In the last two lectures we have talked about the ecology of the veld; that is, the plants and organisms that are found in the veld, and how they got there. We now want to look at the Morphology and Physiology of the plants, mainly grasses, which are found in the veld. Morphology is the study of the forms of the plants, the way in which the stems, roots and leaves grow and develop. Physiology is concerned with what is going on inside the plant, the movement of water and the build up and movement of carbohydrates, proteins, etc, in the plant tissues. All these aspects of plants, including the grasses, have been covered in the Botany Lectures, and it would be a good idea to revise your Botany notes at this stage.
- MORPHOLOGICAL ASPECTS OF VELD MANAGEMENT
The morphology of the grass plant covers the study of the following areas:-
- The Roots of the plant: these are under the ground.
- The Stem of the plant and this includes the normal stem together with specialized stemssuch as rhizomes and stolons.
- The Leaves of the plant. These are important because they are used to identify the varioustypes of grasses.
- The Flowers of the plant, which are also important for identification purposes.
The growth and development of these parts of the grass plant can be divided into three phases, and these are:-
- Phase 1, which is the very early growth and development of the plant, starting from theseed.
- Phase 2, which is the vegetative phase, when a large amount of leaf is produced.
- Phase 3, which is the reproductive phase, when the flowers are formed and seeds areproduced.
During Phase 1 the grass seedling is very small and it is using the maximum amount of light to produce carbohydrates by means of photosynthesis.
These carbohydrates are used for growth and development, particularly of the root system, which will be required to get the maximum amount of moisture and nutrients from the soil. This phase of the plant is particularly sensitive to bad management by over grazing. You can appreciate that the tiny plant can be destroyed, by hoof action, and also by severe grazing. We should pay particular attention to this phase, which occurs at the early part of the growing season: that is, at the beginning of summer. We should ensure that the paddocks get a substantial rest in the early part of the growing season, to ensure the successful establishment of these small grasses.
Phase 2: This is the vegetative phase and it is associated with erect growth of the apex when atremendous amount of vegetative material is produced. The vegetative part that we are particularly
interested in is the leaf area. This is the phase of maximum production and is also the phase at which we should get maximum utilization as far as grazing is concerned. However, it should be pointed out that whenever animals graze a particular veld, the cardinal rule of veld management should apply at all times, and that is to graze half and leave half. If this rule is applied whenever animals graze in a particular paddock, progress can be made as far as restoration of the veld type is concerned and the re-gaining of vigour. It is important to ensure that a certain amount of leaf canopy remains after each and every grazing to enable the plants to recover from that grazing: i.e., to produce the photosynthetic material which is required for normal growth and physiological development. An important aspect of the vegetative phase, or Phase 2, is to ensure that the plants are allowed sufficient rest so that they may retain and regain vigour in the overall herbage production equation.
Then we go into Phase 3, which is the reproductive phase. It is equally important to realize what the reproductive phase means in any normal growth and development curve of a grass or a plant. The reproductive phase is essential for the development of the seed and the normal release of seed. That is the production of seed for the subsequent establishment of grass plants. One cannot hope to establish grass plants successfully if the seeds are not there. You will appreciate from your earlier ecology lectures that this completes one of the phases of successful establishment of a particular species. Remembering that we are at all times trying to ensure that our grass sward is made up of the more palatable species and the perennial species, in order to ensure a good ground cover, good herbage production and, subsequently, a high carrying capacity. This is a necessity as far as successful veld- management goes.
These three phases coincide with the summer period: that is, from early November through to March or April. We will assume that Phase 1 is November and December, Phase 2 is January early February, and Phase 3 is late February and early March. It is essential to be aware of these physical developments so that we can ensure that certain rest periods are, in fact, incorporated into the overall veld management programme. This will ensure that .there is no management of the grasses. This is essential for the successful establishment of the grasses. Having talked about the morphological aspects, it is essential that we consider the rest periods that are required to restore vigour, maintain vigour and restore good quality grasses within the overall composition of the sward. The objectives of resting grass veld can be divided into three main types:-
RESTS BASED ON THE REQUIREMENTS OF THE ANIMALS.
Rests for the accumulation of sufficient grazing material to ensure rapid dry -matter intake Since the daily intake of dry matter by animals is closely related the length of the sward on which they are grazing, animals may be excluded from a pasture for no other purpose than to allow sufficient grazable material to accumulate: this ensures a rapid rate of intake, when the animals are returned to the particular paddock or veld. However, the optimum length of the sward and so the length of the resting period will depend on the type of animal which will graze the particular veld type.
