The effects of postpartum prophylactic oral Ca supplementation on the first Dairy Herd Improvement Association (DHIA) test milk yield, first-service conception and days open within 150 d in milk (DIM) were evaluated on 1,095 multiparous Jersey and Jersey × Holstein cows from 2 commercial herds. After calving, cows were systematically assigned to control (no oral Ca supplementation; n = 553) or oral Ca supplementation (CaOS; 50 to 60 g of Ca as boluses; QuadricalMINI, Bio-Vet Inc., Barneveld, WI; n = 542) at 0 and 1 DIM. Blood samples for serum Ca determination were collected before 1st treatment administration for a subset of cows (n = 768). Productive outcomes and days open within 150 DIM were analyzed by ANOVA with the MIXED procedure, with herd as a random effect and first service conception was analyzed by logistic regression with the GLIMMIX procedure of SAS. Additional variables considered for inclusion in the final model were previous lactation milk yield and DIM, gestation length, dry and close-up period lengths, body condition and locomotion scores at calving and calving easiness. There was a dry period length × treatment (P = 0.006) and a tendency for a DIM at 1st test × treatment (P = 0.08) interaction for 1st test milk yield. For cows with a dry period length larger than the herd mean (60 and 73 d for Herd 1 and 2, respectively), CaOS produced 1.6 kg of milk more than control cows (P < 0.05) whereas for cows with a dry period length shorter than the herd mean, CaOS produced 0.9 kg of milk less than control cows (P = 0.05). When the 1st DHIA test occurred > 15 DIM, CaOS tended to produce 1.1 kg more of milk than control cows. No significant effect of treatment was observed on reproductive outcomes nor interactions with serum Ca were observed in the preliminary analysis. Our results suggest that the response to postpartum oral Ca supplementation may vary according to different peripartum factors and time relative to treatment administration, but further analysis is required to define these interactions.