Odour emissions from livestock operations in Ontario have raised significant public concerns. In this study, downwind odour measurements were conducted during a 7-week period in July and August in 2004. The study involved taking measurements at two poultry farms, six dairy farms, and six pig farms in southern Ontario using both human assessor observed Nasal Ranger field olfactometers and an electronic nose developed in our research laboratory. The factors considered include animal species, distance to the odour source, and weather conditions (i.e. wind speed, temperature, cloud cover, and atmospheric stability). The collected data were studied and the relationships between individual factors and downwind odour intensity were analysed. In addition, the significance of the factors was investigated and ranked. The results show that downwind livestock farm odour should be considered as a complex system and it is suggested that multi-factor analysis be performed. The ranking of the significance of factors to odour intensity can improve the understanding of downwind odour systems and enhance the efficiency of odour evaluation and reduction techniques. Responses of the gas sensors in the electronic nose were compared with the perceived odour intensities, and good agreement was reported.