Associations between dairy cow inter-service interval and probability of conception

Abstract

Recent research has indicated that the interval between inseminations in modern dairy cattle is often longer than the commonly accepted cycle length of 18-24 days. This study analysed 257,396 inseminations in 75,745 cows from 312 herds in England and Wales. The interval between subsequent inseminations in the same cow in the same lactation (inter-service interval, ISI) were calculated and inseminations categorised as successful or unsuccessful depending on whether there was a corresponding calving event. Conception risk was calculated for each individual ISI between 16 and 28 days. A random effects logistic regression model was fitted to the data with pregnancy as the outcome variable and ISI (in days) included in the model as a categorical variable. The modal ISI was 22 days and the peak conception risk was 44% for ISIs of 21 days rising from 27% at 16 days. The logistic regression model revealed significant associations of conception risk with ISI as well as 305 day milk yield, insemination number, parity and days in milk. Predicted conception risk was lower for ISIs of 16, 17 and 18 days and higher for ISIs of 20, 21 and 22 days compared to 25 day ISIs. A mixture model was specified to identify clusters in insemination frequency and conception risk for ISIs between 3 and 50 days. A "high conception risk, high insemination frequency" cluster was identified between 19 and 26 days which indicated that this time period was the true latent distribution for ISI with optimal reproductive outcome. These findings suggest that the period of increased numbers of inseminations around 22 days identified in existing work coincides with the period of increased probability of conception and therefore likely represents true return estrus events.

A description of interestrus and interservice intervals and associated fertility in 16 United Kingdom dairy herds

“…A trend was present for services following even longer intervals (25 to 31 d) to have greater predicted probability of pregnancy than quartile 1. This corresponds with recent observations by Remnant et al (2018) that services after intervals of 25 and 26 d were significantly more likely to result in pregnancy than services following intervals of 17 and 18 d. These findings suggest that some of the mechanisms underlying extended intervals might have a positive effect on fertility. This could support the hypothesis that longer intervals are due to 3-wave cycles as some (although not all) studies have demonstrated higher pregnancy rates for 3-wave cycles (Ahmad et al, 1997;Townson et al, 2002;Bleach et al, 2004).…”

“…If estrous cycle length has changed, these measures may give false impressions of herd performance. Additionally, although some researchers have demonstrated that interestrus intervals outside of a range of 18 to 24 d have a negative association with insemination success, more recent studies have contradicted this, showing improved pregnancy outcomes to insemination following longer interestrus intervals (Royal et al, 2000;Remnant et al, 2018).…”

Reproductive Performance in a Selected Sample of Dairy Farms in Una-Sana Canton, Bosnia and Herzegovina

“…e observed interservice interval of 18-24 days for more than two-thirds of repeat breeding heifers and multiparous animals at the population level indicated a very good oestrus detection. Remnant et al (2018) reported that interservice interval of 19-26 days indicated that this period is the true latent distribution for the interservice interval with the optimal reproductive outcome, suggesting day-22 with the increased probability of conception [25]. However, we found that a total of 9.6% of interservice intervals were longer than 48 days ( Figure 6).…”

Reproductive management of seasonal calving dairy herds

“…Inseminations performed at intervals of 18 to 24 or 36 to 48 days are considered normal or indicative of a missed heat, respectively, while intervals outside of these ranges are abnormal. More recent research has suggested that normal interservice intervals may be more variable, and a range of 18 to 28 days may be more accurate (Remnant and others 2018).…”