Intake and excretion of sodium, potassium, and nitrogen and the effects on urine production by lactating dairy cows.

The present study was designed to describe the relationship between mineral metabolism and urine production by lactating dairy cows. Regression studies were performed to predict urine volume from either observed concentrations of K, Na, and N in urine or observed intakes of K, Na, and N. In addition, empirical equations were derived to assist in the estimation of urinary excretion of K, Na, and N in practical situations. Data used to derive the relationships (n = 67 observations) and to evaluate them (n = 62 observations) were obtained from a wide range of feeding conditions in 10 independent balance trials with lactating cows. Linear relationships of K, Na, and N that were excreted in urine or consumed were fitted against the observed urine production, which explained 89.8% (SE = 4.2 kg of urine/d) and 84.8% (SE = 5.2 kg of urine/d) of the variance. In evaluating these relationships, the observed variation in urine production was predicted with acceptable accuracy. Mean prediction errors were 4.5 and 5.6 kg of urine/d. Urine production could be predicted based on relationships between intakes of digestible Na, K, and N and their excretion in milk and urine. Reliable predictions of urine production are important as attention on the effect of manure production by dairy cows on nutrient management at the farm level increases.