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A difficult calving affects the welfare of the cow and has economic implications for the farm. The degree of calving difficulty can vary from no assistance needed through a slight pull required to surgery being needed. With respect to milk production, it is not clear at which degree of calving difficulty adverse effects occur or for how long they last. Studies usually only consider the milk produced by animals who completed full lactations but the saleable milk production of the whole herd, regardless of each cow having achieved a full lactation, might be a better indicator of the productivity of the cows and the underlying stresses they experience, as well as being more representative of the real losses that producers incur. The objective of this study was to investigate how various degrees of calving difficulty would alter both the cow's milk production and their production of saleable milk over different stages of their subsequent lactation. The calving difficulty scores and the subsequent milk production were retrieved from an experimental dairy farm (in the United Kingdom) for 2 herds that contained 2,430 and 1,413 lactations. To account for milk saleable by the farmer, individual cumulative saleable milk yields, referred to as saleable milk yields (SMY), were calculated at 30, 60, 90, and 300 d in milk unconditional on the animal having achieved the lactation stage of interest. Lactation SMY were obtained based on the real lactation length achieved by the animal. Mean daily milk yields were also calculated for the same lactation stages as an estimate of the cow's milk production (CMP). Calving difficulty impaired milk production of dairy cows in terms of CMP and SMY in both herds, highlighting impaired income for dairy producers as well as detrimental effects to the productivity of the cows and potentially impaired health and survival. The management of the herd affected the presence of an effect of each degree of difficulty on SMY and CMP as well as its magnitude and duration. The analysis of SMY, independently of each animal having achieved a full lactation, could be a more sensitive indicator of the subsequent long-lasting biological stresses than CMP alone.

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