Energy and nitrogen metabolism were examined at weekly intervals during lactation weeks 1 to 8 in Holstein-Friesian cows (no. = 9) offered a diet of hay, maize pellets (whole plant) and concentrates, (barley, maize and soya bean; forage : concentrate ratio 65 : 35), with feeding levels close to ad libitum.
After calving, the cows lost body weight until week 7, with peak milk yield (35 kg/day) recorded during week 3. Dry-matter intakes increased progressively to week 4 then remained relatively constant. Apparent digestibility of dietary energy was unaffected by stage of lactation but the overall value was low (0·653) indicative of the quantity and quality of long hay in the diet. Metabolizable energy intakes ranged between 163 and 202 MJ/day, with little between-week variation after that between weeks 1 and 2 (P < 0·01). Milk energy output was relatively stable during weeks 1 to 4 but then declined progressively for each remaining week, whilst heat energy output was relatively constant throughout. Estimates of body energy retention indicated the cows were in negative energy balance at all times, being greater in week 1 (64 MJ/day, P < 0·01) than weeks 2 to 4 (mean, 35 MJ/day) or weeks 5 to 8 (22 MJ/day). Digestible nitrogen (N) intake was reduced in week 1, whilst apparent N digestibility declined significantly , with little between-week variation after that between weeks 1 and 2 < 0·05) as lactation progressed. The cows were in negative N balance (–19 g/day) during week 1, with zero (week 2) or positive balances noted thereafter.
It is concluded that during early lactation, the extent of body tissue mobilization in average yielding cows can be substantial and prolonged, whilst attainment of positive body N status occurred earlier and was not related to the energy status of the cows.