Bielorai, R., Department of Animal Nutrition, Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Iosif, B., Department of Animal Nutrition, Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Neumark, H., Department of Animal Nutrition, Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel

Six groups of 15 chicks each were fed ad libitum experimental diets that contained between 0.4 and 35 g nitrogen/kg, from 10 to 16 day of age. Soybean meal was the only source of protein, and the marker was magnesium ferrite. The calculated absorption values obtained by regression analysis were higher than the determined apparent values, mainly in the duodenum and upper jejunum, due to elimination of endogenous nitrogen secretion. At the lower ileum, the calculated and the measured (apparent) absorption values were almost identical; thus, with soybean as the protein source in a diet containing a normal level of protein (18-35 N/kg diet), there is no need for endogenous nitrogen correction. Absorption of the endogenous nitrogen in chicks fed ad libitum a nitrogen-free diet was almost completed at the lower ileum (95%).