Two experiments were carried out to study the efficiency of energy utilization by Suffolk × Blackface lambs. In the first experiment, a digestibility trial, the lambs were fed on either a roughage (chopped dried grass), concentrate (whole barley, ground barley and fishmeal), or mixed (roughage and concentrate) diet. The dry matter (DM) and organic matter digestibility coefficients of the mixed diet were slightly, but not significantly, lower than the ‘expected’ values calculated by summation of the separately determined digestibility coefficients of the component feeds. Similarly, the metabolizable energy (ME) value was 1·6% lower than the ‘expected’ value.
In the second experiment, lambs were offered 52·5 kg roughage DM (ME 9·96 MJ/kg DM) and 52·5 kg concentrate DM (ME 11·59 MJ/kg DM) as:(i) a mixed diet (M); (ii) a progressive change from roughage to concentrate (R–C); (iii) a progressive change from concentrate to roughage (C–R); or 105 kg roughage or concentrate DM as either (iv) a roughage diet (R), or (v) a concentrate diet (C). Lambs on treatments R–C, C–R and R and C combined took a similar time to consume their allocation of food and achieved a similar final empty body weight (EBW) and empty body composition. In addition, the total energy retention (ER) of lambs on these treatments was similar to that predicted by a feeding standards model for lambs of a similar liveweight offered diets of the same ME content. Lambs on treatment M consumed their allocation of food faster than those on all other treatments and achieved a slightly, but not significantly, higher final EBW, which contained a higher proportion of fat. The ER was higher than predicted. The higher overall efficiency of lambs on treatment M was attributed to higher daily intakes and differences in the ratio of protein: fat deposited and maintained by lambs on each treatment. The adoption of separate efficiency constants for both deposition and maintenance of protein and fat in current feeding standards would result in a more accurate prediction of animal performance.