(*) R. Rivière Head of the Department of Nutrition of I.E.M.V.T.
Factors determining the value of rangeland
Current methods of rangeland evaluation
Critical points and motifs
Variation factors in voluntary intake
Methods for the measurement or evaluation of quantities ingested
Methods for the evaluation of digestibility
Assessment of the nutritive value of natural rangeland poses complex problems which are closely concerned with the precise definition of:
- edible species of forage vegetation,
- vegetative parts actually consumed,
- amount of forage vegetation consumed by animals,
- energy value of forage consumed, and of the numerous factors that can affect this data.
Entirely satisfactory solutions to these problems have not yet been found and evaluation still remains approximate and rather crude.
However, more exact methods exist, though they are long and often expensive and require the support of specialized laboratories. Thus they may only be used with a certain amount of difficulty in the study of large areas of "on site" rangeland. The above methods are described and appraised.
An assessment of the quality and nutritive value of natural rangeland and its animal carrying capacity poses a certain number of technical problems which are still far from being solved. Yet more than 70 million cattle and 80 million small ruminants in intertropical Africa and Madagascar live in an uncontrolled way and rely for their subsistence solely on the forage provided by natural rangeland.
The productivity of these animals is low, both because they are subject to the vagaries of climate which affect the nature and quality of the rangeland, and because the rangeland is often misused. The density of livestock often results in overgrazing of rangeland, which causes progressive deterioration of the vegetational cover and, in consequence, a deficient supply of animal fodder.
Any improvement of yield from livestock requires the rational utilisation of rangeland; this involves calculating a stocking rate as exactly as possible in relation to the potential of the rangeland, in order that the animals are provided with fodder which is sufficient in respect of both quantity and quality. The stocking rate of an area of rangeland can only be precisely calculated where it is possible to evaluate the nutritional value at a given time, judge its deficiencies, and follow the evolution of its value over a period of time, according to season and manner of use. In this way the stage of growth or season in which the rangeland should be grazed can be specified and the maximum amount of nutritional value obtained.
The nutritive value of rangeland is determined essentially by the benefit that can be obtained from it by livestock - in other words, its ability to satisfy upkeep and production needs (the expression "such rangeland can support x number of animals per hectare, or produce y kg of meat per hectare" is often used).
Nutritive value is conditioned by several factors:
- Productivity: it may be expressed in terms of kg of dry matter or energy per hectare; it may be for a given moment or for a year.
- Palatability of the species of vegetation which occur. Productivity may be concerned with the overall vegetation production or only that of palatable species.
- Potentially consumable quantity, and quantity actually consumed.
- Concentrations in plants of necessary nutritional elements (proteins, vitamins, macro- and oligo-element of minerals) of digested parts.
- Digestibility of the nutrients or the species consumed, which constitutes content in terms of usable energy.
Numerous world-wide, studies have considered the nutritive value of rangeland; and the number of studies concerning tropical rangeland, although carried out only in recent years, have begun to mount up.
In most cases, in temperate regions as well as tropical countries, authors have given priority to problems concerning the energy and nitrogen content of forage and overall productivity, and have neglected those concerning consumption by the animals.
The usual procedure followed in the study of the value of natural rangeland consists of the following points:
1) Identification of floristic composition.
2) Identification of species found palatable by livestock, and their relative importance.
3) Estimate of overall productivity or of productivity of species consumed. Yield is noted either for a given moment, or during the course of the active period of the vegetation, or during the course of the dry season. The use of areas protected from grazing makes it possible to evaluate the total mass of the vegetation (herbaceous biomass) produced over the whole active period of the plants, and thus to estimate the potential yield of the rangeland under study.
4) Identification of food value by means of tables, using the results of the chemical analysis of samples of the major species consumed at different stages of their development.
5) Identification of favourable periods and their duration of use for grazing, based on the preceding results.
6) Calculation of possible stocking rates in terms of the information already acquired and of the average intake by livestock at the rate of 2.5 kg of dry matter per 100 kg of live weight, thus 6.25 kg per UBT (tropical livestock unit).
