ing the net energy of feed components, that the 
net energy values of protein, fats, and carbohy- 
drates are different. True fat has the highest 
net energy value—more than twice that of pro- 
tein and carbohydrates. Protein has much 
higher gross energy than carbohydrates, but 
due to greater metabolic losses in the animal, 
protein has about the same net energy value 
Energy is normally supplied by carbohydrates 
and fats in the diet, and body protein is utilized 
for energy only when the food supply is inade- 
quate (Maynard and Loosli 1956). Protein 
yields its full value only when used for the 
building or repair of protein tissue or products 
(Morrison 1957). 
Other Nutritional Measurements 
Other methods are often employed to gain 
nutritional information about forages. Many 
are less laborious than the classic digestion 
trial or in vivo technique. 
Nitrogen Balance 
A nitrogen balance trial is similar to the 
digestion trial; however, urine and other ni- 
trogenous products are collected and analyzed 
for their nitrogen content. The protein intake in 
the feed is compared to the protein loss in 
excreta, milk, and other sources. The balance 
trial provides a quantitative measure of protein 
metabolism and indicates whether the body 
is gaining or loosing protein. When nitrogen 
intake equals nitrogen loss, the animal is said 
to be in nitrogen balance or equilibrium, and 
is maintaining body condition. A continuing 
nitrogen loss—as animals may show on an 
inadequate diet—indicates the need for protein 
supplements (Maynard and Loosli 1956). 
Nitrogen-Carbon Balance 
A carbon balance is determined in addition 
to the nitrogen balance to provide more data on 
the gain or loss of fat. The carbon and nitro- 
gen content of food, feces, and excreta, and 
carbon in the gaseous output of the animal are 
determined. The amount of carbon gained as 
protein is calculated and subtracted from the 
total carbon gained; the figure obtained indi- 
cates fat gain because almost the entire carbon 
content of the body is contained in protein and 
fat (Maynard and Loosli 1956). 
Replacement Equivalents 
The theory that two feeds are of equal qual- 
ity when they result in the same weight gain, 
milk yield, etc. in animals, has been proposed 
for many years. This theory is the basis for 
Kellner’s starch values and for Scandinavian 
Feed Units. 
Kellner’s starch values, a commonly used de- 
terminant in Europe, are calculated from in- 
6 
formation gained from nitrogen-carbon bal. 
ance trials plus additional data on the fat- 
producing value of a feed. The fat-producing 
value is expressed as the number of kilograms 
of starch required to produce the same amount 
of fat as 100 kilograms of the feed. This use of 
a reference substance instead of the abstract 
measurement of calories provides a decided ad- 
vantage in evaluating feeds (Kleiber 1959). 
Scandinavian Feed Units are based on re- 
placement trials with two groups of six or 
more cows. Barley is used as a reference stan- 
dard in the ration of one group, while the feed 
to be tested replaces barley in the other ration. 
Milk production and body weight is maintained 
at the same level in both groups. The amount 
of test feed which maintains the same milk 
and body weight production as barley is said to 
be the equivalent as a feed (Kleiber 1959). 
The limitation of the replacement equivalent 
methods is that protein requirements for body 
repair and growth are not considered. Kleiber 
(1959) comments “For milk production and 
growth, however, protein is a source of amino 
acids: It is not a fuel—it cannot be replaced by 
carbohydrates or fat.’ For this reason animals 
should be assured of an adequate protein level 
for their physiological activities and for their 
energy requirements. 
Indicator Methods 
Indicators have been used extensively for de- 
termining the digestibilities of feed. Internal 
indicators which occur naturally in the plant 
are lignin, plant chromogens, and nitrogen. 
External indicators which are added to the 
feed include chromic oxide, iron oxide, and 
monastral blue (Harris et al. 1959). The ideal 
indicator or marker will be totally indigestible 
and unabsorbable, have no action on the diges- 
tive tract, pass through the tract at a uniform 
rate, and be easily determined by chemical tests. 
Determining the ratio of the indicator con- 
centration to that of a given nutrient in both 
the feed and in the resultant feces permits cal- 
culation of the digestibility of the nutrient: 
measuring food intake or feces output is unnec- 
essary (Maynard and Loosli 1956). 
Nutrient Requirement Studies 
After the nutritive content of a forage spe- 
cies is determined and its digestibility meas- 
ured, the levels of each nutrient needed to 
meet the demands for growth, fattening, repro- 
duction, and general maintenance must still be 
specified. Nutritional requirements of animals 
can be determined by feeding known amounts 
of individual nutrients at different concentra- 
tions, and measuring weight gain, body size, 
skeletal formation, reproductive ability, and 
milk production. 
