EXAMINATION OF ENERGY-UTILIZATION IN CATTLE OFFERED A FORAGE DIET AT NEAR-MAINTENANCE AND SUB-MAINTENANCE LEVELS OF FEEDING

Eight Friesian calves were reared from birth to the start of the exper iment at a predetermined growth rate of 1.0 kg/d. Four calves (group 1 ) commenced the experiment at 22 weeks of age and the remainder (group 2) at 34 weeks, and feed intake was calculated to provide sufficient metabolizable energy to sustain zero energy balance (E(m)). Between ex perimental days 30 and 56 at maintenance levels of feeding, respirator y exchange measurements were made using open-circuit calorimetry to pr ovide indirect estimates of heat production (H) simultaneous with meas urements of faecal and urinary excretion of energy and N. A datum poin t for E(m) and H was established for each animal giving mean values fo r E(m) of 491 and 537 and for H of 476 and 511 kJ/kg live weight (LW)( 0.75) per d for groups 1 and 2 respectively. Treatment levels calculat ed as 0.25, 0.5, 0.75 and 1.25 of dry matter intake required to sustai n the measured E(m), were imposed on individual animals after day 56 a nd further measurements of H and faecal/urinary balance were made betw een days 72 and 84 followed by measurements of fasting heat production (FHP) during days 86-90 from the two animals in each group which had received the 0.75 and 1.25 levels of intake. Regression analysis of th e treatment levels indicated separate linear models which predicted E( m) at 419 aad 473 kJ/kg LW(0.75) per d for groups 1 and 2 respectively . The incorporation of FHP with partitioning of faecal and urinary ene rgy losses measured during fasting altered the relationship but not th e predicted E(m). Overall predicted E(m) (days 72-84) from all models were 406 and 478 kJ/kg LW(0.75) per d for groups 1 and 2 respectively which were significantly lower (P < 0.05) than E(m) measured during da ys 30-56.