concentration these compounds will destroy many unwanted plants without harming lawn grasses or crop plants.

Fig. 3—Tagged weed-killing chemicals (A) are taken in and transported alike in grassy (B) and broad-leaved (C) plants, but only in the latter are killed.

As in many other instances, beneficial use of the chemicals has far outreached an understanding of how they work. The still scanty knowledge of the process has come almost entirely from tracer studies.

All plants readily absorb selective weed killers (“herbicides”), which are not destroyed within the plants. Resistant plants show no effect of the chemicals, but sensitive plants suffer damage in actively growing roots and shoots. Sugar formation during photosynthesis is disrupted in these plants, and phosphorus movement is retarded. In order to predict what new classes of chemicals might be of value as herbicides, we must await the results of research using radioactive tracers.

Animal Nutrition and Metabolism

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.)

More Tracers in Animal Nutrition Research

Female hormones in microgram amounts[1] 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