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By André Voisin, Catherine T. M. Herriot
ISLAND PRESSCopyright © 1959 Philosophical Library, Inc.
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WHAT IS A HERBAGE PLANT?
Cutting and successive re-growth
A pasture plant must be capable of growing again after it has been cut either by the tooth of the animal or by the blade of the mower.
When this plant is cut it retains very little, and sometimes indeed hardly any of the green aerial part capable, by photosynthesis, of creating the elements necessary for the formation of new plant cells: that is, for the initial re-growth of the plant.
It is therefore indispensable that the plant, at the moment when it is cut, should have, in its roots or at the foot of its stalks, sufficient reserves to allow the formation of a certain green portion which, by photosynthesis, will then permit the normal growth of the plant.
Every new growth, that is to say every re-growth of our herbage plants, takes place at the expense of the organic substances elaborated previously (before cutting) in excess of what was necessary for the maintenance and growth of the plant. These substances have been stocked in the roots and lower aerial portions. If one cuts the plant before the roots and the part not cut have stored up sufficient reserves, re-growth will be difficult and may even not take place at all.
There is a period in which wheat can be grazed without being destroyed
This evolution of reserves in our herbage and forage plants is a question which, unfortunately, has been very insufficiently studied by plant physiologists until now. We know very well that there is a moment in the course of a plant's development when the reserves in the roots are at their maximum and when, in consequence, the conditions for re-growth are optimum. Take our old graminaceous friend, wheat. Grazing wheat as it emerges from the soil destroys it. At harvest-time, when we cut the wheat with its grain formed and ripe, the stubbles of our fields do not produce re-growth. On the other hand, between these two extremes there is a period in which it is possible to graze the wheat and yet allow it to grow again and thus produce a reasonable harvest.
Definition of a herbage plant
We will therefore answer the question asked at the beginning of this chapter by stating that: A herbage plant is a plant which is capable, several times in the course of a year, of accumulating in its roots (and at the foot of its stalks) sufficient reserves to allow it to grow again after every cut.
Let us look quickly at a few points concerning the evolution and nature of these reserve substances which are indispensable to the re-growth of the grass, after cutting with the blade of the mower or shearing with the teeth of the animal.
Evolution of quantities of reserves in the plant
As Professor Klapp tells us (70, p. 350), the production of green matter by our herbage plants is not a continuous process throughout the period of vegetation; but accumulation and expenditure of substance alternate with each other. At the end of the summer and in the autumn the accumulation of reserve substances (as a result of the production of assimilation products by the leaves) permits re-growth in the ensuing spring, followed eventually by development up to flowering and the formation of seeds. An analogous phenomenon takes place after every cut, if the latter does not kill the plant.
Different plants differ enormously in the time and also in the speed of this assimilation and in the storing up in reserve of the substances assimilated.
Alternating rhythm of accumulation and exhaustion of the reserves
The Polish research worker Osieczanski (82, p. 65) has very clearly summarised this alternating rhythm of exhaustion and accumulation of reserves:
"Part of the products of photosynthesis is immediately utilised for the construction of the cells of those organs of the plant situated above and below the soil. Another part of these products of photosynthesis is used to satisfy the physiological requirements (respiration, metabolism). The remainder of these products is put into reserve for a time when there is no synthesis, or at least when the products of this synthesis are completely utilised to satisfy the needs of the plant organs. These reserves allow the plant to survive critical periods, such as, for example, the winter period, during which the balance of the phenomena of assimilation is negative.
"The reserve substances of grass are utilised for respiration, formation of stalks, leaves, seed, roots etc. and in particular for the respiratory processes at low temperatures (below 32° F. [0° C.]) and at high temperatures (above 85°-95° F. [30°-35° C.]); temperatures at which respiration uses up more energy than is supplied by the processes of assimilation. These reserves will also be utilised during periods when the plant is growing strongly as, for example, during tillering or the formation of seed. This will be the case in particular after cutting or grazing when the grass will have to re-create green surfaces supplying the products of assimilation...."
Nature of the reserve substances
Under identical conditions as regards the quantities, or proportion, of reserve substances remaining after cutting, the re-growth of the same plant can vary greatly, concomitant with such other factors as day-length, soil moisture, amount of assimilable fertiliser elements present in the soil, rainfall, etc.
It would therefore be particularly desirable if we had better knowledge of the way in which the reserves are accumulated in our herbage plants: this would help us to use them more profitably.
At present, however, no firm conclusion has been reached even concerning the nature of the reserve substances. Sullivan and Sprague (102) have published a detailed review of the different theories put forward regarding these reserves. We refer the reader to these authors for this bibliographical review, and also for their study of the reserve carbohydrates of rye-grass (vide also Weinmann, 140).
