Efficiency of Test Day Models in Genetic Evaluation with Part of Lactation Information

There has been much interest recently in genetic evaluations for milk production traits in dairy cattle using a Test Day Model (TDM). The advantages of a TDM include more accurate correction for environmental effects on Test Day records (TD), ability to model the lactation curve and estimation of persistency evaluations. In countries not implementing a TDM, the use of part lactation TDs usually involves projecting a 305 day yield. In some cases, especially for young bulls on test with most daughters having part lactations, the projection procedure could result in under prediction of the bulls’ breeding values; which is the so called “rip-dip” effect. While the TDM has the advantage that it avoids the need for such projection, its efficiency in genetic evaluation, especially for young bulls with all or the majority of daughters with incomplete lactations, has not been examined.