Determinants for milk fever An epidemiological study of Swedish dairy cows

Abstract: Milk fever is a worldwide disease, seen mainly in dairy cows. At the onset of lactation the cow is not prepared for the sudden calcium requirement, which causes a drop in the blood calcium and the cow develops hypocalcaemia. Calcium is needed for the correct function of both nerves and muscles in the body. A cow suffering from milk fever can show a variety of clinical signs, such as being recumbent, having a reduced appetite and having cold extremities. In the worst cases of milk fever the cow can develop a comatose state and die. To cope with the loss of calcium the cow has several mechanisms in the body involving the kidneys, intestines and bones. These include increased absorption of calcium from the kidneys and intestines and a resorption of calcium from the bones. Milk fever has been studied for many decades and several risk factors have been identified, e.g. body condition score around calving, diet around calving, age, breed and production. The aim of this study was to analyze suspected and previously stated risk factors and their association with the incidence of milk fever. Data was collected through the milk recording scheme managed by VÄXA Sverige. All herds studied were selected based on the milking year 2015/16 (September-August) and had at least 40 calvings, 20% Swedish Holstein breed and 20% Swedish Red breed. Information about incidence of milk fever and the risk factors in these herds was retrieved from 2006/07 through 2015/16. The risk factors studied were breed, parity, previous milk fever cases, previous lactation length, previous dry period length, calving season, previous calving interval and information about milk production (ECM, fat, protein). A decreasing trend of the incidence of milk fever was observed throughout the years. All of the studied factors had an association with the incidence of milk fever. High risk cows were of Holstein breed, were of high parity, had suffered from milk fever in the previous lactation, had a long previous lactation length, had a dry period length of 70-89 days, calved in spring, had a long calving interval and had a high milk production. The determinant with the strongest association was parity, showing that cows of higher parity have clearly increased odds of developing milk fever. A sixth or higher parity cow had an odds ratio of 277.84 of developing milk fever compared to a first parity cow. The second most important determinant was milk fever in the previous lactation (OR=9.74). Several major factors, which previously have been shown to have an effect on milk fever incidence, could not be studied due to limitations in the available information. Furthermore, only univariable analyses were done although it is known that several of the studied determinants have an effect on each other.