When a new, unfenced roadway was put through a forest in the Ozark Mountains of Missouri, the question arose as to what type of tree could be used for roadside planting without danger of destruction by wild deer. The southern pine was suggested because in the adjoining forests the seedlings of this tree were not taken as browse by the deer. However, when the roadway had been planted with southern pines transplanted from the well-fertilized soils of the forest nursery, the deer that had not been taking this same species in the forest came out to the roadway and completely stripped the fertilized plantings.

Greater consumption by deer of the browse on areas in the forest receiving heavier nitrogen applications has been observed and reported in the Black Rock Forest studies. Other discriminations by wild animals as to quality of vegetation are sufficiently numerous to suggest that there is much to be learned about soil fertility by observing the appetites of wild animals and their selections of feeds according to soil differences.

Domestic animals, too, are very discriminating in their choice of foods, but offerings of herbage of single species only and the confinement of animals by fences prohibit more frequent demonstrations of these abilities. There was wisdom in the husbandman’s recommendation handed down from times past that pastures should be of mixed herbage to be most efficient.

Sweet clover as pasturage is commonly refused by cattle. It is taken by them only under compulsion and after the other herbage in the fence rows and water courses has been completely consumed. Bluegrass and white clover as a mixture are a time-honored pasture herbage, but even these are taken at different rates at different periods in the grazing season, demonstrating the unerring selections by which the animal balances its diet to become sleek and fat quickly in the spring. Bluegrass growing tall and of a luscious green color in spots where there have been animal urines or droppings is also refused, as is the highly nitrogenous sweet clover. Grass in such fertilized spots continues to grow taller while the surrounding short grass is eaten shorter. Can this be due merely to the animal’s fastidious taste?

Fig. 1. Cow chose her grass so as to have a balanced ration. Pasture grasses in mixtures, such as bluegrass and white clover, are not taken at uniform rates.

As a test of this question, a part of the grass area used in a pasture project was fertilized with nitrogen alone to simulate urine addition. This much greener herbage was collected and dried along with that of an untreated plot adjoining. When these two dried harvests were used as the bulk of the diet for rabbits in feeding trials, it was found difficult to maintain them on the grass-hay from soil treated with nitrogen alone. Those on the hay from untreated soil did much better. Sweet clover, too, was fed in a similar manner and the animals “starved to it” to compel its consumption did not survive long enough to suggest other than the starvation interval. These “destructive” aspects of such feeds normally refused by the animal point to the physiological basis for the refusal. They suggest the damaging effects on animal growth of our failure to give heed to the animal’s recommendation by its choices of feeds according to their beneficial physiological effects.

Co-operative field trials with Mr. Poirot, in 1936, for testing the influence by soil treatments on the native flora of the virgin prairie set the stage for annual demonstrations of extraordinary animal behavior since that year. Surface applications of different fertilizers at rates not to exceed 300 pounds per acre of a single fertilizer were made in the early spring. The combinations of fertilizers were at the heaviest rate of all, but even then at only 600 pounds per acre. All treatments were drilled lightly into the soil. A goodly list of fertilizer rates was used, and a total of five acres at one end of a fenced field of 100 acres was covered with these surface applications. Observations of the changes in flora and samplings of unit areas of herbage for yield and chemical analyses were made during the summer. Later in the summer hay was made. The hay from these five treated acres was mixed with that from twenty acres without soil treatment to make one haystack. Three other haystacks were made in the field, each including the hay from twenty-five acres without soil treatment.

In late November each year more than 200 head of cattle were turned into this field to consume the four haystacks. Water and salt were provided in an adjoining pasture at the end of the row of four haystacks opposite the one containing the hay from the area given soil treatments. The cattle soon showed their keen abilities in selection by daily passing the three haystacks from the untreated area and going to consume first the stack in which the five acres of hay with soil treatments was mixed with that from the twenty untreated acres.

Fig. 2. Cattle grazing experiment. Cattle cropped the barley more closely (left) where in drilling out the field corners the fertilizer application was just about doubled.

This 100-acre area has had no soil treatments since 1936. Hay has been made and stacked in similar manner annually since that year. The cattle, too, have behaved similarly each year by demonstrating their first choice of the stack containing hay from the soil given the fertilizer treatments in 1936.

When the stack was first laid down for the eighth demonstration in 1943, it was too small to take the hay from the five acres treated in 1936, and all of the hay in addition from the twenty acres untreated as customarily put into this stack with it. Consequently, a part of the hay from this untreated area was built-on as an extension of the stack. The cattle cut this stack in two at the juncture of these two stackings by consuming first that part which included the five acres formerly treated, and by leaving unconsumed the end made up wholly of untreated hay. They then distributed themselves amongst all the stacks in the field, including this remnant that had no greater lure for them than the untreated hay in the other three stacks.

The delicacy of the appetite of the cattle is clearly demonstrated by giving consideration to the following factors in the case. No more than 600 pounds of fertilizer was put on the surface of the soil. It was subjected to an average annual rainfall of 35 inches during eight years. Nine crops of hay have been removed. The treated hay each year was diluted so as to make up only one part in five of the preferred stack. And in the eighth year the animals were still passing by the three stacks from unfertilized land and selecting the fourth. By the ninth year they were still recognizing the fertilizer effects in the grass, but not in the hay mixed with so much from soils without treatment. Surely, in the light of this evidence the animals’ choice must be recognized as a refinement in detecting differences in the crop coming by way of the soil that chemistry as yet cannot duplicate.

