was concluded that xylanase supplementation of a rye based broiler diet does not change endogenous N-secretions when the supplemental fat is soya oil. However, addition of tallow rather than soya oil increased these N-losses significantly, an effect which was reversed by xylanase addition.