Floristic Quality of Native Tallgrass Pastures in Eastern South Dakota

The number of grass and grasslike species, grass and grasslike mean C, and grass and grasslike FQI were not statistically different among herbicide-grazing intensities, although heavily grazed pastures averaged 6.5 fewer species than moderately grazed pastures (Table 1). The number of forb species, forb mean C, and forb FQI were significantly different among herbicide-grazing intensities (Table 2). Number of forb species was the highest for pastures that were infrequently sprayed and grazed lightly or moderately. Pastures that were frequently sprayed with herbicide and heavily grazed had the fewest forb species. Forb was not different among pastures where herbicides were infrequently used. Pastures that were frequently sprayed with herbicide and heavily grazed had a lower forb than those that were moderately grazed. Forb FQI was 1.5 times greater on pastures that were infrequently sprayed with herbicide compared to those that were frequently sprayed with herbicide. Prairies where herbicide was infrequently used but grazed lightly had 1.25 times greater forb FQI than those that were not grazed. Moderately grazed pastures had 1.26 times greater forb FQI than those that were heavily grazed.

The overall number of species, overall mean C, and overall FQI were significantly different among herbicide-grazing intensities and had rankings similar to the forbs for the different herbicide-grazing intensity combinations (Table 3). The number of exotic grasses and forbs was not statistically different among herbicide-grazing intensities except that frequent use of herbicides averaged 6.8 fewer exotic forbs than pastures that were infrequently sprayed with herbicide (Table 4).

Preserves/wildlife habitat areas were similar to areas managed for livestock grazing in the number of grass and grasslike species, grass and grasslike mean C, and grass and grasslike FQI (Table 5). Pastures managed primarily for livestock grazing averaged 57.7 fewer forb species, had a 14% lower forb mean C, and a 41% lower forb FQI (Table 5). Overall number of species, overall mean C, and overall FQI were 1.8, 1.1, and 1.5 times greater, respectively, for pastures managed as preserves/wildlife habitat compared to those managed for livestock grazing (Table 5). Pastures managed for livestock grazing averaged 2.1 more exotic grasses and 10.7 fewer forbs than areas managed as preserves/wildlife habitat areas (Table 5).

There are several reasons why herbicide use did not affect grass diversity as much as forb diversity (Tables 1 and 2). The most frequently used herbicides on pasturelands in the Prairie Coteau Ecoregion of eastern South Dakota and southwestern Minnesota are those that control broadleaf plants, integrated pest management coordinator, personal communication). State noxious weed laws require the control of invasive species such as leafy spurge (Euphorbia esula L.) and Canada thistle (Cirsium arvense (L.) Scop.), and these species have high infestation rates in counties of this region. In addition, the forb mean C was higher on lands that were infrequently sprayed with herbicides, suggesting that rare and more sensitive forbs are not able to survive frequent herbicide treatment.

It is well documented that increased grazing intensity decreases floral diversity on native rangelands, and our study found likewise. However, grass and grasslike FQI was not significantly different among grazing intensities (Table 1). The reason for this was that the grass and grasslike mean C was not different between grazing intensities even though heavy grazing averaged 6.5 fewer grass and grasslike species compared to moderately grazed pastures. Numerous stocking rate studies show that taller and mid-size grasses are replaced by shorter species. The low standard error of the grass and grasslike mean C indicates that the most commonly found species have similar C values. In addition, the computation of FQI is not density dependent, meaning an area could have a substantial reduction in species diversity without reducing its FQI. Grazing intensity had a significant impact on forb FQI because numerous forb species are sensitive to overgrazing.

