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Weeds are often a problem in
pastures, especially in areas that are overgrazed or overused
(trampled, or fed on during winter to the point that hoof damage
to the sod is extreme enough that the primary forage plants are
killed out and weeds come in). When desirable forage plants are
compromised, weeds start to take over.
Pasture management is something that
must be continually worked at, preferably in a rotational
grazing system, to balance grazing use with grass growth.
Stockmen should also become familiar with the various weeds or
plants in their geographic area that can be health hazards for
cattle, and eradicate them from pastures, if possible.
Toxic Plants
Dr. Anthony Knight, Colorado State University, says some of
the toxic weeds in the intermountain West include various
locoweeds Russian knapweed (similar to yellow star thistle in
its toxicity), water hemlock (growing along streams and wet
areas), houndstongue (an introduced weed that spreads rapidly in
pastures or hayfields—and remains toxic even when dried—causing
photosensitization), some of the senecios or groundsels (which
also cause liver disease and photosensitization) and narrow
leafed milkweeds (not to be confused with broadleafed types).
“Narrow leafed milkweed can be a
problem in hay as well as in pasture. It does not lose its
toxicity when dried. In pastures in the fall, when it’s drying
off after a frost, the animals may eat it when other forage is
scarce. The toxin in narrow leafed milkweed affects the heart
and nervous system and may kill the animal very quickly,” says
Knight.
If you have any water hemlock in
a pasture, even one or two plants, dig them out and get rid of
them, since it takes very little (1 to 2 ounces) of the root or
new leaves in the spring to fatally poison an animal. “Poison
hemlock is not as deadly; animals have to eat a lot more of this
plant to have adverse effects. Usually early spring, as it’s
emerging, is about the only time animals might eat it. The
carrot-like root is also very poisonous. Once it’s mature and
flowering, it is less toxic,” he says.
It’s always a good idea to become
familiar with the plants in your region, to know which ones are
toxic and which are safe for cattle to eat. If you find a plant
in your pasture that you don’t recognize, get some help from
your county extension weed specialist to identify it, especially
if your cattle are eating it, or if there’s a lot of it. “Many
times a livestock owner becomes concerned when he/she finds one
strange plant, but rarely is a single plant going to cause
poisoning (except water hemlock). Poisoning is usually dose
related; the animals must eat a certain amount of it before
poisoning occurs,” says Knight.
Nuisance Plants
Other plants, like thistles, burdock, cocklebur, foxtail,
cheat grass, etc. are not toxic but reduce the value of a
pasture. Some of them cause problems (ulcers or abscesses in the
mouth from eating sharp seedheads, or burrs stuck in the hair
coat). Ulcers in the mouth from the sharp grass awns of cheat
grass or foxtail result in drooling. Microscopic fragments of
burdock burrs may create eye problems if they get caught under
an eyelid. The irritation and ulceration from the tiny “sliver”
scraping the eye may be mistaken for pinkeye. Every region has
different noxious weeds that can come in and take over parts of
a pasture.
“Some of the worst weeds that grow in many parts of the West are
leafy spurge, spotted knapweed, diffuse knapweed, Russian
knapweed, etc. They can be very hard to eliminate, once they get
started,” says Knight.
Controlling Unwanted Plants
“Before you decide to spray weeds, check your county
regulations regarding which herbicides you can use, and how you
should use them. With a really difficult plant like Russian
knapweed, you almost have to use Roundup, to kill all the
vegetation, and then reseed the pasture with appropriate grass
species for your area. It doesn’t work very well to kill the
knapweed unless you can reseed afterward and get a vigorous
pasture grass growing, to provide competition for the weeds and
suppress any recurrence of the knapweed,” explains Knight.
If seeds are blown or carried in,
knapweed will establish in areas that don’t already have
thriving grass cover. “Knapweed also secretes chemicals into the
soil that kill off other plants, and it soon becomes a
monoculture of knapweed. Nothing grows under it.”
Some weeds can be biologically
controlled, by using a specific species of insect that feeds on
the blooms, for instance, so the plant cannot go to seed. “There
are only a few biological controls that have been well tested in
North America. One is a beetle that can effectively control
leafy spurge, and you can usually get those from the state
insectory, through your county extension office. There are some
other things being tried (such as a weevil to control
houndstongue, and a knapweed fly) but these are still
experimental and have not yet been released for public use.
These insects have been brought in from overseas; until these
introduced insects have been thoroughly researched to ensure
they do not become a problem (since we don’t yet know what else
they might kill out) they cannot be released,” he explains.
Sheep and goats are sometimes
utilized to graze weeds. “They are very good at keeping plants
from going to seed and prevent spreading, especially if large
numbers of animals are used at a time to quickly eat the noxious
weeds before they go to seed. A flock of sheep or goats can
devour a large patch of leafty spurge or thistle and stop it
from flowering. But you have to keep after it each year because
the grazing won’t kill the plants. They will come back again.”
Diligently grazing or mowing the weed patches will eventually
allow grasses to become reestablished.
Clipping and mowing can be quite
effective in controlling some types of weeds and thistles such
as Scotch thistle, musk thistle or even spotted knapweed,
because they largely spread by seeds. If you can stop them from
seeding, they tend to die out after a few years because they are
a biennial rather than a perennial. Canada thistle, however,
will still spread even if you mow it to prevent a seed crop,
because it mainly spreads by its root system. “Where Canada
thistle is a problem, it usually requires the use of a selective
herbicide to kill the noxious plant, and then reseed with a good
forage plant. There are several good broadleaf herbicides that
will work for Canada thistle, and you should check with your
county weed regulations to make sure you use one that is
approved for your county,” Knight says.
More information on toxic plants
can be obtained from Dr. Knight’s web site at Colorado State
University:
www.southcampus.colostate.edu/poisonous_plants (which shows
pictures of the various plants, and describes all the signs
caused by eating the plant) and from his book, “A Guide to Plant
Poisoning of Animals in North America”.
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