Points to be Stressed

WHAT MAKES A NUTRITIOUS FORAGE?

J. T. Sullivan, H. L. Wilkins

THE NUTRITIVE value of forages is determined by the presence of substances that are necessary for the health, growth, and productiveness of animals.

Nutritional experiments, or feeding trials, have indicated what are the desirable amounts of many of these substances. A chemical analysis will determine the amounts present in a particular forage and should indicate how forage meets animal requirements.

The high food value of certain feeds is to be expected because of the abundance of such materials as proteins, or vitamins, or minerals. While chemical analysis and feeding trials do not always agree in measuring nutritive values, the chemical method is much more rapid and inexpensive and much of our information about forages is based upon it. This discussion is limited to such information.

The chemical composition of forage crops depends on the conditions under which they grow. Natural factors that have a bearing on the chemical composition and correspondingly on the nutritive value are the kind of plant, whether true grass, legume, or other species; climatic conditions; soil fertility; the weather preceding and during the harvest; the age of the plant at harvesttime; and the season of year.

Young clover, for example, averages higher in moisture, protein, and calcium, and lower in fiber than grass and therefore is considered more nutritive for some purposes. But young grass grown in fertile soil can contain large amounts of protein. And because grass is higher in carbohydrates, it is better adapted for fattening animals and for preservation in silage.

Mature forages show similar differences. Alfalfa hay contains 12 to 16 percent of protein (on the dry basis) and 1.0 to 1.7 percent of calcium; timothy hay contains only 6 to 10 percent of protein and only 0.25 to 0.35 percent of calcium. Mixed herbage has a composition intermediate between grasses and legumes, depending on their relative amounts.

Protein tends to be less in midsummer than in the spring or fall. Temperature, light intensity, length of day, and moisture also control plant development and consequently chemical composition. No one factor works alone. The farmer therefore will do well to keep in mind these natural factors when he considers the influences ( such as crop management and soil fertility) that are at least partly within his control.

In early spring grass is relatively high in moisture and protein and relatively low in fiber. As the season advances, visible changes take place, such as heading and flowering, and at the same time chemical changes occur, such as great carbohydrate storage and also lignification.

When, however, the tops of plants are cut off by the grazing animal or by the mowing machine, these natural developments are interrupted. The plants are stimulated to produce new foliage from shoots near the surface of the ground. At first this new foliage, like young grass, is high in protein and low in fiber, but in time it becomes more and more like older grass.

When the grazing is heavy or frequent, the plants continually produce new shoots and the foliage remains young and of high nutritive value. Grazing also influences the relative amounts of grasses and legumes where they occur together and may be managed so that legumes will thrive, but overgrazing may encourage the influx of weeds. These practices naturally affect the chemical composition of the mixture.

In a grazing experiment at Beltsville, it was shown that the herbage under continually heavy grazing was more nutritious than under light grazing. With a rate of one steer per acre the herbage contained, on a 2-year average, 14.6 percent protein, 0.75 percent calcium, and 0.32 percent phosphorus. With a steer to 2 acres, the herbage contained only 13.0 percent protein, 0.58 percent calcium, and 0.30 percent phosphorus.

These differences were due partly to the stage of maturity of the grass and partly to the higher clover content of the heavily grazed plots. When Napier-grass in Hawaii was cut every 6 weeks it contained as a year-round average 7.9 percent protein, 0.46 percent calcium, 0.72 percent phosphorus, and 29 percent fiber. When cut every 14 weeks, however, it contained only 3.8 percent protein, 0.26 percent calcium, 0.46 percent phosphorus, and as much as 39 percent of fiber. The more frequent cutting promoted the more continuous production of younger and more nutritious shoots.

Soil fertility not, only regulates the amount of crop growth but also influences quality. Soils of high fertility produce forages of high nutritive value and inferior soils often produce forages of known deficiencies. Essential mineral elements are obtained only from the soil and valuable nonmineral substances of plants, such as proteins and vitamins, are produced in quantity only when minerals are adequate.

Most of our knowledge about the effect of soils on plant composition comes from observing changes following the application of fertilizers. These changes are not easy to explain entirely, but the application of fertilizers to the soil has certain general results. It has been stated, "Any increase in the percentage of protein, phosphorus, or calcium that forage crops may contain as a result of fertilizer applications may be considered to be an improvement in the quality of these crops as livestock feeds." Most fertilizer applications do increase one or more of these constituents.

Fertilization affects chemical composition of herbage in two ways.

In one way fertilization may favor the growth of some plants rather than others. For example, clover responds more readily than grass to some mineral fertilizers and as clover becomes more abundant the protein and calcium of the mixed herbage increase.

In another way fertilizers affect the quality of herbage by changing the chemical composition of the individual plant. Plants take up any element present in soluble form in the soil and may take up more when more is present. For example, the application of a fertilizer containing nitrogen will increase the absorption of nitrogen by the roots and this results in an increase of plant protein. Phosphate fertilizers will likewise promote a greater uptake of phosphorus; potash will promote the uptake of potassium, and so on. Lime, which is a salt of calcium, may increase the uptake of calcium.