Grower ostriches that are fed more expensive diets with high energy and high protein to maximise growth can exhibit health problems. Despite this there is an ‘industry view’ that birds can be grown to slaughter weight within 8 months when high energy and protein diets are used, rather than 12–14 months using conventional diets. Given this scenario it is likely that there would be lower total feed costs associated with feeding a high energy and high protein diet for only 8 months compared with feeding a traditional diet for 12–14 months. The purpose of this experiment was to conduct an on-farm trial with grower ostriches (liveweight 48.7–50.0 kg) housed in a feedlot to examine their performance when subjected to four dietary treatments fed over 4 weeks: (i) treatment 1: birds fed a commercial grower diet with 10.7 MJ/kg and 138.0 g/kg of protein; (ii) treatment 2: birds fed a low energy (10.0 MJ/kg) and low protein (126.0 g/kg) diet; (iii) treatment 3: birds fed a high energy (12.5 MJ/kg) and medium protein (136.0 g/kg) diet; and (iv) treatment 4: birds fed a high energy (12.5 MJ/kg) and high protein (143.0 g/kg) diet in a feedlot. Birds fed on the low energy and low protein diet had the highest feed intake compared with the other treatments. Birds on treatment 2 also had a significantly higher (P = 0.01) daily weight gain (277.3 g/bird) compared with treatment 1 (50.9 g), treatment 3 (49.1 g) and treatment 4 (32.0 g), respectively. The results indicate that better daily bodyweight gain is likely if ostrich growers are fed with a lower energy and lower protein diet.