**5. Conclusion**

In this study, there was increase of milk yield level according to previous research [16]. This may result from improvement in breeding, feeding and management conditions (selection for milk yield and culling in the herd etc.). Although, lactation duration was found almost at ideal value, dry period was estimated as higher than the ideal value. In order to make animals more profitable, it is essential to make them be pregnant as soon as possible during the service period in order to shorten the dry period. It can be concluded that Holstein cattle is raised successfully for milk yield under Tunisian environmental conditions. It is concluded that milk yield and lactation length are affected by year and season of calving. Adjusted milk yield (adjusted for lactation length) and lactation length are affected by year into season of calving interaction but actual milk yield is not affected by year by season of calving interaction. Age within parity, also, affected lactation length and milk yield. Negative phenotypic trend in milk yield is alarming and needs further investigations.
