28 CIRCULAR 925, U. S. DEPARTMENT OF AGRICULTURE 
intensities of grazing where other conditions, such as the amount of 
precipitation received and the initial kind, distribution, and amount 
of vegetation, are as nearly alike as possible. The period of comparison 
should be of fairly long duration, since the effects of grazing are cumula- 
tive, usually being small at first and becoming discernible or even 
subject to analysis only after a considerable lapse of time. 
Vegetation at the Desert Experimental Range was studied primarily 
as it occurred within the fairly large subtypes characteristic of winter 
range. These vegetal groups provide convenient units for the observa- 
tion of changes in relation to grazing intensity. Even more important, 
they are natural subdivisions within which grazing practices may be 
modified to bring about improvement in the plant cover. 
SHADSCALE AND WINTERFAT SUBTYPES 
Effects of grazing intensity on the shadscale and winterfat subtypes 
were studied in the experimental range pastures where the vegetation 
was composed primarily of three subtypes, namely, shadscale-winterfat- 
grass, shadscale-grass, and winterfat. Each of these three subtypes 
were represented in some of the range pastures under each of the three 
intensities of grazing. During the experimental studies the sheep 
grazed the entire area of the range pastures and did not noticeably 
show preference for any of the plant subtypes. Estimates of herbage 
yield were made yearly for all species found on the range pastures. 
60 
See 
PRECIPITATION CINCHES J) 
HERBAGE PRODUCTION C€ POUNDS PER ACRE ) 
° 
O 
1935 1937 1939 194] 1943 I945 1947 
Figure 12.—Average herbage production in October of the major grasses—Indian 
ricegrass, galleta, and sand dropseed—in the 20 experimental pastures in relation 
to rainfall during the preceding 15 months, 1935-47. 
