Modelling the Effect of Mastitis on Milk Yield in Dairy Cows using Covariance Structures Fitted to Repeated Measures

The objective of the present study was to study the effect of mastitis on milk yield in dairy cows by accounting the correlations among repeated records on monthly milk yield. The data were collected from monthly milk yield records of 482 dairy cows maintained at three Military Dairy Farms at Agra, Bareilly and Lucknow of Uttar Pradesh (India) for the period from 2011 to 2013. The overall prevalence of mastitis during study period was observed as 24.69%. Mixed modelling using four covariance structures (simple, compound symmetry, autoregressive and unstructured) was used to estimate effect of mastitis on milk yield by adjusting the effects of breed, mastitis, age, calving season, farm, month in lactation and random effects of each cow. On the basis of goodness fit criteria, unstructured covariance structure showed reliable results and estimated 36.76 kg losses due to mastitis in monthly milk yield in dairy cows.