Age at puberty (AGEP) is a moderately heritable trait in cattle that may be predictive of an animal's genetic merit for reproductive success later in life. In addition, under some mating strategies (for example, where mating begins before all animals have attained puberty) animals that attain puberty at a relatively young age will also likely conceive earlier than their herd mates, and thus begin their productive life earlier. Unfortunately, AGEP is challenging to measure because animals must be observed over a period of several months. Our objectives for this study were twofold. The first objective was to produce variance components for AGEP. The second objective was to investigate the implications of a simplified phenotyping strategy for AGEP, when the interval between repeated blood plasma progesterone measures was extended from weekly to monthly, increasing the extent of left, interval, and right censoring. We measured AGEP in a closely monitored population of around 500 Holstein-Friesian heifers, born in 2015 and managed under a seasonal, pasture-based dairy system. Animals were blood tested weekly from approximately 240 to 440 d of age and were deemed to have reached puberty when blood plasma progesterone elevation (>1 ng/mL) was detected in 2 of 3 consecutive blood tests (AGEP_Weekly). To simulate a simplified phenotyping strategy based on monthly herd visits (AGEP_Monthly), we selectively disregarded data from all but 3 blood test events, when animals were around 300, 330, and 360 d of age (standard deviation = 14.5 d). The posterior mean of estimated heritabilities for AGEP_Weekly was 0.54, with a 90% credibility interval (90% CRI) of 0.41 to 0.66, whereas it was 0.44 (90% CRI 0.32 to 0.57) for AGEP_Monthly. The correlation between EBVs for AGEP_Weekly and AGEP_Monthly was 0.87 (90% CRI, 0.84 to 0.89). We conclude that in this population, AGEP is a moderately heritable trait. Further, increasing phenotype censorship from weekly to monthly observations would not have altered the main conclusions of this analysis. Our results support the strategic use of censoring to reduce costs and animal ethics considerations associated with collection of puberty phenotypes.

Università degli Studi di Padova, Dipartimento di Biomedicina Comparata e Alimentazione, Viale dell’Università 16, 35020 Legnaro, Italy valentina.bonfatti@unipd.it (Corresponding Author) DairyNZ Ltd., Private Bag 3221, Hamilton 3240, New Zealand Department of Economic Development, Jobs, Transport & Resources, 5 Ring Road, 3083 Bundoora, Australia La Trobe University, 5 Ring Road, 3083 Bundoora, Australia

Lying behavior may provide predictive information about the timing of calving in grazing dairy cows. We hypothesized that cows would be more active around calving. Standing and lying data from Icetag or Iceqube accelerometers were available from 311 mixed-age and breed grazing dairy cows. Lying behavior during the transition period (experimental days (expday); d -21 to d +34 relative to calving) and on the day of calving (d 0) was examined to calculate mean lying time (LT; h/d) and number of lying bouts (LB; n/d). Relationships between lying parameters and expday were examined using PROC NLIN, then modelled using PROC GLM with cow and expday as fixed effects. The effects of period (i.e., PRE: d -21 to d -3, d 0, or POST: d 3 to d 21) on lying behavior were analyzed using PROC MIXED, including expday and cow as fixed and random effects, respectively. The largest changes occurred on d -3 and d 0 for LT and on d -2 and d -1 for LB. Within cow, there was a decrease (P g< 0.001) in LT between d -3 and d 0 (R2 = 0.57: slope = -1.0 ± 0.06; intercept = 8.5 ± 1.07), and the mean LT was less on d 0 (7.0 ± 0.12 h/d) than (P < 0.05) PRE or POST calving (8.3 and 10.3 ± 0.12 h/d, respectively). The number of LB increased (P < 0.001) during the 2 d before calving (R2 = 0.62: slope = 4.51 ± 0.42; intercept = 14.8 ± 3.72), and the number of LB was greater on d 0 (12.5 n/d; P < 0.05) than PRE or POST calving (8.4 vs 8.0 n/d, respectively). Changes in behavior between d -3 to d -1 indicate that lying parameters may predict calving time in individual grazing dairy cows.