Basis for Listing

Astragalus missouriensis var. missouriensis (Missouri milk-vetch) occurs throughout the Great Plains but is less common along the eastern edge of that region. Its occurrence in Minnesota is at the far eastern edge of the species' range, and it appears to be restricted to just seven counties along the westernmost part of the Minnesota River valley. Historically, A. missouriensis var. missouriensis may have occurred in several more adjoining counties, but apparently it has never been widespread in the state.

One of the main reasons A. missouriensis var. missouriensis is rare is the loss of its prairie habitat. Of the original 7.2 million hectares (18 million acres) of prairie that occurred in Minnesota, less than 2% remains intact (Minnesota's Remaining Native Prairie), primarily due to conversion to agricultural production. Astragalus missouriensis var. missouriensis occurs primarily in dry sand-gravel prairies, which are very rare in west-central Minnesota, and remnant populations are often found in hilly pasture land. Such areas were considered less suitable for cultivation than more mesic prairies and were instead used for livestock grazing. In that sense, grazing prevented the destruction of the species' prairie habitat. However, overgrazing can present conservation threats, and the conversion of pasture land to other uses remains a threat. The glacial till and sand and gravel deposits underlying many hilly prairie habitats are under pressure to be mined because of the demand for road gravel. Because of its limited geographic distribution and threats to its habitat, A. missouriensis var. missouriensis was listed as a special concern species in Minnesota in 1984.

  Description

Astragalus missouriensis var. missouriensis is a low-growing perennial legume with a very short main stem. Leaves and flower stalks are no more than about 10 cm (3.9 in.) long and tend to lie over and spread out from a central point. Plants are densely strigose (with sharp, stiff, straight hairs held tightly against surfaces) on stems, leaves, stalks, and even on the calyx and fruits. The hairs of A. missouriensis var. missouriensis are dolabriform, meaning they are attached at the middle with two free ends. This character is difficult for the untrained eye to see and requires magnification given that the hairs lay so tightly against the plant's surfaces. The white/silvery colored hairs are so dense that the upper surfaces of the leaf, and therefore the overall plant, looks silvery-white to greenish gray in appearance. This characteristic is diagnostic for the species.

The leaves of A. missouriensis var. missouriensis have 9-17 leaflets arranged along 2 sides of the leaf axis, with a leaflet at the tip. The whole leaf is about 4-10 cm (1.6-3.9 in.) long and leaflets measure 7-13 mm (0.28-0.51 in.) long. Leaflets are elliptic to narrowly obovate. At the base of the leaf are leafy stipules. These are lance-shaped, 2-9 mm (0.08-0.35 in.) long, and distinct (separate instead of joined). The pea-shaped flowers are rose-purple in color. They are arranged in a cluster of 3-9 flowers at the tip of the flower stalk (peduncle). As the fruits develop and become heavy, the peduncles droop close to the ground. At the base of each flower is the calyx tube, which is grayish or blackish in color. The fruits are an inflated pod 1.5-3 cm (0.59-1.2 in.) long and 6-8 mm (0.24-0.31 in.) wide, abruptly narrowed into a sharp point. The pods become blackened as they mature.

There are 12 species of Astragalus in Minnesota, but only 6 that overlap in range with A. missouriensis var. missouriensis. Astragalus missouriensis var. missouriensis can be distinguished from the others by the following combination of characters: lack of a notable, erect stem and plant height less than about 15 cm (5.9 in.); presence of dolabriform hairs; calyx tube 6.5-9.5 mm (0.26-0.37 in.) long; flower banner 14-24 mm (0.55-0.94 in.); and leaf stipules distinct. Astragalus missouriensis var. missouriensis might be confused with A. adsurgens var. robustior (prairie milk-vetch), which also has dolabriform hairs. However, A. missouriensis var. missouriensis appears darker gray-green (under the silvery sheen from the hairs), flowers earlier, has a lower stature, and hugs the ground more than A. adsurgens var. robustior. Furthermore, A. adsurgens var. robustior has stems often longer than leaves and flowers, whereas A. missouriensis var. missouriensis is subcaulescent with the stem shorter than leaves and flowers. The flowers of A. missouriensis var. missouriensis are also darker and more intensely colored than those of A. adsurgens var. robustior, and the pods are typically dark in the former and pale in the latter.

  Habitat

Astragalus missouriensis var. missouriensis grows in dry prairies on glacial till, and primarily in dry sand-gravel prairies or in dry, sandy areas within hill prairies. Dry prairies are dominated by grasses, especially midheight and shortgrass species. Forb cover is sparse to patchy. Astragalus missouriensis var. missouriensis is typically found on the upper parts of south- and west-facing slopes. It may also occur near the crest of gentle north-facing slopes and on rocky knolls. Soils are generally thin and of gravelly clay loam, but can be sandy or stony clay loam. Patches of this plant are often found in areas where the soil is exposed or eroded, with sparse vegetation.

There are about 55 recorded locations of A. missouriensis var. missouriensis in Minnesota. Ninety percent of those are located in the Minnesota River Prairie subsection, with a few in the Coteau Moraines subsection. Within those landscapes, A. missouriensis var. missouriensis is associated with bluffs along major rivers and tributaries to major rivers, and with hilly, dissected glacial landforms. There are concentrations of populations in the following areas: south-facing Minnesota River valley bluffs near Granite Falls, bluffs overlooking Lake Traverse in Traverse County, and the Antelope Hills and Yellow Medicine Coteau in western Lac qui Parle and Yellow Medicine counties.

