As production costs increase across the livestock industry, improving feed efficiency (FE) is one of the most crucial tasks for the beef industry to improve economic competitiveness relative to other meat-producing species. Substantial variation in FE may exist between individuals, yet the physiological mechanisms behind this variability are not well characterized. Furthermore, the industry would benefit from a more thorough understanding of the relationships between FE and other traits as well as an evaluation of the repeatability of FE across differing production phases and diet types. Consequently, this research sought to: 1) determine the influence of growing phase FE classification and diet type on performance of steers fed differing finishing phase diets, 2) determine effects of growing phase FE and diet, as well as finishing phase diet on diet digestibility and finishing phase FE, and 3) evaluate the relationship between FE, antioxidant activity and oxidative stress in feedlot steers representing phenotypic extremes for FE. It was hypothesized that relative FE was repeatable across feeding phases and that diet digestibility, antioxidant activity, and oxidative stress may be contributing factors to variation in FE between individuals. Completing the first objective, it was determined that steers classified as highly feed efficient (HFE) in the growing phase maintained greater finishing phase G:F and the relationship was also consistent for mid (MFE) and lowly feed efficient (LFE) growing phase FE classifications. Finishing phase G:F was not directly affected by growing or finishing phase diets but differences in finishing phase performance suggested that differences in finishing phase G:F between FE classifications were driven by differences in ADG among roughage-grown steers, versus differences in DMI that drove G:F variation among corn-grown steers. Additionally, the roughage growing diet and byproduct finishing diet combination appeared to be most advantageous, as those steers excelled in ADG, generating heavier carcasses with no decrease in G:F or marbling score. After completion of the second objective, it was determined that there were no differences in DM digestibility due to FE classification but fiber digestibility appeared to contribute to FE variation while starch digestibility did not. There was a positive correlation between growing and finishing phase diet DM digestibilities in steers fed similar diet types during both feeding phases, suggesting digestibility measured during one feeding phase may be indicative of digestive capacity during a subsequent phase if the diets are nutritionally similar. At the individual steer level, finishing phase G:F was greater in HFE versus LFE steers, though a negative correlation for G:F was detected between feeding phases when steers were roughage-grown and corn-finished. Finally, completion of the third objective revealed that antioxidant activity may play a role in FE as LFE steers, specifically roughage-grown LFE steers, had greater antioxidant activity than HFE steers, conceivably using a greater proportion of energy otherwise available for tissue accretion. Oxidative stress differences were predominately identified among the roughage-grown steers and in that group, HFE steers appeared to have a greater tolerance for oxidative stress than LFE steers as HFE steers had greater oxidative stress markers. Across the studies, G:F was repeatable from the growing phase to the finishing phase, thus growing phase FE appears to be a reasonable predictor of finishing phase FE. Variation in growth traits, diet digestibility, antioxidant activity, and oxidative stress markers were consistently detected between FE classifications, particularly between FE classifications within the roughage-grown groups.