FACTORS INFLUENCING FORAGE QUALITY 
Environmental Influences on Nutritional Value of Forage Plants 
WILLIAM A. LAYCOCK! and DONALD A. PRICE? 
INTRODUCTION 
Environmental conditions such as weather, 
soil, plant competition, and grazing can di- 
rectly or indirectly affect the nutritive value of 
plants for grazing animals. The most obvious 
direct effect is leaching of nutrients from 
plants by rain. Many environmental conditions 
affect the chemical composition of plants indi- 
rectly, often by causing changes in the form or 
in the phenological development of plants. The 
study of these effects is complex because single 
factors rarely act alone; most changes in the 
nutritive status of a plant are caused by nu- 
merous, interrelated factors whose effects 
often cannot be separated. Even when the ef- 
fect of a single factor can be isolated, the 
mechanism of the change in composition of a 
plant may be very complex. These interactions 
must be remembered in any evaluation of plant 
nutrient content and forage value. 
WEATHER 
Precipitation: Leaching 
The amount and distribution of precipitation 
affect chemical composition of plants both di- 
rectly and indirectly. The direct effect, which 
will be discussed in this section, is leaching of 
nutrients from herbage. The indirect effect, 
which is caused by variations in the amount of 
soil moisture available for plant growth, will 
be treated in the “Soils” section. 
Leaching of mature or dry herbaceous plants 
as a result of exposure to rain often results in 
large decreases in protein, phosphorus, ash, 
and carotene. Crude fiber and lignin are not 
leached and thus increase in percentage as 
_‘Laycock is a Plant Ecologist, USDA Forest Ser- 
vice, Intermountain Forest and Range Experiment Sta- 
tion, Ogden, Utah. He is stationed at the Forestry Sci- 
ences Laboratory, Logan, Utah. The Laboratory is 
maintained in cooperation with Utah State University. 
* Price is Director, U.S. Sheep Experiment Station, 
Dubois, Idaho. The Station is maintained by the USDA 
Agricultural Research Service in cooperation with the 
USDA Forest Service and the University of Idaho. 
leaching progresses (Guilbert et al. 19381;. 
Guilbert and Mead 1931; Watkins 1937, 1943. 
Not all species react in the same way to 
leaching. Watkins (1937, 1948) found that cal- 
cium content in various range grasses was Sig- 
nificantly reduced by heavy winter precipita- 
tion in New Mexico, while Savage and Heller 
(1947) found that calcium content of grasses 
in Oklahoma was little influenced by leaching. 
Crude protein of bur clover in California was 
little affected by leaching (Guilbert and Mead 
1931), but crude protein content of native 
grasses in New Mexico was greatly reduced by 
leaching (Watkins 1943). 
For animal nutrition, leaching of nutrients 
from dry or mature herbage is probably most 
significant; however, leaching of considerable 
amounts of nutrients from actively growing 
plants by rain, mist, or dew also occurs. A re- 
view by Tukey (1966) of research on leaching 
of metabolites from aboveground plant parts 
listed 15 elements and inorganic compounds, 8 
carbohydrates, 23 amino acids, and 15 organic 
acids that have been identified from plant 
leachates (table 1). Many constituents impor- 
tant to plant growth and animal nutrition are 
affected by leaching: Calcium, iron, manga- 
nese, nitrogen, phosphorus, potassium, and 
various carbohydrates. Because of this leach- 
ing in the field, chemical composition of plants 
grown in the greenhouse cannot be considered 
representative of the chemical composition of 
the same plants grown in the field. 
The amount leached from leaves depends on 
several environmental influences and upon the 
condition of the plant. Young leaves are less 
susceptible to leaching than are older leaves. 
Injury to leaves by frost, disease, insects, or 
mechanical means increases leaching loss. In 
undamaged living cells differentially permea- 
ble membranes bar free movement of material 
out of the cells. When these membranes are in- 
jured, their differentially permeable character- 
istics are destroyed; thus, there is greater 
movement of metabolites out of the cells when 
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