Selection for body weight in dairy cattle

This thesis deals with selection for body weight (BW) in dairy cattle. The economic efficiency of present breeding schemes might increase further when selection decisions also consider information on BW as BW relates to feed costs and revenues from beef production. However, the practical implementation of such a selection strategy is hindered by limited knowledge on procedures for data recording and genetic evaluation. The aim of this thesis was to study the possibilities and economic relevance of selection strategies for BW in dairy cattle. To obtain information on the optimal moment and frequency of data recording, this thesis analysed repeated BW observations on growing and lactating heifers from experimental herds. A study on growth patterns of heifers from birth until first calving fitted by two mathematical functions suggested that BW at first calving has a high correlation with mature BW.Repeated observations for BW were used to estimate non-genetic effects and repeatabilities during the first three parities. The most important non-genetic effects on BW were feeding, lactation stage, pregnancy stage and age at calving. Observations for BW were highly repeatable within and across parities. Observations at any lactation stage were highly correlated with mean BW during lactation. These results indicated that a single BW observation on a lactating heifer is a good predictor of mean BW during the first three parities.A genetic standard deviation of 29 kg and a heritability of 0.33 were estimated from a large field experiment. The high correlations between BW and conformation scores show that a large-scale genetic evaluation for BW can easily be based on conformation data that are already routinely recorded in most breeding schemes. A study of the dynamic aspects of BW during lactation showed that it is possible to select for mean BW during lactation but that opportunities to select for specific BW change patterns were limited.This thesis also estimated the genetic