Manipulation of the ruminant gastrointestinal tract microbiomes for reduced environmental impact of nitrogen excretion from dairy cows

Abstract

Nitrogen-use-efficiency (NUE) in dairy production systems remains suboptimal, with higher nitrogen (N) excreted in urine and faeces rather than converted into milk protein. This thesis evaluated nutritional strategies to reduce N excretion and improve NUE without compromising animal performance, to examine the impact of these strategies on the rumen microbiome, while also assessing both local (rapeseed meal: RSM) and novel (microalgae) alternative protein feeds. Three complementary experiments were conducted: in chapter 2, lactating Holstein cows were offered diets with three CP levels: Low CP (15%, LCP), Medium CP (16%, MCP) and High CP (17%, HCP), and rumen fluid samples were collected at weeks 0, 4, 8, 12 and 20 post-calving. Reducing dietary CP from 17% to 15% had no adverse effects on intake, milk yield, milk composition and the rumen microbiome, both high-efficiency (HE) and low-efficiency (LE) cows exhibited distinct microbial composition. Functional genes related to amino acid biosynthesis and propionate metabolism were enriched in HE cows, while methanogenesis pathways were enriched in LE cows. Chapter 3 further developed this work by assessing the effect of RSM-based diets with different CP levels (140, 155, and 170 g/kg DM; R140, R155 and R170, respectively) and the inclusion of a conventional concentrate containing both SBM and RSM (155 g/kg DM; C155) on cow performance and the rumen microbiome. Dietary CP level and protein sources influenced production traits, the rumen microbial composition and functions. In chapter 4, an in vitro rumen fermentation approach was used to investigate the effects of substituting SBM and RSM with three microalgae species—Spirulina platensis (SP), Chlorella vulgaris (CLV), and Nannochloropsis gaditana (NG)—at 5%, 10%, and 20% inclusion levels. Microalgae inclusion had minimal effects on fermentation parameters, except CLV at 10 and 20%, which significantly reduced CH4 concentration.

Thesis is embargoed until 31 July 2029.

Date of Award	Jul 2026
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Department of Agriculture, Environment and Rural Affairs, FoodBioSystems DTP & John Thompson & Sons Ltd
Supervisor	Sharon Huws (Supervisor), Conrad Ferris (Supervisor) & Jon Moorby (Supervisor)