Prime Eggs from Prime Cows

The selection of the best dairy heifers is mainly driven by the genetic value of their parents. The phenotype analysis of cows and the daughters of bulls have been used to identify the best genetic value for decades before being replaced by genomic selection of individuals that are not even parents yet. Since it is possible to predict the future value of an individual by its genetic make-up, it becomes feasible to do it as early as the blastocyst stage and to decide which should be implanted or not. Since we know at birth (or before) the genotype of an animal, it is becoming interesting to reproduce these as soon as possible to reduce generation interval and improve selection speed. Nature provides constraints that can be overcome, number of oocyte per cycle and age at puberty. Indeed it is now possible to super-stimulate the ovary at any age and to collect oocyte starting at 6 months by trans-vaginal ultrasonography. The challenge becomes the production of good eggs and embryos capable of implanting and leading to healthy calves. Our understanding of ovarian follicular physiology has been instrumental in designing stimulation protocols that may be adjusted to any physiological context including age and even any individuals to obtain a good response. Therefore, the combination of procedures developed in cows to optimize oocyte quality (FSH coasting) in association with IVF and optimal culture conditions can now results in the production of several female embryos twice a month from 6-12 months of age. The understanding of the chromatin condensation status of the oocyte and the timing of it seems a key factor to ensure further development. The transcriptomic and epigenetic analysis of embryos produced form the same females at different ages were compared and few differences were noted in particular in relation to the animal metabolism. These embryos are as good as the ones from adult animals and can be produced with sexed sperm of bulls of 12 month of age. Such combination permits the production of a complete second generation of 50 females before the mother deliver its first calf.