The effect of twinning on milk yield, dystocia, calf birth weight and open days in Holstein dairy cows of Iran.

Calving records of Iranian Holsteins from April 1998 to September 2006 comprising 16 herds with 104,572 calving events representing 4045 twin births were used to evaluate reported open days, calving difficulties and calf birth weight in single- and twin-births and the relationship exists between twinning and milk production. A logistic regression model was constructed to analyse dystocia for single- and twin-births. In addition, statistical analyses of 305-day milk yield, open days and calf birth weight were performed using the general linear models procedure. The odds of dystocia was greater after twin births [p < 0.0001; odds ratio (OR) = 2.32]. The odds of dystocia decreased from parity 1 to parity 2 and beyond (p < 0.0001; OR = 0.44). Open days were significantly different between single (129.28 days) and twin (144.88 days) births (p < 0.05). Calf birth weights were significantly greater for singletons than twins (43.33 kg vs. 34.36 kg; p < 0.05). In addition, twin-calved cows had greater 305-day milk production than single-calved cows (p < 0.05). In general, development of practical management strategies to cope with the negative effects associated with twinning on dairies is critical, especially if the trend towards increased twinning in the dairy cattle population continues.