Salt as a mitigation option for decreasing nitrogen leaching losses from grazed pastures

BACKGROUND: The main source of nitrogen (N) leaching from grazed pastures is animal urine with a high N deposition rate, particularly between late-summer and early-winter. Salt is a potential mitigation option as a diuretic to induce greater drinking-water intake, increase urination frequency, decrease urine-N concentration and urine-N rate, and thereby decrease N leaching. This hypothesis was tested in three phases; a cattle metabolism-stall study to examine effects of salt supplementation rate, a grazing trial to assess effects of salt (150 g heifer-1 day-1) on urination frequency, and a lysimeter study on effects of urine-N rate on N leaching. RESULTS: Salt supplementation increased cattle water intake. Urination frequency increased by up to 69%, with a similar decrease in urine-N rate and no change in individual urination volume. Under field grazing, sensors showed increased urination frequency by 17%. Lysimeter studies showed a proportionally greater decrease in N leaching with decreased urine-N rate. Modelling revealed that this could decrease per-hectare N leaching by 10-22%. CONCLUSIONS: Salt supplementation increases cattle water intake and urination frequency, resulting in a lower urine-N rate and proportionally greater decrease in urine-N leaching. Strategic salt supplementation in autumn/early-winter with feed is a practical mitigation option to decrease N leaching in grazed pastures.