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 lactation 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 indicated 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 significantly correlated with milk at peak time and 305-day milk yield. Simple linear regression 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 increase of 137.152 Kg, 0.535 day and 0.434 Kg per year, respectively