Angus cows (Bos taurus) with calves and 1/2 Dorset, 1/4 Finn and 1/4 Rambouillet ewes (Ovis aries) with lambs grazed Kentucky bluegrass (Poa pratensis) - white clover (Trifolium repens) in a randomized block design with three replications during 3 yr, to investigate effects of grazing cattle and sheep together and separately on soils, plants and animals. Additionally, changes in stocking rate, seasonal distribution of cool-season forages and varying nutritional requirements of animals were examined. There were 6 cow-calf pairs or 6 ewes with 11 lambs per replication of cattle alone and sheep alone treatments. Six cows plus six ewes, with their offspring, were allotted to each replication of mixed grazed pastures. Initial stocking rates were equilibrated among treatments to 1 cow and calf per .45 ha. Grazing sheep alone increased (P < .05) percentage bluegrass (58%) but decreased white clover (6%) and weeds (36%), compared to pastures grazed only by cattle (30, 16 and 53%, respectively, which was similar to initial composition). Soils in mixed grazed pastures remained higher (P < .05) in plant nutrients, pH, and percent organic matter and were less (P < .05) compacted, compared to soils where sheep or cattle grazed alone. Total forage mass and quality were higher (P < .05) in mixed grazed pastures, compared to pastures grazed by cattle and sheep alone. Daily gain (.23 kg/d), total gain (23 kg), and weaning weights (43 kg) of lambs were higher (P < .01) in pastures grazed by both animal species, compared to sheep alone (.18 kg/d, 19 and 38 kg, respectively). These animals also reached target weaning weights (43 kg) 14 d earlier (P < .05) than those grazed alone. Body condition of ewes was improved by the breeding season, compared to ewes grazed alone. Five measuring techniques were investigated to determine forage mass and botanical composition. Measurements based on small plots permanently located within pastures were as accurate as those based on the entire pasture. Visual estimations of forage mass were lower, compared to hand clipping, but reflected canopy height. Visual evaluations more accurately estimated presence of individual plant species than hand separated samples. Grazing ewes and cows with associated spring-born offspring on bluegrass-white clover improved the balance between plant growth and animal forage requirements and improved sustainability of the soil-plant-animal system. / Ph. D.