Urine features used to survey nitrogen excretion in rabbits

The aim of this work was to estimate liquid and faecal nitrogen (N) excretion from rabbit herds using 2 clinical analyses of urine samples (urinary urea, UU and creatininury, CU) combined with the daily nitrogen intake (DNI) and metabolic weight of growing and lactating rabbit does. In the framework of 6 experiments, 81 growing rabbits, divided into 17 groups, weighing from 1.8 to 2.8 kg, and 18 multiparous lactating does, divided into 2 groups, were reared in metabolic cages. Five experimental groups of growing rabbits and one of lactating does received diets with lower crude protein content (from –8 to –19% less). The urine was collected (4-d and 1-d collection period for the growing rabbits and lactating does, respectively) and the daily weight (DUW: on av. 188±66 g/d), urinary urea (UU: 1012±463 mg/dL) and creatininury concentrations (CU: 46±25 mg/dL) were recorded. Lactating does showed higher DNI (+127%; P<0.001), which was excreted more in the faeces (DFN: +141%; P<0.001) than in the urine (DUN: +35%; P=0.36), compared to the growing rabbits on a daily per-capita basis. Consequently, the faecal-N to urine N ratio was higher for the does compared to growing rabbits (F/U: +93%; P<0.001). The percentage of retained N (PRN) for the lactating does and growing rabbits was not different (50.8 vs. 56.6%; P=0.31). Forward regression models were used to predict the daily nitrogen excretion. Successful r-square fit results were obtained (P<0.005) for the per-capita daily quantity of urinary N (DUN: R2=0.79) and faecal N (DFN: 0.93, mainly depending on DNI). The individual DNI was accurately fitted (R2=0.994; standard error=0.03), considering the 2 model estimates of the DUN and of the DFN, the metabolic weight and the type of animal. Relativising the N excreta as a percentage of the DNI, or as a ratio of the faecal –to urinary–N, led to less stable results of the regression models. The daily N intake, combined with the collection of urine samples and the measurement of urea and creatinine, led to a reliable estimate of the liquid N excretion.