Abstract Orchardgrass was stored at 50% dry matter as large round bales to study short-term effects, and ryegrass (33% dry matter) was ensiled in laboratory silos, which were incubated at 35, 55, and 75°C for up to 46 d to study long-term effects of heat on the chemical composition of grasses. Bale temperature reached 70°C by d 6 postbaling and maintained that temperature for the remaining 7 d of the experiment. In the bales, concentrations of acid and neutral detergent insoluble nitrogen, acid detergent fiber, and lignin increased, whereas hemicellulose decreased over the 13-d storage. Concentration of total amino acids was 99.06 mg/g dry matter on d 0 and decreased linearly at the rate of 1.64 mg/g per d. Total nitrogen remained constant. Concentration of amino acids not soluble in acid detergent was 3.73 mg/g dry matter on d 0 and increased logarithmically (15%/d). Changes in chemical composition of the ryegrass stored at 75°C were similar to those of the orchardgrass. Composition of 35 and 55°C silage was not affected greatly by temperature. Concentration of total amino acids in fresh herbage was 55.86 mg/g dry matter and 47.08, 47.61, and 37.77 mg/g for the 35, 55, and 75°C silage after 46 d of storage. Amino acids insoluble in acid detergent constituted 7.2, 21.4, and 75.0% of the total amino acids in the 35, 55, and 75°C silage on d 46. In summary, heat damage reduced greatly the amount of amino acids present in the forages and altered their distribution.