Breeding and Genetics

To analyze the effects of some environmental factors such as herd, year, season of calving, stage of lactation on mastitis incidence, a total of 101147 monthly test day somatic cell counts collected from 13977 Iranian Holstein heifers calved between 2002 and 2006 in 183 herds were used. The event of mastitis in cows was coded as 1 if their somatic cell count at specific test day was greater than 250000 cells per milliliter of milk yield sample. Although somatic cell score could be also analyzed as a continuous trait, this study aimed to evaluate mastitis incidence in a new approach. Logistic regression statistical method was applied to model the probability of mastitis incidence. In the model, fixed environmental factors of herd, year, season of calving, stage of lactation, milk (linear covariate) and age at first calving (linear covariate) were included. The model was fitted to the data using logistic procedure of SAS programme. The results obtained in this study indicated that all environmental factors had highly statistical significant (p<0.001) effects on mastitis incidence. Spring calvers (April-June) were found to be more susceptible to mastitis than the other cows calving at different seasons suggesting that caving in the seasons other than spring could result in decreasing mastitis in herds. The results also revealed that the probability of mastitis incidence increased (with odds ratio of 1.029) as the age of cow at first calving increased while it decreased when the amount of milk yield (odds ratio of 0.960) during the lactation period increased. Genetic evaluation of animals for somatic cell score could decrease the level of mastitis incidence.