EFFECT OF DRAFT FORCE AND DIET ON DRY-MATTER INTAKE, MILK-PRODUCTION AND LIVE-WEIGHT CHANGE IN NONPREGNANT AND PREGNANT COWS

Eighteen F-1 crossbred dairy cows (Friesian x Boran and Simmental x Bo ran) were allocated to one of three diet groups (H: natural pasture ha y; H + 3: natural pasture hay + 3 kg concentrate; and H + 5: natural p asture hay + 5 kg concentrate) using a stratified random sampling proc edure, with parity, milk production genotype, body weight and body con dition score as blocking variables. Cows on each diet were then alloca ted to three draught forces (7, 11 and 15 kg draught force per 100 kg live weight) in a cross-over design to investigate relationships betwe en 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 th ree 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 cow s 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 lo st live weight both when pregnant and when non-pregnant. During rest d ays, non-pregnant cows on diets H, H + 3 and H + 5 compensated proport ionately 0.12, 0.59 and 0.59, respectively, of the live weight lost du ring working days. Pregnant cows on diet H + 3 and H + 5 compensated p roportionately 0.95 and 1.77 live weight, respectively. Lower total li ve-weight losses during the pregnant period could be attributed partly to relatively greater DMIs and lover mild production, but also to ges tation. Prediction of live-weight change from total DMI above maintena nce, milk yield and work output during periods of 6 working days was p oor (R(2) = 0.18). However, the same parameters explained adequately c hanges in live weight for supplemented and nonsupplemented working cow s (R(2) = 0.38 and 0.79, respectively) during a period of 90 days.