Revenue from dairy farms is directly dependent upon reproductive efficiency because it affects milk production and the number of calves born (Thurmond et al., 1990). Pregnancy loss can have devastating effects on economical success in dairy farms. Normal annual abortion risks have been cited to be 3–5%, once cows are beyond 42 days of pregnancy (Hovingh, 2002), or similarly an observable 2–5% in most dairies (Kirk, 2003). Although, there is little or no indication that milk production is a risk factor for increased pregnancy losses in dairy cattle (Santos et al., 2004), there is little information on the effect of pregnancy losses on subsequent milk production. The aim of this study was to evaluate the impact of different calving classes including eutocia, dystocia, stillbirth and abortion in the first calving on subsequent milk production in Iranian Holstein dairy cows.

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