Introduction:  One of the most important and costly infectious diseases in dairy herds is mastitis, which annually imposes substantial economic losses to dairy farmers and the dairy industry. These losses include a decrease in milk production along with an increase in discarded milk, treatment costs, a decrease in the selling price of milk due to the high somatic cell count (SCC), culling of cows and a decrease in the market value. Due to the high costs caused by mastitis, in recent years, resistance to this condition has been considered as one of the major breeding objectives from economic and animal welfare aspects. Considering the impact of mastitis and the increase in somatic cell count on the decrease in milk production and the economic importance of this issue in the dairy industry, the present study aims to estimate reduction in milk production associated with clinical mastitis in Holstein dairy cows in Isfahan, Iran.
 
Materials and Methods:  The data used in this research included 233,499 records for daily milk production belonging to 16,570 dairy cows collected from three herds in Isfahan Province, Iran from 2011 to 2014 to estimate milk production loss associated with clinical mastitis. Data editing and preprocessing were performed using FoxPro version 6.0. In this research, all analyses were performed using SAS software and the PROC MIXED, PROC MEANS, and PROC UNIVARIATE. Then, based on the coefficient of determination (R²), the model that described the data variation was selected better.
 
Results and Discussion: The results of this study were presented in different sections, including the determination of the cut-off point and reduction of milk production due to clinical mastitis in two ways: field data analysis and lactation curve modeling using the incomplete gamma function. The cut-off point was estimated at 10,000 cells per milliliter of milk, which was in the range estimated in previous studies. Milk production loss for 305 days caused by the occurrence of clinical mastitis by using field data analysis first, second and third‑or‑greater parity cows, respectively. These values correspond to 1.8%, 3.4%, and 2.5% reductions in milk production. The results obtained from the incomplete gamma function output for comparing two groups of healthy and diseased cows in the first parity showed that the average milk production per month of lactation in diseased cows was 5.4%, equivalent to 54 kg less than healthy cows (952.4 in compared to 1006.4 kg). Production loss due to clinical mastitis during different months of lactation was estimated to be between 4.3% and 5.9%. The highest amount of production drop was estimated at the peak of lactation, i.e. in the third month and equal to 70.8 kg. The results for the second calving cows showed that each occurrence of clinical mastitis causes a 7.7% decrease in milk production, equivalent to 85 kg of the average month of lactation. The peak of the difference in milk production between the two groups was related to the second to fifth months of lactation. Also, the results of the comparison of two groups of healthy and diseased cows in the third and above calving periods showed that the average decrease in milk production due to the occurrence of clinical mastitis was 10.9%, equivalent to 120 kg of the average lactation month.
 
Conclusion: Based on the results of this study, the amount of milk production loss over a 305‑day lactation due to clinical mastitis was estimated to be between 195 and 406 kg using field data analysis and 539 to 1202 kg per lactation using the incomplete gamma function model. It was also found that with the increase in parity, the decrease in milk production caused by clinical mastitis increases due to damage to the teat canal during life and previous lactation periods. Despite the dependence of the field data analysis method on the data structure, the incomplete gamma function is better described of the milk production curve and provides a more accurate prediction of milk production decline. The findings of this study can be used to calculate the economic loss caused by the decrease in milk production of cows affected by clinical mastitis and the decision to keep or voluntary culling them.