Energy and nitrogen metabolism in Holstein-Friesian cows during early lactation

Energy and nitrogen metabolism were examined at weekly intervals during lac tation weeks 1 to 8 in Holstein Friesian cows (no. = 9) offered a diet of h ay, maize pellets (whole plant) and concentrates, (barley, maize and soya b ean;forage : concentrate ratio 65 : 35), with feeding levels close to ad li bitum. 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 progressi vely 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 quantify and quality of long hay in the d iet. Metabolizable energy intakes ranged between 163 and 202 MJ/day, with l ittle between-week variation after that between weeks 1 and 2 (P < 0.01). M ilk energy output was relatively stable during weeks 1 to 4 but then declin ed progressively for each remaining week, whilst heat energy output was rel atively constant throughout. Estimates of body energy retention indicated t he 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 (P < 0.05) as lactation pr ogressed. The cows were in negative N balance (-19 g/day) during week 1, wi th zero ( week 2) or positive balances noted thereafter It is concluded that during early lactation, the extent of body tissue mobi lization in average yielding cows can be substantial and prolonged, whilst attainment of positive body N status occurred earlier and was not related t o the energy status of the cows.