12 CIRCULAR 925, U. S. DEPARTMENT OF AGRICULTURE 
covered with pinyon and juniper has been classified on the basis of the 
vegetation found growing in the understory. 
Some vegetation of the southern-desert shrub formation, charac- 
terized by shrubs of low grazing value such as creosotebush, yucca, and 
mesquite, occurs in southwestern Utah and southern Nevada (24) 
Only about 1 million acres of this area is used for winter grazing because 
of scanty rainfall and consequent low, undependable forage production. 
This plant formation does not occur on the experimental range. 
At least four introduced annuals—Russian-thistle, cheatgrass and 
foxtail bromes, and halogeton—have invaded large areas of the winter 
range. Although these plants are not considered in this circular as 
forming subtypes, they have considerable effect on grazing, since they 
have supplanted the better native forage species on many winter ranges. 
They therefore need careful consideration in any program designed to 
improve winter ranges. 
Russian-thistle, the oldest of these invaders, grows profusely in 
depleted winterfat and big sagebrush subtypes whenever summer pre- 
cipitation is ample. Cheatgrass and foxtail bromes have taken over 
large parts of depleted shadscale and black sagebrush subtypes, in some 
places making up more than half the vegetation. It is also prevalent 
in the big sagebrush types where other grasses have been destroyed. 
Halogeton, the most recent intruder, has spread over much winter 
range land in Nevada, Idaho, and Utah, being well adapted to saline 
soils. It grows mainly on overgrazed and depleted ranges, and along 
road shoulders or on other land that has been disturbed, such as aban- 
doned dry-farms or trampled areas. If eaten in quantity, this plant 
is poisonous to sheep. Two of these species, Russian-thistle and 
cheatgrass brome, occur on the experimental range in minor quantities 
FORAGE UTILIZATION 
Although several hundred plant species are found on the winter 
range, only 30 are abundant and palatable enough to be classed as 
important for forage. The importance of a plant as forage is deter- 
mined largely by two factors—its relative abundance and the degree 
to which it is utilized. These two factors are not independent of each 
other, and must be considered together in assessing the importance of 
the different species. 
Most of the grazing on winter ranges, especially during the early 
midwinter period, occurs while the plants are dormant. The grasses 
are cured and dry except for a few basal leaves and stems which often 
remain alive and green during part of the winter. Most of the shrubs 
are completely dormant, although the twigs and stems on a few species 
appear to remain alive and somewhat succulent in most years. The 
forage therefore consists largely of mature, dry herbage. In the late 
winter period, in years w hen plant growth starts early, “the sheep relish 
the new green vegetation, especially that of bud sagebrush and grasses. 
UTILIZATION OF INDIVIDUAL SPECIES 
Ratings of relative abundance within the naturally sparse stands of 
vegetation on winter ranges, and utilization data obtained on all mod- 
