Grassland improvement strategies with imidazolinone herbicides

Strategies that utilize and increase native prairie species can improve fragmented and degraded grasslands in the Great Plains of North America and result in improved forage resources for domestic livestock, increased topsoil conservation, restoration of biodiversity, and enhanced wildlife habitat. One of the primary constraints to establishing native grasses, legumes, and forbs is weed interference. Studies were conducted from 1994 to 1998 to evaluate the use of imidazolinone herbicides for establishing and managing native grasses, legumes, and forbs. In experiments conducted in eastern Nebraska, imazethapyr [2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1 H-imidazol-2yl]-5-ethyl-3-pyridinecarboxylic acid] and imazapic [(±)-2-[4,5-dihydro-4-methyl-4-(Imethylethyl)-5-oxo-1 H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid] applied preemergence at 70 g ha-1 improved the establishment of Illinois bundleflower [Desmanthus illinoensis (Michx.) MacMill.], crownvetch ( Coronilla varia L.), showy partridgepea, (Cassia chamaecrista L.), purple prairieclover (Dalea purpurea Vent.), and spiked liatris [Liatris spicata (L.) Willd.]. Imazethapyr was less injurious than imazapic and also improved establishment of Canada tickclover [Desmodium canadense (L.) DC.] and roundhead lespedeza (Lespedeza capitata Michx.). In other studies, imazethapyr and imazapic improved the establishment and dry matter yield of mixtures of Illinois bundleflower and big bluestem (Andropogon gerardii Vitman) when herbicides were applied in late-spring and annual grasses were controlled. In experiments with a seeded mixture of native mid-grasses, legumes, and asters, imazethapyr improved sideoats grama [Bouteloua curtipendulda (Michx.) Torr.], little bluestem [Schizachyrium scoparium (Michx.) Nash], purple prairieclover, Canada tickclover, and leadplant (Amorpha canescens Pursh.) establishment. Imazapic was more injurious to the planted species than in these experiments, particularly at a site in west-central Nebraska that had lower soil organic matter and higher pH than sites in eastern Nebraska. In experiments conducted on established stands of Illinois bundleflower and big bluestem, imazethapyr and imazapic applied post-emergence up to 105 g ha-1 did not affect forage yields or big bluestem tiller development. Herbicide treatments often resulted in increased in vitro dry matter disappearance and crude protein and decreased neutral detergent fiber and acid detergent fiber. In an additional experiment, tiller development and yields of established indiangrass were suppressed by imazapic