PREDICTING GENETIC TREND BY UNITRAIT AND MULTITRAIT MODELS

The effects of evaluating milk yield in the first three lactations by a single-trait animal model or by a repeatability animal model instead of by the true multitrait animal model were investigated using stocha stic simulation. The models were compared both in terms of how accurat ely they predicted genetic trend when applied to the same dataset and regarding difference in true genetic progress when selecting on breedi ng values predicted by these models. A breeding structure resembling t he Icelandic dairy cattle population was used for the simulation. The single-trait model underpredicted the true genetic trend in second and third lactations quite heavily but only by 2.3% in the first lactatio n, while the repeatability model overpredicted the true genetic trend in the first three lactations by 9.4% on average. The multitrait anima l model, however, estimated genetic trends with a high degree of accur acy. When selection was made on all three lactations with equal econom ic weights, the multitrait model was only about 2.5% superior to the r epeatability model in terms of true genetic trend. The multitrait anim al model was applied to estimate genetic trend regarding milk producti on using real data from the Icelandic dairy cattle population. A total of 61 621 animals were included, of which 38 014 were cows with produ ction records. The genetic trend for milk yield was estimated at 10.4, 10.0 and 9.3 (kg per year) in each of the three first lactations resp ectively.