3H-leucine single-injection method for determining endogenous amino acid losses of broilers

Abstract

Objective A 3H-leucine ( 3H-Leu) single-injection method was proposed for determining the endogenous amino acid losses of broilers. This method was based on the hypothesis that the ratio of the specific radioactivity (SR) of endogenous Leu in excreta (SR e) to that of free Leu in trichloroacetic acid–soluble plasma (SR p) remains constant after a single subcutaneous injection of 3H-Leu into birds fed different diets. Methods Two experiments were designed to clarify this hypothesis. In experiment 1, 40 female broilers were randomly divided into four groups and were force-fed a nitrogen-free diet (NFD), NFD plus enzyme-hydrolyzed casein (EHC), 5% crude protein (CP) and SBM (soybean meal), or 20% CP-SBM. In experiment 2, 24 broilers were randomly divided into four groups and were fasted or force-fed the NFD, 20% CP-SBM, or 20% CP and cottonseed meal (CSM) diet. After the forced feeding, broilers were administered 3H-Leu by a single subcutaneous injection at a rate of 30 μCi/kg of body weight. Blood samples were taken at 5 min, 30 min, 4 h, 24 h, 36 h, and 48 h after the injection. The excreta were totally collected and pooled over the 48-h experiment. Results The ratios of SR e to SR p remained the same for the birds force-fed the NFD, NFD + EHC, and 5% CP-SBM diets in experiment 1 and for the birds fasted and force-fed the NFD diet in experiment 2. The proportions of endogenous Leu to total Leu in excreta were 72.8%, 61.4%, and 57.5% for birds force-fed with the 20% CP-SBM diet in experiment 1 and 20% CP-SBM and 20% CP-CSM diets in experiment 2, respectively. Broilers fed the 5% CP-SBM and 20% CP-SBM diets excreted more ( P < 0.05) endogenous Leu than those fed the NFD and NFD + EHC diets in experiment 1. Broilers fed the 20% CP-SBM diet excreted more ( P < 0.05) endogenous Leu than those fed the NFD diet and fasted and the 20% CP-CSM diet was intermediate ( P > 0.05) in experiment 2. Conclusion The present study verified the hypothesis that the ratio of SR e to SR p remains constant after a single subcutaneous injection of 3H-Leu into broilers and proposes a new method to determine endogenous amino acid losses of broilers.