Abstract
Ruminants are important for global food security because of their ability to convert human-inedible plant matter into nutrient dense meat and milk, thereby complementing protein and micronutrient provision from arable crops. Ruminant products also contain essential fatty acids (FA) that are generally deficient in Western diets. However, there are legitimate concerns with ruminant production, particularly with regards to enteric methane emissions that contribute to global warming. Furthermore, as with arable farming, livestock farming is threatened by climate change due to increased water requirements to counter heat stress, increased frequency of drought and reduced water quality. Small ruminants, such as sheep and goats, have lower water requirements than cattle, and they arealso more tolerant to extreme conditions. As such, they will play a more significant role in global livestock agriculture in the future. This thesis therefore investigates water intake and water use in small ruminant metabolism, an understanding of which is essential for future food security and sustainability.
Hydrogen (H) is central to ruminant metabolism, and H fluxes in the rumen are critical for rumen function, and therefore animal health and productivity. Molecular H (H2) is produced during the microbial fermentation of dietary carbohydrates, and methanogenesis is an important H2 sink, allowing fermentation to continue. Biohydrogenation (BH), whereby plant poly-unsaturated FA (PUFA) are saturated by rumen bacteria, has been proposed as an additional H+ sink. Animals have two sources of H: water and the diet. Understanding the role of water-derived H in these fundamental metabolic processes and the relative contributions of water- and feed-derived H to ruminant FA could lead to novel methods to improve productivity and reduce the climate impact of ruminant production. Understanding the contribution of water-derived H to ruminant FA will also help calibrate a novel palaeoclimate proxy based on the stable H isotope composition of ruminant FA preserved in archaeological pottery.
As such, this thesis adopts a stable isotope approach using 2H-labelled drinking water (DW) to study body water dynamics and the incorporation of water-derived H into rumen and body tissue FA in sheep. Fifteen Lleyn ewes were fed at maintenance on a dry forage diet and given ad libitum access to one of three isotopically distinct DW. The DW intakes were recorded daily and blood, urine and
rumen fluid samples were collected for isotopic analysis, allowing body water half-life to be determined. Compound-specific isotope analysis (CSIA) of the H in feed, rumen and faecal FA allowed a mass balance equation to be constructed to investigate the sources of H in BH in vivo, building on previous in vitro studies. This revealed that the only H sources are the dietary PUFA and rumen waterderived H, suggesting that BH is not a H2 sink in the rumen. Molecular analysis and CSIA of serum, adipose and muscle FA demonstrated that the sources of circulating FA include absorption from the gastrointestinal tract and mobilisation from adipose. However, 2H-enrichment of serum FA was greater than both of these sources in the two 2H-enriched DW groups, leading to the suggestion that the lipogenic activity of the liver may be more significant than is often considered in ruminants fed at maintenance. Finally, CSIA of the FA in all sample types revealed that the C16:0 FA has the largest response to DW isotopic composition, suggesting that this FA should be targeted for palaeoclimate reconstruction. The results from this thesis improve understanding of H fluxes in ruminants and could lead to novel strategies to enhance the nutritional value and sustainability of ruminant products.
| Date of Award | 30 Sept 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Mélanie Roffet-Salque (Supervisor), Michael Lee (Supervisor) & Daniel Enriquez Hidalgo (Supervisor) |
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