7) Estimate of the evolution of the rangeland in terms of its manner of exploitation.
8) Drawing up of a report of the techniques for the most judicious rangeland exploitation and for mapping.
This procedure, extensively used for twenty years in French-speaking tropical countries, has made it possible to inventory a considerable area of rangeland (nearly 1.4 million square kilometres, of which 900,000 square kilometres is Sahelian rangeland) and has permitted a better knowledge of this rangeland, which up till that point had been the object of only occasional and very superficial studies.
It must also be admitted that this method is not entirely satisfactory. This is not a criticism of rangeland experts. The problem is very complex and the present lack of precise information and resources hardly permits of other methods.
The results obtained from the procedure are perhaps sufficient for an appraisal of extensive areas of savannah, but the intensification of livestock raising requires more precise techniques, which current research may enable us to define.
I will not make any comments on the procedure used in the evaluation of productivity and biomass. This is a problem for specialists, and is not in my field of interest. It is, however, necessary to recognise that the problem is many-sided and not at all easy to resolve. The question of the dynamics of rangeland is particularly difficult to study because unpredictable variations may occur in the evolutionary process, due to vagaries of climate, to exploitation methods, and to various factors governing deterioration.
My observations as a nutritionist will be related to factors of interest to the animal and to the benefit it can obtain from rangeland:
- appraisal of palatable species,
- quantities of forage and other vegetative organs actually consumed,
- assessment of food value.
Appraisal of palatable species
The appraisal of palatable species and their relative frequency in proportion to the total vegetation of an area of rangeland is one of the most difficult aspects of the problem to resolve. Certain criteria exist, of course, which may provide information and which may make it possible to state that this or that particular plant is not edible for livestock. This is the case for highly scented plants such as certain Cymbopogon (proximus and giganteas).
But very often palatability is only a very relative notion: plants may be eaten when young and rejected when they reach a more advanced stage (Imperata), or the reverse (the case of Cymbopogon); plants thought to be little palatable may be in demand if no others of greater palatability exist, and refused if the reverse is the case. The floristic composition of rangeland is thus an important feature of this phenomenon.
Study of animals makes it possible to collect valuable data on this subject, although each area of rangeland represents a special case, and it is not usually possible to follow herds for the whole of the growth period of the vegetation in order to evaluate palatability at different stages of the development of vegetation; and such animals are not always available to rangeland specialists.
The questioning of graziers may be of assistance, but information obtained in this way is liable to lack precision.
Finally, certain anatomical and chemical characteristics of plants (anatomy of laminae of foliage, sugar and nitrogen contents, extent of lignification...) may provide some indication of palatability and make it possible to confront the problem in the absence of herds and graziers. Nevertheless, the appraisal of these characteristics often remains approximative and subjective, and may be the origin of considerable errors in rangeland evaluation.
Identification of vegetative components consumed
The food value of forage is normally determined for the whole of the above-ground parts of reputedly palatable species, cut at a level of 5 or 10 cm above the ground. The animals are selective in their grazing, choosing by preference certain components before others. In some cases no problems arise - especially in regard to young forage plants which are eaten entirely - although when components of the vegetation have reached maturity, inflorescences are often preferred over the rest of the plant and the leaves over the stem. New shoots which grow from the base of perennial plants are similarly preferred. It is extremely difficult to predict, for many cases, in the absence of livestock, which plants the animal will consume.
Where trees and bushes exist on the rangeland it is practically impossible to ascertain the amount of leaves and seeds which will be absorbed by livestock. Nevertheless they do provide a not inconsiderable amount of nutritive value which is able to balance, more or less, the amount available, because of the relatively high nitrogen content of this above-ground rangeland vegetation.
A procedure exists permitting the identification of vegetative plants and organs consumed; the procedure is valid also for the assessment of palatable species. It makes use of holes cut into the oesophagus of animals, may only be applied in stations, and is not really suitable for studies of rangeland over wide areas, or where a rangeland specialist has only a few months at his disposal for the inventory of a large area.
