Phenotypic Study of Lactation Curve in Iranian Holsteins

A total of 136,250 monthly test day milk records collected from 13,625 Iranian Holstein heifers (three times a day milking) calved between 1991 and 2001 and distributed over 264 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 = W0 + W1t + W2e-0.05t) 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 W0 had a negative and significant (p<0.05) relationship with parameters W1 and W2 while it was positively sig-nificantly correlated with milk at peak time and 305-day milk yield. Simple linear regres-sion analysis of adjusted means of a 305-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 137.152 Kg, 0.535 day and 0.434 Kg per year, respectively.