Degree of utilization and livestock production
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Degree of utilization is the proportion of the current year's forage production that is consumed and/or destroyed by grazing animals. It may refer to a single species or to the vegetation as a whole and it can be expressed in quantitative terms such as percent (25, 50, 75, etc.) or in qualitative terms such as light, moderate, heavy, over or destructive. Its synonyms are degree of use, utilization or use, all of which appear in the literature. Degree of use and animal production are inversely related. As use increases, individual animal productivity decreases, and vice versa.
Many investigations of all kinds of pastures, both native and artificial, in various parts of the world have been made and all have shown a curvilinear relationship between degree of use and animal production per hectare. One model developed from thirty years of data on animal weight gain per individual and per hectare as related to ungrazed herbage on the central plains of Colorado, U.S.A. is presented here merely to illustrate the relationship and to point out a high ranking range management fundamental (Bement, 1968). Bement's measure of "ungrazed" forage is a reflection of degree of use and stocking rate.
The utilization-animal relationship may be illustrated by plotting the two animal responses (production per animal and production per hectare) over forage remaining ungrazed at the end of the grazing period as in Figure 8. Up to point "A" in Figure 8, forage supply is not limiting and an animal receives its daily forage requirements and only genetical or other such factors limit production. Beyond this point, forage supply becomes limited and production per animal is reduced because of nutritional stress which becomes worse as the amount of ungrazed forage becomes less due to excessive utilization and stocking.
Point "B" as illustrated in Figure 8 shows when maximum animal production per hectare is reached. This point is always reached when the ungrazed forage per hectare is less than where the optimum, individual production per animal is reached (Point "A"). However, at point "B" individual animal performance and the range vegetation are both stressed due to over utilization. If this continues, the advantage of attaining maximum production per hectare will be short-lived because the vegetation and forage supply will deteriorate and Point "B" will shift down the production scale.
The optimum point of forage availability and degree of use for sustained maximum production, the objective of range management, therefore, lies between Points "A" and "B". And this common balance point comes at the cross-over of the two graphs shown as Point "C" in Figure 8. It can be noticed that production per hectare rapidly declines once the optimum stocking rate (Point "C") is reached. Therefore, undergrazing with less animals can produce a greater total production than overgrazing with more animals. Clearly, the philosophy "more livestock, more animal products" is not true once overgrazing is reached.
Succession
Manipulations that assure proper plant nutrition also assure superior livestock production, and represent a huge step towards desertification reversal. But, as with the word "nutrition," there are other things that one needs to know. Plant succession also plays a major role in either desertification or its reversal, and domestic livestock are right in the middle of it.
Succession is the replacement of one community of plants by another one. There are two kinds of succession, progressive and regressive. The latter is commonly called "retrogression" which will be used in this text. There are two kinds of progressive succession, primary and secondary. Primary succession is the original and simultaneous development of vegetation and its associated soils. The sere was the harmonious development of soils and vegetation, each influencing the other. The nature of the sere and its end product depended on the climatic, edaphic, physiographic, pyric and biotic factors involved in face of chemical and mechanical weathering. The end product was either any of the kinds of rangelands previously mentioned or forests or other kinds of biomes
The sere stopped when an equilibrium between all the factors named above was established. This stage of development is called "climax" by some ecologists and "natural potential" by others. Regardless of its name, the equilibrium stage of primary succession is the highest natural form of development that the environmental factors are capable of producing.
Mother Nature is very forgiving. She always tries to restore what man has destroyed. This is the case of secondary succession. A disturbance of climax will cause retrogression, but, if given the opportunity, nature will make every effort to restore the damage via secondary succession. Secondary succession is much faster than primary because the soil is already formed, although deteriorated and needing restoration as well.
The Climax Rangelands
The Americas' explorers described an abundance of rich and lush natural pastures. Like a sea of grass, some said, which was probably inspired by vegetation shown in Figure 9. Some described the grasses as being up to their stirrups and higher in places. Tall grass prairies like that shown in Figure 10 dominated the Pampas. Chile's annual ranges were dominated by perennials. There was no shortage of forage for the indigenous peoples' llama and alpaca in the Altiplano. The dry and humid tropical savannahs were free of brush and wars were fought for their control in Venezuela. Even the arid areas flourished with grasses and shrubs palatable to livestock.
These rangelands were and still are the Region's heritage. Settlers came with their livestock and one of the world's most colorful and largest livestock industries was built. Having no precedents to follow, the industry's builders thought that the bountiful vegetation would regenerate and last forever. It did not. Misuse has caused retrogression and declining productivity. The heritage is sick, needing medical attention.
Retrogression and Secondary Succession
Retrogression or degeneration is the replacement of a community of plants of higher ecological order with a community of lower ecological order. Disturbance of the stabilized climax through overgrazing or cultivation causes retrogression. Vegetation deterioration is followed by soil degradation. The latter begins with loss of organic matter and structure breakdown followed by erosion. Extreme disturbance can expose parent material and lichen stages where primary succession began. If disturbance is compensated or eliminated in time, succession moves back towards climax. This is called secondary succession which is any succession after the first and primary succession. It usually begins with higher stages of soil development and plant communities, therefore, improvement through secondary succession can be quite rapid.
