Variation in Estrual Cycles of Holstein-Friesian Cattle

The lengths of intervals between estrual periods were studied in a herd which consisted of six sire lines of breeding and in which four systems of mating were practiced. The distribution and mean lengths of 766 estrual cycles and 191 return intervals from 221 heifers differed significantly (P < 0.05). The mean lengths were 22.3 days for estrual cycles and 25.3 days for return intervals. A repeatability estimate of estrualcycle lengths was calculated for the heifers in each sire line and system of mating. Repeatabilities ranged from zero to 0.29 for the sire lines and from 0.02 to 0.21 for the systems of mating.