Metabolizable protein requirements of Dorper crossbred ram lambs

The net protein (NP) and metabolizable protein (MP) requirements of Dorper crossbred ram lambs grown from 35 to 50 kg BW were assessed in a comparative slaughter trial. Thirty-five intact male lambs (34.1 ± 0.6 kg BW) of F1 crosses of Dorper × thin-tailed Han sheep were used: 7 lambs of 35 kg BW were slaughtered as the baseline animals at the start of the trial, and the remaining 28 lambs were randomly divided into 4 groups of 7 lambs each. Three of the 4 groups were fed a pelleted mixed diet (concentrate/roughage = 44:56, DM basis) for ad libitum intake or 65% or 50% of ad libitum intake, and they were all slaughtered when the lambs fed ad libitum reached 50 kg BW. The lambs from the fourth group were also fed ad libitum and slaughtered at 43 kg BW as the intermediate group. Total body N and N retention were measured. The daily NP requirement for maintenance of the ram lambs was 1.56 g/kg metabolic BW (BW0.75) or 1.64 g/kg metabolic shrunk BW (SBW0.75), whereas the daily MP requirement for maintenance was 3.69 g/kg BW0.75 or 3.89 g/kg SBW0.75, with a partial efficiency of MP utilization for maintenance of 0.42. The MP requirement for growth ranged from 19.4 to 77.1 g/d for ADG from 100 to 400 g BW, and the partial efficiency of MP utilization for growth was 0.75. The NP and MP requirements of Dorper × thin-tailed Han crossbred ram lambs differed from the recommendations of the American and British nutritional systems.