Assessment of the quantity of forage consumed
The nutritional value of any given forage vegetation depends largely on 2 factors:
a) the amount of voluntary intake of the animals, expressed as a quantity of dry matter and recorded either according to the live or metabolical weight of the animal W 0.75.
b) the amount of energy and nitrogen contained in a unit of weight (kg of dry matter).
Up to the present moment, consideration of voluntary intake has given rise to only a very few studies m tropical regions; and normally it is satisfactory to reckon on an estimated average intake of 2.5 kg of dry matter per 100 kg of live weight, as I stated previously. Studies carried out in several temperate countries demonstrate that this value is effected by numerous factors and is subject to considerable variation.
Insufficient knowledge of these variations inevitably results in errors in the assessment of the value of rangeland, and it is essential to take them into consideration. The animal's forage ingestion capacity is a basic factor of nutritive value: forage vegetation may have high energy value and yet not be able to satisfy the needs of the animals because they cannot consume sufficient quantities.
These factors relate on the one hand to the animal and on the other to forage vegetation and the environment.
Considerable disparity in the level of ingestion between the different species of animals may be observed. Thus cattle consume more than sheep, if intake is expressed in dry matter per kilo of metabolic weight (it is the reverse if expressed in terms of live weight); and goats have a much greater ingestion capacity than cattle or sheep.
Weight and age
With regard to cattle, ingestion capacity of the same matter (expressed in kg of dry matter per unit of weight) decreases in relation to the increased age and live weight of the animal, whereas with sheep this variation is hardly perceptible.
Level of production
This influences above all animals of high productivity. Cows that produce milk have a greater intake than cows that do not. The same holds for cows at the beginning of pregnancy, although the appetite generally decreases towards the end, when the foetus constricts the rumen. A head of cattle being fattened on pastureland consumes less than a cow of the same weight which gives milk; the appetite seems to diminish at the end of the fattening period when energy needs are greatest. Work and exercise stimulate the appetite. Intake on rangeland is greater than at the feeding trough.
Considerable variations in ingestion, often inexplicable, may be observed in homogeneous groups of animals. They may be the result of differences in eating time, feeding preferences, or genetic factors.
State of health also influences the level of ingestion. Animals with organic problems or infections generally have diminished appetites.
The appetite of a ruminant is conditioned by the capacity of its rumen and the activeness of its microorganisms, upon which digestibility and speed of passage of food depends. The composition of the nutriments and the age of the plants, which exert an influence on the quality of the forage, are thus most important factors at the level of ingestion. The membranous carbohydrate content increases with the age of the plants, and as the plants grow older so their membranes become more and more resistant to microbic attack. Also, the microbic activity of the flora depends on the proportion and the nature of the flora's cellular make-up, and in particular on the amount of nitrogenous matter, which decreases with age. The low nitrogenous content level of forage vegetation does not permit a sufficient multiplication or proliferation of the bacteria of the rumen, leading to a decrease in these organisms, which in turn causes a lessening of digestibility and the congestion of the rumen by undigested substances. It is thus the amount of membranous carbohydrates and nitrogen which determines digestibility to a large extent, that is to say, the speed of digestion and the proportion of indigestible matter, and consequently the quantity ingested.
The example set out in the table below was studied in the Ivory Coast, and illustrates clearly the role of the age of forage vegetation in voluntary intake.
Results of an experiment on the digestibility of Panicum maximum
It may be noted that intake increases from the 21st to the 28th day and afterwards decreases. Variation observed during the first week is the result of large resources of water of 3-week forage, which gives it a greater mass.
We may thus state that intake varies in the same way as digestibility. Yet the correlation is slight and it is not possible to deduce the level of ingestion from a knowledge of digestibility.
With regard to cattle, intake during the dry season from pasture consisting of standing straw, with low digestibility, generally does not exceed 1.4 to 1.6 kg of dry matter per 100 kg of live weight. This decrease in intake, related to the decrease in energy and nitrogen value of forage, aggravates in a dramatic way the feed situation of Sahelian livestock, for which there is available only the most meagre forage vegetation, consisting of straw, during the dry season. There comes a time when the combined effect of these variations creates a situation where the animals can no longer satisfy the two requirements for their upkeep - energy and nitrogenous matter - and this provokes a spectacular process of loss of weight, which may lead to death.
