Thompson, D. J. 2013. Yield and nutritive value of irrigated tall fescue compared with orchardgrass: in monocultures or mixed with alfalfa. Can. J. Plant Sci. 93: 799-807. Orchardgrass (Dactylis glomerata L.) is commonly grown for irrigated forage production in interior British Columbia. Tall fescue [Schedonorus phoenix (Schop.) Holub.] is also adapted to the area but no comparative trials have been reported. Three varieties of each grass species were grown in monocultures or in mixtures with alfalfa at three irrigated sites throughout southern interior British Columbia. Study objectives included comparing the forage yield and nutritive value of the following groups: (1) tall fescue and orchardgrass monocultures, (2) tall fescue and orchardgrass mixtures with alfalfa and (3) grass-alfalfa mixtures with monocultures. In monoculture, tall fescue yield was 9% greater than orchardgrass (significantly greater yield at 3 of 6 site-years), though forage nutritive values were similar. Mixtures of the two grasses with alfalfa had similar yields, but those containing tall fescue had superior nutritive value. Alfalfa contributed a greater percentage to total yield and had higher survival when mixed with tall fescue. Tall fescue is a viable alternative to orchardgrass for irrigated forage production in monoculture and may be more suitable for mixtures with alfalfa. Our findings demonstrate a functional diversity effect; grass-alfalfa mixtures over-yielded the mean of the alfalfa, orchardgrass, and tall fescue monocultures by 12%.