Effect of dietary protein to energy ratio of milk replacer on growth and body composition of pre-weaned lambs reared artificially

This study aimed to determine the effect of changing the dietary crude protein to metabolisable energy ratio (CP:ME) of milk replacer on growth and body composition of pre-weaned lambs. Thirty-two Romney twin-born ram lambs were selected with four lambs being slaughtered at 24 h post-partum to estimate initial body composition. The remaining twenty-eight lambs were assigned to one of three nutritional treatments. Treatments consisted of either (i) a commercial milk replacer (CMR, n = 10); (ii) a high protein milk replacer (HPM, n = 9) or (iii) a mix of normal milk replacer and milk protein concentrate (MB, n = 9). All lambs were fed at 2.1 times their maintenance requirement. The CP:ME ratio of MB was adjusted twice-weekly to match the lambs CP and ME requirements for growth and maintenance over time. All lambs were slaughtered at 22 kg live weight (LW). The LW and chemical composition of carcass and viscera plus blood were determined. The combined CP:ME ratio of milk and pellets was greater for the HPM lambs than MB lambs, which was greater (P < 0.0001) than CMR lambs. Daily ME intake was greater (P < 0.05) in MB compared to HPM and CMR, which did not differ (P > 0.05). Average daily LW gain was higher (P < 0.05) in both HPM and MB lambs than CMR lambs. The ME and CP intakes per kilogram LW gain were greater (P < 0.05) for CMR and HPM lambs, respectively than MB lambs. Omental fat content at slaughter was lowest (P < 0.05) and gut fill was greatest (P < 0.05) in HPM, while CMR and MB did not differ (P > 0.05) for either parameters. Fat content of the carcass and viscera plus blood was lowest (P < 0.05) in HPM whilst, there was no difference (P > 0.05) between CMR and MB. Lambs fed HPM had lower (P < 0.05) daily fat deposition than CMR, which was highest (P < 0.05) in MB. Daily dry matter deposition was greater (P < 0.05) in MB than both CMR and MB, which did not differ (P > 0.05). In conclusion, adjusting the CP:ME ratio of milk replacer to match the lambs’ theoretical requirement improves growth performance of artificially reared lambs. Further studies are required to determine the optimal CP:ME ratio effect of milk replacer, its cost effectiveness and to examine post-weaning growth effects.