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Title: VEGETATION RESPONSE TO STOCKING RATE IN SOUTHERN MIXED-GRASS PRAIRIE

Author
item Gillen, Robert
item ECKROAT, JOHN - OK STATE UNIV
item MCCOLLUM, F. - TEXAS A&M UNIV.

Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/1999
Publication Date: 9/20/2000
Citation: N/A

Interpretive Summary: Forage production on Great Plains grasslands is affected by grazing management, fire occurrence, climate, and soils. Producers generally use grazing and fire as their primary tools for forage management. Grazing is most often managed by varying the stocking rate or the number of animals present per unit land area for a specified time. We studied the effects of grazing at moderate to very high stocking rates in the spring-summer perio on the health and vigor of mixed-grass prairies over 7 years (1990-1996). The moderate rate was the level of grazing currently recommended by the Natural Resources Conservation Service for sustainable management. The highest stocking rate tested was double the recommended moderate rate. Precipitation was near normal during the study period. Total standing and dead vegetation declined as stocking rate increased but live standing vegetation was not affected by stocking rate. Total plant production and the percentages of the total production contributed by major plant species or species groups (sideoats grama, shortgrasses, and broad-leaved plants) were not changed by stocking rate. Tallgrasses increased at lower stocking rates over the study period but these grasses contributed less than 5% of the total vegetation. Threeawn grasses, often believed to increase under heavy grazing, were not affected by stocking rate. For practical purposes, the prairie vegetation showed little change over the 7-year study period. The vegetation may have been altered by previous heavy grazing and 7 years was not sufficient time to show significant response to more moderate grazing.

Technical Abstract: Stocking rate directly influences the frequency and intensity of defoliation of individual plants which, in turn, impacts energy flow and plant succession in grazed ecosystems. The objective of this study was to determine the effect of cattle stocking rate on standing crop dynamics and plant species composition of a southern mixed-grass prairie over a 7-year period (1990-96). Long-term mean precipitation is 770 mm per year. Growing conditions were generally favorable for the study period. Yearling cattle (initial weight 216 kg) grazed at 6 stocking rates, ranging from 23 to 51 AUD ha-1, from 14 April to 23 September 15 (162 days). The currently suggested yearlong stocking rate is 25 AUD ha-1. Herbage standing crop was measured in July and September every year while species composition was determined in July in even years. Total and dead standing crop declined as stocking rate increased but live standing crop was unaffected by stocking rate. Slopes of regression lines relating standing crop and stocking rate were constant over years, indicating no response for plant productivity. The major vegetation components, sideoats grama [Bouteloua curtipendula (Mich.) Torr.], shortgrasses, and forbs were not affected by stocking rate. Tallgrasses increased at the lower stocking rates but these grasses contributed less than 5% of the total standing crop. Red and purple threeawn (Aristida longiseta Steud. and A. purpurea Nutt.) increased at all stocking rates from 1990 (5.2%) to 1996 (13%). Stocking rate had little overall impact over the 7-year study period. The vegetation may have been in equilibrium with previous heavy stocking rates so that little change would occur at those rates. Increases in grazing sensitive species at lighter stocking rates may occur over longer time intervals.