The performance of primiparous and multiparous beef cows rearing single and simulated-twin calves

One hundred and eighteen Hereford-cross heifers (n = 70) and cows (n = 38) and their offspring were used to evaluate the impact of simulated twinning on weaned calf production and reproductive performance. Twin-calf rearing w as simulated by the adoption of a second calf born within 24 h of the birth of the dam's natural calf. In each of 4 yr, cows calved in winter (January -February), were turned out in spring (late May) to rotationally graze gras s-legume pastures until weaning in the fall (late September). From calving until turnout cows were given a forage-based diet consisting of either gras s-legume silage or hay offered foe choice, plus concentrate, the amount of which was 75 or 100% higher for dams in the simulated-twin (ST) than the si ngle (S) treatment. All calves had access to the conserved forage and were given the concentrate ration as creep feed. Estrus detection was performed twice daily beginning 3 wk before and continuing for 4 wk into the breeding season, which consisted of Al for the first 25 d and bull service for the remaining 22 d, In years 2 and 3, the postpartum anestrous period was deter mined by serum progesterone concentrations. At weaning, S calves weighed mo re than natural or adopted calves in the ST treatment because weight gain w as greater for S calves from birth to turnout. However calf production effi ciency (total kilograms calf weaned per kilogram dam weight) was greater fo r primiparous and multiparous ST than S groups (primiparous 0.85 vs. 0.33 a nd multiparous 1.62 vs. 0.49; P < 0.05). The ST treatment had no negative e ffect (P > 0.05) on reproductive performance. As a research tool, this stud y suggests there are large potential benefits to beef cows raising two, as opposed to one calf. Simulated twinning poses severe management challenges and is not being proposed as a commercial option. However, advances in repr oductive technology that could control twinning (e.g., transfer of an embry o a week following AI or twin-transfer of sexed embryos) mean that in futur e, twin rearing of beef calves may become a feasible option and this study provides valuable information on the post-natal management of cows rearing two calves.