Fertility, a key measure of reproductive efficiency (RE), is an important component of overall herd profitability in modern dairy industry on global level. It depends on accurate heat detection, proper breeding time and EPD. Estrus detection failure and improper timing of insemination, however, are the main responsible components for compromised RE. Enormous reproductive management protocols have been designed for estrus synchronization in postpartum dairy cows. Each protocol has its own merits and demerits. The success of any commercial dairy farm is based in part on the calving interval of the cows. In order to maximize the economic profitability of the farm, cows must return to ovarian cyclicity, express estrus behaviorand be bred within 85 days postpartum to keep calving interval close to 365 days. Two major physiologic factors which influence reproductive success in the postpartum dairy cow are: ovarian cyclicity and uterine health.As parturition is very traumatic event and the ability to control ovarian and uterine events in the postpartum cow could play an important role in the subsequent fertility. Synchronization is a central part of successful production.Like synchronization, resynchronization in a suitable manner can also be a good management tool for optimal conception. Correspondingly, EPD is also a major requisite, as identification of nonpregnant animals is required for resynchronization or culling as per policy at the earliest. Two basic direct EPD methods, rectal palpation (RP) and transrectal ultrasonography are of fundamental importance. During RP, pregnancy can be diagnosed around day 35th post AI by only a skilled person. Although RP is widely used simplest and economical method, it may cause pregnancy termination. Ultrasonography is a valuable method which not only used for early PD, but also fetal viability and fetal sex as well. This method is cost effective; skill oriented and needs high degree equipment maintenance. In this scenario, there is still continues field demand and industry thirst for an early, precise and non-invasive EPD method. Acceptable method must have a high negative predictive value (NPV), economical, involved less technicalities. Progesterone profile, in a specific time frame, is another method with its own merits and constraints. The PAGs are other reliable EPD markers in ruminants detectable after the third week of breeding in cattle in both milk and blood.The main objective of the first experiment was to evaluate OVP0 (standard Ovsynch; n= 58) protocol with other two modified treatments OVP5 (n = 55) and OVP7 (n = 54) in postpartum dairy cows through the measurement of PR and plasma P4 concentration as a postpartum reproductive management tool. We hypothesized that the introduction of P4 insert (CIDR) in Ovsynch protocol for FTAI would enhance the wave synchrony and ultimately PR as compared to standard Ovsynch protocol. Briefly, cows in OVP0 group were given GnRH on d0 and d9 (d0 was the day of 1st GnRH). Seven days later to 1st GnRH all cattle were administered PGF2α analogue and were bred through FTAI approximately 16 h after the 2nd GnRH. In OVP5 group CIDR was inserted on d2 of GnRH (d0) while in OVP7 group CIDR insert was done on d0. In both groups CIDR was removed on d7. Size of pre-ovulatory follicles before AI was also measured through US and P4 profiles on d30 and d60 post AI. Pregnancy rate (PR) in OVP0 group was 39.7% on d30, 36.2% on d60 and 32.8% on d90 post AI in comparison with OVP5 (45.5% on d30, 43.6% on d60 and 41.8% on d90 post AI) and OVP7 (42.6% on d30, 37% on d60 and 37% on d90 post AI). Although, statistically non-significant (P = 0.48 on d30, 0.39 on d60 and 0.61 0n d90), overall improved PR was observed in OVP5 and OVP7 respectively. The overall pregnancy loss was lower in OVP5 (8.0%) as compared to OVP0 (17.0%) and OVP7 (13.0%) groups (P = 0.62). The size of pre-ovulatory follicles was 15.19 ± 0.17, 15.30 ± 0.21 and 15.24 ± 0.19 mm in OVP0, OVP5 and OVP7 groups respectively (P > 0.05). P4 profile was significantly (P < 0.05) higher in both OVP5 (7.75 ± 0.38 ng/mL) and OVP7 (7.58 ± 0.26 ng/mL) as compared to OVP0 (6.52 ± 0.32 ng/mL) group on d30 post AI. On d60 post AI, P4 profile was 6% higher in both P4 added groups (OVP5 and OVP7) as compared to OVP0 group. Statistically this difference was not significant in OVP0 (6.37 ± 0.49 ng/mL), OVP5 (6.75 ± 0.36 ng/mL) and OVP7 (6.80 ± 0.41 ng/mL). On the basis of results of experiment 1, it is concluded that the modified Ovsynch protocol can be used as a postpartum reproductive management tool for optimal PR. Results of current study have shown an advantage of P4 supplementation as a part of the Ovsynch protocol in cyclic animals either for 5 or 7 days as compared to standard Ovsynch protocol. This finding shows that there is some beneficial impact of P4 supplementation on luteal