Rests to provide out-of-season fodder
Rests may be applied to allow for the accumulation of material for conservation as hay or forage, either for planned use during the non-growing season that is the winter period, or as a drought reserve. The conservation of summer growth for winter grazing is important in Africa. In certain situations crop residue in the form of maize stover can be used as a winter forage, but it should be pointed out that maize stover only serves as an alternative source of winter forage but is not a substitute for a sound veld management system.
RESTS BASED ON THE REQUIREMENTS OF THE INDIVIDUAL PLANT
Rests to allow for seed production
Seeding rests are perhaps the most commonly applied rests in natural grasslands. They form the
basis of resting programmes in North American ranges, and have been widely advocated in our veld management system. These rests are applied from the time the stem apexes of fertile culms rise above grazing height, until the seed has matured and shed. These rests have often been advocated as an integral part of veld grazing systems, but such rests are likely to serve an important purpose in perennial grassland only when the density of the sward is low, or when the species composition is poor. To a degree at least, these rests can be timed to allow only certain species to produce seed.
Rests to promote seedling establishment
For a seedling rest to fulfill its intended function, attention should be given to the seedling derived from the seed produced during the seeding rest. Little attention has been paid to this factor for veld improvement, although the widely advocated two- camp system designed for semi-arid areas has incorporated this principle. Here, an autumn seeding period is followed by a winter grazing to trample the seed into the soil, and a spring rest for seedling establishment. There is however, a need for much careful study of the fate of the seeds produced during the seeding rest.
Rests to promote vegetative growth of established plants
Where plant density is sufficiently high but tufts are small, canopy density can be increased by encouraging the development of lateral tillers through adequate resting. Since the ability of a parent tiller to support a large number of developing daughter tillers is partly a function of its vigour, resting should form an integral part of any such programme. Rests should generally be combined with cutting or grazing treatment designed to stimulate the activity of those tiller points situated at the basal nodes of the parent tillers.
Rests to promote rapid growth
Generally this aspect has been considered in terms either of the reserve carbohydrate status of the plant or the leaf area available for photosynthesis. These two aspects are clearly linked, but have generally been considered independently.
Reserve carbohydrate status of the plant
The basis of the resting programme designed o ensure an adequate overall carbohydrate status of a summer-growing perennial forage plant is that rest should be given whenever the supply of storage carbohydrates in the stem base and roots has been depleted through the demands for energy by new growth at a time when active leaf tissue is not available. Such a situation arises each spring and each time the sward is heavily grazed or cut. Rests are frequently applied in autumn to allow the plants an undisturbed period of photosynthesis. Since growth at this time is slow, a large proportion of the carbohydrate which is produced is transported to the stem base for storage and is used by the plant to supply its needs for respiration during winter and for growth during the following spring. It is recognized also that the levels of carbohydrate reserves may fall during the period of stem elongation and seeding, and that the stimulation of new growth by defoliation of the plant at this time is likely to impose excessive strain on the plant. To retain a vigorous sward, therefore, utilization should be delayed until the reserve levels within the plant have been restored.
Since most of the carbohydrates on which plant growth depends are produced by the leaves, it is important that adequate rests be provided after intensive defoliation to allow for accumulation of leaf material. If, however, grazing is light so that an adequate amount of active leaf remains on pasture at all times, rest periods will, theoretically, not be necessary, because the continued presence of leaf area allows for the sustained production of carbohydrates to provide the energy requirements of growth. The plants will not be required to draw on their supply of reserves and so vigour is maintained.
In practice, however, grazing complicates such an ideal situation.
RESTS DESIGN TO PROMOTE CHANGES IN THE COMPOSITION OF THE COMMUNITY
Where there is a delicate balance between two communities which are at different levels in the secondary sphere, a grazing programme may be designed to favour either of the two communities. In theory, the community which is generally the more advanced of the two will benefit from repeated resting, while the more pioneer community will benefit from longer grazing periods and shorter rests.
This approach is similar to that used in the range lands of North America, where the range is classified according to its present condition relative to the potential of the environment. Species are categorized as decreases, increases, and invaders. The value of the species in each category being related to its position relative to the climax vegetation of the area.
Rests designed to eliminate or control bush encroachment
Long rests may be designed specifically to allow for the accumulation of large quantities of herbage, either to provide direct competition for developing bush seedlings or to provide sufficient fuel for a hot burn to kill such seedlings. This procedure has long been a part of the management programme in areas prone to bush encroachment.
