Reproductive performance of lactating dairy cows is a key profit generator for dairy producers. During the last 6 decades, as milk production has increased significantly, pregnancy per artificial insemination (P/AI) of lactating dairy cows has decreased. Moreover, high milk production is associated with the increased metabolic clearance rate of progesterone (P4) and estradiol due to greater feed intake and greater blood flow through the liver. Previous data from our laboratory and others have indicated that cows with the greatest circulating concentrations of P4 during ovulatory follicle development had the greatest P/AI. Furthermore, timed-AI programs that control follicle and corpus luteum (CL) development increase circulating concentrations of P4 during the growth of the ovulatory follicle and synchronize ovulation, and improve fertility of lactating dairy cows following timed-AI. The mechanism involved on the effects of levels of P4 during growth of the ovulatory follicle on fertility remains unclear. Our overall hypothesis of this dissertation is that treatments that increase serum concentrations of P4 during the growth of the ovulatory follicle increase P/AI and reduce pregnancy losses in lactating dairy cows. The three main objectives are: (1) determine the effect of three fertility treatments on serum concentrations of P4 during the growth of the ovulatory follicle and P/AI of lactating dairy cows; (2) develop a simpler fertility treatment for dairy operations limited by labor or other logistical constraints that cannot use currently fertility treatments; and (3) determine the effect of high vs. low P4 during ovulatory follicle development on fertility of lactating dairy cows. Research in Chapter 3 focused on the comparisons of fertility treatments that increase the percentage of cows with an accessory CL during the growth of the ovulatory follicle. We hypothesized that fertility treatments that use pre-synchronization with PGF2α and GnRH would increase: percentage of cows with an accessory CL at time of ovulatory follicle development, circulating levels of P4 and P/AI compared to pre-synchronization with only PGF2α. Results indicated that fertility treatments with pre-synchronization with GnRH were able to increase percentage of cows with accessory CL; However, P/AI was similar to a pre-synchronization with only PGF2α. Interestingly, enhancing P4 during growth of the ovulatory follicle was associated with enhanced PAI of in a large number of lactating dairy cows (~2500). In Chapters 4 and 5 experiments, we hypothesized that combining PGF2α and GnRH (PG+G) in a pre- synchronization strategy one week before Ovsynch would result in similar P/AI compared to other fertility treatments. Results indicated that our simpler pre-synchronization program had similar outcomes compared to fertility programs. Thus, it may offer a reasonable alternative to more complex fertility programs to enhance P/AI. The hypothesis of Chapter 7 study was that reduced circulating levels of P4 during ovulatory follicle would decrease P/AI and increase pregnancy loss during gestation. P4 was manipulated to reach high or low circulating concentrations during the pre-dominance phase and dominance phase of the ovulatory follicle. Low P4 during ovulatory follicle development increased double ovulation rate in lactating dairy cows. This resulted in a higher P/AI at d 23 post-AI. However, low P4 also created greater losses during post-attachment period to 56 days post-AI. Most of these losses post-attachment was primarily due to unilateral double ovulation/ twins.
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