ADJUSTMENT FACTORS FOR PREVIOUS AND PRESENT DAYS OPEN CONSIDERING ALL LACTATIONS

Previous and present days open of the first three lactations were fit simultaneously as fixed effects using a mixed model containing herd-year-seasons and groups of sires (fixed effects), sires within groups, and cows nested within sires (random effects). Best linear unbiased estimates showed that, as present days open increased from 20 to 305 d, milk yield increased by 1024, 1221, and 1015 kg and fat yield increased by 30, 39, and 33 kg in first, second, and third lactations, respectively. As previous days open increased from 20 to 305 d, milk yield increased by 1041 and 1097 kg and fat yield increased by 35 and 43 kg in second and third lactations, respectively. Multiplicative adjustment factors were developed to adjust production data for the mainly environmental effects of previous and present days open. Small differences were found between the various sets of factors. One set of factors could be used for all lactations with only small differences in adjusted records.