Page 885 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Poisonous Plants of the United States Chapter | 61 843

VetBooks.ir grazing and also to provide a place to move the identified locoweed-infested ranges but, rather, should ensure ade-
quate forage is always available. Improper use of some
loco-eaters. This practice appears to reduce the impact of
grazing systems can cause livestock to graze locoweed.
locoweed on these ranges.
Many minerals, feed additives, and clay minerals have Rest-rotation grazing systems are designed to force live-
been investigated to prevent poisoning, but none have stock to uniformly graze all forage in a pasture. This
been effective. The poisonous plant literature is filled caused cattle and horses to start grazing spotted loco-
with statements that native livestock are less likely to be weed in western Utah. Changing to a three-herd,
poisoned than new, inexperienced livestock. Locoweed four-pasture deferred rotation grazing system stopped
poisoning does not follow this general trend. Cattle that locoweed poisoning by reducing the grazing pressure
are familiar with locoweed will likely select it first. Early and allowing the cattle to select alternative forages in
observations by C.D. Marsh as early as 1909 suggested preference to white locoweed. The heavy grazing
that black cattle and black-faced sheep were more pressure associated with short-duration grazing systems
inclined to be poisoned by locoweed than were white- may also induce poisoning problems.
faced cattle and sheep. In a grazing study comparing Conditioned food aversion can be used as a manage-
breeds, Brangus steers consumed more locoweed than did ment tool to train animals to avoid grazing locoweed. In
Hereford and Charolais steers. The gregarious nature of the conditioning protocol, animals are brought into a pen
Brangus cattle may have facilitated the social acceptance and fed fresh-picked locoweed, and then lithium chloride
of locoweed among the steers. (an emetic that causes gastrointestinal distress) is adminis-
Livestock should be denied access to locoweeds dur- tered by stomach tube. The animals associate the induced
ing critical periods when they are relatively more illness with the taste of the plant and subsequently avoid
palatable than associated forages. On short-grass prairies eating it. Naive animals that are unfamiliar with the target
of northeastern New Mexico, stocker cattle should not be plant form strong and lasting (.3 years) aversions fol-
turned onto locoweed-infested rangelands until warm sea- lowing a single dose. Averted animals must be kept sepa-
son grasses start growth in late May or early June. Cattle rate from nonaverted animals on locoweed areas to
on rangeland year-round should be removed from prevent social facilitation from extinguishing the aver-
locoweed-infested areas in the spring when it is green and sions. Aversion conditioning may be feasible where losses
growing and warm-season grasses remain dormant. They are heavy and persist year after year.
can be returned to locoweed-infested pastures in summer
when warm-season grasses are abundant.
Most locoweed species are endemic, growing only in Summary
certain habitats or on specific soils. Fences could be con- Locoweed is the most widespread poisonous plant prob-
structed on soil or vegetation boundaries to provide sea- lem in the western United States. Knowledge of sites
sonal control of grazing. Reserving locoweed-free where locoweeds grow and environmental conditions
pastures for grazing during critical periods in spring and when they cause problems is necessary to manage live-
fall can prevent locoweed poisoning. stock and prevent poisoning. Locoweeds are relatively
Locoweed-free areas can be created by strategic herbi- palatable, and many locoweeds are the first plants to start
cide use. White locoweed is most susceptible to growing in the spring and they may also regrow in the
Clopyralid, but Picloram and Escort are also effective. fall. Livestock generally prefer the green-growing loco-
However, natural population cycles should be considered weeds to other forage that is dormant in the fall, winter,
to determine the practicality of spraying large areas and and spring. The most effective management strategy is to
the potential lifetime of control. With the abundant seed deny livestock access to locoweeds during critical periods
bank in the soil, locoweeds are sure to germinate and when they are more palatable than associated forage.
reestablish when environmental conditions are favorable. Reserving locoweed-free pastures or controlling existing
Animals that start eating locoweed may influence locoweed populations with herbicides can provide “safe”
others to start. Social facilitation or peer pressure is a pastures for critical periods. Watching animals closely
very strong influence inducing others to start eating loco- and removing those that begin eating locoweed can pre-
weed (Ralphs et al., 1994). Graham developed the “eat vent further intoxication and prevent them from influenc-
and pull” management strategy, whereby livestock should ing others to start. Finally, conditioned food aversion is
be watched closely and removed if they start eating loco- an effective practice to train animals to avoid eating loco-
weed to prevent poisoning and prevent them from influ- weeds, and it may be economical where losses are large
encing others to start. and persistent. Good range management and wise grazing
Grazing pressure can also force cattle to begin grazing strategies can provide adequate forage for livestock and
locoweed when they run short of desirable forage avoid critical periods of the year when locoweed is rela-
(Ralphs et al., 1994). Ranchers should not overstock tively more palatable than associated forages.