Changes in vegetation under yearlong grazing were compared with those under Santa Rita grazing system, a rotation system designed for southwestern US rangelands where 90% of the forage is produced in mid-to late-summer. The study was conducted in Arizona from 1972 to 1984. In 1984 there were no significant differences in grass densities, forb densities, shrub densities, or shrub cover on pastures grazed yearlong or in the Santa Rita rotation. The authors attribute annual variation in grass species densities to precipitation, which reduced shallow-rooted species in gravelly and sandy soils with low water holding capacities such as Santa Rita threeawn (Aristida glabrata), tall threeawn (Aristida hamulosa and A. ternipes) and slender grama (Bouteloua filiformis). The results of this study contrast with those of an earlier experiment at the same location, which showed increased density in perennial grasses under deferred grazing. The authors suggest that high initial grass density from strong rainfall and moderate grazing in the present study, compared to drought conditions and prior heavy grazing in the earlier study, contribute largely to the differences found.
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