Anabolic steroid implants are commonly used in the beef industry to increase BW gain in feedyard cattle. However, there are a limited number of implants approved for use in replacement heifers. Objectives were to use growth-promoting implants to add value to low-BW heifers, and to determine the relationships among hormones and cow efficiency. Charolais × Balancer heifers (n = 65; 179 ± 30 kg; 255 ± 12 d of age) were used to determine the influence of androgenic and(or) estrogenic implants on serum concentrations of insulin-like growth factor-I (IGF-I), prolactin (PRL), cortisol (CORT), and reproduction. Heifers were blocked by BW and assigned to 1 of 4 implant treatment groups: 1) control, no implant (CON; n = 16); 2) trenbolone acetate (TBA; 200 mg of TBA; n = 15); 3) trenbolone acetate plus estradiol (TBA+E2; 40 mg TBA and 8 mg E2; n = 17); or 4) zeranol (ZER; 36 mg ZER; n = 17). Heifers were implanted on d 0, and blood samples collected on d 0 (15 Nov), 106 (1 Mar), and 195 (29 May) of the experiment. Heifers were AI on d 220 and exposed to an Angus bull for 28 d starting 12 d after AI. Calf birth date, and birth weight were recorded. At weaning, dam and calf weights were determined and cow efficiency was calculated by dividing calf adjusted 205 d weight by dam weight at weaning. Treatment did not influence (P > 0.10) calving rate (41.5%), Julian calving date (96.8 ± 2.5 d), cow efficiency (0.47 ± 0.23), concentrations of IGF-I (78.4 ± 3.9 ng/mL), PRL (83.5 ± 5.81 ng/mL), CORT (36.3 ± 9.9 ng/mL), or PRL:CORT (ratio = 2.19 ± 0.24). Serum PRL and IGF-I increased (P < 0.01) from d 0 to 195. Serum cortisol increased (P < 0.01) from d 0 to 106, and concentrations were similar from d 106 to 195. Multiple regression analyses indicated PRL and IGF-I at d 195 described 30% of variation in adjusted 205 d calf weights; 35% of variation in cow weight at weaning was explained by PRL at d 0 and 106; and PRL:CORT at d 0 and IGF-I at d 195 accounted for 44% of variation in cow efficiency. Anabolic implants did not affect heifer reproduction. Serum PRL, CORT, and IGF-I of heifers at weaning, yearling, and pre-breeding may serve as biomarkers of cow efficiency.