Feeding salicylates containing willow leaves to cattle modulates urea metabolism and mitigates urine-derived ammonia and nitrous oxide emissions from soil

Ruminants on grazing lands have a great impact on ammonia (NH₃) and nitrous oxide (N₂O) emissions released from livestock production. Willow leaves are an established supplement in ruminant nutrition and are rich in salicylates and tannins, which may have a mitigating effect on NH₃ and N₂O emissions. We hypothesised that willow leaf supplementation in cattle nutrition affects nitrogen (N) and urea metabolism and mitigates urinary NH₃ and N₂O emissions from soil. Eight weaned Holstein bull calves were kept on pasture and supplemented with willow leaves or alfalfa hay in a crossover design. In a respiration chamber, feed intake, faeces and urine excretions were recorded and analysed for total N and N-metabolites. Urea-N recycling was measured by the intravenous administration of a ¹³C urea tracer and a series of blood sampling. Cattle urine and artificial mimics supplemented with different salicylates were incubated with standard soil to measure NH₃ and N₂O and the N and O isotopic signatures. Despite a decline in urea turnover and N digestibility in rations supplemented with willow leaves, the leaves had no effect on microbial protein synthesis or the growth rate. Urine excretions with reduced urea but increased hippuric acid, phenolic acids, and salicylate concentrations in cattle fed willow leaves mainly inhibited bacterial denitrification processes involved in N₂O release from soil and mitigated NH₃ and N₂O emissions by 14 and 81 %, respectively. The results highlight intrinsic and extrinsic mechanisms that define both the nutritional significance and emission mitigation potential of supplementing cattle in pastures with willow leaves.