Page 911 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

866 SECTION | XIV Poisonous Plants

VetBooks.ir from 5% in early growth up to 13% at flowering in management strategy to prevent grazing of broom snake-
weed and prevent toxicosis is simple: ensure adequate
August (Ralphs et al., 2007).
Snakeweeds are both abortifacient and toxic. In west
feed is available. This can be accomplished by maintain-
Texas in the 1930s, 10 60% of cattle ranches experi- ing range in good condition and moving animals when
enced abortions, and cows retained placentas, which led proper utilization is reached. Although this sounds rela-
to infection and death of the cows. Many calves were tively simple, there are thousands of acres of rangeland
born small and weak. Threadleaf snakeweed was fed to on which broom snakeweed is dominant; thus, how can
cattle, sheep, and goats, causing damage to the liver and these ranges be restored or utilized by livestock or
kidneys, but no abortions were produced. Dollahite and wildlife?
Anthony (1957) and others found that threadleaf snake- Thacker et al. (2008) studied the invasion of broom
weed did cause abortions, retained placenta, and weak snakeweed in two plant communities during a 5-year
calves, and it was more toxic on sandy soils. Low levels period following disturbance and evaluated vegetation
of snakeweed cause abortions and high levels are toxic. change in a state-and-transition model. In a sagebrush/
Clinical signs of poisoning include anorexia, mucopuru- bunchgrass community that burned, bluebunch wheat-
lent nasal discharge, loss of appetite and listlessness, diar- grass became the dominant species, and in the sage-
rhea, and then constipation and rumen stasis, which may brush-only dominated community that burned, broom
lead to death. snakeweed quickly established and dominated the com-
There appears to be a relationship between nutrition munity. Snakeweed cover increased from 2 to 31% during
and fertility problems caused by broom snakeweed. Smith the 5-year period.
et al. (1991) summarized research in rats at New Mexico Broom snakeweed can be controlled by the common
s
State University and concluded that increasing amounts of rangeland herbicides: Tordon (0.25 0.5 lb/ac) is most
snakeweed in rat diets reduced intake, which led to mal- consistent, Escort (3 6 oz./ac) is a promising herbicide,
nutrition and contributed to diminished fertility and and 2,4-D (1 2 lb/ac) can be applied for two successive
increased fetal mortality. Edrington et al. (1993) con- years when soil moisture is not limiting growth (Whitson
firmed that increasing amounts of snakeweed in rat diets and Freeburn, 1990). Better control is obtained in fall
reduced intake and contributed to problems of malnutri- after flowering, when the carbohydrate stream is going
tion and toxicity. However, they determined that the over- down and carries the herbicide to the roots.
riding factor in reducing fertility and reproduction was Ralphs and Banks (2009) used intense short-duration
the impaired hormone balance and disruption of blood grazing pressure and high stocking rates to force cattle to
flow to the uterus and developing embryos. Ewes on a graze snakeweed in an attempt to reduce snakeweed dom-
high-quality alfalfa diet (18% crude protein) consumed inance. This intense targeted grazing pressure actually
snakeweed for up to 25% of the ration with no adverse reduced snakeweed plants, but this method of snakeweed
effects on estrus; whereas ewes fed blue grama hay (11% control has yet to be recommended, especially consider-
crude protein) would not consume rations containing ing the health of the animals and the potential for reestab-
more than 10% snakeweed, and 43% of these ewes did lishment of snakeweed. Although the long-term success
not show estrus and did not breed. In heifers fed snake- of targeted grazing to control snakeweed is yet to be
weed as 15% of a balanced diet before breeding and dur- determined, one should strongly consider seeding with
ing early gestation, there was no effect on progesterone cool-season grasses following episodes of overgrazing or
levels or conception rates. During the last trimester of targeted grazing such as reported by Ralphs and Banks.
gestation, snakeweed added up to 30% of this same diet Defoliation studies to mimic grazing suggested that spring
did not cause abortion or lower calf birth weight. In a treatments put snakeweed at a disadvantage compared to
grazing trial on snakeweed-infested crested wheatgrass, fall clipping (Ralphs, 2009). Although there was little dif-
cows in the last trimester of gestation were forced to ference in the effect of clipping cool-season grasses in the
graze snakeweed as a biological control. Snakeweed con- spring versus the fall, there was a distinct effect on
sumption averaged 10% of bites over the day and peaked snakeweed.
at 20% of bites in the evening grazing periods. There Snakeweed is readily killed by fire, and prescribed
were no signs of toxicity or abortions, even though their burning is an effective control in the early stages of the
feed intake was severely restricted (Ralphs et al., 2007). population cycle while there is sufficient grass to carry a
fire and respond to the released resources. In the latter
part of the population cycle, grasses will be crowded out,
Management and Treatment
and spraying with herbicides may be the only alternative.
Broom snakeweed is usually not palatable to most large In this case, spraying must be followed by seeding to
ungulates; cattle will not graze snakeweed unless all other ensure a weed-resistant plant community is established
vegetation is depleted (Ralphs et al., 2007). Thus, the that will resist reinvasion of snakeweed. Cool-season