Effect of sodium chloride intake on urine volume, urinary urea excretion, and milk urea concentration in lactating dairy cattle.

Milk urea nitrogen (MUN; mg of N/dL) has been shown to be related to excretion of urinary urea N (UUN; g of N/d) and total excretion of urinary N (UN; g of N/d) in dairy cows. In the present experiment, it was hypothesized that MUN and the relationship between MUN and UUN or UN is affected by urine volume as a result of dietary sodium chloride intake. Twelve lactating Holstein-Friesian dairy cows (mean ± SD: milk production 28.1±3.23 kg/d and 190±41 d in milk), of which 4 were fitted with catheters in the urine bladder and jugular vein, were randomly assigned to 4 dietary levels of sodium chloride (3, 9, 14, and 19 g of Na/kg of DM) according to a triple 4×4 Latin square design. Cows were fed at 95% of ad libitum intake, excluding salt addition. Milk was analyzed for MUN and protein content; urine was analyzed for total N, urea, and creatinine content; feces were analyzed for total N and DM content; and blood plasma was analyzed for urea and creatinine content. Creatinine clearance rate (CCR; L/min) and renal urea reabsorption ratio were estimated based on plasma concentrations of urea and creatinine, and total excretion of urea and creatinine in urine. Intake of DM and N, milk production, and milk protein content were (mean ± SD), on average, 21.4±1.24 kg/d, 522±32.0 g/d, 25.4±2.53 kg/d, and 3.64±0.186%, respectively. A linear relationship was found between Na intake and urine production [urine (kg/d; mean ± SE)=7.5±4.33+0.136±0.0143 × Na intake (g/d)] and between Na intake and MUN [MUN (mg/dL; mean ± SE)=13.5±0.35-0.0068±0.00104 × Na intake (g/d)]. Despite the decrease in MUN with increased Na intake, UN excretion increased linearly with Na intake. Excretion of UUN was not affected by dietary Na content. A linear plateau relationship was observed between CCR and renal urea reabsorption. An increase in CCR coincided with an increase in calculated renal urea reabsorption until a CCR breakpoint value (mean ± SD) of 1.56±0.063 L/min was reached. We conclude that Na intake is negatively related to MUN, whereas UUN is not affected. Variation in mineral intake levels that affect urine volume should, therefore, be taken into account when using MUN as an indicator of UUN in dairy cattle.