Estimates of heterosis for in vitro embryo production using reciprocal crosses in cattle

In vitro embryo production and exploitation of heterosis are two methods of increasing productivity and accelerating genetic progress in many cattle production systems. However, it is not known if heterosis exists in bovine embryos produced in vitro. Tests for heterosis in in vitro embryo production were conducted in two experiments using reciprocal crosses. In the first, gametes from Bos taurus and Bos indicus were used; in the second, gametes from dairy and beef breeds of Bos taurus were used. In each experiment, both parental groups were used as sperm and oocyte donors, producing crossbred and purebred embryos. Oocytes obtained from abattoir-derived ovaries underwent in vitro maturation and in vitro fertilization with frozen semen. Embryos were cultured to blastocyst stage and observed. In the first experiment, higher (P < 0.05) rates of blastocyst formation were found for Bos taurus both as sires and as dams. Approximately 36% of the purebred Bos taurus oocytes and 21% of the purebred Bos indicus oocytes developed to blastocyst. Crosses averaged 16% resulting in a heterosis estimate of 45%. Ovaries from Bos indicus cows had more harvestable oocytes than did those from Bos taurus cows (P < 0.05). No evidence for heterosis was found for crosses within Bos taurus. Oocytes from beef cows had a higher rate of blastocyst formation than did those from dairy cows (30 vs. 24%, P < 0.05). These seemingly disparate results concerning heterosis were discussed in light of the period of genetic isolation of the parental populations in the two experiments.