The desire for ecosystem goods and services (i.e., an economic benefit such as beef production or a service providing habitat for flora and fauna) dictate management decisions regarding herbicide use and grazing intensity. Our data set represents the common pastureland uses in this region and our findings of higher FQI values for forbs and for overall species on areas managed as preserves/wildlife habitat areas compared to pastures managed for livestock grazing (Table 5) was similar to pervious findings. Higgins et al. (2001) surveyed 63 tallgrass prairie remnant sites in eastern South Dakota and found that private land had an FQI of 39 compared to 57 for preserves. In northeastern Kansas, Jog et al. (2006) found warm-season hay meadows had an FQI of 32 and warm-season pastures managed for livestock grazing had an FQI of 21. Landowners whose primary objective is livestock grazing tend to use herbicides for weed control more frequently and stock heavier than managers of preserves. To conserve the floristic diversity of remaining native prairie tracts, the cultural and economic incentives behind these management decisions must be understood.

Livestock producers tend to accept exotic grasses whereas managers of preserves dislike exotic grasses because they reduce native species biodiversity. Our data showing 2.1 more exotic grass species on pastures managed for livestock grazing versus areas managed as preserves/wildlife areas (Table 5) support this view. Exotic forbs are a real concern for producers and managers. Pastures that were frequently sprayed had fewer exotic forb species and producers that managed land for livestock grazing also had fewer exotic forb species (Table 5). These data suggest that cultural factors such as “what the neighbors think” along with legal obligations to control noxious weeds are likely responsible for the fewer exotic forbs found on lands where livestock grazing is the primary objective.

Judicious use of herbicide application should be practiced instead of frequent broadcast applications. Fuhlendorf et al. (2009) showed that broadcast application of herbicides on Oklahoma rangeland where forbs comprised 23% of the herbage production did not increase grass and beef production. Their work suggests that a higher economic threshold of weedy broadleaf plants exists. Such thresholds are unknown in the Prairie Coteau Ecoregion, but historic climax plant community theory for this region would indicate that forbs make up approximately 5 to 15% of the plant community in terms of biomass (NRCS 2010). The perception that broadleaf plants reduce grass production in this region may be incorrect.

Economic incentives from conservation agencies need to focus on reducing stocking rate to a light grazing intensity in order to maintain or increase plant diversity. Such incentives exist through the Natural Resources Conservation Service’s Conservation Stewardship Plan. Practices such as deferred grazing and rotational grazing with proper stocking rates, pays producers who qualify (NRCS 2010). These payments are necessary to offset economic drivers to graze heavy.

On the other hand, managers of preserves/wildlife production areas should be aware of the benefits of using livestock to apply periodic disturbances. Pastures that were infrequently sprayed with herbicide and lightly grazed had the highest species richness, mean C, and FQI (Tables 1-3). As long as grazing is kept at a light intensity (<25% utilization of annual herbage production) and applied at the right time, it can be a useful tool to reduce competition of exotic cool-season grasses.

• Number of grass and grasslike, forb, and overall species (n), grass and grasslike, forb, and overall mean coefficient of conservatism, and grass and grasslike, forb, and overall floristic quality index (FQI) from 30 native pastures categorized by primary land use. Pastures were surveyed from 2006 to 2009 in eastern South Dakota and southwestern Minnesota.
• Number of grass and grasslike species (n), grass and grasslike mean coefficient of conservatism, and grass and grasslike floristic quality index (FQI) from 30 native pastures categorized by herbicide-grazing intensity. Pastures were surveyed from 2006 to 2009 in eastern South Dakota and southwestern Minnesota.
• Number of forb species (n), forb mean coefficient of conservatism, and forb floristic quality index (FQI) from 30 native pastures categorized by herbicide-grazing intensity. Pastures were surveyed from 2006 to 2009 in eastern South Dakota and southwestern Minnesota.
• Number of exotic grasses and forbs from 30 native pastures categorized by primary land use. Pastures were surveyed from 2006 to 2009 in eastern South Dakota and southwestern Minnesota.
• Number of overall species (n), overall mean coefficient of conservatism, and overall floristic quality index (FQI) from 30 native pastures categorized by herbicide-grazing intensity. Pastures were surveyed from 2006 to 2009 in eastern South Dakota and southwestern Minnesota.
• Number of exotic grasses and forbs from 30 native pastures categorized by herbicide-grazing intensity. Pastures were surveyed from 2006 to 2009 in eastern South Dakota and southwestern Minnesota.