To determine the effects of administration of a low dose of slow-release bST (Posilac, Elanco, Greenville, IN) on hormone concentration and conceptus development, a total of 190 suckled beef cows were exposed to the 7-d CO-Synch+CIDR fixed-time AI (TAI) protocol. Cows were blocked by days postpartum, BCS, breed and randomly assigned to receive one of the following treatments: (1) 2 injections of 325 mg bST, one at TAI and a second injection 14 d after TAI (D-bST, n = 40); (2) 1 injection of 325 mg bST at TAI and a placebo (saline) injection 14 d after TAI (TAI-bST, n = 48); (3) a placebo injection at TAI and one injection of 325 mg bST 14 d later (14D-bST n = 49); and (4) 2 injections of placebo, one at TAI and a second injection 14 d after TAI (Ctrl, n = 53). Pregnancy was determined via transrectal ultrasonography 35 d after TAI and conceptus development was assessed by measuring crown to rump length (CRL) on d 35 and crown to nose length (CNL) on d 65 after TAI. Blood samples were collected on d 0, 7, 14, 21, 35 and 65 relative to TAI to determine concentrations of and IGF-1. Plasma concentrations of pregnancy-specific protein B (PSPB) were also assessed on d 35 and 65 after TAI. Individual calf birth weight and sex were determined at birth. Procedure GLIMMIX of SAS was used to analyze all data with the appropriate models. There were no differences (P = 0.767) among treatments on pregnancy to TAI (48.7 ± 0.5%). Administration of bST at TAI increased (P < 0.01) plasma concentration of IGF-1 on d 7, 14 and 21. However, CRL and CNL (12.11 ± 0.4 mm and 17.09 ± 0.2 mm, respectively) did not differ (P = 0.231) among treatments. Concentration of PSPB did not differ (P = 0.18) among treatments and between days (P = 0.30; 2.69 ± 0.11 ng/mL), and gestation length (282 ± 9 d) did not differ (P = 0.49) among treatments. In addition, calf birth weight was similar (P = 0.52) among treatments. We conclude that administration of 325 mg bST during the time of TAI to suckled beef cows enhanced concentrations of IGF-1, but failed to improve pregnancy rates, fetal size, PSPB concentrations, and had no effect on calf birth weight.