Determination of urinary creatinine excretion and estimation of urinary nitrogen and purine derivatives in Wagyu, high-marbling fattening cattle

To improve the efficiency of nitrogen (N) utilisation in Wagyu (Japanese Black cattle), which are characterised by a distinct muscle-to-fat ratio, accurate estimation of urinary N and purine derivatives (PD) excretion is essential for researchers to evaluate and optimise feeding strategies. This study aimed to evaluate the use of creatinine-based spot urine sampling as a practical alternative to total urine collection for estimating urinary N and PD excretion in fattening Japanese Black cattle. Two experiments were conducted: a 4 × 4 Latin square design with four heifers and a 3 × 3 Latin square design with three heifers. These experiments evaluated the effects of various dietary treatments and sampling times on urine volume, creatinine, N, and PD concentrations. A total of 125 daily urine samples were collected from all seven heifers to determine creatinine excretion. Results showed that daily creatinine excretion in Japanese Black cattle averaged 24.7 mg/kg BW per day, lower than the 26.3 mg/kg BW per day calculated from literature data for other breeds (P < 0.001). This difference can be attributed to breed-specific variations in muscle mass and fat deposition related to feeding practices aimed at improving marbling. The highest estimation accuracy for N (R² = 0.92, RMSE = 8.5 g/day) and PD (R² = 0.38, RMSE = 16.9 mmol/day) was observed in spot urine samples collected between 0500 and 0900 h. These findings, based on comprehensive total urine collection from seven heifers, suggest that spot urine sampling using creatinine can reduce the labour and cost associated with total urine collection, making it a viable approach for monitoring N metabolism in large-scale fattening operations.