duration, dry period and service period between optimal limits [6,7]. The yields of farm animals are the result of the combined effects of genotype and environmental conditions. I n o r d e r t o i n c r e a s e t h e y i e l d l e v e l , i t i s n e c e s s a r y t o o p t i m i z e t h e e n v i r o n m e n t a l conditions and to improve the genetic structure of the animals. In order to enhance productivity of a dairy animal, it is necessary to develop an understanding of the factors affecting its milk production. Environmental factors can be classified as factors with measurable effects (age, year, season, milking frequency, etc.) and factors with immeasurable effects (infectious diseases, parasitic infestations, etc.). The measurable effects can be determined and used in the management of the farm [8]. Environmental factors affecting variability in daily milk yield are widely documented in dairy cattle [9][10][11]. The 305 days milk yield of Holstein cows was 5905 kg in Tunisia [12], 5353 kg in Morocco [13], and between 4597 and 6464 kg in Turkey [14][15][16]. Environmental factors such as year of calving, season of calving and age at calving affect productivity [17]. Many researchers [6,14] reported that the effect of calving season on 305 days milk yield was as significant and indicated that milk yield was higher in autumn and winter. Unlike, Bilgiç and Aliç [16] and Pelister et al. [18] reported that effect of calving season on 305 days milk yield was non-significant. Although, effect of lactation number on 305

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