Can grass build up a reserve of growth hormones?
In general, one considers as reserve substances all the fats and the nitrogen-free extract. As mentioned above, it is essential that the plant contains in its roots and the part not cut the maximum possible of these reserve substances. But these indispensable substances are probably not sufficient. Our herbage plants have also a stock of other substances which allow them to grow away again after being cut. Here, it is probably one or more hormones which allow the growth of the plant to be set in motion once more. R. O. Whyte (144), who is a plant physiologist, reminds us of this in well-chosen words:
"The physiologist studying herbage plants cannot fail to wonder at the remarkably small effect which repeated removal of leaves and damage to tender growing points of the plant has on the physiological behaviour of the plant and on its development.
"It does not appear out of place therefore to put forward a few hypotheses: is it not possible, when a plant goes to seed every year or every two years, that all (or almost all) the growth (or re-growth) hormone is removed in the seed? There would then be no more hormone left to revive the meristematic activity at the base and lead to formation of new tillers. Might it not be that in a herbage plant, only a proportion of the hormonal content is removed with the part cut off and enough remains at the base to meet the needs of the new tiller growth? The higher the concentration of hormone remaining, the more active the new tillering of the grass ..." (vide also Söding, 98).
If these hypotheses are correct, it would obviously be of interest to know the fluctuations which take place in the reserves of this re-growth hormone in the parts not cut, and how we could augment these reserves by our different methods of cultivation (fertilisers, for example). Unfortunately we have no answers as yet to these important questions.
Comparison of the quantities of reserve substances and of their distribution in two gramineae
As McIntyre (73a) has reminded us, the recuperation of plants from defoliation is dependent on:
(a) the extent to which the photosynthetic surface has been eliminated;
(b) the extent of stored material which is accessible to the animal;
(c) the rapidity with which the plant can replace its reserves.
Professor Klapp has studied the evolution of reserve substances in the course of the development of cocksfoot and smooth-stalked meadow grass with different numbers of cuts per annum. In addition, he determined the distribution of the reserve substances between the cut, green, aerial part and the roots and the base of the green part left by the cut.
The short summary of the data (Table 1) is taken from Klapp's table and clearly shows the difference in the behaviour of cocksfoot and smooth-stalked meadow grass in the face of frequent cropping close to the ground.
We see that where three and four annual cuts are made, cocksfoot retains only 29 and 39% of the reserves in its possession when it is cut only once per annum. This proportion is reduced to only 40 and 55% respectively in the case of smooth-stalked meadow grass.
It is understandable, therefore, that repeated increase in the frequency of cutting will weaken cocksfoot much more than smooth-stalked meadow grass. This corroborates Weinmann's judicious observation (140): "The effects of repeated defoliation are cumulative, and progressively deplete the reserves more and more ..." (vide p. 24).CHAPTER 2
THE CURVE OF GRASS GROWTH
Kinetics of plant growth
WHEN a plant emerges from its seed, it grows slowly to begin with and then accelerates its growth until it reaches the flowering stage, when the growth slows down again.
In their admirable work Principles of Plant Physiology (9, pp. 322-325)
Bonner and Galston have provided us with an explanation of the kinetics of growth:
"Suppose we follow the growth of an intact plant through its life cycle by means of measurements of height or of total dry weight. We shall find, in general, that the dry weight of the seedling plant first tends to decrease slightly following germination, as the reserves of the seed are depleted.
"This is followed as photosynthesis becomes established in the new leaves, by a rapidly increasing growth rate, which finally becomes constant at some relatively high level (Fig. 1). The growth rate during this period is often remarkably rapid. The bamboo stem may grow as much as 24 in. [60 cm.] per day, and staminal filaments of certain grasses have been observed to elongate as much as 0·11 in. [3 mm.] per minute over short periods of time. Growth continues at this rapid rate until the approach of maturity at which time its rate slowly declines and approaches zero. The dry weight of the plant may even decrease in the final stages of senescence.
"The 'S', or sigmoid, shape of the curve is typical of the growth of the plant as a whole, as well as of the growth of living organisms generally.
"The sigmoid growth curve of an entire organism is the resultant of the individual sigmoid curves of each of its component organs. For example, during the later phases of the growth of a plant, increase in dry weight may be largely manifested in the developing seeds and fruit, the vegetative organs contributing but little.
"In all of these instances we may distinguish three stages which together make up the socalled 'grand period of growth':
"1. An early period of slow growth.
"2. A central period of rapid growth.
"3. A final period of slow growth."