From the above discussion, it is clear that the objects of resting are many and varied, as is their timing and duration. The current terminology applied to resting does not however differentiate between these rests. This is unfortunate because when we try to implement a veld management system, we do not necessarily take into account the various rest periods required, but merely infer that such rests do occur during the non-grazing period.
- PHYSIOLOGICAL ASPECTS OF VELD MANAGEMENT
Production of Carbohydrates:This is based on the assimilation of carbon dioxide for the productionof energy from the carbohydrates. These are produced in the leaves and translocated to the various organs of the particular plant, in particular the roots. This forms the main storage organ of the grass plants.
Utilisation of Light Energy: A plant can utilize a maximum of 15% of the energy which it receivesfrom the sunlight, and an average of 10%. This is due to the fact that the leaves are unable to utilise all the light energy received. What is this due to? It is due to a leaf area, the angle of the sun, the amount of damaged leaf, which has resulted from the grazing or the mowing or whatever the case may be. All these factors tend to add up to reduce the effectiveness of a successful harnessing of light energy.
Interception of Light Energy: Most of the light striking a sward is not intercepted by the leaf canopy,and ends up on the ground totally wasted, without being used for any purpose whatsoever. We can capitalize on this loss of energy by ensuring that there is a sufficient leaf area to intercept as much light as possible.
Leaf Area: Since the growth rate of a plant is directly proportional to the amount‘ of leaf which it has,it would be of benefit to increase the amount of leaf area. Increasing the leaf area results in an increase in the amount of light which is intercepted, resulting in an increase in the utilisation of light energy. Maximum light interception is achieved when the canopy intercepts 100% of the light, but as you can appreciate, this is not possible because of the natural situation in the veld.
Growth Rate: All the above factors are directly related to the growth rate. Therefore to getmaximum growth rate, the above factors have to be at an optimum level. The physiology of growth is extremely difficult when we start using mathematical formulae to try to indicate what is happening. It is sufficient for this course that we understand that the physiology is important for the following reasons.
- The successful interception of light.
- The utilisation of light energy to produce the energy substrates.
- The successful trans-location, of the energy to the main organs of the plant.
Carbohydrate Reserves: Root or stubble reserves play an important part in the recovery of plants,either after drought, in the winter period or after very heavy utilization. They should, however, be looked upon not for production as such. These reserves are also used during the winter as substrates for respiration. The reserves accumulate only when the carbohydrate production is in excess of the current demand for growth. It is generally considered that there are three main “sinks” in the plant into which the carbohydrates, formed by the leaves, are channeled; these sinks vary in their strength:
- The stronger is the one associated with the growing part above the ground level such as the stem apexes and the leaf initials.
- When the requirements of these stem and leaf meristems have been met, the requirements of the root meristems are then satisfied.
- If there is still carbohydrate left over, this may be deposited in the form of reserves in the crown or the roots. In practice, it seems that carbohydrates are stored only under conditions when growth is very slow, but assimilation continues rapidly irrespective of the growth rate. The ideal time to graze the plant would be in the accumulation phase, or Phase 2 of the normal growth and development curve. If a plant is cut or grazed heavily, all the roots stop growing within 24 hours. If, however, a small proportion of the leaves are removed, root growth will continue but at a slower rate. Removal of more than 50% of the leaves may stop root growth from 3 to 10 days, this period getting longer with the more severe defoliation. Above about 80% defoliation, the roots start dying back from their tips so that over utilization tends to destroy the amount of functional root system there is. It can be seen that it is essential to ensure that there is sufficient rest period after grazing for root recovery and carbohydrate restoration. The root growth is more sensitive to heavy utilization and to the lack of reserve than the top growth. The root system is far more sensitive to defoliation than the aerial parts are. A common characteristic of grass is that the tops to roots ratio is 3:1, but if you cut off the tops and you change the ratio, to say, 1:3, the first thing that the grass is going to do is to change the ratio, back to 3:1. If you cut back to 2:1 then this will also re-establish the ratio 3:1. The plant re-establishes 3:1 ratio relatively quickly. In the first case, where the ratio was 1:3, the roots die back and the plant is forced to reduce the root size before being able to reestablish the 3:1 ratio. Current studies of this problem indicate that the 3:1 ratio can be established in about 10 days. Hence it is essential to ensure that there is at least a minimum of a 10 day rest period between successive grazings.