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 1997 and 2007 and distributed over 264 herds were used to study the effect of some environmental factors influence lactation curve parameters as well as production characteristics. Wilmink's function (Yt = W0 + W1t + W2e-0.05t) was fitted to individual lactation. Least squares analysis of variance indicated that the herd, year and month calving had a significant effect of all traits under consideration. Correlation analysis showed that the parameters 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 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.