nium soils. The plants that most commonly 
cause poisoning are those selenium accumula- 
tors that grow just as well as soils lacking se- 
lenium. Among the most frequently grazed of 
these selenium-accumulating plants are various 
species of Aster, Atriplex, Castilleja, Sideran- 
thus, and Gutierrezia. Fortunately, most of 
these plants are not very palatable, and they 
become even less palatable as their selenium 
content increases. Selenium also can be concen- 
trated by accumulator plants and returned to 
the soil upon leaf drop; thus, it is concentrated 
in the soil around these plants. As a result, rel- 
atively palatable, nonaccumulating plants, 
such as grasses that grow close to the peren- 
nial accumulator plants, become toxic. 
Concentrations of silica in some grasses is 
suspected of contributing to urinary calculi 
(water belly) in cattle and sheep (Ellis 1956). 
A highly significant correlation has been found 
between the amount of silica in forage and the 
incidence of urinary calculi in Montana (Par- 
ker 1957). 
The only other mineral worth mentioning 
that is accumulated by plants is molybdenum. 
Either an excess or a deficiency of this mineral 
can be toxic. Members of the Leguminosae 
family can accumulate high levels of molyb- 
denum. Livestock can be poisoned if they con- 
tinuously graze legumes with a high content of 
this mineral. 
Repellents 
Chemical constituents in plants that are sim- 
ply distasteful to livestock might be viewed 
somewhat as a mixed curse. On the one hand, 
they adversely affect palatability; on the other, 
if it were not for unpalatable plants on unman- 
aged ranges, there might not be any plants. We 
do not know precisely why some plants are 
taken more readily than others. However, we 
do know that palatability varies with the in- 
terrelationship of plant, animal, and environ- 
mental conditions. 
Organic and inorganic composition, aroma, 
morphology, succulence, harshness, hairiness, 
leaf-to-stem ratio, associated species of plants, 
and even weather conditions can influence how 
readily an animal will graze a certain plant 
(Joint Comm. A.S.A. et al. 1962). Some of 
these are obviously chemical characteristics— 
organic and inorganic compounds. Since they 
adversely affect palatability, they might be 
considered repellents. 
For example, a high content of certain es- 
sential oils is thought to adversely affect palat- 
ability. This is probably at least one of the rea- 
sons why Artemisia tridentata is not eaten 
more readily by livestock. Low palatability of 
A. tridentata is unfortunate, for it is one of the 
most abundant and widespread of our range 
shrubs, and is recognized as a nutritious win- 
66 
ter browse (Smith 1957; Short et al. 1966). 
However, Cook et al. (1952) suggest that its 
metabolizable energy is lower than that indi- 
cated by its digestible energy values; inaccur- 
ate estimates of energy are obtained because of 
the high content of essential oils. Essential oils 
are not only responsible for some palatability 
differences between species, but also for some 
of the differences in palatability between 
plants of the same species. Approximately 20 
percent more essential oils were found in un- 
grazed Juniperus scopulorum and J. utahensis 
than in nearby grazed foliage of the same spe- 
cies (Smith 1950). 
Although lignin is not ordinarily considered 
a repellent factor, there is some evidence sug- 
gesting it might be. Studies with both sheep 
and cattle have shown that the amount of or- 
ganic material eaten decreases about 6 percent 
for each 1-percent increase in lignin content of 
the forage (Forbes and Garrigus 1950). Lignin 
also appears to actually decrease the overall di- 
gestibility of the constituents that are ordinar- 
ily digestible. 
No doubt, many other chemical substances 
act as repellents. Some of these probably ren- 
der unpalatable otherwise nutritious forage. 
Others, however, sometimes perform a very 
useful function. Many poisonous plants are not 
readily eaten because of the presence of sub- 
stances animals find objectionable. This pres- 
ence, of course, minimizes livestock losses on 
well-managed ranges. 
STRUCTURAL FEATURES 
Mechanically Damaging Features 
Several structural characteristics of range 
plants are injurious to livestock. These charac- 
teristics are in the following categories: 
Sharp, stiff, or barbed awns and floret bases; 
spiny fruit or seed coat; barbed or spiny 
stems; and abundance of hairs or fibers. These 
features seldom kill, but the affected animals 
are usually weakened. Secondary effects are 
often more severe than the direct injury. For 
example, puncturing of the skin of animals by 
plants with sharp floret bases, awns, or spines 
allows entrance of screwworms and fungi 
(Stoddart and Smith 1955). Most troublesome, 
perhaps, is entrance of the fungi that causes 
actinomycosis, or lumpy jaw. 
Certain grasses, particularly on our dry 
western ranges, possess both sharp floret bases 
and awns. These include various species of 
Stipa and Aristida (Durrell et al. 1950; USDA 
Forest Service 1937), as well as Bromus rigi- 
dus and Avena fatua (Stoddart and Smith 
1955). After the plants mature, the awns and 
florets penetrate the soft skin of parts of the 
animal’s mouth and eyes, causing irritation. 
The barbed bristles at the base of the florets of 
Setaria lutescens easily penetrate the skin and 
