Emergence of seeded forbs in established stands of Geyer's larkspur on Colorado rangelands

Schroeder, Jesse D., author
Meiman, Paul J., advisor
Paschke, Mark W., committee member
Brummer, Joe E., committee member
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Larkspurs (Delphinium spp.) are considered by many to be the most damaging poisonous plants on rangelands in the western United States. Larkspurs are palatable and acutely toxic to cattle resulting in a consistently large number of annual cattle deaths on western rangelands in the United States. Attempts to avoid the toxic effects of larkspurs often result in missed opportunities to harvest considerable amounts of high-quality forage and dictate management of infested rangelands. Herbicide application can effectively reduce larkspur, but also reduces other herbaceous plants expected to compete most directly with larkspur for resources, so recovery of larkspur following herbicide application is common. A field study was conducted in the foothills of northern Colorado to determine whether seeded forbs would emerge after being sown into existing stands of Geyer's larkspur (Delphinium geyeri), and whether pre-seeding application of two herbicides at light-rates would reduce initial competition from larkspur and increase emergence of seeded forbs. Seedling emergence of native forbs was compared to introduced forbs in sprayed (2 different herbicides) and unsprayed stands of Geyer's larkspur. Experimental plots were randomly assigned one of nine possible treatment combinations and replicated 3 times in each of 3 locations. The treatments consisted of all possible combinations of seeding (native forb mixture, introduced forb mixture, and unseeded), herbicide (2,4-D LV4, picloram, and unsprayed) and location (1, 2, and 3). Larkspur density was consistently reduced by herbicide at all locations regardless of seed mixture and no difference was detected between the two herbicides. Perennial grasses were unaffected by herbicide and seeding treatments. Treatment effects on larkspur canopy cover were not obvious. At 2 of the 3 locations, larkspur canopy cover in unsprayed plots was similar to one or both herbicide treatments. At the third location, larkspur canopy cover in plots treated with picloram was less than 2 of the 3 unsprayed plots and less than all three plots treated with 2,4-D. 2,4-D reduced canopy cover of non-target forbs compared to the unsprayed plots at all three locations, and also compared to the picloram treatment at two locations. Canopy cover of non-target forbs in plots treated with picloram was similar to unsprayed plots at 2 of the 3 locations, and reduced at the other. Sub-shrub cover was greatest in unsprayed plots and reduced by picloram and 2,4-D at 2 of 3 locations. Seedling density of seeded forbs was very low and dependent on location, herbicide, and seed mixture but the effects were variable and subtle. Results suggest that both herbicides reduced larkspur and other existing broadleaf species. The picloram treatment seemed to be more effective at reducing larkspur while leaving greater non-target forb cover than 2,4-D. However, there are indications of a slight reduction in seedling density of seeded forbs in the picloram treated plots.
2013 Spring.
Includes bibliographical references.
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