The biological variation in feed efficiency is regulated by multiple physiological mechanisms relevant to energy use in livestock species. The current study examined the associations between body composition, feeding behavior, linear body measurements and plasma metabolites with different measures of feed efficiency in growing heifers, finishing steers and mature pregnant cows. Our findings indicate that inclusion of body size measurements in prediction models of gain and intake improved the models’ accuracy and might account important differences related to eating capacity. Among the traits evaluated, feeding behavior possessed stronger associations with efficiency measures and displayed differences between efficient and inefficient animals. The associations between the traits studied herein varied across the efficiency measures used and beef cattle stage of production. Thus, selection criteria and performance evaluation based on efficiency measures should account for these traits, combined with animal’s stage of production and system’s outputs of interest.