1. Herbivores adopt foraging strategies to maximize efficiency in diverse, resource-constrained environments. However, the effectiveness of these strategies may be more constrained by their capacity for energy cost rather than their ability to acquire resources. The swift utilization of resources during acquisition is crucial for optimizing energy conversion efficiency in animals. Nonetheless, the energy expended in this process inherently limits food conversion efficiency (FCE), an aspect that remains insufficiently explored in current research. 2. In this study, we introduced a concept framework that integrates harvest rate (HR) and energy expenditures (EE) into evaluating herbivore FCE. Utilizing high-resolution tri-axial accelerometry within a grazing treatments platform, we analyzed the behaviors of herbivores (Ovis aries) to determine the energy costs and time allocation for both lamb and dry ewe groups. 3. Our analysis demonstrated an inverse correlation between HR and EE, exhibiting both positive and negative influences on FCE. Notably, the impact of EE was more pronounced in larger-sized grazers (dry ewes), while HR significantly influenced smaller-sized grazers (lambs). However, the interaction effects between these variables tended to neutralize the variations in FCE observed across both groups. 4. Our research highlights how the behavioral patterns of grazers, in terms of resource acquisition and relative energy costs, are pivotal in determining resource utilization efficiency. Additionally, it reveals the trade-offs in these behaviors, which transition from being beneficial to restrictive as the body grows. This finding substantiates the theory that the behavior of herbivores is a reliable predictor of their efficiency in resource utilization.