Phenotypic and genetic correlations between measurements of feed intake, body weight and production within and between 4 periods in the first 15 weeks of lactation have been estimated. Measurements were available on 358 dairy heifers during 2 week periods starting at 2, 5, 9 and 13 weeks after calving. Heifers were progeny of 38 sires. The diet consisted of 6 kg concentrates and roughage ad libitum. Genetic parameters were estimated by REML fitting an animal model. Heritability of roughage intake decreased from 0.32 in week 2 to 0.18 in week 13. Fat protein corrected milk production (FPCM) had a heritability of 0.33 in week 2 which increased to 0.47 in week 13. Heritability of feed conversion changed from 0.17 to 0.29 from week 2 to 13. The largest differences in heritability were found between week 2 and week 5. The genetic correlation between roughage intake and feed conversion changed from 0.24 in week 2 to −0.57 in week 13. The genetic correlation between measurements in week 2 and 9 was 0.47 for energy intake, 0.80 for roughage intake and 0.99 for FPCM production. Estimated genetic correlations and heritabilities have been used to determine the relative efficiency of single trait selection using measurements in one single period. It is concluded that for an accurate evaluation of animals for feed intake and feed conversion measurement during at least two periods are needed.