Page 910 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Poisonous Plants of the United States Chapter | 61 865

VetBooks.ir originating from the crown each year without becoming and will invade deeper soils and more productive habitats.
It is very competitive with desirable grasses and greatly
woody. The stems originate from the crown and are
suppresses forage production.
unbranched, giving rise to the common name of broom
snakeweed, with the stems resembling the straight straws A two-tier root system allows the deep taproot to
of a broom. Leaves are narrow and linear and alternately extract deep-stored water, and the extensive adventitious
arranged on stems. These species are of the Composite roots near the surface take advantage of the light sporadic
family; thus, they have two types of flowers. Disc flowers rain showers. It has little leaf stomatal control; thus, it is a
are tightly clustered in the center of the head, whereas luxuriant water user for maximum growth. It grows early
long yellow petal ray flowers radiate out from the head in the spring and depletes moisture from the entire soil
(Lane, 1985). profile before warm-season grasses break dormancy.
Broom snakeweed is one of the most undesirable plants in
Broom Snakeweed (Gutierrezia sarothrae the various regions of the west. On many localized range-
(Pursh) Britt. & Rusby) lands of the southwest, it is the most significant problem
limiting forage and livestock production.
Broom snakeweed has more than three florets (usually
Broom snakeweed is short-lived and experiences dra-
seven) per involucre or flowering cluster. Broom snake-
matic population cycles, which appear to be related to cli-
weed is one of the most ubiquitous range plants and is
matic patterns. Although it is very competitive for soil
widely distributed throughout North America, ranging
moisture, it is not particularly drought tolerant. Broom
from the cold temperate climate of Canada to sub-tropical
snakeweed populations die off in drought and from insect
areas of Mexico and from the subhumid Great Plains to
depredation, but it is one of the first plants to germinate
the montane Rocky Mountains and Sierras and the arid
and establish when rains resume, forming wide-spread
Great Basin. It ranges in elevation from 50 to 2900 m,
monospecific stands (Ralphs and Sanders, 2002).
rainfall from 20 to 50 cm, and temperatures from 4 to
21 C. It is a principal component of the following plant

communities: desert creosote, desert grassland, short-
grass prairie, salt-desert shrub, sagebrush, Toxicology
pinyon juniper, and mountain brush. Its wide tolerance
The snakeweeds are toxic and abortifacient to cattle, sheep,
limits allow it to inhabit a broad range of environments,
and goats. Abortions and retained fetal membranes in cattle
and it can dominate and cause significant problems as
are among the most serious problems in livestock. In 1985,
both a noxious and a poisonous weed in the short-grass
McGinty estimated losses in excess of $15 million annually
prairies and sagebrush plant communities.
to the cattle industry in Texas alone and more than $30
million when losses in New Mexico and Arizona were
Threadleaf snakeweed (G. microcephala included. This does not account for indirect losses, such as
(DC) Gray) loss of usable forage, management changes, increased calv-
Threadleaf snakeweed has one floret per flowering clus- ing intervals, or added veterinary care.
ter. Threadleaf snakeweed is restricted to the Mojave, Snakeweed contains toxic and abortifacient com-
Sonoran, and Chihuahuan deserts of the southwestern pounds. Extracts of a saponin fraction from threadleaf
United States and northern Mexico. It grows in dry, desert snakeweed caused abortions in rabbits, goats, and cattle at
habitat and favors sandy soils, and it has a greater toler- low doses and caused death at high doses. Molyneux
ance for arid climates than does broom snakeweed. et al. (1980) identified some major monoterpenes and ses-
quiterpenes in the essential oil fraction of snake-weed,
including α-pinene, myrcene, linalool, cis-verbenol,
Ecology
trans-verbenol, verbenone, geraniol, caryophyllene, and
Broom snakeweed is a native range shrub found through- γ-humulene. Several furanoditerpene acids and flavones
out semiarid rangelands of the western United States. from the resinous exudate in trichomes on leaves of
Although a minor component of pristine plant communi- broom snakeweed have been identified. These diterpene
ties, it can predominate on rocky ridges, gravely slopes, acids were structurally similar to ICA, the abortifacient
and immature infertile soils. Ralphs and McDaniel (2011) compound in ponderosa pine (P. ponderosa Laws) nee-
provide an up-to-date review of broom snakeweed ecol- dles. Gardner et al. (1999) speculated that some of the
ogy, including seed ecology, population cycles, toxicol- furanoditerpene acids may be abortifacient, whereas
ogy, and management. Broom snakeweed increases and others may be toxic, and the relative concentrations deter-
may dominate rangelands following disturbances such as mine whether animals are poisoned or abort. The crude
overgrazing, fire, and drought (Ralphs and Banks, 2009). resin content of broom snakeweed, which includes the
Broom snakeweed will increase where it is established diterpene acids along with other monoterpenes, increased