Soil – plant – animal relationship
The plants derive the minerals from soil, and the animals from the plants / feed they consume and there is a dependent interrelationship between soil, plant and animals, which is may not be linear always. Several factors regulate the transfer of minerals from soils to plants and from plants to animals. Soil characteristics (pH, moisture), the type of plant (green fodder, legume and mature straws etc.), the physiological status of the animal (growing, pregnant, lactating) and the accompanying feed, all of these collectively or individually contribute to the mineral uptake and utilization. The mineral content of soils depends not only on the parent material but on a complex of pedogenic factors like laterization, calcification and salinization. Translocation further occurs by processes of surface erosion, leaching, evaporation and redeposition of minerals on the surface. Of the total mineral concentration in soils, only a fraction is taken up by the plants. The availability of minerals in soils depends upon their effective concentration in soil solution. Several factors influence the uptake of minerals by crops and pastures from the soil. These include
(i) soil acidity
(ii) soil moisture
(iii) soil temperature
(iv) plant variety
(vi) organic matter and microbial activity of soil.
For trace mineral absorption, the pH has the most marked effect on the availability. Alkaline soils lead to an increased biological availability of some trace elements such as Se and Mo. With decreasing soil pH, Se is less available, but the uptake of some cationic metals like Cu is increased. Soil leaching, erosion and long- term cropping lead to depletion of trace minerals. Crop management and climatic conditions also influence the eventual trace mineral level in feeds. Fertilization and / or heavy rainfall can result in lush pasture growth and the dilution of some trace minerals. Water near factories effluents also carry toxic minerals and produce antagonistic effect on certain essential minerals.With increased soil pH, there was drastic decrease in manganese (Mn) content.Water logging of a soil results in conversion of an aerobic to an anaerobic environment in the root zone area. The concentration of nitrogen in the plant tends to decrease and that of phosphorus increases with increasing moisture level, but no definite trends are seen for other minerals. The soil temperature and season can influence the uptake of minerals and growth of pastures. At low temperatures, the mineral uptake is slower possibly because of depressed root extension and membrane permeability. With the advent of green revolution, deficiencies of micronutrients were observed widely in several Indian soils and crops. The soil in tropical climate is mostly deficient in P. Zinc deficiency has been widely reported in rice, wheat, maize, groundnut, cotton and their residues in the intensively cultivated irrigated areas. In India, Bihar, Andhra Pradesh, Tamil Nadu, Madhya Pradesh and Haryana as well as all Indo-Gangetic Alluvial plains showed extensive deficiency of Zn in soil. While soil-plant-animal relationship may point towards area problem of specific mineral deficiency, the relationship is not linear in many situations. Hence soil, plant and livestock sometimes do not respond equally to certain top dressing of soil. For example soil enhanced with Mg by epsom or Ca by using gypsum may not be able to elevate the systemic Mg or Ca level in animals.
Feeds / fodders are the main source of minerals for livestock. Grazing animals receive certain level of minerals from water and soil ingestion. Of the minerals present in soil only a fraction is taken up by plants depending on geophysical / chemical conditions as explained above. Plant mineral content is dependent on other factors like type of soil, plant species, stage of maturity, pasture management and agro-climatic conditions. Mineral concentrations and availability are mainly affected by four interdependent factors, viz.
(i) genus, species, or variety of crop,
(ii) type and mineral concentration of soil
(iii) climatic or seasonal conditions
(iv) stage of plant maturity.
Plant varieties growing on the same soil under the same environmental conditions show marked differences in mineral uptake. Legumes are superior in Ca and Mg uptake from soil compared to the grasses. In general, legumes are higher in calcium, copper, zinc, iron, and cobalt than grasses. In contrast, grasses tend to be higher in manganese and molybdenum than legumes when grown on the same soil. Most of the trace mineral concentration was higher in pasture legume species than other grasses. Research has shown that even variety within a species affects mineral composition. Straws and stovers are deficient in most of the minerals. They also contain excess of silica, oxalate and tannins which may interfere in the utilization of other minerals / nutrients. Plant requirement of certain minerals (Mn, Zn, K) may exceed animal requirements and certain minerals may be required at higher levels in animals (Na, Cl, I, Co and Se). Mature plants are low in minerals as most of the minerals may get accumulated in seeds due to translocation. This is true in straws, which are the major roughage source for animal feeding in many areas.
Requirements for most minerals are not constant but are influenced by a number of dietary and physiological factors that affect absorption and tissue demand,e.g. an animal’s need for calcium is relatively low during the dry period; however, lactation more than doubles the calcium requirement. The dietary requirement for minerals and vitamins differ among animals based on their stage and class (gender, age, physiological state, heat stress, etc.).
Environmental effects (soil type and fertility, season, plant maturity, etc.) also produce differences in mineral and vitamin concentration within the plant, which ultimately lead to variation in mineral availability. Additionally, interactions among minerals and vitamins supplied by feed and water in the animal’s diet influence the relative bioavailability of these nutrients for absorption and utilization. For example: molybdenum and copper interact in such a way that “ties-up” copper rendering it unavailable for absorption, resulting in the appearance of a dietary copper deficiency. It is therefore important to evaluate the nutritional composition of feedstuffs and water so that the diet can be properly formulated to meet the animal’s nutrient need. Assessing the consequence of mineral deficiencies in animals is difficult because slightly lowered weight gains in calves, reduced milk production and/or decreased reproduction rates may occur without visible signs of deficiency. At the same time, excess mineral consumption may cause reduced performance without obvious signs of toxicity.
Requirements and maximum tolerable concentrations for some minerals are shown in Table 9.3. Many essential minerals are usually found in sufficient concentrations in practical feedstuffs. Other minerals, however, are frequently insufficient in diets and supplementation is necessary. The maximum tolerable concentration for a mineral has been defined as “that dietary level that, when fed for a limited period of time, will not impair animal performance. Supplementing diets at concentrations in excess of requirements not only greatly increases cost, but mineral loss in cattle excreta may also cause problems in soils and/or groundwater.