Prairie plants often growing in association with A. missouriensis var. missouriensis include Bouteloua curtipendula var. curtipendula (side-oats grama), B. hirsuta var. hirsuta (hairy grama), B. gracilis (blue grama), Schizachyrium scoparium var. scoparium (little bluestem), and Solidago nemoralis (gray goldenrod). It can also co-occur with Xanthisma spinulosum var. spinulosum (cutleaf ironplant) and Dalea candida var. oligophylla (western white prairie clover), and occasionally Astragalus flexuosus var. flexuosus (slender milk-vetch), all of which are special concern species.

  Biology / Life History

Like most legumes, A. missouriensis var. missouriensis is insect pollinated. Fruits may persist on the plant into the next growing season (Great Plains Flora Association 1986). Many species of Astragalus have a toxic alkaloid in their tissues, and some accumulate selenium, both of which can cause illness in livestock with prolonged grazing. However, the plants also have forage value and cattle will graze on the species, especially in the early spring when other forage is scarce. Astragalus missouriensis var. missouriensis does contain the alkaloid, but does not accumulate selenium (Davis 1982; Ralphs et al. 2002). It is not known as an especially problematic species for grazing livestock (Kantrud 1995; Sedivec and Barker 1998).

  Conservation / Management

The remnant habitats for A. missouriensis var. missouriensis are mostly in hilly, grazed landscapes (either currently grazed or grazed in the past). Given this setting, the species faces several threats. Overgrazing causes erosion, which may degrade dry prairie habitats. However, A. missouriensis var. missouriensis seems to benefit from grazing to some extent, perhaps because grazing removes the competition of taller plants for light. Also, the species is able to grow in areas of thin, eroded soils. Nonetheless, there are other threats in pasture habitats. The common practice of managing pastures with herbicide to control Cirsium spp. (thistles) threatens prairie forbs (Minnesota County Biological Survey 2007). Many pastures are also heavily invaded by non-native cool-season grasses such as Poa pratensis (Kentucky blue grass) and Bromus inermis (smooth brome), which outcompete native species for light, space, nutrients, and/or water. Other non-native herbs such as Euphorbia virgata (leafy spurge) can also be a problem. Astragalus missouriensis var. missouriensis is further threatened by invasion of woody species into its hilly habitats. Especially problematic are Rhus glabra (smooth sumac) and Juniperus virginiana (eastern red cedar). In certain areas, J. virginiana is rapidly invading the species' bluff habitats. If left unchecked, these woody species will shade out available habitat for A. missouriensis var. missouriensis and other prairie forbs.

Prescribed fire is recommended as a management tool to keep cover of invading shrubs and trees in check, and help reduce the dominance of non-native cool-season grasses. Mechanical brush removal may be necessary in areas where woody invasion is severe. Hilly habitats in southwestern Minnesota are also threatened by habitat conversion. The sand and gravel deposits often present in the moraines and outwash deposits where A. missouriensis var. missouriensis grows are in high demand for use in road construction. Mining the gravel substrate obviously destroys the prairie habitat and the associated plant populations. The remnant habitats supporting this and other rare dry prairie species must be protected from aggregate mining if the species are to survive in Minnesota. Several programs and resources are available to land managers and landowners to help protect and manage remaining prairie parcels including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook.

  Best Time to Search

The best time to search for A. missouriensis var. missouriensis is throughout the growing season, since growth form, leaves, and flowers are all fairly distinctive.

  Conservation Efforts in Minnesota

Six populations of A. missouriensis var. missouriensis are found on public land (Upper Sioux Agency and Big Stone Lake State Parks; Yellow Bank Hills and Bonanza Prairie Scientific and Natural Areas; and Skalbekken County Park). One additional population on private land is protected by a conservation easement. Presumably, some of these prairies are being managed for the benefit of the native prairie vegetation and associated wildlife. However, the effects of current management on A. missouriensis var. missouriensis are not being monitored.

  References and Additional Information

Davis, A. M. 1982. Crude protein, crude fiber, tannin, and oxalate concentrations of 33 Astragalus species. Journal of Range Management 35(1):32-34.

Haddock, M. 2007. Kansas wildflowers and grasses: Missouri Milk-Vetch. . Accessed 14 February 2010.

Kantrud, H. A. 1995. Native wildflowers of the North Dakota grasslands: Missouri Milk-vetch (Astragalus missouriensis). Northern Prairie Wildlife Research Center Online, Jamestown, North Dakota. . Accessed 14 February 2010.

Minnesota County Biological Survey. 2007. Native plant communities and rare species of the Minnesota River Valley counties. Division of Ecological Resources, Minnesota Department of Natural Resources, St. Paul. 153 pp.

Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the prairie parkland and tallgrass aspen parklands provinces. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 362 pp.

Ralphs, M. H., J. D. Graham, and L. F. James. 2002. A close look at locoweed poisoning on shortgrass prairies. Rangelands 24(2):30-34.

Sedivec, K. K., and W. T. Barker. 1998. Selected North Dakota and Minnesota range plants: Missouri Milkvetch. North Dakota State University, Fargo, North Dakota. . Accessed14 February 2010.

The Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence. 1,402 pp.