Quantifying spatial variation in pasture and crop biomass can help to direct management practices and improve farm productivity, through accurate and informed movements of grazing rotations, crop and pasture nutrient management and yield production. Visual pasture assessments are often subjective, and require a degree of expert knowledge/experience. Quadrat-based plant harvesting determinations of biomass is labour intensive and costly when accurate quantification of pasture and crop variability over large areas is required. Furthermore, quadrats of 1m² or less are not easy to relate to remotely sensed images with spatial resolutions ranging from 4 to 400m². Active optical plant sensors that measure the red and near-infrared radiation reflected from the vegetation canopy offer the potential of deriving objective estimates of the photosynthetically-active plant biomass and may also hold the key to quantifying biomass over large areas with high spatial resolution. Crop Circle™ (Holland Scientific) is one example of an active plant sensor that measures red and near infrared light reflected from the canopy/soil matrix. In this paper we present the results of a project to assess the accuracy of Crop Circle™ for biomass assessment in a variety of agricultural environments including: native, improved and irrigated pasture; and forage and cereal crops.