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Six experiments were conducted to investigate the effect of a feed supplement on performance of grazing Belgian Blue double-muscled heifers with an initial weight and age of 195±43 kg and 190±52 days. Treatments included were: Exp. 1: supplementation with beet pulp (BP): 2 kg/day/head vs. ad libitum intake; Exp. 2: supplementation ad libitum with BP vs. a mixture of BP and soybean meal (SBM; BP/SBM ratio of 80/20; FW (fresh weight) basis); Exp. 3: supplementation with 4 kg/day/head of a mixture of BP/SBM (80/20; FW basis) vs. BP/formaldehyde treated SBM (BP/FSBM); Exp. 4: supplementation with 4 kg/day/head of a mixture with a similar protein content (125 g DVE per kg dry matter), consisting of 80/20 BP/SBM vs. 92/8 BP/FSBM; Exp. 5: supplementation with 3 kg/day/head of a mixture of BP/SBM (80/20; FW basis) vs. BP/DDGS (dried distillers grains and solubles; 70/30, FW basis); Exp. 6: supplementation with 3 kg/day/head of 80/20 BP/SBM vs. maize silage and SBM, based on a similar protein concentration in the dry matter as the 80/20 BP/SBM supplement, and fed at a similar amount of dry matter as in the BP/SBM group. Supplementing BP ad libitum did not affect daily gain (0.54 vs. 0.48 kg) and partial feed conversion (3.62 kg on average) compared with 2 kg/day. Supplying SBM besides BP increased growth rate compared to BP (0.87 vs. 0.62 kg/d; P < 0.001), but partial feed conversion was similar. Supplying FSBM did not affect growth rate and partial feed conversion (P > 0.10), but blood urea levels were reduced by FSBM (P < 0.05). DDGS tended to increase growth rate (0.77 vs. 0.59 kg/d; P < 0.10) compared to BP/SBM, without effect on partial feed conversion. Replacing BP by maize silage did not affect daily gain, but partial feed conversion tended to be higher (3.21 vs. 3.60 kg/kg body weight gain; P = 0.062). Increasing the supplement (80/20 BP/SBM) level from 3 to 4 kg daily, corresponding to 1.02 and 1.18% of the mean body weight, respectively, resulted in a tendency (P = 0.121) for an increased growth rate. Grazing Belgian Blue double-muscled heifers of less than one year of age necessitate extra protein beside an energy supplement to improve their performance. DDGS can replace SBM and BP can be replaced by maize silage.