position of forage plants. In any study, the 
possible effects of leaching and of the presence 
or absence of competing species and the mea- 
surable environmental factors must be consid- 
ered. Information from such studies would en- 
able range managers to predict the effect of 
specific environmental changes on the nutritive 
value of native plants. It would also permit in- 
telligent selection for seeding of species best 
suited to furnish needed nutrients to animals 
in areas where the range of environmental fac- 
tors is known. 
SUMMARY 
Weather, soil, competition, and grazing are 
highly interrelated factors influencing the 
chemical composition and nutritive value of 
plants for grazing animals. Many of these fac- 
tors affect the chemical composition of plants 
indirectly by hastening or delaying plant de- 
velopment. The percentages of many desirable 
components such as protein, phosphorus, and 
carotene are high when plants are young. As 
plants mature, the percentages of these compo- 
nents decrease and less digestible components 
such as crude fiber, lignin, and nitrogen-free 
extract increase. In temperate climates these 
changes associated with plant maturity occur 
concurrently with an increase in temperature 
and a decrease in soil moisture during the 
growing season. Below-normal precipitation 
that causes deficient soil moisture hastens 
these seasonal changes; cool, rainy weather 
may prolong or reinitiate the green, nutritious 
stages of growth. 
Precipitation may also have a more direct ef- 
fect by leaching nutrients from plants. Protein, 
phosphorus, ash, and carotene are often 
leached from dry mature plants, leaving the 
indigestible crude fiber or lignin. Leaching can 
also take place in younger, actively growing 
plants; the amount leached depends on age of 
leaves, damage, amount and duration of rain, 
light, temperature, and nutrient status of the 
plant. 
In many plants, percentages of protein and 
phosphorus increase as soil or air temperature 
increases up to 80°F. The effect of tempera- 
ture on other nutrients is variable and differs 
among species. 
Shaded plants generally produce less her- 
bage and have less total carbohydrates and ni- 
trogen-free extract than plants grown in full 
sunlight; however, they usually have more 
protein and lignin. Some of these differences 
are not caused directly by the shade; however, 
shaded areas generally have higher soil mois- 
ture, and plants grown under shade are often 
retarded in stage of development and are more 
succulent than sun-grown plants. 
The depth, texture, and nutrient status of 
the soil influence the chemical composition of 
plants, but these factors are closely interre- 
lated with both climate and plant form. Plants 
grown under conditions of soil and climate that 
lead to more leafy growth form will generally 
be more nutritious because the content of pro- 
tein, phosphorus, and other desirable nutrients 
is higher in leaves than in stems. Protein, 
phosphorus, and nitrogen-free extract of 
plants have been found to increase with alti- 
tude, but some of these differences probably re- 
sult from higher precipitation and a delay in 
stage of development at higher elevations. 
Grazing affects the nutrient content of 
plants by altering plant form or state of devel- 
opment. It may also change the nutrient status 
of the whole plant community by changing the 
species composition of the community. A de- 
cline in range condition often, but not always, 
results in a decline in nutritive value of availa- 
ble forage. Less palatable species that may pre- 
dominate on ranges in poor condition may be 
more nutritious than some: of the more palat- 
able plants. For most species, removal of her- 
bage by grazing often reduces the yield of dry 
matter. However, such treatment may increase 
protein, phosphorus, and other desirable nutri- 
ents because it delays maturity and prolongs 
growth or initiates regrowth if soil moisture is 
adequate. If the clipping does not have a severe 
effect on dry-matter production, total protein 
yield may be increased. However, on arid 
ranges, clipping in midgrowth may reduce 
yield but may yet not initiate regrowth because 
of limited soil moisture. 
The botanical composition of a range is not a 
good indicator of either botanical or chemical 
composition of diet of livestock because ani- 
mals are highly selective as to the species and 
parts of plants eaten. This selectivity often en- 
ables animals to hold the nutrient level of their 
diet relatively constant during periods when 
the nutritive value and digestibility of plants 
are decreasing. 
The chemical composition of plants may be 
affected by associated species. Plants grown in 
pure stands may differ in form, chemical com- 
position, and digestibility from the same plants 
grown in mixed stands. Plants with nitrogen- 
fixing bacteria in nodules may increase the ni- 
trogen content of the soil and thereby increase 
both the amount and protein content of the 
herbage produced. 
In most of the studies reported, effects of in- 
dividual environmental factors were con- 
founded with those of other influences or with 
stages of plant development. Carefully con- 
trolled studies are needed to define the effects 
of these factors, both alone and in combina- 
tions, on the chemical composition of forage 
plants. 