Let us now see how this universal, biological curve applies in the case of a grass growing up again after defoliation.
The curve of re-growth in grass
The curve of re-growth in grass is also sigmoid in shape, that is S-shaped, the characteristic and universal form of growth in all living organisms, as we have just seen (Fig. 1).
At first the grass, having only its reserves and an infinitesimal number of chlorophyll workshops at its disposal, grows slowly and with difficulty. Then it succeeds in creating a sufficiency of green cells, the photosynthesis of which will furnish building material for the rapid creation of other green cells, that is, of a large mass of grass per unit time. This is the blaze of the grass's growth. Towards the end of this period of rapid growth the grass renews its reserves and then slows down its synthesis of green cells in order to devote all its efforts to the production of flowers and seed.
This is what is shown in Fig. 2, where we have reproduced the typical sigmoid curve showing, in this instance, the quantity (in lb. or kg.) of green grass present per acre (or hectare) as influenced by the number of days which have passed since the grass was grazed, that is, since it was sheared with the animal's teeth.
In practice, the curve is much less regular. The increase in weight of the dry matter presents a serrated curve; but, on an average, this S-shaped curve is a good representation of the actual re-growth of the grass.
We have assumed two seasons in which the growth is different. For the sake of simplicity, the growth of grass in August-September is taken as being twice as slow as in May-June.
This relationship, of course, is theoretical: it varies with the region and the prevailing climatic conditions in any season. Nevertheless, one may say that it is more or less the average relationship in many regions of North-West Europe where grass growth is almost half as rapid in August as in May: this means that with well-conducted rational grazing the rest period for the grass between two successive rotations will have to be twice as long in August as in May (vide Voisin, 128 and 129).
The optimum times in this connection (subject to annual climatic variations) are, on the average, 18 days in May and 36 days in August (vide Voisin, 134).
We assume that during these optimum rest periods there has been a regrowth of 4200 lb. harvestable grass per acre [4800 kg./ha.].
We see then that:
1. With a rest period of half the optimum time, production is reduced to a third 1400 lb./acre [1600 kg./ha.] against 4200 lb./acre [4800 kg./ha.].
2. With a rest period equal to one third of the optimum, production is reduced to a tenth 430 lb./acre [480 kg./ha.] against 4200 lb./acre [4800 kg./ha.].
3. With a rest period half as long again as the optimum, production is only increased by 20% 5060 lb./acre [5760 kg./ha.] against 4200 lb./acre [4800 kg./ha.].
Productivity curve of grass
What I shall arbitrarily describe as the "productivity of grass" is the daily quantity of grass re-growth per acre (or hectare), underlining the fact that this is a restricted conception of productivity.
We assume that during these optimum rest periods there has been a regrowth of 4200 lb. harvestable grass per acre [4800 kg./ha.].
Two productivity curves will make quite clear to us the necessity for observing optimum rest periods so that the grass may be allowed to do its work with maximum productivity.
In Figs. 3 and 4 we have shown the lb./acre [kg./ha.] of grass produced daily as a function of the number of days of re-growth at the two periods of the year under consideration here, namely, May-June and August-September.
In actual fact, it is the same curve in both graphs but with different scales.
In each case a net maximum manifests itself corresponding with the maximum productivity of the grass, viz., 18 days in May-June (Fig. 3) and 36 days in August-September (Fig. 4).
These curves make it even more clear to us how low is the productivity of grass during short rest periods corresponding more or less to those pertaining between two bites where cattle grazing is continuous.
It is again emphasised that the rest period of 18 days, which corresponds with maximum productivity in May-June, corresponds only with low productivity in August-September. To obtain maximum productivity during this latter period one must double the resting time to 36 days. This demonstrates the necessity of varying the rest period of grass according to the season in order to obtain maximum productivity (subject to satisfying the demands of the cow).
As we shall see later in more detail, these prolonged rest periods make for a considerable increase in annual production of grass and of the nutritive elements per acre (per hectare).
When we choose an optimum rest period it will be necessary to go beyond, rather than to stay within, this optimum period for many grasses.
In actual fact, when, in May-June, we prolong the rest period by 9 days beyond the optimum, productivity falls to 191 lb./acre [214 kg./ha.], while if we reduce the optimum period by these same 9 days the productivity falls to 159 lb./acre [178 kg./ha.].
Of course, there is reason to take the nutritive value of the grass into account, and the next chapter will provide us with some data on this point.
Excerpted from Grass Productivity by André Voisin, Catherine T. M. Herriot. Copyright © 1959 Philosophical Library, Inc.. Excerpted by permission of ISLAND PRESS.
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