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