Estimation of genetic parameters for udder quarters milk yield and milking traits in Holstein cattle

Improving the genetic potential of dairy cows should take into account, among others traits, animal health, udder conformation and suitability for milking. This study aimed to estimate the genetic parameters for traits related to milk yield and quarter-level milking in primiparous cows. The study included 1 891 primiparous Polish Holstein-Friesian cows born in 2011-2012. Cows were kept in 23 free-stall barns and milked using milking robots (Lely Astronaut A4). Cows were fed a Partial Mixed Ration diet. The amounts of concentrates were dosed individually, depending on the level of milk production, in the robot and the feed station according to Lely's standard recommendations. Genetic parameters were estimated for the following traits: milk yield (MY, kg), milking time (MT, s), milking speed (MS, kg/min) - average milk flow rate of all milkings throughout the day of robot operation, estimated as daily MY/MT, electrical conductivity (EC, mS/cm), attachment number (AN, n) - number of attachment of the milking cups, attachment time (AT, s) - time of teat-cup attachment. These parameters were estimated based on 45 9607 daily records using Wombat software. The average daily milk yield was 28.53 ± 8.24 kg; rear udder quarters produced on average approximately 1 kg more milk per quarter than front quarters. The rear quarters also required longer milking time (≈ 30 s) and teat-cup attachment time (≈ 10 s). The highest heritability was observed for EC in the left rear quarter (0.427 ± 0.067). The largest common environmental effects were observed for EC (0.733 ± 0.054) and MS (0.508 ± 0.077), whereas AN (0.289 ± 0.034) and AT (0.321 ± 0.036) had the lowest values. An influence of genetic and environmental factors on the analysed traits was demonstrated by the varying heritability and common environmental effect indices across udder quarters. Estimated h² values for EC, MT and MS ranged from 0.206 to 0.503, depending on udder quarter and day of lactation. The lowest h² values, estimated for the whole udder or for individual quarters across lactation, were observed for AT and AN. It may be more appropriate to include in the selection the values of genetic parameters estimated without division into udder quarters and lactation phase.