Knowledge of the processes involved with succession, ether progressive or regressive, better prepares one to analyze and synthesize occurrences in the field. The first step in the process is migration, which is the movement of the progeny of a plant to a new location, whether it be far or close to the mother plant. Migration can occur sexually in the form of seeds or asexually (vegetatively) in the case of some species through stolens (above ground stems which roots at the nodes) rhizomes (below ground stems which send out roots and above-ground shoots at the nodes), tillers (basal buds which form new stems) or bulbs.
Ecesis is the successful establishment of a seed or vegetative reproduction organ. Whether or not ecesis takes place in respect to the species involved depends upon the situation. For example, in an overgrazing situation, ecesis of unpalatable and worthless species is favoured whereas ecesis of palatable species is opposed and the pasture will eventually be dominated by worthless species.
On the other hand, in a properly grazed deferred-rotation grazing system, ecesis of the palatable species is favoured which gives them an opportunity to increase.
Aggregation is the expansion and grouping of newly established plants in the secondary succession or retrogression process. The forces that limit or favour aggregation are the same as those related to ecesis. Competition is the general struggle in which plants and plant species compete for light, space, water and nutrients. Naturally, those species which are genetically superior in respect to leaf and root production will dominate if given the opportunity. Climax is characterized by a dominance of species and ecotypes of species which in most cases are the most productive forage species that an environment can naturally produce. Finally, reaction is the influence that plants and plant material have on the environment, particularly the microclimate where the plants actually live.
The influence of plants on the microclimate may be beneficial or disastrous depending on the circumstances. As a general rule, soil and surface temperatures and evaporation increase and rainfall infiltration decreases with retrogression, making a desert like (xeric) microclimate in which only desert or xerophytic species can live. The reverse happens with secondary succession and the end result is a more humid (mesic) microclimate favourable to mesophytic species, but unfavorable to xerophytes. In secondary succession, reaction creates a home that is "too" good for the species of lower ecological order but "just" right for species of higher order. Assuming all other factors to be equal, there is no barrier to the migration, ecesis and aggregation of the latter and they will move in and dominate. Thus, reaction is not only essential to secondary succession, it is also a catalyst.
When the supply of desirable species becomes limited, the animals then turn to the next most palatable species, which are usually less productive and nourishing and less desirable in respect to soil and water conservation. While the desirable species are decreasing, these species increase to a point, but with continued overuse, they also weaken and die. These species are termed "increasers" or "less desirable" (Figure 11).
Only unpalatable species and grazing evasive species can survive such a system of overgrazing and eventually these will invade and they are termed "invaders" or "undesirable". Invaders are species that were absent or present in very small amounts in the original vegetation, which invade following disturbance or continued overgrazing. Invaders are less productive than increasers and are of very little value as regards soil and water conservation. Some are excessive consumers of water, giving nothing in return, and livestock refuse to graze some species. Eventually the animals are forced to eat some species of invaders or die from starvation, which the productive animals usually do, leaving the least productive and demanding animals which manage to survive, giving little production in return.
The terms decreaser, increaser and invader are used in the United States, where the ecology of individual species are fairly well known. This is not well known in Latin America and the Caribbean and the classification of desirable, less desirable and undesirable is more applicable at this time. These are described as follows:
Desirable:
Highly productive and palatable species that provide good
environmental protection.
Less Desirable:
Species that are less productive and palatable than desirable
species and which provide less environmental protection.
Undesirable:
Species that yield very little, if any, forage that is not
particularly palatable. They impair the ecosystem. They can also
be noxious.
An example of secondary succession is given in Figure 12, which illustrates the opposite of retrogression and the fact that desirable species can regain dominance or at least increase with years of proper management. It is doubtful that the original climax will ever be completely re-established, although a climax type of vegetation is a worthy goal. Also, there are some situations where a subclimax is apparently more nourishing than climax, thus yielding higher livestock returns. Nevertheless, the fact that there is a natural phenomenon in which nature tries to restore the climax gives the manipulator a very valuable and most useful tool. He or she knows that in many situations, species composition, plant cover and forage production can be improved along with soil and water conservation through natural and inexpensive secondary succession. It is his or her duty to devise, plan and execute the manipulations required to activate this. Neither retrogression nor secondary succession follow well-defined patterns. Retrogression patterns depend to a large extent on the kinds of livestock involved.
Retrogression under cattle grazing will likely consist of an increase and/or invasion of short grasses, fortes and browse. There will likely be an increase of tough grasses and browse with sheep and grasses and fortes with goats. Destructive grazing by all three will result in either bare ground Or a kind of vegetation that neither one can or will eat.
Box (1972) pointed out that secondary succession sometimes "jumps" or "bounces" into a new state which may be just as stable as the climax, but different. It is unlikely that it will ever return to the pristine state because it represents a new stable interaction which may be just as productive as the original vegetation. Chile's annual ranges associated with a Mediterranean climate is an example. Introduced species which behave like native species are called "naturalized species." Desirable naturalized species have spread in some areas, creating a new range ecosystem with a different set of actors.
Figure 11. An example of retrogression and range condition related to percent of climax remaining.
