percent of the animal’s weight of green plant 
material is consumed. Practically all species of 
this genus are at least partly toxic. Although 
several glycosides and some alkaloids have 
been found in these plants, their main toxicity 
is attributed to resinoids. Various species of 
Ledum, Rhododendron, and Kalmia, and Men- 
ziesia ferruginea, all in the Ericaceae family, 
contain toxic resinoids that are known to cause 
livestock losses. 
Photosensitizing Compounds 
Photosensitizing compounds occur in a few 
range plants. When these plants are eaten, the 
animals become hypersensitive to light rays 
over a broader part of the spectrum than in 
the case of ordinary sunburn. The reaction is 
somewhat similar to extreme sunburn in un- 
pigmented or lightly pigmented skin not cov- 
ered by a dense coat of hair. Although the pho- 
todynamic action is rarely fatal in itself, sec- 
ondary effects can cause death. 
Various species of Tetradymia are perhaps 
the most prominent plants causing photosensi- 
tization in livestock. Symptoms of “bighead” 
are usually apparent in sheep within 24 hours 
after toxic quantities of Tetradymia are eaten. 
Other range plants that contain photosensiti- 
zing compounds and that are potentially inju- 
rious to livestock include Hypericum perfora- 
tum, Agave lecheguilla, Nolina texana, and 
various species of Panicum, Lantana, and Tri- 
folium. 
Other Compounds 
Although not specifically related to the 
groups just mentioned, several plants that con- 
tain toxic properties very dangerous to live- 
stock should be mentioned here. Quercus spp., 
primarily Q. havardi and J. gambeli, can poi- 
son cattle early in the growing season if the 
animals eat little besides oak leaves. In this 
case the toxin has been identified as “Gallotan- 
nin.” 
Another plant with somewhat unusual toxic 
properties is Pteridiwm aquilinum. This species 
and its varieties occur almost worldwide and 
contain the enzyme, “Thiaminase,” which ad- 
versely affects horses by breaking down thia- 
mine, producing a thiamine deficiency. The 
toxic effect on ruminants is more complex, and 
the exact nature of the toxin has yet to be de- 
termined. 
Certain legumes contain estrogenic com- 
pounds in sufficient quantities to affect the fer- 
tility of animals. For example, the estrogen 
“Coumestrol’” has been isolated from Trifolium 
repens (Bickoff et al. 1958). Ewes grazed on T. 
repens and Lotus corniculatus pastures con- 
ceived 3 weeks later, and their fertility was 
much less than that of ewes grazed on blue- 
grass (Engle et al. 1957). 
The polypeptides and amines found in some 
fungi can also poison livestock. Perhaps most 
prominent among these are certain fungi like 
Amanita spp., which contain toxic peptides, 
and Claviceps spp. (ergot), which contain both 
amines and alkaloids. 
Several important range plants contain lit- 
tle-known or unknown toxins that can cause 
severe livestock losses. Foremost among these 
is the group of plants called locoweeds, this 
group consists of several different Astragalus 
and Oxytropis species. Most important are A. 
mollissimus, A. lentiginosus, A. wootonii, A. 
earlei, and O. lambertii. Toxins in these plants 
have some of the characteristics of an alkaloid. 
The locoweeds are one of the few plants to 
which livestock can become addicted. Usually 
locoweeds are unpalatable, but if animals are 
driven to eat them because of hunger, the ani- 
mals begin to crave them. This craving can be 
fatal unless addicted animals are removed 
from ranges where these plants occur. 
Cattle grazing Festuca arundinaceae pas- 
tures are sometimes afflicted with ‘fescue 
foot ;”’ symptoms include lameness and a pre- 
disposition to dry gangrene at the extremities 
(Yates 1962). These symptoms are similar to 
those cause by ergot poisoning, but the fungi 
ergot is not involved. However, toxic metabol- 
ites produced by the mold Fusarium nivale are 
suspected of contributing to this disorder 
(Yates et al. 1968). 
Hymenoxys oderata and H. richardsonii 
have killed many sheep, but the toxin they con- 
tain has not yet been identified. Oxytenia acer- 
osa, which once killed many cattle in south- 
western Colorado, also contains an unknown 
poison. 
MINERALS 
Mineral excesses and deficiencies in plants 
ordinarily are caused by soil imbalances rather 
than by species characteristics. However, some 
plants have the ability to concentrate certain 
minerals in their foliage, which then becomes 
toxic. 
Selenium is very toxic to livestock. Soils rel- 
atively high in selenium occur sporadically 
throughout the Western United States and 
Canada. Selenium uptake by plants depends on 
a number of conditions—the most important is 
the amount of selenium in soluble form in the 
soil. The particular chemical form of the sele- 
nium and the quantity of other soil elements, 
such as sulphur, also influence uptake. 
Certain selenium-accumulating plants, such 
as Stanleya, Xylorrhiza, Oonopsis, and a num- 
ber of species of Astragalus, grow only on soils 
high in selenium content. Although these 
plants seldom poison livestock, their presence 
is a reliable indicator of the presence of sele- 
65 
