Estimating the Risk of Hypomagnesaemic Tetany in Dairy Herds

摘要

A model of magnesium dynamics in sheep has been adapted and improved as part of a programme to develop a tool that will assist dairy farmers to assess the risk of animals in a milking herd suffering from hypomagnesaemic tetany. The terms in the modelfor hindgut absorption have been altered to better represent the process of magnesium absorption and secretion in this region of the gastrointestinal tract, a term representing lactation has been introduced and the equation for urinary magnesium flux hasbeen altered so that it no longer saturates at high plasma magnesium concentrations. Because the concentration of magnesium in cerebrospinal fluid (CSF) is strongly related to the onset of tetany, we have introduced a CSF compartment. We have also madeallowance for diurnal feeding patterns. The improved model, scaled to represent dairy cattle, gives good agreement with a published experiment in which plasma and CSF magnesium concentrations were measured in animals on a magnesium-deficient diet. To obtain an estimate of the risk of tetany, we included diurnal feeding patterns and information about animal variation in the model parameters and carried out Monte Carlo simulations. If at any time during a simulation run the value of the magnesium concentration in CSF were less than 0.6 mmol/1, the simulation was assumed to represent an animal at risk. With sufficient simulations we obtained an estimate of the risk of tetany for a herd as a whole. A sensitivity measure was used to determine which parameters significantly affected magnesium concentration in CSF and hence the importance of knowing their distributions. To make the risk estimate more specific to the herd under investigation, we calibrated the model using herd samples of milk and estimated urine magnesium flux. As the latter is a model output, a fitting procedure was required to use this information to improve the estimates of the sensitive model parameters. Although we have initially used a very simplistic fitting procedure, the calibratedmodel gave very reasonable estimates of risk.