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Title: INTERACTIONS AMONG WESTERN RAGWEED AND OTHER SANDHILLS SPECIES AFTER DROUGHT

Author
item REECE, PATRICK - UNIV. OF NEBRASKA
item BRUMMER, JOE - COLORADO STATE UNIV.
item Northup, Brian
item KOEHLER, ANN - UNIV. OF NEBRASKA
item MOSER, LOWELL - UNIV. OF NEBRASKA

Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2004
Publication Date: 10/15/2004
Citation: Reece, P.E., Brummer, J.E., Northup, B.K., Koehler, A.E., Moser, L.E. 2004. Interactions among western ragweed and other sandhills species after drought. Journal of Range Management. 57(6):583-589.

Interpretive Summary: Forage production and condition of native rangelands in the Great Plains can change yearly, and are often related to precipitation. Western ragweed is a perennial broadleaf that grows in open stands, is considered an indicator of poor pasture condition, and may increase in response to drought. We conducted a study to determine whether there were relationships between western ragweed and other species of plants found close to ragweed shoots on excellent condition sandhills prairie in north-central Nebraska, in good and poor rainfall years. We clipped either ragweed shoots or plants near ragweed shoots, from 1.1 yd^2 plots, in May, June, and July 1991 (good rainfall year) and 1992 (poor rainfall year), and measured forage produced by ragweed and its associated species. Production by ragweed and its associated species in 1991 was 77% greater than in 1992. Western ragweed had little effect on production by associated species during 1991, but small amounts (168 lb/acre) reduced production by the dominant grasses (sand bluestem, switchgrass, prairie sandreed) on plots clipped early in 1992. Ragweed stands were not affected by production of associated species, growing season, or most clipping treatments. Western ragweed was least competitive in stands of grass clipped in late summer. These results suggest that ragweed is able to maintain vegetative buds under stressful conditions. If ragweed increases during a grazing season, pasture condition can be improved by delaying the start of grazing until July of the following year.

Technical Abstract: Forage production and species composition of semi-arid rangelands varies annually, and is often related to variable precipitation. Relationships among western ragweed (Ambrosia psilostachya) plants, an indicator of poor pasture condition, and associated species following drought-induced population flux were evaluated. Western ragweed or plants of associated species were clipped from 1.0 m^2 quadrats at regular intervals in early-May, June or July 1991 (good rainfall year) and 1992 (poor rainfall year) on high-seral sandhills prairie in Nebraska, and effects on production and community determined. Peak standing crop of ragweed and associated species was 77% greater during May-October 1991 than 1992, though July-August precipitation was similar. Under above-average precipitation in 1991, up to 435 kg/ha of ragweed had no effect on associated grass species. In contrast, 189 kg/ha of ragweed reduced rhizomatous C4 grass production in 1992. The later dates of ragweed removal produced some improvement in grass yield during 1992, but no effects during 1991. Western ragweed was least competitive in stands where grasses were clipped in July. Ragweed densities were independent of associated species, seasonally constant, and similar among treatments within years. Ragweed densities in years following clipping treatments and severe defoliation were unaffected by preceding-year responses of ragweed or associated species, indicating an ability to maintain primordia with limited aboveground plant growth. If western ragweed densities increase under grazing, the negative effects on grasses can be reduced by deferral of grazing until July of the following year.