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Oxidative processes deteriorate the quality of meat products. High tannin sorghums (HTS) contain flavonoid oligomers known as proanthocyanidins of condensed tannins. These compounds act as antioxidants in vitro, but their effectiveness in vivo remains unclear. We tested the hypothesis that moderate amounts of dietary HTS could reduce markers of oxidation on muscle of rats without having detrimental effects in growth. We used 2 groups of 38 male Sprague Dawley rats at 5 and 13 wk of age each. Each age group was fed 4 diets in a completely randomized design. The younger group was fed the experimental diets for 10 wk (10W); whereas the older group was fed for 2 wk (2W). The diets were modified from the NIH-07 diet and contained HTS and corn at ratios of 0:50 (SO, control), 20:30 (S20), 35:15 (S35), and 50:0 (S50) as a percentage of the diet. Growth and the efficiency of gain were assessed periodically measuring BW, ADFI, ADG, and G:F. Oxidation in muscle was measured in fresh tissue and after 6 d of aerobic-refrigerated storage. Muscles evaluated were LM and soleus (SM). Fresh liver was also evaluated. Thiobarbituric acid-reactive substances (TBARS) and carbonyl content were used as markers of lipid and protein oxidation, respectively. No differences in BW, ADFI, ADG, and G:F were observed in 2W rats. Greater (P < 0.05) ADFI and ADG were observed in 10W-S35 group between d 1 and 7 and greater BW (P = 0.049) was observed in group 10W-S35 at d 70 compared with 10W-S0. No differences were observed between SO and any HTS diet in G:F in 10W and 2W rats. No differences in TBARS or carbonyls were observed in liver. No differences in TBARS were observed in fresh and aged LM and SM. When LM samples were aged for 6 d, decreased carbonyl contents (P < 0.01) were observed in 10W-S35 and 10W-S50 diets compared with 10W-S0. Reductions in carbonyls were also observed in aged SM between 2W-S50 and 2W-S0 (P = 0.013). We concluded that inclusion of 35% HTS in the diet increased intake and growth rate of young, fast-growing rats without changing the efficiency of gain. Feeding HTS reduced markers of protein oxidation in rat muscle after 6 d of refrigerated storage. If similar results are observed in animals such as swine or cattle, the use of HTS as animal feed should be reassessed. Key words: growth, muscle, oxidation, rat, sorghum, tannin