Forage Quality and Animal Production: Some Statistical Problems 
MEREDITH J. MORRIS and JACOB L. KOVNER 14 
Many of the problems associated with the 
evaluation of forage quality and with animal 
production are problems in measurement. Some 
of the population parameters are difficult to de- 
fine. When defined, they become even more dif- 
ficult to measure. For our purposes, let us as- 
sume that the quality of forage is reflected in 
the yield of animal product, which therefore 
becomes a complete measure of quality. 
Output per animal is determined by the fac- 
tors shown in figure 1 (Barnes 1965). The nu- 
tritive value of forage is a function of chemical 
composition and digestibility. The nature of 
the product produced during digestion is im- 
portant in determining the nutritive value of a 
forage, but it is so difficult to measure that it is 
seldom considered in nutritive value expres- 
sions. The rate of voluntary intake is deter- 
mined by acceptability of the forage, rate of 
digestion and rate of passage, amount of for- 
age available, and environmental effects on the 
animal. Thus, the two most important charac- 
teristics in the evaluation of forage as related 
to animal production are nutritive value and 
rate of intake. 
AT TTT TT 
[= | NUTRITIVE VALUE [J 
ATT Tf 
DIGESTIBILITY AND 
NATURE OF DIGESTED 
PRODUCTS 
Seo 
Pe 
| 
SN PD BP CUTPUT PER 
ACCEPTABILITY = ANIMAL 
(QUALITY ASPECT) tH 
ARATE 
RATE OF PASSAGE [| 
7 A BP GG EF 7 as BD GB 
FORAGE AVAILABLE |} Le | RATE OF INTAKE i 
PER ANIMAL PER 
UNIT TIME 
ENVIRONMENTAL " CTD pPviant 
"ANIMAL 
EFFECTS ONTHE | 
ANIMAL ") 
CITT] PLANT AND 
ANIMAL 
Figure 1.—The determinants of animal output (Barnes 
1965) 
If output per animal depends on nutrient in- 
take, then those factors responsible for the 
high voluntary intake of forage are as impor- 
‘Principal Biometricians, Rocky Mt. Forest and 
Range Exp. Sta., USDA Forest Serv.; central head- 
quarters is maintained at Fort Collins, Colo., in co- 
operation with Colo. State Univ. 
32 
tant as the energy attributes in determining 
pasture quality. Since forage quality, as re- 
flected by its rate of digestion, usually controls 
voluntary intake within the limits set by avail- 
ability, the rate of forage digestibility is prob- 
ably the most significant single nutritional at- 
tribute of any pasture (Roberts 1967). On na- 
tive ranges acceptability and availability of 
forage may become limiting factors, particu- 
larly in some plant species. However, if that 
occurs, the rate of intake then approaches or 
reaches zero for those species. 
Now let us examine briefly some problems 
associated with the factors already mentioned. 
NUTRITIVE VALUE 
Chemical Composition 
It is generally agreed that net energy is the 
most precise measure of the value of the en- 
ergy component of any forage. Net energy is 
the useful or available energy used for the 
maintenance and production requirements of 
the animal. Blaxter (1956) has reviewed the 
methods for determining net energy value. Un- 
fortunately, net energy measurements are ex- 
pensive in terms of equipment and labor, so 
data are accumulated slowly. Thus, people 
have looked for other measures, such as appar- 
ent digestibility, that can be correlated with 
net energy and are easier to take. 
Procedures for determining the chemical 
constituents of forages are fairly well stan- 
dardized. Where difficulties occur, they can 
usually be traced to improper laboratory tech- 
nique (Dietz and Curnow 1966) or to poor 
sampling procedures. Stalleup and Davis 
(1965) have reviewed the numerous attempts 
by researchers to quantify relations between. 
forage nutritive value and chemical constitu- 
ents. Success has been limited, partly because 
of the inadequacy of classical chemical frac- 
tionation schemes to measure nutritive entities 
(nutritionally ideal chemical fractions). An- 
other problem has been the use of empirical 
statistical models that do not consider basic 
facts concerning digestive processes (Chalupa 
and McCullough 1967). Lucas (1962) stated 
that the ideal measure of forage quality should 
be expressed in terms that would permit the 
estimation of digested end products absorbed 
per animal per unit of time, and should be 
meaningful to all kinds of forage-consuming 
animals under all environmental conditions. He 
then derived a steady-state deterministic model 
