known to be eaten by animals does not give ad- 
equate indication of the nutritive content of 
the diet. Sheep and cattle prefer leaves and 
tender stems and reject the more fibrous, coars- 
er stems. Consequently, the forage eaten is of 
much better quality than chemical analyses of 
the entire plant would indicate (Cook et al. 
1948; Weir and Torell 1959). In general, pro- 
tein, phosphorus, cellulose, and gross energy 
are higher in the parts consumed than in the 
total current growth. 
Where forage is plentiful, selectivity enables 
animals to maintain nutrient levels of their 
diet relatively constant even though the nutri- 
tive value of the plants decreases with matur- 
ity. This maintenance of level of nutrition has 
been found on pure stands of crested wheat- 
grass (Cook and Harris 1952) and on mixed 
stands of grasses and shrubs (Cook et al. 1962: 
Edelfsen et al. 1960). If utilization becomes 
heavy enough, animals can no longer be so se- 
lective, and the nutritive level and the digest- 
ibility of the diet may eventually decrease. This 
decrease is often less pronounced or occurs 
later than the corresponding decrease in the 
nutritive value of the entire plant. Arnold 
(1962) found that the digestibility of forage 
selected by grazing sheep on phalaris-subterra- 
nean clover pastures in Australia did not 
decline until almost 3 weeks after a substantial 
decline in digestibility of the same species 
clipped and fed to penned sheep. If animals 
switch to less palatable but highly nutritious 
species, such as some of the shrubs, when utili- 
zation of the palatable plants becomes heavy, 
the nutritive content and digestibility of the 
diet may increase with increased degree of uti- 
lization (Cook et al. 1962). 
COMPETITION 
The presence or absence of competition from 
other species evidently can affect the chemical 
composition of plants. However, ‘carefully con- 
trolled studies to determine exactly what 
changes are caused by competition have not 
been conducted. In California, VanDyne 
(1965) found wide differences among the mor- 
phology, chemical composition, and digestibil- 
ity of annuals grown in pure stand plots and 
among the same species collected from range- 
lands. The areas compared presumably were on 
the same type of soil, but the pure stands were 
seeded on areas where other grasses had been 
grown. Thus, some of the differences in compo- 
sition obtained may have resulted from site, 
soil, or microclimatic differences as well as 
from competition. VanDyne found that many 
grasses on the range had a greater percentage 
of weight in stems and leaves and less weight 
in heads than the same species in pure stands. 
For the brome species this resulted in a 
higher overall protein content for the range 
plants than for those in the pure stands. Avena 
barbata plants from the pure stands contained 
more protein and less silica than range plants. 
Brome plants from the pure stands had higher 
silica content. Ether extract was higher in 
range plants for all species. Other constituents 
were quite variable, and 50-percent differences 
were not uncommon. VanDyne concluded that 
chemical composition and _ digestibility of 
plants from pure-stand plots cannot be used to 
make inferences about nutritive value of these 
plants under range conditions. 
The presence of plants with nodules contain- 
ing nitrogen-fixing bacteria can increase the 
nitrogen content of the soil and thereby in- 
crease the yields of other plants. Burton et al. 
(1953) found that Coastal Bermuda grass in 
Georgia produced three times as much hay per 
acre when bitter blue lupine was seeded into 
the sod and when the mature growth was sub- 
sequently disked into the sod as similar stands 
of grass without lupine. Lawrence (1958) 
found that fallen leaves of young alder in 
Alaska, which has nitrogen-fixing nodules, 
added as much as 140 pounds of nitrogen to the 
soil per acre each year. As a result of the in- 
creased fertility, growth of associated species 
was stimulated. No data on chemical composi- 
tion of foliage was presented in either study. 
However, available nitrogen in the soil is the 
major influence on nitrogen content of herbage 
(Whitehead 1966); consequently, it might be 
speculated that the increased nitrogen in the 
soil in both instances increased the protein 
content of the foliage of the plants associated 
with the nitrogen-fixing species. 
Dietz et al. (1962) found that sagebrush 
samples collected from the Poudre range in 
Colorado were lower in protein, ash, calcium, 
and phosphorus than samples collected from 
the Mesa Verde area in southern Colorado. The 
Poudre site had a mixed stand of sagebrush, 
grasses, forbs, and bitterbrush on south-facing 
hillsides. The Mesa Verde site was an alluvial 
fan that supported an almost pure stand of 
sagebrush. The higher nutritive value of sage- 
brush at Mesa Verde was attributed to less 
competition from other vegetation and to bet- 
ter soil conditions. However, the two areas are 
widely separated, and differences in climate or 
other factors could have caused the difference 
in composition. 
CONCLUSIONS 
In almost all studies reported, the effects of 
individual environmental factors were con- 
founded with other influences or with stages of 
plant development. Carefully controlled field 
and growth-chamber studies are needed to de- 
fine the effects of environmental factors, both 
alone and in combination, on the chemical com- 
43 
