Bovine mastitis is regarded as the most costly disease of dairy cows. Estimating its economic impact therefore gives farmers and veterinarians an insight into the costs of the disease at herd level and helps them make appropriate decisions regarding its control. The aim of this study was to determine the costs of clinical mastitis in Friesian × Bunaji crossbred dairy cows. Passive data collected between 2000 and 2015 was retrieved from the Dairy Research Programme of the National Animal Production Research Institute, Shika-Zaria, Nigeria and this was used to determine the input parameters for a simulation model. The parameters included the lactation and seasonal prevalence of clinical mastitis, average daily milk yield of cows, average illness period and the proportion of cows in each parity. Stochastic (Monte Carlo) simulation modelling of milk yield losses due to clinical mastitis was done using Microsoft ® Excel with @Risk 7 add-in. The cost of a case of clinical mastitis at a base risk incidence of 35.2% was ₦5,005.85 ($15.87). The costs increased by 7.5% in a herd with 10% higher milk yield, while revenue generated was higher by 10.2%. The cost was 1.64% higher in a herd with fifty per cent of it cows in first parity than a herd with fifty per cent of its cows in third parity. 1.01% higher net revenue was generated from herds with fifty per cent of it cows in third parity than the herd with fifty per cent of its cows in first parity. Improving milk production potential of cows resulted in more cases of clinical mastitis, although the increase in revenue overshadows the added costs of clinical mastitis, successful control of mastitis will also significantly reduce production costs and improve the farm revenue.

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