estimation of genetic parameters, proposal of an udder health index and on comparision of different selection strategies to include this trait in iranian holstein cows' breeding programs

genetic and phenotypic parameters related to production traits, udder conformation and somatic cell count for the iranian holstein cows were estimated. monthly records of somatic cell counts (scc) were primarily transformed into somatic cell scores (scs) and the the average level of lactation period, with correction for milk volume, was calculated. by use of these estimates an udder health index for iranian holstein cows was proposed which consisted of lscs, fore teat placement as well as udder depth. this index indicates that selection for lower values of lscs, lower udder depth and closer teats should result in increased mastitis resistance. at the end, a restricted selection index method was employed to compare different selection strategies and to obtain proper weight for mastitis in the breeding goal. through the definition of the different breeding goals, the simultaneous genetic progress for milk yield as well as the resistance to mastitis were explored. it was finally revealed that with selection for milk yield only, susceptibility to mastitis will increase. but by inclusion of mastitis phenomenon in the breeding program, it would be possible (in the meantime that a continuation of increase in annual milk production is planned) to stop the increase in the trend of mastitis spread.