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The aim of the study was to investigate the genetic variation of daily rumination time (min) and its correlation with test-day milk production (kg). Data for the analysis consisted of 91,589 records for rumination time and milk yield from 398 cows (age: 43.21 ± 16.11 mo), collected from September 2014 to October 2015 in two Italian Holstein herds (TAD and MIL). There were 493 calvings, and data distribution across parities was 46.4%, 26.7% and 26.7% for first, second and later parities, respectively. DIM classes were defined as one class for every 30 d, resulting in 11 classes, and there were a total of 378 herd-test-day contemporary groups. The average rumination time was 513.51 ± 115.84 min, and the average milk yield was 33.59 ± 9.18 kg.

Pedigree information was available for 11,634 animals. A Repeatability Animal Model was fitted using the AIREMLF90 software. Herd, yr/mo of calving, and DIM …