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

Authors: THOMPSON, JOSH - Queen'S University Belfast; CARBALLO, CRISTOBAL - Agri-Food And Biosciences Institute; YAN, TIANHAI - Agri-Food And Biosciences Institute; Zeller, Wayne; HUWS, SHARON - Queen'S University Belfast; SAFOI, L. - Agri-Food And Biosciences Institute; SOUTHAM, ANDREW - University Of Birmingham; LUDWIG, CHRISTIAN - University Of Birmingham; LLOYD, GAVIN - University Of Birmingham; STERGIADIS, SOKRATIS - University Of Reading; THEODORIDOU, KATERINA - Queen'S University Belfast

Submitted to: Animal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/27/2025
Publication Date: 11/5/2025
Citation: Thompson, J., Carballo, C., Yan, T., Zeller, W.E., Huws, S., Safoi, L., Southam, A.D., Ludwig, C., Lloyd, G.R., Stergiadis, S., Theodoridou, K. 2025. Supplementation with willow containing condensed tannins shifted nitrogen excretion from urine to faeces in yearling ewes. Animal. https://doi.org/10.1016/j.animal.2025.101698.
DOI: https://doi.org/10.1016/j.animal.2025.101698

Interpretive Summary: Ruminants can suffer from poor nitrogen use efficiency (NUE) with 70% of total ingested nitrogen (N) excreted. Much of this excreted N is in the form of urinary urea, which is easily converted to ammonia leading to negative impacts on environmental and human health. Dietary supplementation with plants containing condensed tannins (CTs) may offer a nutritional solution to the problem of poor NUE. CTs are known to bind to the protein in the rumen preventing degradation allowing more N to reach the hindgut for uptake towards maintenance and growth improving efficiency. However, if this N is not absorbed in the hindgut, it results in a shift in N excretion from urine to feces. This study tested the effect of CTs through a 21% dry matter (DM) dietary inclusion of two willow varieties mixed with grass silage. The variety of willow treatments, Beagle (BG) and Terra Nova (TN), had an overall CT inclusion of 1.20 and 0.09% DM with the control (SIL) diet containing no CTs. The higher CT-containing treatment (BG) had a lower absorbed N to N intake ratio relative to SIL and TN. Despite this, there was no differences in the proportion of retained N to N intake between all feed treatments. For N excretion, BG had a greater fecal N to N intake ratio relative to SIL and TN. While proportion of urinary N to N intake was different by treatment and tended to be lower for BG and TN relative to SIL, no significant interactions occurred between treatments. Overall, no differences in the proportion of total manure N (fecal N + urinary N) to N intake were observed between feed treatments. In terms of N excretion apportioned to either feces or urine relative to total manure nitrogen excretion, BG had increased amount of nitrogen excretion in feces equal to the decrease in nitrogen excretion observed in the urinary form relative to SIL. However, BG was responsible for significant reductions in nutrient digestibility relative to SIL and TN. This resulted in the ratio of digestible energy to gross energy intake to be lower for BG relative to SIL and TN. Therefore, at an inclusion of 1.20% CTs from BG could result in a shift in nitrogen excretion from urine to feces with potential to minimize ammonia production and mitigate against the negative environmental impact of the ruminant industry but, as a trade-off, mildly reduces energy efficiency.

Technical Abstract: 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.