As the global demand for animal products is rising, these should be produced with less environmental impact. Climate-induced shifts increase feed costs due to increased need for resources (land and water). However, protein is poorly utilized by ruminant livestock and 20-35% of ingested N is excreted via urine and faeces. Therefore, alternative feeds associated with reduce competition for land and constitute industry by-products, are required. Willow (Salix sp.) fodder is used in biofuel production but leaves and branches, up to 18mm diameter, are considered as waste. Willow fodder tree has a high protein content (well above from that required for livestock maintenance) and contains condensed tannins (CT) which bind to proteins, reduce their ruminal degradation and shift N excretion from urine to faeces (environmentally friendly N form). Although willows fodder shows good potential in animal nutrition, data on its nutritive value are scarce. The aim of this project is to evaluate the nutritive value of willow and assess its potential to improve N use efficiency in dairy cows; by also assessing its impact on milk quality. This will be achieved via the following studies: 1. In vitro assessment of the effects of willow CT on rumen digestive capacity. Different willow varieties will be screened using the ANKOM Gas Production System. Measurements include: gas production/composition, fermentation of end products (VFA/ammonia) (QUB), biochemical profile (QUB), and CT quantification and profiling (URE). 2. Optimization of the ensiling process. Two willow varieties with the most potent ammonia inhibitory properties (study 1) will be selected. Raw material will be harvested in two seasons and willow silage (WS), with or without inoculum, will be prepared and stored for 3 months. The most efficient ensiling method will be used for the production of fodder for the animal trials (QUB). 3. Assessment of the effect of WS inclusion in the diet of dairy cow and ammonia emissions. A dairy cow trial will be contacted at AFBI, including the following assessments: (1) Determination of optimum replacement rates of grass silage. The inclusion rates of 2 WSs will be gradually increased with an interval of 7 days until a maximum inclusion rate is determined. (2) Quantifying the effect of WS inclusion on feed intake, milk production, nutrient digestibility, energy/N utilisation efficiency and CH4 emissions. Twelve lactating dairy cows, in a 3(diets)*3 (periods) Latin-Squared design, will be blocked into 4 groups of 3 cows based on live weight, body condition score, parity, lactation stage and milk production. Cows will be randomly allocated, within each group, into 3 diets: control (no WS) and 2 diets (50% and 100% of the maximum level). Each period will include a feeding phase of 19 days in cubicle accommodation, followed by a 9-day digestibility/chamber measurements (6-day period in individual metabolism units plus a 3-day period in indirect open-circuit respiration chambers). Feed intake and faecal/urine outputs will be measured in the final 8 days, while gaseous exchange will be measured on the final 2 days in chambers. (3) Measurement of ammonia emission rate from slurry. A sub-sample of faeces/urine collected during the digestibility/measurement phase will be placed in respiration calorimeter chambers for 2 days to measure ammonia in two different temperatures (5, 200C; to simulate winter and summer temperatures). 4. Reveal how rumen microbes influence metabolic pathways related to N use efficiency and milk quality. Basic milk composition analysis (QUB), FA profiling (URE), metagenomics (QUB) and 500MHz NMR-based metabolomics techniques (URE) using samples from the animal trial will be used to identify rumen microbes, their genes, and relevant metabolic pathways, which promote N use efficiency and milk quality.