Estrous activity in nulliparous heifers with divergent genetic merit for fertility traits

This observational study determined the effect of genetic merit for fertility traits on estrous activity and duration and interestrous interval in nulliparous dairy heifers. We also compared estrous activity between estrous events with or without conception and determined the effect of genetic merit for fertility traits on age and BW at time of first detected estrous activity event. Activity monitoring devices (Heatime, SCR-Engineers) were fitted to Holstein-Friesian heifers aged 7 to 9 mo with positive or negative genetic merit for fertility traits (POS FertBV: average +5%, n = 275; NEG FertBV: average -5%, n = 249) and activity data were collected to the end of the first breeding period (15-17 mo). An estrous event was defined as when the activity change index exceeded 19.2 activity units (AU) for ≥6 consecutive hours. In total, 2,434 estrous events were identified (POS FertBV: n = 1,454; NEG FertBV: n = 980). Estrous event duration was defined as the period when the threshold was first exceeded and when activity dropped below threshold, without another event starting within 24 h of the end of the previous event. This definition included occurrences where activity crossed the threshold multiple times in a day; these were classified as a single estrous event. A second measure, high activity duration, was defined as the total time activity exceeded the threshold. To characterize estrous activity, baseline activity was determined from the previous 7 d of activity. Peak activity and total activity (area under the curve of activity above baseline) were calculated. A fifth variable, interestrous interval, was calculated as a proxy for estrous cycle length. Total activity and peak activity were greater in POS than NEG FertBV heifers (total activity: 548 vs. 464 AU, SED = 19.6 AU; peak activity: 72 vs. 65 AU, SE of the difference [SED] = 1.5 AU). The POS FertBV group had a mean estrous event duration and high activity duration of 15.1 and 15.0 h, compared with 14.1 and 14.1 h for the NEG FertBV group (SED = 0.30 and 0.29 h, respectively). Inter-estrous interval did not differ between POS FertBV and NEG FertBV heifers (19.5 vs. 20.0 d, SED = 0.49). Estrous events associated with conception were shorter than those not associated with conception (mean ± SEM, high activity duration: 13.0 ± 0.25 h vs. 13.9 ± 0.31 h, estrous event duration: 13.1 ± 0.25 h vs. 14 ± 0.32 h) and had less total activity (408 ± 15.2 vs. 487 ± 18.2 AU). The POS FertBV heifers were more likely to have their first estrous activity event than NEG FertBV heifers by a given age (hazard ratio [HR] = 1.26, 95% CI = 1.0-1.6) or BW (HR = 1.35, 95% CI = 1.1-1.6). The outcomes of this study provide evidence that positive genetic merit for fertility traits is associated with more overt estrous expression. Therefore, estrous expression traits may have potential to be used as earlier-in-life predictors of genetic merit for fertility.