Extending part lactations to a total lactation is important in order to increase the accuracy of sire evaluation and to take into account low producing heifers which are culled before the end of their lactation. Without extending part lactations, these low producers would not be considered in sire evaluation, and this would result in an overestimation of the breeding value of their sires. There are two classes of methods for extending part lactations in the literature the regression method and the method of ratio factors. Regression coefficients and ratio factors are found to be dependent on various environmental factors. Age, lactation number, calving interval and in a few cases herd level are taken into account in extending procedures.