Eighteen F1 crossbred dairy cows (Friesian × Boran and Simmental × Boran) were allocated to one of three diet groups (H: natural pasture hay; H+3: natural pasture hay + 3 kg concentrate; and H+5: natural pasture hay + 5 kg concentrate) using a stratified random sampling procedure, with parity, milk production genotype, body weight and body condition score as blocking variables. Cows on each diet were then allocated to three draught forces (7, 11 and 15 kg draught force per 100 kg live weight) in a cross-over design to investigate relationships between work output, live-weight changes, dry-matter intake (DMI) and milk yield. Each cow worked for 36 days in early lactation (from calving to 90 days) and for a further 36 days in late lactation (from 250 to 340 days post partum). Work output was similar for cows on each of the three diets. Cows on the H diet consumed more hay than cows on H+3 and H−5 diets. Hay and total DMI, milk yield and milk fat were similar across draught forces and during working and resting days when the cows were not pregnant. Similar results were obtained when cows were from 82 to 172 days pregnant. Pregnancy did not affect the ability of cows to perform work at different intensities. During working days cows lost live weight both when pregnant and when non-pregnant. During rest days, non-pregnant cows on diets H, H+3 and H+5 compensated proportionately 0·12, 0·59 and 0·59, respectively, of the live weight lost during working days. Pregnant cows on diet H+3 and H+5 compensated proportionately 0·95 and 1·77 live weight, respectively. Lower total live-weight losses during the pregnant period could be attributed partly to relatively greater DMIs and lower milk production, but also to gestation. Prediction of live-weight change from total DMI above maintenance, milk yield and work output during periods of 6 working days was poor (R2 = 0·18). However, the same parameters explained adequately changes in live weight for supplemented and non-supplemented working cows (R2 = 0·38 and 0·79, respectively) during a period of 90 days.