The Effects of Fertilization on the Quality of Forages

A sound program of fertilization, then, can increase the nutritive value of a mixed forage in two ways—by the introduction of highly nutritious forage plants, such as the legumes and the elimination of less desirable plants, and by an increase in the individual plant of such mineral nutrients as calcium, phosphorus, cobalt, and other of the micro-nutrient elements.

We can assume that many other constituents, some that are unknown and consequently not subject to laboratory measurements, may be modified by fertilization. These factors, both known and unknown, all contribute to the nutritive quality of forages. Therefore, many workers in an attempt to evaluate the nutritive quality of one forage against another have fed the two forages to animals. The growth response has then been used as an over-all measure of nutritive value.

This over-all measure, which has been termed a bio-assay of soil fertility, is subject, however, to many limitations. A successful or positive result from such an experiment might actually be considered fortuitous.

Two situations that would result in negative conclusions when positive ones would have been justified will be discussed. Many other combinations of circumstances could be considered.

If a forage (assume a pure species) is produced at different fertility levels of the soil, it is assumed on the basis of the foregoing discussion that there may be some measurable difference in their nutritive quality. It may be that these differences are due to constituents unknown with respect at least to their ability to influence animal growth or health, or it may be that some constituent such as protein is the variable. Suppose that a particular fertility level of the soil is associated with a forage containing 20 percent of protein as compared to 15 percent in a forage from a lower fertility level. The two forages are fed to lambs and the growth response is taken as the criterion of the nutritive values of the two forages. But the difference in protein levels would probably not be reflected in a difference in growth of the two groups of lambs, because the optimum protein requirement for this animal is about 10 percent, and both forage samples exceed this. Thus, a negative result would be obtained from the experiment, although there was an actual difference in the composition of the forages.

A second set of circumstances that might give rise to negative results is based on suboptimum, rather than superoptimum, concentrations of a nutritive constituent. Assume that each forage in this case contains 6 percent protein but that there exists a difference in the phosphorus content, one forage containing less than the minimum required for growth of sheep. The difference in phosphorus levels in this case would probably not be detected because of the limiting factor in both forages, the low protein content.

Both these circumstances could be repeated in the relationship between any two or more constituents of a plant that are required in some minimum quantity by animals. It is obvious that the possible combinations resulting in negative conclusions are manifold. The examples illustrate furthermore a basic requirement of such bio-assays, that the nutrients, known or unknown, be fed at suboptimum levels if differences are to be detected and also that nutrients other than those under test be fed in adequate amounts.

Requirements of this kind imply that specific nutrients be under consideration rather than all of them at one time. It is probable that future effort in the field of evaluation of the nutritive quality of plants will be directed largely toward this kind of an approach, which will necessarily be predicated largely on preliminary laboratory examination of the composition of the plant.

E. W. Crampton, D. A. Finlayson, and their co-workers at Macdonald College in Quebec have contributed materially to our knowledge in this sphere through their numerous publications. They conclude that some factor or factors other than quantitative differences in total protein, energy value, fiber, or minerals ( calcium and phosphorus) of mixed forages are responsible for observed differences in nutritive value. In the experiments upon which their conclusions are based, however, the fertilized forage contained nearly four times more white clover than the unfertilized forage did. A corresponding reduction in weeds and undesirable grasses occurred in the fertilized forage. Hence it is not possible to differentiate between the effect of fertilizers on the botanical composition of the mixed herbage and on the chemical composition of individual species. In other words, one cannot conclude from these experiments that the nutritive value of individual forage species has been altered by fertilizing the soil.

W. A. Albrecht and his associates at the Missouri Agricultural Experiment Station have also attempted to measure this improvement in the biological factors in forage plants that follow the use of fertilizers, particularly liming materials and superphosphate. Their general approach, like Dr. Crampton's, has been to use animal growth as an index of the changes in crop quality in response to soil treatment. They assume that the animal should respond to changes in the composition of the forage even though some of the changes cannot be detected even by ordinary chemical analyses.