Mastitis and the shape of the lactation curve in Norwegian dairy cows. 
An investigation of the shape of the lactation curve and the mastitis incidence was conducted to identify whether management interventions of the lactation curve constitute a potential for reducing incidence of mastitis at herd level.  Lactation curves were estimated to describe the variation of daily milk yield during the 305-d lactation period in Norwegian Red cows.  Associations between mastitis incidence at herd level and lactation curve characteristics such as production level at onset of lactation, magnitude and time of peak milk yield, and increase and decrease of milk yield rates were studied.  Data from 250,303 lactations occurring during 2005 and 2006 from 14,766 herds were obtained from the Norwegian Dairy Herd Recording System.  Besides veterinary treatments, the records included information on monthly test-day milk yields.  The shapes of the lactation curves at herd level were parameterized using a modified Wilmink model in two separate mixed model analyses.  In the first analysis a subset of lactations with no records of veterinary treatments was used.  Lactation curves from herds with high (>0·31 cases/305-d lactation) and low (<0·07 cases/305-d lactation) herd mastitis incidence rate were parameterized and compared for three separate strata of parity.  The result showed that high herd mastitis incidence rate was associated with a low intercept (P<0·05), a steep slope before peak milk yield (P<0·01) and a rapid decline after peak milk yield (P<0·01).  In the second analysis a subset of high-yielding lactations with veterinary treatments of mastitis only and lactations with no records of veterinary treatment were compared.  This was done to investigate whether the findings at herd level were also reflected at cow level.  These results showed that lactation curves from lactations with mastitis cases were associated with a steep slope before peak milk yield (P<0·05) in second and later parities and a rapid decline after peak milk yield (P<0·01) in all three parity groups.