Post-natal growth and development of Simmental calves derived from in vivo or in vitro embryos.

Large fetuses arising from embryos produced in vitro have been shown to exhibit altered organ development in utero, but it is not known whether this persists post natally. Post-natal growth and development was examined in 18 Simmental bulls derived from in vivo frozen-thawed (n = 6), in vitro frozen-thawed (n = 6) or in vitro fresh (n = 6) embryos and reared together post weaning on an ad libitum diet until slaughter at approximately 13 months old. Calves weighing less than 60 kg at birth (n = 11) were classified as normal, and heavier calves (n = 7; all from in vitro embryos) as oversize. Lifetime growth rates and slaughter weights apparently were unaffected by embryo source or birthweight. Mean (+/- s.e.m.) post mortem liver and kidney weights were unaffected by embryo source, but hearts of bulls from in vitro frozen embryos were heavier than those of bulls from in vivo frozen embryos (2.7 +/- 0.04 v 2.3 +/- 0.07 kg, P<0.025). Heart weight per kilogram body weight at slaughter for the 7 perinatally oversize males (4.01 +/- 0.08 g) exceeded that of the other 5 bulls from in vitro embryos (3.60 +/- 0.10 g kg(-1); P<0.04) and the 6 in vivo males (3.56 +/- 0.12 g kg(-1); P<0.02). Overall, one-third of the variation in heart weight at slaughter (r2 = 0.35; P = 0.01) was due to variation in birthweight. This is the first study to demonstrate birthweight-related developmental effects on post-natal organ weight following the transfer of embryos produced in vitro.