Comparisons between nulliparous heifers and cows as oocyte donors for embryo production in vitro

The aims of the present study were to compare (1) Holstein-Friesian heifers versus early postpartum lactating cows, and (2) different age categories of crossbred beef heifers versus cows, in terms of oocyte yield, morphological quality and developmental competence. Four experiments were designed to test the associated hypotheses. In Experiment 1, ovum pick up was carried out twice weekly for a period of 5 weeks on Holstein-Friesian heifers (n = 8) and early postpartum cows (n = 8). Oocytes were submitted to in vitro maturation (IVM), fertilization and culture. Significantly more follicles were punctured on the ovaries of heifers than cows (10.4 versus 7.8, P < 0.001). This was reflected in a significantly higher number of total oocytes (4.7 versus 2.8, P < 0.001) and grade 1-2 oocytes recovered/animal from heifers than from cows (3.0 versus 1.8, P < 0.05). There was no significant difference in the percentage of oocytes cleaving after fertilization, or in the percentage reaching the blastocyst stage between heifers and cows. In Experiment 2, oocytes were obtained by manual aspiration from the ovaries of slaughtered crossbred beef heifers (under 30 months, n = 1241) and cows (over 4 years old, n = 1125), and processed in vitro as above. No significant difference was observed between the two groups in terms of the number of aspirated follicles or oocytes recovered. A significantly higher proportion (P < 0.01) of cow oocytes than heifer oocytes reached the blastocyst stage (Day 8: 46.5% versus 33.4%). In Experiment 3, ovaries were separated according to age of heifer into three groups: (1) 12-18 months, (2) 19-24 months and (3) 25-30 months, and compared with cow oocytes. There was no significant difference in the blastocyst yield between the different age groups of heifers. Irrespective of heifer age, the blastocyst yield on Day 8 was significantly lower than that from cow oocytes (35.0, 35.2, 36.5 and 48.3%, respectively, P < 0.05). In Experiment 4, a significantly higher proportion (P < 0.001) of presumptive zygotes derived from abattoir-derived cow oocytes reached the blastocyst stage following culture in vivo in the ewe oviduct than those derived from heifer oocytes (Day 8: 53.1% versus 25.2% for cow and heifer oocytes, respectively). In conclusion, the origin of the oocyte has a significant impact on its subsequent developmental potential. These results would suggest that in an in vitro production system, cow oocytes should be preferentially used over those from heifers in order to maximize blastocyst development.