Studies have suggested that average inter-service (and inter-ovulatory) intervals in dairy cows may be longer than typically expected. Some authors suggest fertility may vary with follicular wave number. This study aimed to identify associations between inter-service-interval (ISI) and the probability of pregnancy in dairy cows. Data from 312 UK dairy herds were analyzed. There were 257,396 insemination records from 75,745 cows. The intervals between subsequent inseminations in the same cow in the same lactation were calculated. Inseminations with a corresponding calving recorded were deemed successful, allowing the calculation of the average probability of pregnancy at different ISIs. A random effects logistic regression model was constructed to predict the probability of pregnancy for ISIs (16–28 d). Univariable analysis showed a peak probability of pregnancy of 44% with an ISI of 21 and 22 d; the distribution across the range of ISIs is tabulated below. Preliminary multivariable analysis showed that the probability of pregnancy was significantly (P < 0.05) lower for inseminations carried out at 16–18 d and significantly higher for those carried out at 21–22 d when compared with 25 d. These results suggest that pregnancy is most likely following inseminations carried out at the expected ISI of 21 d. ISIs of less than 19 d result in a lower probability of pregnancy, suggesting that these ISIs reflect inseminations of cows not truly in estrus or that estrous cycles of shorter length are less fertile. ISIs greater than 24 d are less likely to result in pregnancy than those of 21 d but are more likely to result in pregnancy than ISIs of 18 d or less, suggesting that these ISIs may represent true estrus events at an interval longer than typically expected. These longer ISIs may be a result of embryonic death extending the inter-estrus interval or may represent longer than expected true estrous cycles. These results suggest a need to reconsider the expected cycle length of the modern dairy cow, both in research and when reviewing farm insemination records. Table 1.

ISI, d	Probability of pregnancy	n
16	0.27	657
17	0.24	853
18	0.30	1,374
19	0.38	2,844
20	0.43	5,977
21	0.44	9,667
22	0.44	10,515
23	0.42	9,239
24	0.41	6,501
25	0.40	4,682
26	0.39	3,496
27	0.35	2,472
28	0.37	1,817