Understanding how much urine a cow produces each day is important for nutrition research. Traditionally, scientists estimate urine output by measuring the concentration of creatinine—a compound made in muscle—found in the urine. A standard “fixed” value of 29 milligrams per kilogram of body weight per day (mg/kg BW·d⁻¹) is often used to calculate this. But new research suggests this one-size-fits-all number may not be accurate.
A recent study published in JDS Communications looked at 16 lactating Holstein cows and measured how much creatinine they actually excreted daily. Researchers ran two separate feeding experiments using different diets and feeding conditions. Their goal was to check whether the fixed creatinine value is reliable across cows and diets.
Key Findings:
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Big differences between cows: Creatinine output ranged from 14 to 29 mg/kg BW·d⁻¹—much wider than expected. This means the “average” value of 29 is too high for many cows.
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Diet didn’t change creatinine levels: Whether cows were fed high or low protein, with or without supplements, their creatinine output stayed the same.
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Body weight isn’t the whole story: Cows with similar body weights had different creatinine levels. That’s because body weight includes fat and doesn’t reflect muscle mass, which is what actually produces creatinine.
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Study average was lower than the standard: The average creatinine rate in this study was 22 mg/kg BW·d⁻¹—not 29. This lower number could mean researchers are underestimating how much nitrogen cows excrete if they stick to the old number.
Why This Matters:
Estimating urine output using a fixed creatinine value may lead to errors in understanding how nutrients—especially nitrogen—are used or wasted in dairy cows. This is especially important in studies that measure environmental impact or feed efficiency. Overestimating urine volume could lead to wrong conclusions about how much nitrogen cows are really excreting.
What’s Next?
More accurate methods are needed—ideally based on actual muscle mass, not just body weight. While tools like body condition scoring or ultrasound may help, they aren’t yet widely used for this purpose. Until better methods are developed, researchers and nutritionists should be cautious when using a fixed creatinine value in experiments or on-farm monitoring.
This study reminds us that cows aren’t all the same, and even subtle differences between animals can influence data. Understanding those differences is key to better nutrition, animal welfare, and environmental outcomes.
