Conservation agriculture (CA) has appeared in America since 1970 as an alternative practice to conventional tillage to limit soil degradation. Despite its expansion around the world, socioeconomic analyses of its adoption, as well as its impact on agricultural yields, still suffer from imperfect identification of CA adopters. The present study therefore proposes a new composite index for measuring the adoption of CA among maize and soybean farmers in the province of Québec, Canada. A model of partial adoption of CA both at parcel and farm levels is developed to build the composite index; and experts’ judgements and the Analytical Hierarchy Process are used for weight elicitation of principles of CA. Data from 144 maize and soybean farmers are also used to assess the level of adoption of CA in Québec. The new composite index improves on the measure of adoption of conservation agriculture, as it can be used to discriminate among farmers according to the level of adoption of principles of CA. Indeed, the new composite index shows that 77.08%, 21.53% and 1.39% of maize and soybean farmers, respectively, are partial adopters, full adopters and non-adopters of CA, whereas the traditional binary indicator indicates that 83.33% and 16.67% of maize and soybean farmers, respectively, are adopters and non-adopters of CA. The results also show that many maize and soybean farmers (38.89%) have shown a certain flexibility in the adoption of CA.