The objectives of this study were to determine frequency of stillbirth and twin birth, and the individual and environmental factors affecting them in commercial dairy herds located in northeast of Iran. Parity of dam, season of calving, sex of calf, the number of calves born per calving event and survival of calf up to 24 h after birth were recorded for all calving events that occurred between 21 March 2009 and 20 March 2010 in nine industrialized Holstein dairy herds. Out of 3595 calving events, 155 (4.3%) stillbirths and 79 (2.2%) twin births were recorded. Logistic regression with herd as a random effect was used to evaluate the effects of calving season, sex of calf, parity of dam and twinning on stillbirth. Also, the effects of calving season and parity of dam on twinning was evaluated using the same technique. Season of calving and parity of dam had significant effects on twinning rate. Odds of a twin birth was greater during spring (OR = 2.39; p = 0.033) and autumn (OR = 2.27; p = 0.038) than winter calving. Multiparous cows were more likely to produce twin births compared with primiparous cows (OR = 6.17; p < 0.001). Frequency of stillbirth was significantly associated with sex of calf and twinning. Odds of stillbirth was greater for twin births compared with singleton births (OR = 3.13; p = 0.004). Male calves were more likely to be dead at birth than female calves (OR = 2.93; p < 0.001). The seasonality of twinning would be relevant to the season of conception. Also, some factors such as conceptus sire, nutrition, or maternal body condition have been speculated to affect the seasonal variation in twinning. Generally, our results show that individual and environmental factors affect stillbirth and twin birth rate in Holstein dairy cows.

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