INCREASING FORAGE YIELDS ON INTERMOUNTAIN WINTER RANGES. 19 
The utilization of the three grasses under the three intensities of 
grazing was interrelated. In lightly stocked pastures the sheep utilized 
65 percent of the choice Indian ricegrass and only 25 and 9 percent of 
galleta and sand dropseed, respectively. With moderate stocking the 
sheep ate more of the desirable portions of Indian ricegrass and also 
considerable quantities of galleta and sand dropseed. In _ heavily 
stocked pastures sheep used all the edible parts of the Indian ricegrass 
and were forced to eat much more of the galleta and sand dropseed 
herbage in addition. 
Under heavy grazing, the desirable forage species were closely grazed 
and often seriously injured or killed. Asa _ result subsequent yields 
were often impaired. Sheep were then forced to eat large amounts 
of herbage from the less desirable species. 
Shadseale, which is protected by thorns or spines, was utilized about 
equally under light and moderate grazing and only slightly more under 
heavy grazing. 
The average utilization by sheep of winterfat, bud sagebrush, and 
Indian ricegrass, the three key species, under moderate grazing was 
fairly close to the predetermined plan. The average percentages of 
utilization obtained for winterfat, bud sagebrush, and Indian ricegrass 
were 55, 52, and 76 percent, respectively. Wide variations occurred 
from year to year, however, and utilization of the individual species 
varied independently. For example, the lowest utilization obtained 
under moderate grazing for winterfat was in 1937-38, while the lowest 
utilization of bud sagebrush occurred in 1943-44 when utilization of 
both Indian ricegrass and winterfat was relatively high. In 1945-46 
when utilization of winterfat was just below average, that of both bud 
sagebrush and Indian ricegrass reached a maximum. 
Because wide fluctuations in utilization occur from year to year, 
utilization standards established as grazing guides should provide for 
allowances to compensate for variations in vegetation and production. 
PRECIPIEATION AND EIS INFLUENCE 
ON VEGETATION 
Range plant growth and herbage production are markedly influenced 
by both annual and seasonal fluctuations in precipitation. Low and 
extremely variable precipitation is typical of the winter ranges through- 
out the Intermountain region. Annual precipitation on winter ranges 
is usually less than 10 inches a year. At the Desert Experimental 
Range mean annual precipitation (January to December) was 6.64 
inches for the period 1934—47. This low annual precipitation is the 
chief factor limiting plant growth and consequently herbage production. 
PRECIPITATION RECORDS 
Precipitation records at the Desert Experimental Range began in 
1934. For comparison with records of longer duration, data for the 
experimental range and for weather stations at Modena and Deseret, 
Utah, are charted in figure 8. These two stations are located relatively 
near the experimental range, one 55 miles south, and the other 75 miles 
northeast. 
