Repeatability of the Estrous Cycle Length in Dairy Cattle

Abstract

Breeding records for the Kentucky Agricultural Experiment Station dairy herd were studied covering a period of 18 yr. (1928–1946). Estrous cycle lengths were tabulated for 278 cows and included 3,776 intervals or an average of 13.6 intervals per cow. The mean cycle length was 32.4 days with a range from 1 to 549 days. About 11.6 per cent of the intervals were 53 days or over. There were no cases of known pregnancy or observed abortion included in these long intervals. The modal length was 22 days and this cycle length represented about 12 per cent of the intervals. In order to get a mean of approximately 22 days it was necessary to omit all intervals of 36 days or more. The standard deviation, after eliminating these long intervals, was 4.6 days. In this case 82.8 per cent of the intervals fell between 17 and 26 days. Estrous cycle lengths following heat periods at which the cows were bred averaged 35.7 days, whereas following heat periods at which the cows were not bred, the cycles averaged 30.6 days. The difference between these means was highly significant. Among cows which were not bred at the previous heat, the percentage of cycles in the 17- to 26-day or “normal” group was 15.6 percentage units higher than for cows which were bred. The percentage of intervals in the 27- to 33-day and 53-day or over group for the cows not bred at the previous heat was 8.2 and 5.0 percentage units lower, respectively, than for those in the group following service. These differences also were statistically highly significant. The analysis of variance, when applied to the data, showed that there was significantly greater variation in cycle length between cows than there was within cows. The composition of the mean square indicated a repeatability for single records of 0.069, or 6.9 per cent for length of estrous cycle. When all intervals of 36 days or over were omitted, the variance due to cow was about 1/25th as great as when all intervals were considered. The repeatability was still only 0.081. The breeding efficiency of cows following cycles of approximately 3 wk., 6 wk. or longer was significantly higher than for cows bred following cycles of 1 to 16 or 27 to 33 days. © 1951, American Dairy Science Association. All rights reserved.