This field study investigated whether the administration of a single dose of gonadotropin-releasing hormone (GnRH) to dairy cows without a corpus luteum (CL) 4 weeks after calving can improve reproductive performance. Holstein dairy cows underwent ultrasonography to assess the presence of ovarian structures at 29.2 ± 5.2 days post-partum, and cows were divided into two main groups based on the presence (CL group, n = 230) or absence (non-CL group, n = 460) of a CL. The non-CL group was further randomly divided into two subgroups based on the administration of GnRH (non-CL GnRH group, n = 230) or no GnRH (non-CL control group, n = 230). Subsets of cows from non-CL control (n = 166) and non-CL GnRH (n = 175) groups received a second ultrasonography at 44.5 ± 5.4 days post-partum to assess CL formation. The percentage of cows with CL at the second ultrasonography was greater in the non-CL GnRH group (70.9%) than in the non-CL control group (53.0%, p = 0.0006). The hazard of the first post-partum insemination by 150 days in milk (DIM) was higher in the CL group than in the non-CL control group (hazard ratio [HR]: 1.36, p = 0.001). The probability of a pregnancy to the first insemination was higher in non-CL GnRH (odds ratio [OR]: 1.50, p = 0.04) and CL groups (OR: 1.55, p = 0.03) compared to the non-CL control group. Furthermore, the hazard of pregnancy by 210 DIM was higher in non-CL GnRH (HR: 1.30, p = 0.01) and CL (HR: 1.51, p = 0.0001) groups than in the non-CL control group. In conclusion, administration of GnRH to dairy cows without a CL 4 weeks after calving was associated with an increase in ovulation and improved reproductive performance.