Genetic and nongenetic factors associated with lactation length in seasonal-calving, pasture-based dairy cows

Lactation yield estimates standardized to common lactation lengths of 270-d or 305-d equivalents are commonly used in management decision support tools and dairy cow genetic evaluations. The use such measurements quantify the (genetic) merit individual cows fails penalize that do not reach length, indeed reward lactate for more than length. objective present study was nongenetic factors associated with length seasonal-calving, pasture-based cows. A total 616,350 records from 285,598 Irish were used. Linear mixed models associations between calving month, parity, age at calving, previous dry period difficulty score, heterosis, recombination loss, breed, herd size, as well estimate residual variance components median edited data set 288 d, 27% achieving lactations least 305 d. Relative January, February, March, April was, on average, 4.2, 12.7, 21.9 d shorter, respectively. a first parity 7.8, 8.6, 8.4 shorter second, third, fourth cows, Norwegian Red Montbeliarde had, 4.7- 1.6-d Holstein-Friesian heritability estimate, coefficient variation, repeatability 0.02, 1.2%, 0.04, Based standard deviation estimated (3.3 d), ranked top 20% would be expected have 9.2 longer bottom 20%, demonstrating exploitable variability. Given vast array an approach which combines improved practices selective breeding may efficient effective strategy lengthen lactations.