Post-partum interval to oestrus in monozygous twin cows and possible effects of maternal bonding

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Abstract
Every cow among 19 sets ofmonozygous twins was separated from its calf at about 72 h postpartum and added to a grazing herd which was machine-milked twice each day. The average postpartum interval to first oestrus under these conditions was 41 days. The within-twin set correlation coefficient was 0.67. One member of each of another 6 twin sets retained its calf for 7-9 weeks post-partum but each calf was muzzled at 72 h post-partum to prevent suckling. These calves were fed warmed milk while their dams were machine-milked. The cows maintained many normally observed maternal responses towards their progeny. The average post-partum interval to first oestrus for the 6 cows with muzzled calves was 58 days compared to 36 days for their twin mates. Suckling may not be essential to the maintenance of a maternal bond which could contribute to the longer period of post-partum anoestrum frequently observed in suckled cows.