Number of oocytes retrieved per donor during OPU and its relationship with in vitro embryo production and field fertility following embryo transfer

The association of OPU-IVEP is an important instrument to drive genetic progress. In vitro embryo production (IVEP) has remarkably expanded in the last decade compared to in vivo embryo production. Because of the high repeatability of oocyte retrieval within oocyte-donors, studies exploring the relationship between the number of oocytes recovered per OPU section with IVEP efficiency, as well as with field fertility (pregnancy results following embryo transfer; P/ET) are extremely important to guide cow-donor selection and optimize field reproduction efficiency and the herd's genetic gain. Based on this rationale, our group conducted a retrospective analysis of a large database comprising IVEP records from several cattle breeds, including Bos indicus and Bos taurus for either beef or dairy purposes. A total of 205,140 oocytes recovered from 7,906 OPU procedures of 6,902 donors (5,227 beef and 1,675 dairy) of Brazilian farms were analyzed. Beef breeds analyzed were Nelore (Bos indicus) and Senepol (Bos taurus) and dairy breeds were Gyr (Bos indicus) and Holstein (Bos taurus). According to our analysis, the IVEP in beef cattle had a great improvement throughout the last years, with a remarkable increase in numbers of pregnancies per OPU compared to late 90' s (averaging only 1 pregnancy per OPU in 1998 vs 2,4 in 2014). As for the distribution of oocytes retrieved, both Bos indicus beef (Nelore = 27.2) and dairy (Gyr = 23.8) breeds seem to yield greater average numbers of oocytes per OPU compared to Bos taurus (Senepol = 21.8; Holstein = 19.3). Despite these differences across genetic groups, outstanding donors can be found in all breeds and the number of oocytes retrieved per donor seems consistent across time. For both beef cattle breeds studied, it appears that number of oocytes retrieved at OPU had a negative but minor effect on both cleavage and blastocyst rates, especially for Senepol breed. Conversely, in dairy breeds the number of oocytes recovered per OPU had essentially no effect on cleavage rates, but we captured a trend for lower blastocyst rates with greater numbers of oocytes per OPU. For both, beef and dairy breeds the number of blastocyst per OPU was greater when higher number of oocytes were recovered per OPU, regardless of genetic group. Pregnancy rate following ET in Nelore breeds was lower in donors with greater amounts of oocytes retrieved per OPU. In contrast, in the Senepol breed and both dairy breeds (Gyr and Holstein) pregnancy rates after ET seems to increase when the number oocytes recovered per OPU increases. In addition, the semen utilized had a major impact of IVEP efficiency: top ranking sires yielded outstanding blastocyst rates, while poor performers produced very low blastocyst rates. The season of the year also had effect on IVEP, with Bos indicus breeds showing less variation in IVEP results throughout the year. In conclusion, despite the evolution of IVEP in the last two decades, the number of oocytes recovered per OPU had a minor effect both on blastocyst rate and pregnancy rates after ET. However as more oocytes are collected, the number of produced blastocysts improves. Thus, it seems important to identify donors with greater oocyte recovery-per-OPU potential, especially in cattle breeds yielding fewer oocytes per OPU, such as Holstein, to assure greater IVEP efficiency. It is also clear that cattle breed, semen used during IVEP and season of the year can potentially influence IVEP and field fertility results. A holistic approach controlling the quality of the performed OPU, consistency in lab routines, as well as selecting donors with high genetic value (through genomics) and greater oocyte population (through AMH assays or ultrasound) are highly advisable.