How Nutritious Are Various Feedstuffs?
An endless phase of animal nutrition research deals with efficiency of
rations, that is, the pounds gained by the animal per pound of feed
consumed. The standard form of such research is to feed groups of
animals on different rations for several weeks or months and determine
average change in weight per pound of feed used.
In recent years scientists have used chemical tests to compare the
amount of calcium in the diet against the amount excreted. The apparent
digestibility of such minerals has thus been computed for different
rations. Yet one important source of error in these chemical tests
plagued researchers.
There is a “turnover” in nutrients fed to animals; elements in feed are
absorbed into the animal’s body, retained for a time, and later
excreted. For example, a cow actually loses more calcium (through milk
and excreta) during the first six months of milk production than her
normal ration contains. As long as the amount of recycling was unknown,
scientists could not tell, for instance, how much calcium in alfalfa hay
could be digested by simply measuring incoming and outgoing calcium.
Formerly scientists could study the problem only by withholding all
calcium from the diet. Under this unnatural condition all outgoing
calcium came from the animal’s body.
With radioactive calcium in a steer’s diet (or injected into the blood),
scientists can quickly tell how much of the excreted calcium comes from
the animal’s blood and organs under normal conditions. In a typical
instance a ration thought to have 24 per cent digestible calcium,
chemically determined, was found to have 38 per cent by the tracer
technique.
The tracer method shows that milk contains phosphorus, only 20 per cent
of which may come from the feed and 80 per cent from the cow’s bones.
With eggs, about 65 per cent of the phosphorus is provided by feed and
35 per cent by the hen. Radioactive tracers permit measurement of such
“biological pathways,” as the biochemist calls them.
Can Lean Meat be Estimated “on the Hoof”?
The proof of the ration, one might say, is in the cutting. That is, the
worth of a particular feed was formerly unknown until the carcass had
been cut and priced.
Because of the time and expense, researchers in the past have merely
tested groups of animals on a ration for a few weeks and then estimated
the total gain by weighing and measuring. The main drawback to such a
method is that it measures total growth only. In meat animals, knowing
total growth is less important than knowing how much gain is in the more
valuable lean meat, how much is in fat, and how much merely water.
Techniques based on atomic energy have provided a new approach without
adding radioactive contamination to the animal.
Of the “background radiation” that has existed since the earth was
formed, part comes from cosmic rays (from outer space) and part from
radioactive materials in the earth itself. One of these naturally
radioactive isotopes is radioactive potassium, which is present to a
small but significant extent in food, in human bodies, and in
construction materials.
While some chemicals such as carbon, hydrogen, and oxygen go into almost
every kind of substance in living things, potassium plays a special role
in animals: it lodges almost exclusively, not in bone or fat or water,
but in lean meat.
Biological and medical researchers are now cooperating to build
“whole-body” radiation counters. A human being or an animal is actually
enclosed by these huge devices, some of which are so sensitive they
measure nearly every ray that emerges from the body. These counters will
help answer many questions, but here only their use to measure
radiopotassium in meat animals is explained. The animal is fed a test
ration containing no added radioactivity. At intervals of a week or
more, the animal is weighed and is also tested for natural
radioactivity. Weighing tells total gain, while radiopotassium counting
shows how much gain is in the desired lean meat. This method is
remarkably simple, and since no radioactivity is added to its diet, the
animal can still be marketed.
Does Thyroid Affect Milk—Egg Production?
Recognition of the significance of the thyroid gland in animals, the
association of iodine with the thyroid, and the availability of an
excellent radioisotope of iodine have resulted in increased study of
this important gland. Chemical tests had hinted at a link between the
thyroid gland and the production of milk and eggs. Using radioactive
iodine, scientists learned that thyroid activity increases with the
onset of milk and egg formation. In hot weather, when yield of milk and
eggs decreases, activity of the thyroid gland diminishes.
It may be that a dairy breeder can soon select calves for potential milk
production because of thyroid activity as measured by radioactive
iodine. At present he must let the animals grow and produce milk for
several years before he chooses those to use in herd improvement. (See
Fig. 4.)