Impact of oocyte donor age and breed on in vitro embryo production in cattle, and relationship of dairy and beef embryo recipients on pregnancy and the subsequent performance of offspring: A review.

Genomic selection combined with in vitro embryo production (IVEP) with oocytes from heifer calves provides a powerful technology platform to reduce generation interval and significantly increase the rate of genetic gain in cattle. The ability to obtain oocytes with developmental competence from calves has been recognised for more than 60years. There is renewed interest in the potential of this reproductive technology as the beef and dairy industries seek to identify and multiply animals that satisfy consumer demand for efficient utilisation of natural resources, with minimal environmental impact and high product quality. Heifer calves show a good follicular response to FSH stimulation and oocytes can be recovered by ovum pick-up (OPU). However, the efficiency of OPU/IVEP remains lower for calves compared with peripubertal heifers and cows, in both indicus (Zebu, Bos indicus ) and taurus (Bos taurus ) breeds. Indicus cattle generally have more follicles, yield a greater number of oocytes, and have a better IVEP outcome, compared with taurus cattle. The treatment of prepubertal heifers with bovine somatotrophin (bST) and propylene glycol before OPU/IVEP has yet to show a major improvement in embryo production. Holstein (taurus) dairy heifers derived from different reproductive technologies (AI, MOET, OPU/IVEP) showed a similar age at puberty and first-lactation milk production. OPU/IVEP Holstein embryos transferred to beef or dairy cows likewise yielded heifers with the same performance. The gains in productivity that can be achieved with strategic use of OPU/IVEP in heifer calves make this a relevant and highly important reproductive technology in cattle breeding. Ongoing optimisation of the technology is needed for the potential of OPU/IVEP in young donors to be fully realised.

Use of embryo transfer to alleviate infertility caused by heat stress.

Heat stress (HS) has a pronounced deleterious effect on fertility in dairy herds throughout the world, especially in hot and humid summer months in tropical and subtropical areas. Summer HS reduces feed intake and increases negative energy balance, induces changes in ovarian follicular dynamics, reduces estrus detection rates and alters oviductal function leading to fertilization failure and early embryonic death. Furthermore, oocytes harvested from lactating cows during summer HS have a decreased ability to develop to the blastocyst stage after in vitro fertilization when compared with oocytes harvested during winter. The present manuscript describes the detrimental effect of HS on reproduction, with emphasis on preovulatory oocytes and carry over effects of HS on embryo development and P/AI. Embryo transfer (ET) has been an effective tool to reestablish fertility during HS because it bypasses the damage to the oocyte and early embryo caused by hyperthermia. Therefore, a management strategy to maintain increased fertility throughout the year would be to produce embryos during the cooler months, when oocyte quality is greater, and use them to produce pregnancies during the periods of HS, when oocyte quality is compromised. However, this strategy only can be implemented using cryopreserved embryos, what is still limiting. During the warmer months, the use of heifers or non-lactating cows as oocyte or embryo donors may facilitate embryo production, mainly because of the lesser deleterious effects of HS comparing to lactating cows. Also, genetic selection of donors for thermoregulation ability is one potential strategy to mitigate effects of HS and increase embryo production during the warmer months. These alternatives enable the transference of fresh embryos with more efficiency during HS periods. Additionally, the application of timed ET protocols, which avoid the need for estrus detection in recipients, has facilitated management and improved the efficiency of ET programs during HS.

Bovine embryo transfer recipient synchronisation and management in tropical environments.

Numerous studies have shown that it is possible to manipulate follicular and luteal dynamics, thereby eliminating the need for oestrus detection in embryo transfer (ET) programmes. Fixed-time ET (FTET) protocols are based on the use of gonadotrophin-releasing hormone (GnRH) and prostaglandin (PG) F or progesterone/progestogen (P4)-releasing devices and oestradiol. The FTET protocols increases the proportion of recipients transferred, and therefore pregnancy rates, compared with the use of PGF followed by ET 7 days after oestrus. Furthermore, the addition of equine chorionic gonadotrophin (eCG) to the P4 and oestradiol-based FTET protocols results in an even higher proportion of recipients transferred, and thus higher pregnancy rates. The beneficial effect of eCG treatment may be related to increased growth of the dominant follicle and increased plasma P4 concentrations during the subsequent luteal phase. In Bos taurus x Bos indicus recipients, pregnancy rates were positively correlated with the diameter of the corpus luteum (CL) and the number of CL at ET. When repeat-breeder Holstein cows were used as recipients, FTET protocols increased number of recipients transferred and pregnancy rates compared with the traditional PGF-based synchronisation protocols. In conclusion, the use of FTET protocols eliminates the need for the detection of oestrus and results in a greater proportion of recipients transferred and satisfactory pregnancy rates. Thus, FTET optimises the use of recipients, reducing labour and animal handling and facilitating the use of ET.