Intake and excretion of sodium, potassium, and nitrogen and the effects onurine 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 studie s were performed to predict urine volume from either observed concentration s 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 e xcretion of K, Na, and N in practical situations. Data used to derive the r elationships (n = 67 observations) and to evaluate them (n = 62 observation s) were obtained from a wide range of feeding conditions in 10 independent balance trials with lactating cows. Linear relationships of K, Na, and N th at were excreted in urine or consumed were fitted against the observed urin e 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, th e observed variation in urine production was predicted with acceptable accu racy. Mean prediction errors were 4.5 and 5.6 kg of urine/d. Urine producti on 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 produc tion by dairy cows on nutrient management at the farm level increases.