Two experiments evaluated effects of amount of supplemental alfalfa hay on intake and utilization of dormant, tallgrass-prairie forage by beef steers and on performance of cows grazing tallgrass prairie during winter. In Exp. 1, four supplemental alfalfa levels (.23, .47, .70, and .94% BW.steer-1.d-1) were evaluated in a 34-d, randomized complete block design experiment using 16 steers (291 kg). Voluntary tallgrass-prairie hay intake decreased linearly (P = .02), whereas total DMI increased linearly (P < .01) with increased alfalfa. Dry matter digestibility was unaffected (P > .10) by treatment, although NDF digestibility decreased (linear, P = .03) and passage rates of indigestible ADF and Cr EDTA increased (linear, P = .02) with increased alfalfa. In Exp. 2, supplemental alfalfa (.48, .72, or .96% BW.cow-1.d-1) was fed to 113 pregnant Hereford x Angus cows (502 kg) from November 27 until calving (average calving date = March 7). Cumulative weight loss from the beginning of the experiment until just after calving was lowest with .96% BW alfalfa (quadratic, P = .09), and cumulative condition loss was decreased linearly (P = .02) with increased alfalfa. Although treatment did not alter (P > .10) pregnancy rates, increasing the amount of alfalfa supported shorter intervals to conception (P = .03). Cows fed .96% BW alfalfa weaned heavier calves (quadratic, P = .04) than other groups. Results indicate that improvements in performance of beef cows in moderate body condition were greater when the amount of supplemental alfalfa was increased from .48 to .72% BW than when it was increased from .72 to .96% BW.(ABSTRACT TRUNCATED AT 250 WORDS)