Supplementation with willow containing condensed tannins shifted nitrogen excretion from urine to faeces in yearling ewes

Ruminants have poor nitrogen use efficiency, with 70% of total ingested nitrogen (N) excreted. Much of this excreted N is in the form of urinary urea which is easily hydrolysed to ammonia. Increasing ammonia production has negative impacts on environmental and human health. Dietary supplementation with condensed tannins (CTs) could be a solution as they bind to the N source (protein) in the rumen, preventing degradation, allowing more N to reach the abomasum for digestion and absorption. However, if this N is not absorbed, it results in a shift in N excretion from urine to faeces. The aim of this study was to evaluate the effect of willow containing CTs through a 21% DM dietary inclusion of two willow varieties (Salix Beagle: BG and Salix Terra Nova: TN) mixed with grass silage. Willow treatments: BG and TN had a CT concentration of 1.20 and 0.08% DM, with the control (SIL) containing no CTs. Twelve replacement yearling ewes were used in a 3 (treatment) × 3 (period) Latin square design experiment with a single period lasting 28 days and the overall experiment lasting 12 weeks. Structural arrangement of CTs showed BG had a greater (P < 0.001) proportion of prodelphinidin (62.7%) compared to TN (5.42%). The higher CT-containing treatment (BG) showed a 13 and 12% lower absorbed N-to-N intake ratio relative to SIL (P < 0.001) and TN (P < 0.01), respectively. There were no differences in the proportion of retained N to N intake among feed treatments (P = 0.44). BG had a 22 and 17% greater faecal N to N intake ratio relative to SIL (P < 0.001) and TN (P < 0.01), respectively. BG led to a 19% higher nitrogen excretion in faeces and a 19% lower nitrogen excretion in the urinary form compared to SIL (P < 0.01). However, BG reduced (P < 0.01) the nutrient digestibility, compared to SIL and TN. This resulted in the ratio of digestible energy to gross energy intake to be 8 and 6% lower for BG relative to SIL (P < 0.01) and TN (P < 0.01), respectively. Feed treatments had an effect on blood and urine metabolites involved in protein metabolism. Isobutyrate was 17 and 28% higher in BG and TN relative to SIL (P < 0.01). While in urine, creatinine was 61% lower in TN relative to BG (P < 0.05). CTs from the willow Beagle variety could redirect nitrogen excretion from urine to faeces, offering a promising strategy to reduce ammonia emissions and mitigate the environmental footprint of the ruminant industry.