Raceme Milkvetch - Astragalus racemosus
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Raceme milkvetch occurs near the margin of its range in Montana, where several, mostly small populations have been found in Carter and Fallon counties. Its response to grazing is unknown, however it accumulates selenium and may be toxic to livestock. Accurate population and trend data are lacking.
Raceme Milkvetch is a coarse, herbaceous perennial with few to many erect stems, 15-70 cm high, and arising from a woody taproot and branched rootcrown. The pinnately compound leaves are 4-15 cm long with 11-31 narrowly to broadly elliptic leaflets. Foliage is thinly to densely covered with short hairs. Inflorescences with 15-70 densely clustered flowers arise from the axils of the upper leaves. The nodding, whitish flowers are 16-21 mm long with a partially reflexed upper petal and purplish tinged lower petals. The calyx is 8-11 mm long and glabrous or with scattered hairs. The pendant, oblong-elliptic pods have a basal, 3-7 mm long stalk, are glabrous, triangular in cross-section, with 3 sides that are nearly flat and equal-width, and 15-30 mm long.
Our plants are variety racemosus.
Flowering occurs in late June-early July; fruits persisting.
The combination of coarse, erect stems, short pubescence, and pendant pods, triangular in cross-section, is diagnostic of this species in the n. Great Plains. The more common Astragalus drummondii is similar but has pods dorsiventrally compressed with 2 parallel grooves on the bottom, and has foliage with long spreading hairs.
MT, ND south to UT, NM, TX and OK (Lesica 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX).
Observations in Montana Natural Heritage Program Database
Number of Observations:
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(Observations spanning multiple months or years are excluded from time charts)
Raceme milkvetch typically grows in grasslands on heavy soil derived from shale. It is an indicator of - and confined to - selenium soils (Great Plains Flora Assoc. 1986), typically occurring on "gumbo" in "low sites with an abundance of alkali" (Barr 1983). Soils with a high selenium content are found locally in Montana on substrates derived from marine shales (Vanderhorst et al. 1998). Vegetation at these sites varies from grasslands to sagebrush steppe and greasewood communities.
The largest and most robust population in Montana occurs on the gentle slopes of a low relief shale ridge. This site supports a high quality Elymus lanceolatus/Stipa viridula grassland with eroded areas dominated by Eriogonum pauciflorum, and has among the highest diversity of forbs observed in Carter County (Vanderhorst et al. 1998). Another large population was documented in Stipa comata/Carex filifolia grassland at the contact zone between shale and sandstone, with Stipa comata/Carex filifolia dominant on the sandstone and wheatgrasses (Elymus lanceolatus and/or Pascopyrum smithii) on the shale.
In the breaklands of southeastern Carter County, it occurs sparsely among sandstone and shale outcrops dominated by a mix of sagebrush steppe and Stipa comata/Carex filifolia grasslands. It has also been occasionally documented in shrub dominated communities, including Artemisia tridentata ssp. wyomingensis/Elymus lanceolatus and Sarcobatus vermiculatus/Elymus lanceolatus. Another selenium indicator, Stanleya pinnata (prince's plume), grows in the same habitat at two sites in Carter County (Vanderhorst et al. 1998).
Ecological Systems Associated with this Species
Raceme milkvetch accumulates selenium and may cause poisoning of livestock. However, it is apparently rarely eaten by livestock (Great Plains Flora Association, 1988). No browsed plants have been observed during surveys in Montana, as populations documented were in rangelands without evidence of recent livestock use (Vanderhorst et al. 1998). The site of the largest population has an unusually high forb diversity and cover of Nasella viridula (green needlegrass) indicating excellent condition for that range site. It is possible that livestock are excluded from areas known to have selenium soils or that grazing in those areas is timed seasonally to avoid poisonous plants (Vanderhorst et al. 1998). The range literature does not indicate whether raceme milkvetch is an increaser or decreaser under grazing. The fact that its largest known numbers occur in areas of high quality range suggests that it warrants some attention from range managers and livestock operators.
- Additional ReferencesLegend: View Online Publication
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- Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.