Repeatability of the Estrous Cycle Length in Dairy Cattle

Much has been published concerning the estrous cycle of dairy cattle. Asdell (2) and Roark (13) have given excellent reviews of literature on the subject. McNutt (11) observed 19 animals and found the average cycle to be 21 days. Hammond (6) states the length of the normal estrous cycle in the cow averages 19.5 days but varies from 17.5 to 24 days. Werner et al. (17) observed the estrous cycles of 35 dairy heifers and found that 65.9 per cent of 82 cycles ranged between 18 and 24 days. Alba (1) reported the modal cycle length as 20 days for unbred heifers and 21 days for cows. He found that 85 per cent of the cycles in heifers fell between 18 and 22 days, whereas in cows 84 per cent fell between 18 and 24 days. The mean for heifers was found to be 20.23 days with a standard deviation of 2.33 days. For cows the mean was 21.28 days with a standard deviation of 3.68 days. Lasley and Bogart (8) found the mean cycle length for beef cattle to be 19.6 0.12 days, the modal length being 20 days, and 79 per cent falling between 17 and 23 days. Roark (13) recorded the length of 504 estrous cycles of 110 cows and found that about 80 per cent of the cycles fell between 18 and 24 days. The mean cycle length was 21.4 days. Chapman and Casida (4) found that intervals between heats varied from 2 to 200 days. The mode was 21 days both for cows which were bred at the previous heat and for those which were not bred. However, the means of the two groups differed. The non-copulatory cycles averaged 32 days while the copulatory cycles averaged 37 days. When all cycles over 33 days were excluded, the mean for the non-copulatory group was 21 days and for the copulatory group it was 22 days, the differences being significant.