Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/1999
Publication Date: N/A
Citation: Interpretive Summary: Western ragweed (Ambrosia psilostachya DC.) is one of the most common forbs (or weeds) of the mixed and tallgrass prairies. Western ragweed is seldom grazed by cattle and is usually considered to be a major competitor with desirable grasses for water, nutrients, and light. It is important to better understand the relationships between western ragweed, perennial grasses, and other forbs to make better decisions about grazing management and herbicide applications. We hypothesized that as the amount of western ragweed increases the production of grass decreases. In tallgrass prairie there was no correlation between the amounts of western ragweed and perennial grasses on a given area. In mixed prairie, which receives less precipitation, the production of grasses actually increased as the amount of western ragweed increased. This was contrary to our expectations and indicates that western ragweed may not be a strong competitor at low to moderate population levels. Ragweed density naturally decreased over the summer whether plots were grazed or ungrazed. The decrease was greater in grazed plots. Ragweed densities were similar in both prairie types. Individual ragweed stems were taller and more slender in tallgrass prairie. This is likely due to greater competition for light. These results indicate that chemical control of western ragweed may not be needed as often as previously thought because the weed is not highly competitive with desirable grasses as long as weed densities are moderate. This allows greater multiple use management because western ragweed seed is the single most important food item for quail in the southern plains.
Technical Abstract: We studied the ecology of western ragweed on mixed and tallgrass prairies to determine the relationships between the standing crop of ragweed, other forbs, grass, and total herbage. The effects of moderately heavy grazing and rest on ragweed density, growth, and morphology were also evaluated. The study consisted of 8 sites, 4 in tallgrass and 4 in mixed prairie, each containing an exclosure and a plot open to grazing. Ragweed stems were collected every 2 weeks during the growing season to develop growth curves. Density of western ragweed was determined in early June, mid-July, and early September. Western ragweed, other forbs, and grasses were clipped individually in early September to determine standing crop. Ragweed standing crop and density were positively correlated with grass and total standing crop in mixed prairie, but no significant relationships existed in tallgrass. Ragweed density did not differ by vegetation type. Density decreased over the growing season under both grazing treatments, but was reduced to a greater extent in grazed plots. Ragweed stems were taller in ungrazed plots compared to grazed plots, but stem weights were not significantly different. In ungrazed plots, ragweed grew taller at the cost of potential leaf growth, as shown by lower stem weights per unit of height.