Tasco-Forage, an extract from the brown seaweed Ascophyllum nodosum, has increased antioxidant activity in both plants and animals. Turf and forage grasses exhibited increased amounts of αtocopherol, ascorbic acid, β-carotene, and increased activities of superoxide dismutase, glutathione reductase, and ascorbate peroxidase in response to exogenous application of Tasco. Endophyte (Neotyphodium coenophialum [(Morgan-Jones and Gams) Glenn, Bacon, and Hanlin]-infected tall fescue (Festuca arundinacea Schreb.) can increase oxidative stress. Both high environmental temperatures and increased body temperatures increase respiratory rates, which increase free radical production. Steers that grazed infected fescue had diminished immune function and vitamin E, Cu, and Se status, indicating less ability to deal with oxidative stress. Two applications of Tasco-Forage (3.4 kg/ha in water solution) to infected fescue during the growing season improved steers' immune function, and the effect lasted through cross-country transportation and a 160-d feedlot finishing period. Grazing infected fescue reduced serum cholesterol, but the effect was reversed by Tasco, indicating effects on lipid metabolism. Applying Tasco to low-endophyte fescue seemed to have an immune-dampening effect on steers, at least during the grazing season. At slaughter, marbling was greater in retail cuts of meat and meat had a longer shelf-life if steers had grazed the Tasco-treated fescue, regardless of the endophyte. Direct supplementation to steers with Tasco-EX (extract) during the final 14 d in the feedyard also extended shelf-life of strip loins. Supplementation of Tasco-EX or Tasco-14 (meal) to porcine reproductive and respiratory syndrome-stressed pigs during the nursery phase improved weight gain and feed intake within 35 d. Supplementing Tasco-EX in drinking water failed to reduce morbidity and mortality in transported heifers stressed by Pasturella haemolytica. The mode of action of Tasco is not clear, but antioxidants and specific vitamins may be involved. Supplementation with certain antioxidants can have beneficial effects, but inappropriate use can have detrimental effects. Although positive effects on stress tolerance and carcass composition are apparent, further research is needed to elucidate these relationships and to provide predictable responses.