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Daily rumination time in Italian Holstein cows: heritability and correlation with milk production
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 months), 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 30d 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, year/month of calving, and DIM classes within parity were treated as fixed effects, while herd-test-day, permanent environmental, and the additive genetic cow effects were treated as random. Rumination time was longer in pluriparous than in primiparous cows and showed a decreasing trend across DIM. On average, at the beginning of the lactation, pluriparous cows ruminated 75 min longer than primiparous. As expected, pluriparous cows had a higher production levels across DIM than primiparous, with a peak around DIM class 2 and 3 (i.e., 60-90 d). The herd with the highest daily rumination time had the lowest milk production yield: the fixed effects solutions were 569.5 min and 25.8 kg (Herd TAD; rumination time and milk yield, respectively) and 446.4 min and 31.9 kg (Herd MIL; rumination time and milk yield, respectively). The heritabilities for test-day milk yield and daily rumination time were 0.13 (SE = 0.06) and 0.32 (SE = 0.09), respectively. Although the negative phenotypic correlation observed, genetic correlation between the two traits was 0.38 (SE = 0.47); this high standard error is possibly the consequence of the dataset dimension. So far, rumination time has been used as a key monitor of dairy cow health at farm level. Investigating its genetics aspect and the relationship with other important yields and health traits may turn this management tool in a new informative selection criterion for the dairy cattle breeding strategies.
Keywords: Rumination time, Milk production, Genetic variation