Ninety-six Hereford steer calves that weighed 249(s.e.4·7)kg were allocated randomly to four treatments of 24 steers each in a randomized complete block design with four blocks. The steers grazed dormant native range forage that contained 41 to 60 g of crude protein per kg dry matter during the 120-day wintering trial and were given 0·91 kg of supplement per head per day. The supplements used contained 175 or 350 g crude protein per kg dry matter and 0 or 165 mg monensin per kg supplement; a factorial arrangement of treatments was used in allocating the supplements. Weight gains of steers given the high-protein supplements were greater than those on the low-protein supplements during periods 1(0 to 61 days) and 2 (62 to 120 days), and over the total trial (P < 0·01). Monensin increased weight gains during period 1 and the total trial (P < 0·01). The protein level × monensin interaction was not significant for either period or the total trial (P > 0·15). The results indicate that inclusion of monensin in protein supplements for growing cattle grazed on low-quality dormant native forage may allow some reduction in protein content of the supplement.