| Grazing and rangeland development for livestock production |
|Management of rangelands and other grazing lands of the tropics and subtropics for support of livestock production. Technical Series Bulletin No. 23|
|IV. The elements of productive grassland management.|
There are two points of view in dealing with soil nutrient deficiencies: (1) the deficiencies affecting plant growth, and (2) those that affect performance of livestock. The simple production of more forage is useful where its nutritive content meets animal needs. However, there is good evidence that some plants may make substantial growth as to dry matter, but of such low content of protein and essential minerals that the feed has limited nutritive value. This situation is widespread on both dry ranges and humid pasture
a. Low protein forage. Much standing dry forage on dry grazing lands is so low in protein content (6% or less) that the energy feed constituents cannot be utilized beyond the available protein, even though eaten and passed through the animals digestive tract. The fostering of forage legumes in grazing lands, where feasible, is the most efficient way of supplying the necessary nitrogen (made available through root nodules) for stimulating growth of associated grasses, as well as for meeting the protein needs of grazing livestock. The alternative to fostering growth of legumes in grazing lands is to provide supplemental protein concentrate feeds particularly to the breeding herd and young stock. Increasing legumes in the forage is the preferred method in most situations.
b. Mineral deficiencies in soils of rangelands in semi-arid and subhumid regions. Phosphorus is widely deficient in such soils, and consequently in the forages grown on them. This deficiency may be great enough to hamper growth of legumes, even though most tropical forage legumes have stronger "feeding power" for soil phosphorus than temperate zone forage legumes. The magnitude of the soil phosphorus deficiency should be evaluated when introduction of new legumes in grasslands is contemplated. It may be necessary to apply phosphate fertilizer in preparation for seeding such legumes. Wherever the phosphorus content of forage is deficient for livestock, it may be necessary to supply supplemental phosphorus in a mineral mix (with salt) as a positive measure. This can meet livestock needs, in an inexpensive way even though the need for phosphorus by legume forages is not met.
In humid regions deficiencies in soil calcium, magnesium, potassium and sulfur are often found, and these adversely affect growth of forage legumes, as well as the performance of grazing livestock. In general, soil amendments are needed to correct these deficiencies so that legumes can be grown successfully, and these will also more nearly meet nutritional needs of livestock grazing on such forage. Low grade forage (wiry grasses and weeds) will grown on soils deficient in these minerals, but nutritive values of such forage are insufficient to support breeding stock and young animals even when total feed volume appears adequate. Fortunately, for virtually all soils (except some sands) in the semiarid and subhumid regions, calcium, magnesium, potassium and sulfur are adequate for both forage plant growth and for grazing livestock.
Trace element deficiencies are not uncommon in tropical and subtropical soils, both in regions of limited rainfall, and in humid regions. These deficiencies may involve only one, or several "trace" elements. These elements are required in very small amounts by both plants and animals, which accounts for the term "trace" elements, The elements are boron, zinc, molybdenum, copper, manganese, and iron. To these, must be added cobalt which is a requirement for animals, but not for forage plants. The correction of these deficiencies for livestock is easily accomplished by including small amounts of all of them in a salt-mineral mixture. However, when these deficiencies affect plant production, it becomes highly desirable to identify the specific soil groups in which individual deficiencies occur, and to develop practices to correct them. An example is provided in Australia where a large region that was quite unproductive responded quickly to a few grams of molybdenum per hectare, to produce abundant growth of a nutritious forage legume.
The identification of trace element deficiencies and development of practical methods for their correction is closely tied-in to the establishment and growth of legumes. Forage legume species differ in their ability to extract trace elements from soils. At least one legume species is capable of extracting copper from a deficient soil, and will succeed when others fail.
Because the general knowledge on the occurrence of trace element deficiencies in soils of the tropics and subtropics is still fragmentary, it would be desirable to mark this as a fruitful area for investigation wherever Forage legumes are unthrifty. The differences between the capabilities of different legume forages to extract trace elements from the soil, also needs more study to enable better matching of legume with kinds of soils. Fortunately, when the nutritional needs of forage legumes have been met, the mineral needs of livestock feeding on them also will be less critical. The exception is cobalt which is uniquely required by livestock, and must be supplied whenever deficient.