Creatinine as an index substance for estimating the urinary excretion of nitrogen and potassium by grazing sheep

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Abstract
The use of creatinine as an index of urinary nitrogen and potassium excretion of grazing sheep was examined in three experiments. Separate relationships between creatinine excretion and liveweight were derived for ewes and wethers in experiment 1. These equations were used in experiment 2 to predict the urinary nitrogen and potassium excretion of sheep maintained in metabolism cages from the nitrogen : creatinine and potassium : creatinine ratios in urine samples taken at 0900 and 1630 hours. These estimates were compared with the measured urinary nitrogen and potassium output. It was concluded that urinary nitrogen excretion of one sheep on one day could be estimated with a random error of ± 17% and potassium output with an error of ± 24%. Both estimates were subject to bias. In experiment 3, changes in the nitrogen : creatinine and potassium : creatinine ratios of six grazing wethers were measured; a diurnal trend in the nitrogen : creatinine ratio was observed. The reliability of two possible sampling procedures was examined and it was concluded that under grazing conditions the technique could only be used to detect large differences in urinary nitrogen and potassium excretion.