Part 4

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.)

More Tracers in Animal Nutrition Research

Female hormones in microgram amounts accelerate fattening of cattle and sheep. Before this method can be used on animals for human consumption, however, it must be determined that no possible human injury can result from any residue. With chemical tests the measurement of such tiny amounts was impossible. Even with radioactive carbon-14, doses of hormone 1000 times normal dosage were required before the hormones in the flesh were measurable.

Fig. 4—Future high-producing milk cows may be selected as calves, because of the measured activity of their thyroid glands. A minute amount of iodine-131 is fed and within minutes has concentrated in the thyroid. High concentration means high thyroid activity, which in older animals means high milk production.

Recently an isotope of hydrogen (tritium or H³) was linked to hormones, and these were fed in normal amounts to cattle. Tests 90 days afterward showed less than one part per billion in the meat, a big step toward cheaper fattening of meat animals with hormones.

The use of tranquilizers has been proposed for reducing the customary loss of weight in cattle being shipped to market. These chemicals, also, are used in such minute quantities that residues could not be detected except with radioactive tracers. Tritium can now be used by health officials to study the effects of tranquilizers.

Insects

Where and How Fast Do Insects Travel?

Radioactive isotopes have been used to study insects, their life cycles, dispersion, mating and feeding habits, parasites, and predators. Several hundred such studies have been made on dozens of insect species.

With radioactive tracers even the smallest insect becomes more easily followed. As one example, nearly half a million mosquito larvae were tagged with radioactive phosphorus in Canada. Some of the adults from these larvae were later found as far as seven miles away, but most were recovered within one-eighth mile.

In a companion study grasshoppers were labeled with the same isotope. Their average rate of movement was only twenty-one feet per hour, and after seven days their position was based entirely on random motions plus prevailing winds. It seems that grasshoppers have no ability to move toward food.

How Far Do Insects Carry Pollen?

This question is of practical importance in knowing how far to separate seed-fields to maintain pure varieties of plants. In the past it was studied by the laborious method of growing a plant having a dominant “marker” gene for some visible trait surrounded by plants without the marker. Seeds from plants at various distances from the marked plant were grown the following year to see how far the genetically marked pollen had been carried. Since such plants are normally cross pollinated, it was difficult to obtain strains genetically pure for presence or absence of the marker gene. Also, considerable testing and bookkeeping were involved.

With tracers the answer may be found in a few days. A plant is injected with radioactive phosphorus; after a few days its pollen is highly radioactive. Flowers at various distances from the tagged plant may be checked daily for radioactivity. In one study with alfalfa, radioactive pollen was carried as far as thirty feet by bees, but more than one-third was deposited on plants adjacent to the labeled one.

Are Predators Used to Destroy Insects?

With insect pests, as with plant diseases, biological control is more economical than artificial control. The use of insecticides too often results in destruction of helpful insects along with pests. Limited success has been achieved in breeding certain plants for resistance to insects.

Two important uses of biological control in agriculture have been made in recent years: importing an insect from Australia to eradicate a weed in California and disseminating ladybird beetles to control certain scale insects.

Fig. 5—Identifying predators that destroy unwanted insects.

Bumblebee has not eaten radioactive aphids, but mantis has.

Helpful parasites and predators must first be identified before they can be used. In the case of small or nocturnal insects, this can be exceedingly troublesome. Tagging the pests with radioisotopes in order to identify the predators which consume them is much simpler because the most efficient predators contain the most activity.

With such techniques entomologists have studied insects and animals which prey on unwanted aphids, mosquitoes, blackflies, and roaches. Such experiments may lead to a deliberate increase of certain predators to control injurious insects.

Radioactive labeling is also valuable in studying helpful insects. In one case the indolence of drone bees was indicated by finding that even with adequate syrup in their cage they still received identical syrup from worker bees in an adjoining cage!