Grazing-Induced Modifications to Peak Standing Crop in Northern Mixed-Grass Prairie

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) 3 stocking rate (light: 16 steers 80 ha-1, moderate: 4 steers 12 ha-1, and heavy: 4 steers 9 ha-1) study was initiated in 1982 on northern mixed-grass prairie. Here, we report on the final 16 years of this study (1991-2006). Spring (April + May + June) precipitation explained at least 54% of the variation in peak standing crop. The percentage of variation explained by spring precipitation was similar between stocking rates with short-duration grazing but decreased with increasing stocking rate for season-long grazing. April precipitation explained the greatest percentage of the variation in peak standing crop for the light stocking rate (45%), May precipitation for the moderate stocking rate (49%), and June precipitation for the heavy stocking rate (34%). Peak standing crop was 23%-29% greater with light (1 495 +/- 66 kg ha-1, mean +/- 1 SE) compared to moderate (1 218 +/- 64 kg ha-1) and heavy (1 156 +/- 56 kg ha-1) stocking rates, which did not differ. Differences in peak standing crop among stocking rates occurred during average and wet but not dry springs. Neither the interaction of grazing system and stocking rate nor grazing system alone affected standing crop across all years or dry, average, or wet springs. Grazing-induced modification of productive capacity in this northern mixed-grass prairie is attributed to changes in species composition with increasing stocking rate as the less productive, warm-season shortgrass blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Griffiths) increases at the expense of more productive, cool-season midheight grasses. Land managers may need to substantially modify management to offset these losses in productive capacity.