Countries with different herd structures and climates use different combinations of herd, year, and season to define the effect of their climatic conditions and management. Estimates of variance components and Akaike information criterion values resulting from 8 analytical models differentiated by different combinations of herd, year and season effects were compared for milk and fat yields. Data consisted of pedigree and records of 11,850 Holsteins freshening in 38 dairy herds of Isfahan, Iran, between January 1994 and October 2001. Heritability estimates were lowest for the models that used herd-year combination as a random effect. Moreover, the use of these models is not recommended due to their low goodness of fit, at least for countries that have too much variety in herd size. The models that considered herd-year or herd-year-season as a fixed effect to define contemporary groups fit the data best. It is worthwhile rechecking the frequency of animals in herd-year-season groups in order to control the minimum number of animals in contemporary groups. Countries with 4 seasons and small herds may consider herd-year and 4-month season as 2 separate fixed effects to observe contemporary groups sufficiently and to make a better correction for seasonal changes or considering herd-year-season as a random effect.

Countries with different herd structures and climates use different combinations of herd, year, and season to define the effect of their climatic conditions and management. Estimates of variance components and Akaike information criterion values resulting from 8 analytical models differentiated by different combinations of herd, year and season effects were compared for milk and fat yields. Data consisted of pedigree and records of 11,850 Holsteins freshening in 38 dairy herds of Isfahan, Iran, between January 1994 and October 2001. Heritability estimates were lowest for the models that used herd-year combination as a random effect. Moreover, the use of these models is not recommended due to their low goodness of fit, at least for countries that have too much variety in herd size. The models that considered herd-year or herd-year-season as a fixed effect to define contemporary groups fit the data best. It is worthwhile rechecking the frequency of animals in herd-year-season groups in order to control the minimum number of animals in contemporary groups. Countries with 4 seasons and small herds may consider herd-year and 4-month season as 2 separate fixed effects to observe contemporary groups sufficiently and to make a better correction for seasonal changes or considering herd-year-season as a random effect.