The objective of the present study was to determine the effect of resynchronization with either GnRH or P4 on Day 23 (controlled internal drug release device containing progesterone; CIDR) on pregnancy rate, cumulative pregnancy, and embryonic and fetal losses in CIDR-GnRH synchronized Nili-Ravi buffaloes. Buffaloes (n = 181) of mixed parity, lactating, 181 ± 73 days postpartum, a body condition score (BCS) of 3.2 ± 0.5 (scale of 1-5), and 450-600 kg weight were subjected to synchronization and resynchronization. All buffaloes received CIDR on Day -9.5. In addition, GnRH was injected 36 h after CIDR removal, and fixed-time artificial insemination (FTAI) was performed 18 h later (Day 0). On Day 23, buffaloes were randomly assigned to receive one of the following treatments: 1) CON (n = 63), 2) P4 (n = 55), and 3) GnRH (n = 63) for resynchronization (2nd AI). Pregnancy rate and embryonic and fetal losses were monitored by serial ultrasonography on Days 30, 45, 60, and 90 after synchronization (1st AI), respectively. The pregnancy retention rate in GnRH-treated buffaloes remained significantly and consistently higher (P < 0.05) than in the CON group at Days 30, 45, 60, and 90 after 1st FTAI. Based on the pregnancy diagnosis, on Day 30 post 1st AI, buffaloes in the CON, P4, and GnRH groups received: 1) Artificial insemination on detected estrus (AIDE; n = 37), 2) CIDR-GnRH protocol (CIDR; n = 27), and 3) Ovsynch protocol (OVS; n = 23), respectively. The pregnancy rate in resynchronized buffaloes did not differ (P > 0.05) between the OVS and CIDR groups, whereas the cumulative pregnancy rate in GnRH + OVS buffaloes (81%) after the 1st and 2nd FTAI when determined on Day 64 was higher (P < 0.05) than that in CON + AIDE (59%) buffaloes. The embryonic losses were significantly lower (P < 0.05) in GnRH-treated (18%) buffaloes than in CON (42%) buffaloes on Day 45 post 1st AI. Fetal losses were fewer and did not differ (P > 0.05) due to treatments on Day 60 or 90 post- 1st AI. In conclusion, 1) the
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pregnancy rate and cumulative pregnancy rate in GnRH + OVS buffaloes were higher than in CON + AIDE buffaloes when determined on Day 64 after synchronization and resynchronization and 2) embryonic and fetal losses were lower in GnRH-treated buffaloes than in CON buffaloes when determined from Day 31 - 90.