tissue competency.The main objectives of the second study were i) to examine the effect of standard CIDR-EB protocol in terms of PR and pregnancy loss in postpartum dairy cows, ii) to investigate the CIDR insertion on RESYNCH protocol for insemination of open lactating cows on d33 post AI without any delay for resynchronization purpose and to compare P4 concentrations before and after CIDR insertion as well as luteal tissue cross-sectional area in CONTROL and RESYNCH groups. This experiment was executed on two different geographical locations as CONTROL [(N = 134); (n = 70 location A; n = 64 location B)] and RESYNCH [(N = 144); (n = 90 location B; n = 54 location B)]. For P4 profile blood sampling was done on d14 (on the CIDR insert day), d16 and30 in CONTROL (n = 27) and RESYNCH (n = 30) group which was non-significant (P = 0.53) on d14 before CIDR insert in both groups and significantly higher in RESYNCH group on d16 (P = 0.004) and d30 (P = 0.0002) post AI. Luteal tissue cross sectional area was measured on d30 post AI which was significantly (P = 0.03) higher in CONTROL group. The pregnancy rate was non-significant on d30 post AI in CONTROL and RESYNCH groups (43 versus 48%; P = 0.05) while overall PR on d60 (44 versus 73%; P = 0.002) and d90 (43 versus 72%; P = 0.002) was significantly lower in CONTROL versus RESYNCH group on location A. Similarly, on location B, PR was numerically lower on d30 in CONTROL as compared to RESYNCH (44 versus 54%; P = 0.28) group and overall PR on d60 (53 versus 72%) and on d90 (50 versus 70%) was significantly higher (P = 0.033; 0.025) in RESYNCH group. Overall PR on both locations was 43 versus 50% (P = 0.26), 49 versus 73% (P = <0.001) and 46 versus 72% (P <0.001) in CONTROL and RESYNCH groups on d30, 60 and 90 pot AI respectively. The overall pregnancy loss was recorded as 12.5 versus 4.0 % (P = 027) in both groups. On the basis of results, in second experiment, standard CIDR-EB based synchronization protocol is a practicable estrus synchronization protocol for postpartum dairy cows which has resulted in similar fertility rate at different geographical locations. RESYNCH is, at the same time, also a viable protocol for early rebreeding in open cows after EPD to reduce the days open in lactating dairy cows. The main objectives of third experiment were, i) to compare G7G-Ovsynch (PG-GnRHGnRH-PG-GnRH) protocol with modified G7G-Ovynch (PG-GnRH-GnRH-PG-PG-GnRH) and standard Ovsynch protocols separately in postpartum Holstein cows, ii) to determine the pregnancy status by PAGs (milk, plasma), plasma progesterone and US as pregnancy diagnosis tools andto evaluate the effect of different factors like parity, Progesterone, fetal sex and sire effect on circulating PAGs in both milk and plasma. It was hypothesized that G7G-Ovsynch protocol with two PGs will improve PR and PAGs based EPD would be an alternate early pregnancy marker. In study one PR on day 31 was 43.3% and 50% (P = 0.87), on day 59, PR was 36.8% and 33.3% (P = 0.82) and on day 87, it was same as on day 57 post AI in G7G-Ovsynch (n = 19) and MG7G-Ovsynch (n = 18) protocol respectively. In study 2 PR on day 30 was 42 and 52% (P = 0.16), on day 60 post AI, PR was 37% and 45% (P = 0.23) and on day 90, PR was 36% and 44% (P = 0.28) in the Ovsynch (n = 110) and G7G-Ovsynch (n = 102) protocols respectively. In milk PAGs method sensitivity was 100%. Positive predictive value (PPV) was 92%. NPV was 100 % (P < 0.05). Sensitivity of plasma PAGs was recorded as 98% with 92% PPV and 83% NPV. Sensitivity of plasma P4 was 98% with 98% PPV and 89% NPV. There was a positive correlation among P4 profile and PAGs concentration in both milk (r = 0.460) and plasma (r = 0.421). A strong correlation between milk and plasma PAGs (r = 0.846) was also observed. Other parameters including BCS, cyclicity did show any impact on pregnancy, neither independently nor in interaction with synchronization protocol. Similarly, no effect of sire, fetal sex, parity and estrus synchronization protocol on milk or plasma PAGs was observed. In conclusion, although nonsignificant, increase in PR in G7G-Ovsynch makes it a protocol of choice in postpartum cows and PAGs, a viable and noninvasive method, as EPD tool either in milk or plasma is as feasible as US provided that exact corresponding breeding date is available. The common objective of the present thesis, all three experiments, was to compare various FTAI based estrous synchronization protocols in terms of PR with secondary objective of experiment 3 to evaluate PAGs as EPD method in postpartum dairy cows as reproductive management tool and on the basis of PR rates, it can be concluded that OVP5, G7G-Ovsynch and CIDR-EB based RESYNCH protocols are viable methods of postpartum reproductive management for early pregnancy establishment and PAGs are useful pregnancy markers for EPD." xml:lang="en_US