RESUMO
The effects of high (2.5% of DM) versus normal dietary sodium chloride (NaCl) intake on renal urea-N kinetics and urea-N metabolism were investigated in 9 rumen-cannulated and multi-catheterized lactating dairy cows in a crossover design with 21-d periods. It was hypothesized that urinary urea-N excretion would be greater, and blood urea-N concentration lower in response to greater diuresis induced by high NaCl intake. Also, urea-N transport across ruminal and portal drained viscera (PDV) tissues was hypothesized to be affected by dietary sodium intake. A second experiment was conducted using 8 lactating cows in a crossover design with 14-d periods to test high NaCl (2.5% of DM) versus high KCl (3.2% of DM) intake on milk yield and milk urea-N concentrations. Experiment 1 showed that despite greater diuresis there was no effect of high NaCl intake on urinary urea-N excretion or blood urea-N concentration. The high NaCl intake did not affect rumen ammonia concentrations, total rumen VFA concentrations, ruminal venous - arterial concentration differences for ammonia, or ammonia absorption indicating that high NaCl did not adversely affect ruminal fermentation and microbial protein synthesis. High NaCl intake did not affect the total amount of urea-N transport from blood to gut, but ruminal venous - arterial concentration differences for urea-N were lower with high NaCl and ruminal extraction of arterial urea-N was numerically smaller, indicating that the ruminal epithelial urea-N transport was lower with high NaCl. Energy corrected milk yield was greater with high NaCl (3.2 ± 1.5 kg/d); however, milk urea-N concentrations were not affected by treatment. In experiment 2, ECM was greater with NaCl (1.4 ± 0.31 kg/d) compared with KCl (30.2 and 28.8 ± 0.91 kg ECM / d, respectively). Milk urea-N concentration was lower with KCl, suggesting a urea-N lowering effect in milk not evident with high NaCl intake. In conclusion, the present data show that dietary Na intake of 12-13 g/kg DM was followed by greater diuresis but did not impact urea-N excretion or blood urea-N concentration. High NaCl intake did not affect the total amount of urea-N transfer across PDV tissues. Energy corrected milk yield was greater with high NaCl compared with both control and feeding KCl, however, with KCl milk urea-N decreased.