Prediction of endogenous urinary nitrogen of goats

Three databases were constructed to estimate endogenous urinary N (EUN) in nonlactating and lactating goats. The first database consisted of 22 observations in which urinary N (UN) was measured with nonlactating goats fed diets very low in N concentration (0.032-0.33% of DM). A log-log weighted linear regression of EUN (g) on BW (kg) indicated that 0.75 was an appropriate power of BW for which UN, the estimate of EUN, could be expressed. The intercept, which represented an estimate of EUN, was 0.122 g/kg BW^0.75. The second database for nonlactating goats, with means from 186 treatment-experiment combinations, was split into two groups, one for equation development (n=121) and a second for evaluation of the equations (n=65). With the development set, UN (g/kg BW^0.75) was regressed on total N intake (TNI; g/kg BW ^0.75) or apparently digested N intake (DNI; g/kg BW^0.75). After removing observations with relatively high residual S.D. from the development set, equations were: UN=0.092 + (0.288×TNI) (n=79; R ²=0.59) and UN=0.165+(0.340×DNI) (n=79; R²=0.59). The intercepts, 0.092 and 0.165 g/kg BW^0.75, are estimates of EUN when TNI and DNI are zero, respectively. At zero DNI, truly digested N intake should equal metabolic fecal N; thus, the DNI estimate of EUN may be applicable to nonlactating goats in zero or positive N balance with feed intake above maintenance and appropriate to use in summation equations to predict N requirements without need for further adjustment factors. Prediction equations for lactating goats with feed intake above maintenance were: UN=0.182+(0.235×TNI) (n=33; R²=0.65) and UN=0.160+(0. 354×DNI) (n=33; R²=0.72). In summary, based on databases from publications on goat feeding and nutrition, EUN of nonlactating goats with feed intake above maintenance was estimated at 0.165 g/kg BW^0.75 by regressing UN against DNI; EUN of lactating goats based on DNI seemed similar to that for nonlactating goats.