Mastitis, ketosis, and milk fever in 31 organic and 93 conventional Norwegian dairy herds.

The aim of this study was to investigate differences in disease incidence between organic and conventional herds. The study was based on data from the Norwegian Dairy Herd Recording, which includes the Norwegian Cattle Health Recording System. All herds certified for organic farming in 1994 with a herd size of more than five cow-years were included. Conventional herds were matched on size and region, and from these, three herds were randomly selected for each organic herd. This resulted in a study group of 31 organic and 93 conventional herds with data from 1994 through 1997. The study unit was the cow within a lactation. Factors influencing disease incidence were studied by means of a generalized linear model approach. Management system had a highly significant effect on disease incidence. Odds ratios for organic compared with conventional herds were as follows: mastitis, 0.38; ketosis, 0.33; and milk fever, 0.60. Other significant factors that emerged in modeling the three diseases were year and lactation category for mastitis; lactation category, maximum milk yield, and season for ketosis; and lactation category and milk yield for milk fever. There was no marked difference in milk somatic cell count (SCC) between organic and conventional herds. However, cows in organic herds had lower SCC in lactation two and greater counts in lactations six and higher.