Variation in Estrual Cycles of Holstein-Friesian Cattle

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Abstract
The lengths of intervals between estrual periods were studied in a herd which consisted of 6 sire lines of breeding and in which 4 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 estrual-cycle 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. An analysis of variance showed estrual-cycle length in heifers to be affected significantly by the season of birth and the system of mating. The summer- and spring-born heifers had the longest cycles, 25.0 and 25.2 days, whereas the winter- and fall-born had cycles averaging 22.2 and 20.9 days, respectively. Daughters of inbred dams had the longest mean cycle lengths, 24.9 days for the outbred daughters and 25.6 days for the inbred; daughters of outbred dams averaged 20.6 for those that were outbred and 22.2 for those that were inbred. An analysis of variance of the 1st estrual-cycle length followed calving in outbred and inbred cows showed a significant interaction of sire line and system of mating . Repeatability estimates of the 1st estrual-cycle length following calving from one calving interval to another of 0.11 (P<0.10) for the outbreds and 0.06 for the inbreds were calculated.