Hogs, as grain eaters, point out by their selection of food that soil treatments register in the differences in the grain, as was demonstrated in forages by the cattle choices. Mr. Virgil Burk, formerly the county agent of Johnson County, Missouri, reported the remarkable discrimination of hogs in “hogging down” corn on the farm of Mr. Cliff Long. These animals, when turned into a large field, went to its opposite end to take grain first where the soil had previously been limed; and they passed daily through the untreated corn in going to and from the water and protein supplement.

The hog has long been credited with discriminating ability as a balancer of its diet. It was Professor Evvard, of Iowa State College, the creator of the self-feeder idea and observer of the hog as a balancer of its diet, who said, “The pig, if given a chance, will make a hog of itself in less time than you will.” This uncanny capacity of the hog to exercise choice was used to demonstrate differences in the quality of corn grown on plots with different soil treatments on Sanborn Field, Columbia, Missouri. Since a number of these adjoining plots, in their crop rotations, were to have corn, it was planted across them, several corn hybrids being chosen for the experiment. This arrangement made it possible to sample the corn according to each hybrid and also for the hogs to select according to each plot with a different soil fertility treatment.

It was significantly interesting that, regardless of the distribution of the corn among the different compartments in the self-feeder, there was each time the same order of consumption of the grain relative to the soil treatments. Since the corn consisted of mixtures of hybrid breedings in each feeder compartment, a segregated selection by the hogs according to the various hybrids was impossible. The weights of the animals were not kept, nor was any measure attempted of the greater efficiency in making pork through animal selection. But this greater efficiency was reported for rats by Dr. Curt Richter who demonstrated that when permitted choice of the separate ingredients of a common feed mixture, they made the same gains on less feed than when compelled to take the mixture.

As confirmatory of the choice of food by hogs, observations were made of native rats which were not given open access to whole grains because the corn was stored in bags. Samples in small bags of different hybrids and from different plots—though considered carefully stored—were attacked by rats. The bags happened to be piled so that every one was accessible, and the choice of the rats was demonstrated by bags which were found to be cut and those which were left unharmed. Surprising as it may seem, the choice of bags cut by the rats agreed closely by plots and soil treatments with the choices by the hogs. The rats did not distinguish between the hybrids on the same plots within the limited number of choices available in this demonstration. This suggests that the soil on which grain grows, rather than the pedigree of its breeding, is the basic factor that is reflected in the delicacy of the appetite in food selection by animals.

Just what particular chemical compound in the food helps the animal to discriminate and whether it does so by smell, as well as by taste, are still open questions. Very delicate differences are even recognized by chicks, as was demonstrated by Dr. Weston A. Price in testing their ability to discriminate the differences in contents of Vitamin A and “activator X” in butter. Even for butter as an animal physiological product—seemingly far removed from the soil—of which three grades varying in contents of these two catalytic agents were weighed out daily, put into different positions in the brooder, and weighed back regularly, the chicks selected and ate about two and one-half times as much of the butter high in the “activator X” as of that which was low. The selection was in accordance with the better survival and growth as brought on by these differences in the butter in other tests. This selection by the chicks occurred when neither Dr. Price nor any member of his staff could distinguish between the butters.

Fig. 3. Hogs chose the best corn. Choice by hogs of corn grown on treated soil exactly marked out this area as they daily ate it before taking corn grown on untreated soil.

The ancient art of agriculture has handed down practices often frowned upon by its youthful science. But when it is now reported that vitamins are generated through microbial activities in the cow’s paunch, we may appreciate the soundness of the old practice of hogs following fattening steers to gather the undigested corn. Interdependence of animals, as demonstrated in the importance of providing animal products in the diets for carnivorously inclined pigs and chickens, while cows and sheep are truly herbivorous, suggests the possible use of the animal appetite as a new tool for the assay of feeds, and for the measure of the levels of soil fertility producing them.

Fig. 4. Effect of diet on rabbits left. Rabbit fed hay from unfertilized soil. Right, rabbit fed hay from treated soil. Note differences in size of bones and appearance.

That the animal’s delicacy in selecting food serves for its better body growth is clearly suggested in work to date. That the soil’s contribution of fertility in more generous and in better-balanced amounts is provocative of these choices has not been widely considered. Animal assays of soil fertility to much more refined degrees than are possible by chemical tests are now being used to learn just what treatments a soil needs in order to make its herbage products an efficient forage or hay. In the final analysis, pasture studies must submit to this criterion which the animal itself imposes.

More studies will be required before we can use the animal appetites, animal growth, and animal reproduction as refined techniques for the most complete assay of soil fertility. To date, however, animal behaviors have given some very encouraging results to suggest that the student of soils has in the animal itself a detecting instrument, the delicacy of which approaches that of the chemist’s spectrograph. When the physiology behind the appetite is more generally understood and when organic differences in the body are connected with, or provocative of distinct manifestations of the appetite, we shall be in better position to handle our soils for more efficient health and growth of the animals. What this promising approach can do for human health is still an unexplored field. Increasing food problems on a national scale suggest that we should call in all aids to make our soils more efficient as food producers. The delicacy of animal appetite in food selections is one such help that warrants early and wider acceptance.