b. Reduced nutritive value of forages.
Certain nutritional deficiencies in feeds for ruminant livestock maintained on grazing lands are the result of mineral deficiencies in soils. Phosphorus is often widely deficient in forages of semi-desert and savanna regions, because the soil content is low. This is aggravated by the predominance of forage species (mostly grasses) that have low phosphorus contents. The introduction of forage legumes that accumulate phosphorus from the soil is often useful. A more positive practice that has been proven quite effective on rangelands of several continents is to provide stock with mixed salt-mineral mixtures rich in phosphates. A more sophisticated practice is the addition of soluble phosphate compounds to drinking water. Correction of phosphate deficiency by any means results in more effective utilization of forage; it improves reproduction and growth of grazing stock. The calcium and magnesium content of forages grown on acid soils in humid regions may be deficient from the standpoint of live stock needs, but this is less likely to occur in semi-desert or savanna grasslands. The content of certain essential "trace" elements needed in very small amounts by both plants and animals may be below acceptable levels in forages. These deficiencies result from the soil's low content of such "trace" elements as boron, zinc, copper, molybdenum, manganese, and iron. Cobalt also is highly essential for livestock, but not for plants.
As the inherent deficiencies of specific soil areas for individual "trace" elements are identified, their correction may make spectacular improvements in the vigor and productivity of the grazing livestock. Some soil areas are deficient in one element; and others have different deficiencies. In some regions where "trace" element deficiencies were suspected but not individually identified, the supplying of a mixture of all essential trace elements in a general purpose mixture with salt has produced great improvement in livestock performance, and resulted in more efficient use of available forages.
In general, the leguminous forages tend to be richer in all minerals that are essential for livestock, as well as being higher in protein content, and generally are more palatable and nutritious than other types of forage. Those management practices that foster an increase in forage legumes in the available herbage generally improve the value of the feed for support of livestock. This is so important that in some regions (Australia, for example), the entire grassland improvement program is built around the forage legume component.
The most sensitive segment of livestock herds to any nutrient deficiencies (protein, phosphorous, calcium, magnesium, and the "trace" elements) are the breeding females, lactating animals, and young stock. The rates of conception and reproduction, growth of the young while nursing, and weight gains after weaning are significantly improved by access to feeds with adequate mineral content. Older animals are less affected.