H. Farhangfar - Department of Animal Science, Faculty of Agriculture, University of Birjand, Birjand, Islamic Republic of Iran.
H. Naeemipour - Department of Animal Science, Faculty of Agriculture, University of Birjand, Birjand, Islamic Republic of Iran.

A total of ۱۳۶,۲۵۰ monthly test day milk records collected from ۱۳,۶۲۵ Iranian Holstein heifers (three times a day milking) calved between ۱۹۹۱ and ۲۰۰۱ and distributed over ۲۶۴ herds were used to study the effects of some environmental factors influencing lacta-tion curve parameters as well as production characteristics. Wilmink’s function (Yt = W۰ + W۱t + W۲e-۰.۰۵t) was fitted to individual lactations. Least squares analysis of variance in-dicated that the herd, year and month of calving had a significant effect on all traits under consideration. Correlation analysis showed that the parameter W۰ had a negative and significant (p<۰.۰۵) relationship with parameters W۱ and W۲ while it was positively sig-nificantly correlated with milk at peak time and ۳۰۵-day milk yield. Simple linear regres-sion analysis of adjusted means of a ۳۰۵-day milk yield, days to reach peak yield as well as peak milk yield in the first year of calving also revealed that there was a phenotypic in-crease of ۱۳۷.۱۵۲ Kg, ۰.۵۳۵ day and ۰.۴۳۴ Kg per year, respectively.

Iranian Holsteins, Lactation curve, Wilmink’s function