Dietary energy:protein ratio influences the efficiency of utilisation of dietary protein by growing turkeys.

1. The following experiment was designed to confirm that the efficiency of dietary protein utilisation (ep) can be reduced when the AMEn to digestible crude protein (DCP) ratio of the feed falls below a critical level. In addition, whether the efficiency would be improved at high DCP contents through the addition of oil or starch was determined.2. A 14d trial (starting at 21 d of age), using BUT 6 turkeys, was designed to measure the effects on ep of four feeds (12.1 MJ AMEn/kg) with AMEn:DCP ratios ranging from 45 to 72 MJ AMEn/kg, and two additional feeds with the same DCP as the highest protein feed, one with an AMEn:DCP ratio of 59, through the addition of canola oil, and the other with a ratio of 51 MJ/kg supplemented with both starch and oil. Twenty birds constituted each experimental unit, with two replications of each sex being used per dietary treatment.3. Ten birds per sex were sampled at the start of the trial and a further five per pen at the end for carcass analysis. Each carcass including feathers was ground and subsampled for water, protein and lipid determination.4. A split-line regression described ep on the four feeds unsupplemented with additional energy (R = 0.81) with breakpoint at 60.6 MJ AME/kg DCP, maximum ep at 0.736 g/g, and a slope of 0.017. No improvement in ep resulted from supplementing the high protein feed with oil or a mixture of oil and starch, but feed intake and protein and lipid retention increased significantly (P < 0.05).5. The argument that insufficient energy is available to enable high protein feeds to be utilised efficiently was not corroborated by the evidence from this trial.