Three greenhouse and two field plot experiments were conducted to determine the influence of seaweed extract and other plant growth regulators on forage quality and persistence of endophyte-infected (Acremonium coenophialum Morgan-Jones and Garns) and endophyte-free tall fescue (Festuca arundinacea Schreb.). The influence of plant growth regulators on forage quality of endophyte-infected tall fescue and performance of wether lambs was evaluated in two grazing trials. Greenhouse and field experiments were randomized block designs. Results from greenhouse studies varied but indicated increased (P ≤ .05) root strength and decreased (P ≤ .01) pressure required to extract moisture from leaves due to treatment. Analysis from greenhouse and field studies indicated little effect on fiber components, nonstructural carbohydrates, and crude protein. In tall fescue grazed by lambs, pyrrolizidine alkaloid concentration was decreased (P ≤ .05) between seaweed extract treatments. Ergovaline tended (P ≤ .08) to decrease in all treated forage and was 4.26, 3.78, 3.59, and 3.48 µg g⁻¹ (S.E. = .28) for control, 1703, and 3406 g seaweed extract ha⁻¹, and 19.1 L VTMix ha⁻¹, respectively. Lambs grazing treated forage in July had increased (P ≤ .05) ADG (.03 vs a mean of .11 kg gain per d for control vs treatments, respectively). Linear trends (P .13) of increased serum vitamin A and increased (P ≤ .10) whole blood Se due to seaweed extract treatment were observed. Serum Se values were 241, 264, and 274 ng/ml (S.E. = 13) for control, 1703, and 3406 g seaweed extract ha⁻¹, respectively. Results indicated that plant growth regulators may have potential to increase plant persistence and decrease toxicity of tall fescue. / M.S.