Sixteen ram lambs (5 m.o. old, 45 +/- 1.5 kg) received a control diet (50% concentrate, no locoweed, n = 4), locoweed (20% locoweed for 21 d, n = 4), MUSE (2 mL i.m. of MUSE containing 5 mg selenium and 50 mg vitamin E/mL, n = 4) on Days 21 and 35([Day 0 = first day of trial]), or locoweed + MUSE (n = 4). The rams were maintained in individual pens (3 x 9 m) with free access to feed, water, salt and shade. On Day 7 after initiating locoweed, serum alkaline phosphatase (AP) increased (P < 0.01), and serum thyroxine (T4) decreased (P < 0.01) in locoweed-fed rams. Effects on serum AP and T4 remained constant in rams during the 21 d of locoweed feeding. Treatment with MUSE did not influence (P>> 0.10) AP or T4. Locoweed-fed rams had reduced (P < 0.05) intake and body weight for the 2-wk period after locoweed feeding ended. The MUSE regimen or diet had no effect on intake or body weight (P>> 0.50). Neither locoweed nor MUSE affected serum LH before or after GnRH administration on Day 22 (P>> 0.10). On Day 50, however, area under the LH curve (AUC) was 966 units in locoweed-fed rams and 1,373 units (+/- 154) in controls (P = 0.09). Serum testosterone (T) was reduced in locoweed-fed rams before and after (P < 0.05) GnRH on Day 22. On Day 50, the T AUC was numerically lower (P = 0.14) in locoweed-fed rams (1,252 units) than in controls (1,539 +/- 130 units). Conversely, MUSE treatment resulted in increased (P = 0.02) T AUC on Day 50 (1,148 and 1,643 +/- 130 units in control and MUSE-treated ram lambs, respectively). During the 6-wk period after locoweed feeding, serum immunoglobulin G averaged 14.0 and 18.6 (+/- 1.1) mg/mL in control and locoweed-fed rams (P < 0.01), respectively. Twenty percent dietary locoweed for 21 d exerts adverse effects on feed intake, growth, and reproduction in young ram lambs and MUSE was not effective in reversing these effects.