COMPARATIVE PRODUCTIVITY OF COWS AFTER RECEIVING TWO EMBRYOS AT TRANSFER
Embryos were transferred surgically to each uterine horn of 31 recipient cows. Fourteen (45%) became pregnant. Nine of the pregnant recipients calved twins for a rate of 64% twins per pregnancy. Embryos were obtained from nine donors by nonsurgical flushing, with the resulting single and twin calves offspring of six of these. Donors were crossbreds of varying percentage of Charolais, Chianina, Maine Anjou, Simmental and Holstein; service sires were Charolais, Limousin or Maine Anjou and the calves were therefore 2-, 3- or 4-way crosses. Recipients included a wide range of crosses of breeds, varying in milk yield and mature weight. In addition to the twins produced by transfer, an additional six sets of twins were born during the same period to cows bred by artificial insemination (AI). Single and twin calves produced by AI service were sired by bulls of breeds similar in mature weight to that of the dam. At calving, 79% of the twinning cows required assistance compared to 29% of the single bearing cows. The most common problem observed for cows bearing twins was presentation of both calves at the same time, although this problem was easily corrected in all cases. Calf mortality was low and not significantly different between twin- and single-bearing dams. Days to first ovulation were similar for twin and single nursing cows, 39 and 34 d postpartum, respectively. Transfer twin calves were 16.6 kg lighter at birth and gained weight at a significantly slower rate (249 g d−1) from birth to weaning than transfer calves born as single. Milk production of transfer twin-bearing dams as measured by average machine yield at 5, 12 and 20 wk of lactation was significantly greater (340 kg 200 d−1) than for dams bearing a single transfer calf. Total mass of weaned calf per cow was 151 kg greater for dams with transfer twins than those with transfer singles and 165 kg greater for AI dams with twins than AI dams with singles. Within the transfer twin group, milk yield of dam significantly influenced total weight of calf produced (0.02 kg kg−1 milk) while weight of dam did not. Key words: Embryo transfer, beef, twins, recipients, milk yield, reproductive performance