Effect of sodium chloride intake on urea concentration in milk from dairy cows

A reliable indicator of nitrogen (N) excretion by dairy cattle is required to easily estimate N excretion on farm and to evaluate N excretion mitigation strategies. Milk urea nitrogen (MUN, mg/dl) has been shown to be positively correlated to excretion of urea and total N in urine in dairy cows. However, a significant proportion of variation in urine N-excretion (UN) remains unexplained by MUN content. In the present experiment, it was hypothesized that urine volume is affected by dietary salt intake and affects MUN content and the relationship between MUN and UN. Twelve lactating Holstein Friesian cows (milk production 25.4 ± 2.53 kg/d and 207 ± 41.3 DIM), of which 4 were fitted with catheters in the urine bladder, were randomly assigned to 4 dietary levels of salt (3, 9, 13, and 18 g Na/kg DM) in a 4 × 4 Latin square design. Cows were fed at 95% of ad libitum feed intake to ensure equal N-intake across dietary levels of Na. During the last 2 d of each one-week treatment period, milk was sampled and analyzed for MUN. Urine and feces of catheterized cows were collected quantitatively during the last 2 d of each treatment week. Urine was analyzed for total N and urea, and feces for total N and DM. Data were analyzed with the PROC MIXED procedure of SAS in which the blocking factors cow and period were included as random and fixed effects, respectively. Dry matter and N intake were 21.4 ± 1.24 kg/d and 522 ± 32.0 g/d, respectively, and equal across treatments. A significant negative linear correlation was found between intake of Na and level of MUN: MUN = 12.8 ± 0.44 - 0.70 ± 0.075 × 100 g Na/d. Based on the 4 catheterized cows, for every 100 g increase in Na consumption, a significant linear increase was found for urine production (13.7 ± 0.87 L/d), UN (5.3 ± 2.07 g/d) and urinary non-urea N excretion (4.2 ± 0.57 g/d). However, excretion of urinary urea N was unaffected (1.2 ± 1.62 g/d) by Na intake level. It is concluded that salt intake level affects MUN without an effect on urinary urea excretion. Level of salt intake should hence be considered when using MUN as an indicator of urinary urea excretion or UN.