Effect of different feeding regimens on energy and protein utilization and partitioning for maintenance and growth in pre-weaned lambs reared artificially

Estimation of metabolizable energy (ME) requirement for maintenance (MEm) and growth (MEg) in pre-weaned lambs have been limited to milkonly fed lambs. This study aimed to determine energy and nitrogen metabolisability of milk and pellets when fed together, compare the growth and chemical body composition of lambs fed varying levels of pellets in addition to milk, and to estimate MEm, MEg, and the CP:ME ratio requirements for growth. The study included 32 twin-born Romney-cross ram lambs. Four lambs were slaughtered at 24 h post-partum to estimate initial body composition and the remaining 28 were assigned to 1 of 4 treatment groups of 7. Group 1 was fed milk replacer (MR) only; group 2 was fed MR and allowed ad libitum access to pellets; groups 3 and 4 were offered 30% and 60%, respectively of the average pellet intake of the ad libitum group the previous day while being fed MR. Milk replacer was fed as a proportion of the lamb's live weight (LW). Lambs from each treatment were placed in metabolic cages at 17 kg LW for 4 d to allow for total fecal and urine collection. All lambs were slaughtered at 18 kg LW. The ADG, ADG: ME ratio, stomach and liver weight, and rumen papillae lengths increased (P < 0.05) with increasing pellet intake. Increasing daily ME intake increased (P < 0.05) both daily energy and protein deposition but had no effect (P > 0.05) on fat deposition. However, the total chemical body composition was unaffected (P > 0.05) by dietary treatment. Digestibility of energy and N decreased (P < 0.05) with increasing ME intake. Percent energy and N retained for growth were 96% vs. 71% and 72% vs. 30% for milk and pellets, respectively. The MEm and MEg values obtained were 0.40 MJ ME/kg LW0.75 . d(-1) and 13.8 MJ ME/kg ADG, respectively. The CP: ME ratio of MR and pellet was 11.1 and 15.7, respectively. However, a simulation model suggested that lambs require a CP: ME ratio of 13.1 at 5 kg and 10.9 at 18 kg LW, indicating that protein intake may be limiting to lamb growth in early life and in excess by 18 kg LW. In conclusion, increasing pellet intake was associated with decreased N retention. The inclusion of pellets, however, improved the efficiency of ME utilization for growth in pre-weaned lambs and was beneficial for rumen development. The MEg was higher than previously recommended values and the CP: ME intake of lambs does not match their requirements which may warrant further studies.