North American prairies are often subject to livestock grazing. There needs to be an appropriate stocking rate and grazing regime to ensure grasslands are not degraded and that floristic diversity is maintained. In tallgrass prairie, high stocking rates tend to lead to increases of mid- and short-grasses, whilst tall species decline. In this study, cattle grazing system and stocking rate effects on standing crop and species composition of tallgrass prairies at Oklahoma State University Research Range (36º22’N, 99º04’W), central USA, were assessed from 1989 to 1993.
Twelve prairie pasture units (14-26 ha in area) dominated by big bluestem Andropogon gerardii, little bluestem Schizachyrium scoparium and indiangrass Sorghastrum nutans, were established.
These were randomly assigned, short duration rotation (6 units) or a continuous grazing system (6 units). Within each grazing system the units were randomly allocated one of six stocking rates (cattle/unit from 10 to 22). Stocking rates ranged from 51.5 animal-unit-days/ha (AUD/ha) to 89.8 AUD/ha (i.e. moderate to very heavy for such grasslands); cattle grazed from late April to late September (average 151 day grazing season). All units were burned on 1 April 1990 and 20 March 1993.
Relative vegetation composition was measured (within 100, 0.1m² quadrats in a grid pattern in each unit) between 15 August-1 September 1989 and 1993. Plants were grouped as: big bluestem, little bluestem, indiangrass, ‘midgrasses’, ‘shortgrasses’, annual grasses, and forbs. Total standing crop was estimated in late September (clipping 45, 0.1 m² quadrats/unit, and scaling up to unit area).
The continuous and rotational grazing regimes trialled affected vegetation similarly over time. Standing crop declined as stocking rate increased (and over time from 1989 to1993); decreases of big bluestem, indiangrass and forbs were greatest at lower stocking rates. Concurrent increase in switchgrass occurred in both grazing systems. In 1993 (after 5 years grazing) shortgrasses were positively related to stocking rate (i.e. they were more abundant at higher rates) under both grazing systems.
Good growing conditions (above average precipitation) and the high seral state (i.e. a stable climax community) of the pasture vegetation may have reduced the responses to grazing treatments.
If using or referring to this published study, please read and quote the original paper, this can be viewed at: https://www.uair.arizona.edu/holdings/journal/issue?r=http://jrm.library.arizona.edu/Volume51/Number2/