The aims in this thesis were to investigate more comprehensive breeding goals in dairy cattle, to investigate the possible importance of genotype by environment interaction and to identify possible sources of genetic variation in feed utilisation. On the basis of a literature review, longevity was examined as component of the breeding goal, and four linear type traits (foot angle, udder depth, teat length and angularity) with the closest genetic association with longevity were used as index measurements. Economic values were derived for protein, fat and milk yield and for longevity using dynamic programming. The derived values were in genetic standard deviations relative to protein yield 1.0, 0.2, -0.2 and 0.8, respectively. Three completely additive indices were derived, assuming that the breeding goal was for: (i) yield only (YIN), (ii) longevity only (LIN) or (iii) yield and longevity, hence profit (PIN). Selection on PIN was expected to give a 5% higher annual rate of genetic progress in economic merit compared with selection on YIN, and PIN was robust to most assumptions made in the calculation. Genotype by environment interaction was investigated for a range of traits. Selection (S) and Control line (C) cows, housed and managed at the Langhill Dairy Cattle Research Centre, have been offered ad libitum complete mixed diets, with proportions (in total DM) of concentrates, silage, brewers' gains of either 20:5:75 (LC; 1.0 ton concentrate per annum) or 45:5:50 (HC; 2.5 ton concentrate), over a full lactation. No diet x genetic line interactions were observed for a number of traits, describing milk production, feed intake, efficiency and body tissue mobilisation. However, regression coefficients of milk yield and condition score on pedigree index for fat plus protein yield were significantly different between LC and HC. Phenotypic and genetic variances were generally larger on HC than on LC, but difficulties in separating the permanent environmental variance from the additive genetic variance might have obscured some of the comparisons.