Response in morphology, plant mass, and quality to variation in daylength and temperature was determined for American and Swedish cultivars of red clover (Trifolium pratense L.) and timothy (Phleum pratense L.). These characteristics were evaluated for seven weeks in growth chambers in which a combination of 10-, 15-, or 20-h daylengths and temperatures of 10 or 20°C were used. In a field study, changes in stage of maturity and quality of American and Swedish cultivars of alfalfa (Medicago sativa L.) red clover, timothy, and smooth bromegrass (Bromus inermis Leyss.) were studied at four locations in Iowa, Minnesota and Wisconsin in 1984 and 1985. Stage of maturity and concentrations of N, in vitro dry matter digestibility (IVDMD), and fiber were evaluated for eight weeks. Increased daylengths and temperatures enhanced maturity and total dry weight. Herbage weight was relatively more stimulated than root weight, however. Maturity and plant mass of red clover and timothy were more affected by increases in daylength than by increases in temperature. Swedish cultivars required longer days than American cultivars to reach comparable maturity stages, resulting in lower maturity and productivity for Swedish cultivars. Maturity and composition differed more for red clover than for the other species indicating that red clover cultivars were genetically more diverse. Grasses were more mature and developed faster at locations with low temperatures, whereas legumes were more mature at high temperatures. Maturity of all species was negatively affected by dry conditions in 1985. Alfalfa and smooth bromegrass were less affected than timothy and red clover possibly because of higher tolerance to drought. Increased dry matter and maturity resulted in diluted concentrations of N and IVDMD, whereas fiber concentrations increased. Hot and dry conditions in 1985 resulted in a smaller range and a smaller rate of change over time in forage quality compared with results in 1984. Average N and IVDMD concentrations were negatively affected by extended daylengths and increased temperatures, respectively. It is concluded that development, growth, and composition of genetically diverse forage legumes and grasses respond differently to environmental variations.