Standing crop and response of key plant species (primarily grasses) were evaluated following burning in mid-May and mid-June during two drier than average years, on mesic northern Mixed Prairie at Samuel H.Ordway Memorial Prairie, South Dakota, north-central USA.
Two sites on loam soils were selected; a ‘silty range’and an ‘overflow range’. Kentucky bluegrass Poa pratensis was common on both, indicating a decline in range condition (attributed partly to cutting for hay). ‘Excellent condition’ silty ranges are dominated by western wheatgrass Agropyron smithii, needleandthread Stipa comata,and green needlegrassStipa viridula; andoverflows by big bluestem Andropogon gerardi.
In 1980, plots (25 x 50 m) were established (randomized block design). Treatments (3 replications) comprised burning in mid-May (before warm-season grass emergence) or mid-June (warm-season grasses 5-10 cm height), and an unburned control. Ten, 0.25 m² quadrats per plot were randomly located and vegetation clipped (28 July-1 August) in 1980. Three per plot were clipped (27 July-3 August) in 1981. Material was dried and weighed, by species or vegetation class: Kentucky bluegrass; big bluestem; needlegrasses; western wheatgrass; sedges Carex spp.; other grasses and forbs.
Green needlegrass, western wheatgrass, Kentucky bluegrass (cool-season grasses) and big bluestem (a warm-season grass) were further evaluated by measuring longest leaf length of a sample of plants. Green needlegrass basal area was measured and seed heads counted for this and big bluestem.
Average annual precipitation (1940-1970) is 502 mm. During this study, cool-season precipitation was 33% below average (both years); warm-season precipitation was slightly below average.
Standing crop averaged higher in burn plots on overflow range (e.g. 1981 growth: May burn - 489 g/m²; June burn - 454 g/m²) compared with the control (387 g/m²). On the silty range it averaged around 240 g/m², regardless of treatment.
Big bluestem standing crop was greatest in mid-May burn plots (313 g/m²; June burn - 278 g/m²; control 233 g/m²).
Kentucky bluegrass standing crop (e.g. 1981 growth on silty range: May burn - 63 g/m²; June burn - 101 g/m², vs. control - 136 g/m²) and leaf length (silty site: burn plots 23-24 cm vs. control 33 cm; overflow site: burn plots 44-49 cm vs. control 53 cm) were reduced with burning (May or June).
Leaf length, basal area and inflorescences of other cool-season grasses were also reduced.
On the silty site, forb standing crop was greatly reduced by mid-June burning (May burn 58 g/m²; June burn 8 g/m²; control 19 g/m²).
If management aims to reduce Kentucky bluegrass, mid-May burning (prior to warm-season tallgrass emergence) in dry years may be effective. Reductions in production of native cool-season species can be expected on silty ranges. Mid-June burning in dry years is not recommended.