His PhD concerned the use of sewage sludge as a fertiliser for grassland and involved both field experimentation and chemical analysis. As a post-doctoral research assistant, he extended his empirical experience to forest hydrology and modelling. Following his first permanent appointment in 1984, he initiated both empirical and modelling research on the production and nitrogen flows in grazing livestock systems. Since moving to Denmark in 1995, his research has exclusively been based on farm-scale modelling, initially focusing on ammonia emission from livestock production systems. Latterly, this focus has extended to all aspects of the farm nitrogen cycle losses, to greenhouse gas emissions, and to the balance between crop and animal production and environmental pollution. This means he has a broad understanding of the processes ongoing in livestock, animal housing, manure storage, crops and soils.

He has been a member of the UNECE Task Force on Emissions, Inventories and Projections since 2004 and is co-chair of Expert Panel on Agriculture and Nature. As such he shares responsibility with his two co-chairs for the chapters in the EMEP/EEA Air Pollutant Emission Inventory Guidebook that relate to agriculture. He has been a member of the UNECE Task Force on Reactive Nitrogen since 2007. He also retains his special interest in grassland, and is a member of the British Grassland Society.