Page 884 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

842 SECTION | XIV Poisonous Plants

VetBooks.ir of the seeds. Seedlings of Astragalus lentiginosus pro- that green up and start growth early in the spring, flower,
set seed and go dormant in summer, and then resume
duced in embryo culture without seed coats did not con-
growth in fall. Livestock generally prefer the green, grow-
tain swainsonine or the fungus. Plants produced from
whole seed contained Undifilum in both foliage and root ing locoweeds to dormant grass. Sheep preferred the
tissues. Simulated environmental stresses, including high regrowth foliage of Green River milkvetch to dormant
temperature, low and high pH media, nutrient-deficient grasses during late fall and early winter on desert range in
media, and polyethylene glycol-amended media, impacted eastern Utah. Horses selected green spotted locoweed
both dry mass and swainsonine levels. instead of dormant grasses in spring in Arizona (Pfister
Research results have shown that inhibition of et al., 2003). Cattle readily grazed Wahweap milkvetch in
α-mannosidase is relatively transient and quickly revers- proportion to its availability on desert winter range in
ible once animals stop eating locoweed (Stegelmeier southeastern Utah. In a series of grazing studies in north-
et al., 1994). Blood serum clearance of swainsonine is east New Mexico, cattle readily grazed white locoweed in
rapid (half-life of 20 h); thus, the effects of locoweed March May but stopped grazing it in June as warm-
should be reversible if tissue damage has not become season grasses became abundant and white locoweed
extensive and permanent. This suggests that intermittent matured and became coarse and rank. Stocker cattle
grazing of locoweed should be an effective means of grazed white locoweed on short-grass prairies in May and
reducing locoweed poisoning. There is also an apparent early June, but the weight loss continued throughout the
threshold dosage where severity of cell damage is more summer, even though they were not eating locoweed. On
time dependent than dosage dependent. Once the thresh- mixed-grass prairies on the eastern foothills of the Rocky
old dosage is reached, which appears to be relatively low Mountains in northern Colorado, cattle ceased grazing
(0.35 mg/kg in the rat), eating more locoweed does not white locoweed when it matured following flowering in
accelerate the toxicosis. Therefore, increasing animal mid-June and became rank and unpalatable. However,
numbers on loco pastures and reducing time of grazing is they continued to graze it throughout the subsequent sum-
also a logical method to reduce adverse effects. mer when abundant summer precipitation caused loco-
Many locoweeds are biennials or perennials that flour- weed leaves to remain succulent (Ralphs et al., 2001).
ish periodically under optimum environmental conditions.
Historically, losses are regional and sporadic, with large
Prevention of Poisoning and Management
regional economic impact. Individual cases of significant
losses are frequent and reported in James and Nielsen Recommendations
(1988). In a single case, 25% of more than 500 mother Prevention of poisoning remains a matter of management
cows of a university foundation herd either aborted or strategy adapted to individual grazing programs to mini-
apparently resorbed their fetuses after grazing pastures mize grazing of locoweed plants (Graham et al., 2009).
with O. sericea. Necropsy of aborted fetuses showed path- Currently, no broad management schemes or methods of
ological lesions characteristic of locoweed poisoning, and treatment are known to generally prevent locoweed poi-
50 calves born alive showed out-ward signs of toxicity soning. Management strategies for individual operations
(James, personal communication, 1994). Follow-up com- have been developed once the grazing practices and
munication indicated that locoed calves were immunolog- options are identified, allowing utilization of the particu-
ically compromised, and 3 or 4% of these calves died lar range and yet minimizing losses. It was determined
after weaning. The remainder of the loco calves remained that cattle generally rejected woolly loco even under
approximately 200 pounds less than calves not exposed to extreme grazing conditions, but once they were forced to
locoweed even though after the first 28 days in the feed- start eating it, they continued to graze it and became
lot, the rate of gain was approximately the same. intoxicated. Ranchers should watch for these “loco eaters”
and remove them to clean pastures. Shortage of feed with
high grazing pressure, social facilitation (loco eaters
Conditions of Grazing teaching non-loco eaters to eat loco), or supplementing
The early literature suggested that locoweeds were dis- with alfalfa hay or cubes may compel cattle to start graz-
tasteful and animals were forced to start eating them ing woolly locoweed. White locoweed is more
because of hunger. However, once started, animals palatable than woolly locoweed and is green before spring
seemed to become addicted to locoweeds. Recent research grasses begin to grow in northeastern New Mexico.
showed that locoweeds are not addicting but, rather, are Therefore, cattle readily graze white loco in early spring
relatively more palatable than associated forages during while grasses are dormant, and once green grass starts to
various seasons of the year. grow, cattle switch off of loco. Recommendations include
Preference for locoweed is relative to what other for- creating loco-free pastures through spraying, fencing, or
age is available. Many locoweeds are cool-season species selection of low loco-infested pastures for early spring