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32 pairs of monozygons twins were fed ad lib. with a complete diet feed during the first 20 weeks post partum. The heritablity (in broader sense) for milk yield, feed intake and energy deficit was 0.39, 0.17 and 0.19 resp. the genetic and phenotypic correlations between daily milk yield and feed intake were 0.14 and 0.26 resp. Therefore selection for feed intake should be based on progeny groups of A. J. bulls. To estimate the feed intake under field conditions the content of ketone-bodies in milk at first and second test day p.p. can be combined with the milk yield and fat- and protein content. The correlation between estimated and actual energy deficit in 101 cows was r=0.17. In field data with about 7500 cows there was a genetic varience of the acetone content in milk in cows with two and more lactations with h2=0.17 ± 0.09 and h2=0.09 ± 0.06 for the first and second test day p.p resp., but not in heifers.