An experiment zoas conducted to assess differing methods of twin pregnancy establishment in Hereford × British Friesian beef cows and heifers. The experiment was 2 × 2 × 2 × 2 factorial design in which the factors were (i) source of embryos (in vivo or in vitro produced); (ii) pregnancy status of recipient (inseminated or non-inseminated); (Hi) method of embryo transfer (surgical or cervical); and (iv) uterine location of a native and transferred embryo, or two transferred embryos (both located in the ipsilateral, or one in each of the ipsi and contralateral uterine horns). Pregnancy and twinning rates for 285 animals used for embryo transfer were initially diagnosed at day 56 after induced oestrus by transrectal ultrasonography. Subsequently, calving rate and birth weiglit at calving were recorded.
Pregnancy rates at day 56 after induced oestrus were similar for both surgical and cervical transfers (58·6% v. 55·2%), as was the case for twinning rate (36·2% v. 30·0%). Similarly, there were no differences between these two methods of transfer (50·0% v. 46·9%) and (26·1% v. 17·7%) for calving and twin calving rates respectively. Recipients which had two embryos located in the ipsilateral uterine horn had higher (P < 0·001) pregnancy rates (66·6% v. 47·3%) but similar twinning rates (32·6% v. 33·4%) at day 56 after induced oestrus to recipients which had one embryo located in each horn. A greater (P < 0·05) percentage of recipients with two embryos originally located in the ipsilateral horn calved (56·0% v. 41·0%) but fewer (P > 0·05) produced twins (17·8% v. 25·7%) than was the case for recipients which originally had one embryo located in each horn. In vivo produced embryos resulted in higher (P < 0·001) pregnancy rates (74·4% v. 39·7%) and twinning rates (48·3% v. 18·0%) at day 56, and higher (P < 0·001) calving rates (64·5% v. 32·7%) and twin calving rates (36·3% v. 7·6%) than did in vitro produced embryos. Inseminated (Al + ET) recipients had slightly greater (P>0·05) pregnancy rates (61·6% v. 51·6%) and twinning rates (36·9% v. 28·7%) than non-pregnant recipients which received two embryos. A greater (P<0·05) percentage of inseminated recipients (Al + ET) calved (54·3% v. 42·0%) than was the case for non-pregnant recipients which received two embryos. The percentage producing twins at calving were similar for these two methods of twin pregnancy establishment.
Embryo survival to day 56 after induced oestrus averaged 45·0% and was found to be non-independent of its co-twin. From day 56 to parturition foetal loss averaged 21·0% and foetal survival was found to be independent of the fate of its co-foetus. Twin foetuses located in the same uterine horn were lighter at birth than twin foetuses located in separate uterine horns (33·0 v. 35·2 kg; P < 0·05).