GRAZING-INDUCED MODIFICATIONS TO PEAK STANDING CROP IN NORTHERN MIXED-GRASS PRAIRIE

Authors: Derner, Justin; HART, RICHARD - RETIRED ARS SCIENTIST

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2007
Publication Date: 5/15/2007
Citation: Derner, J.D., Hart, R.H. 2007. Grazing-induced modifications to peak standing crop in northern mixed-grass prairie. Rangeland Ecology and Management 60:270-276.

Interpretive Summary: Livestock grazing can affect the production of aboveground biomass through modifications to the plant community composition resulting from some plants increasing in abundance with grazing while other plants decrease. Land managers have traditionally used different stocking rates (light, moderate and heavy) and grazing systems (season-long continuous or short-duration rotational grazing) to accomplished desired goals and objectives in semi-arid rangeland ecosystems. However, precipitation, and especially spring precipitation in the northern mixed-grass prairie, is often the primary driver of vegetation responses in these rangelands. We evaluated the influence of spring precipitation, stocking rate and grazing system on current year peak standing crop of aboveground biomass in a 22 year study in northern mixed grass prairie. Peak standing crop was greater under light compared to moderate and heavy stocking rates across the study period, with this difference attributable to differences between stocking rates in years with average and wet, but not dry, springs. Peak standing crop did not differ between season-long and short-duration rotational grazing systems for either moderate or heavy stocking rates across years. Over the 22 years of this study, livestock have modified the composition of vegetation with heavier stocking rates increasing the abundance of the shortgrass blue grama which is less productive than the mid-height grasses that are more prevalent with light stocking rates.

Technical Abstract: Selective grazing can modify the productive capacity of rangelands by reducing competitiveness of productive, palatable species and increasing the composition of more grazing-resistant species. A grazing system (season-long and short-duration rotational grazing) X stocking rate (light: 2.7 ha•AUM-1, moderate: 0.8 ha•AUM-1 and heavy: 0.6 ha•AUM-1) study was initiated in 1982 on northern mixed-grass prairie that was previously very lightly grazed. Peak standing crop, harvested in late July/early August, ranged from 56 to 2299 kg•ha-1 over the 22-yr study period (1982-2003). April+May precipitation explained at least 51% of the variation in peak standing crop across stocking rate and grazing system treatments, with the highest value (67%) observed for the light stocking rate. Mean peak standing crop was greater under light (1512±80 kg•ha-1, mean±1SE) compared to moderate (1298±81 kg•ha-1) and heavy (1130±64 kg•ha-1) stocking rates across years with season-long grazing. Differences in peak standing crop among stocking rates occurred during years with average and wet, but not dry, springs. In average and wet springs, peak standing crop was significantly greater in light (1655±88 and 1935±124 kg•ha-1 for average and wet, respectively) compared to heavy (1224±67 and 1430±169 kg•ha-1) stocked pastures with moderately stocked pastures intermediate (1434±99 and 1557±148 kg•ha-1). Peak standing crop did not differ between season-long and short-duration rotational grazing systems for either moderate (1272±55 vs. 1170±49 kg•ha-1) or heavy (1106±42 vs. 1127±42 kg•ha-1) stocking rates across years. This long-term study illustrates the importance of stocking rate rather than grazing system affecting peak standing crop of northern mixed-grass prairie. This reduced productive capacity is attributed to changes in species composition with increasing stocking rate as the less productive, warm-season perennial shortgrass blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Griffiths) increases at the expense of more productive, cool-season perennial mid-height grasses. Land managers may need to substantially modify management to offset these losses in productive capacity which has altered the structure and function of this rangeland ecosystem.