An examination of energy utilization in lactating dairy cows receiving a total mixed ration based on maize silage

Six multiparous Holstein-Friesian cows were offered a total mixed ration ba sed on maize silage in a repeated measure design to evaluate the partition of gross energy (GE) during early to mid lactation. Four measurements were made at 6-week intervals with energy and nitrogen balances carried out in o pen-circuit respiration chambers over 6 days during lactation weeks 6, 12, 18 and 24. The intakes of total diet dry matter (DM) corrected for volatile losses (VCDM), organic matter (OM) and GE declined significantly (P < 0.01 ) as lactation progressed, although apparent digestibility of these functio ns was not altered, resulting in a significant (P < 0.01) decline in digest ible nutrient intake at each stage of lactation. Methane and urine energy l osses were not significantly affected, resulting in significantly (P < 0.00 1) higher amounts of digestible energy (DE) partitioned to methane and urin e as lactation progressed with associated significant reductions in metabol izable energy (ME) intake (MEI) (P < 0.01) and ME as a proportion of DE (P < 0.001) and GE (q) (P < 0.05). With advancing lactation there was a signif icant (P < 0.001) increase in the amount of ME partitioned to heat (HP/MEI) , but no significant change in the amount partitioned to milk and tissue. I ndividual values for diet metabolizability (ME/GE) at actual (production) l evels (q(c)) (mean 0.625 MJ/MJ) were corrected to an equivalent value at ma intenance (q(mc)) (mean 0.666 MJ/MJ). The overall ME intakes (MJ/day) were: ad libitum, 246, corrected for level of feeding effect, 263, with a predic ted ME requirement according to AFRC (1993) (MER93) of 242. Substitution of the calculated q(mc) into the predictive equations (AFRC, 1993) resulted i n a mean maintenance requirement of 57.6 MJ/day (0.464 MJ/kg M-0.75/day) wh ilst the mean value derived from the linear model describing the experiment al data was 82.5 MJ/day (0.664 MJ/kg M-0.75/day). The mean efficiencies of utilization of ME for milk production derived from AFRC (1993) and the line ar regression model were 0.653 MJ/MJ and 0.625 MJ/MJ respectively.