Species of vegetation play, in addition, an important role with regard to ingestion. This has been demonstrated in France by Demarquilly, who was able to measure variations from 1.7 up to 2.8 kg of dry matter per 100 kg of live weight according to the nature of the forage vegetation studies at an identical stage of development.
An experiment similar to the above, carried out at Minankro, demonstrated that Brachiaria ruziziensis was consumed at a lesser rate than Panicum maximum, compared at the same age and with roughly the same composition.
Temperature variations modify the ingestion level of animals. In temperate climates, a raise in temperature to above 21°C causes a decrease in intake, and a lowering of the temperature has the reverse effect.
In tropical regions, the climate has an indirect effect, in several respects:
- In great heat, the animals have a tendency to seek the shade of trees, and they do not graze during the hottest hours. Animals which are brought back to the village in the evening and enclosed do not have enough grazing time, and thus ingest less forage. Night-time grazing enables animals to palliate this low intake level.
- In the dry season pasture becomes sparse, and animals forced to search for food require more time to consume the same quantity of dry matter than is needed with lush pasture.
- Long journeys, made necessary by the infrequency of sources of water, tire the animals and reduce grazing time.
Intake of dry matter depends partly on the availability of water. In general, ruminants absorb, by drinking or through the moisture component of nutriments, 2 to 4 kg of water per kilo of dry matter ingested. If one reduces by half the normal water intake of an animal, voluntary intake of dry matter is reduced by 30 per cent.
Problems in the supply of water are thus also a variable factor in the forage intake of animals in the Sahel in the dry season. However, a decrease in water intake causes a reduction in the volume of urine, a decrease in the excretion of nitrogenous matter and urinary urea, and an increase in ureic nitrogen of the plasma and in nitrogen retention.
The quantity of forage consumed can thus vary considerably, and it is almost impossible, with our present state of knowledge, to make a precise assessment of the various types of tropical rangeland without measuring this quantity.
However, we can affirm that during the dry season on pasture composed almost entirely of straw-type vegetation, animals do not consume the established average base quantity of dry matter; and stocking rates estimated on the basis of this value will always be, for this season, lower than the rates which can actually he supported by the rangeland. This nevertheless does not mean that the energy requirements of the animals will be satisfied.
The average value of 2.5 kg of dry matter for 100 kg of live weight has been used in the definition of a minimum energy value (in FU); and of the concentration of nitrogenous matter, which must be provided by the forage in order to satisfy the needs of the animals, 0.45 FU and 25 g of DNM per kg of dry matter.
kg of dry matter
These quantities are sufficient to satisfy the needs of a Livestock Standard Unit (LSU) of 250 kg. This may reflect reality fairly closely when the forage vegetation is young, in the springtime. But it is not so for straw-type vegetation which may fall as low as 1.5 kg of dry matter for each 100 kg of live weight, when the preceding minimum values quoted are no longer valid.
What methods are used in the measurement of intake?
This measurement is precise and simple for animals fed at the feed trough, but not for animals grazed on rangeland. For the latter purpose a direct and several indirect methods exist.
Two identical plots are used: the first is cleared, in order to estimate the quantity of forage available; after grazing the second, the leftover forage is cleared. Differences in weight and composition represent forage ingested, provided that there has not been, in the meantime, either growth of grass (insignificant if the grazing period is short) or deterioration; trampling will in any case create some errors of measurement.
a) Numerous research studies have shown that a correlation exists between the ingestion level and the concentration of nitrogen in the faeces. It is thus possible to calculate intake to a fairly accurate degree by means of a regressive equation based on measurements taken of stalled animals and on amounts of nitrogen found in faecal matter.
b) The use of " markers " makes it possible to assess the quantity of food ingested. The " marker " is an indigestible substance which is found in its original state in excretion. Either natural substances are used, such as lignin or silicon, or artificial "marker" is added to the food or fed in some way to the animal; chromium oxide is most frequently employed.
Natural " markers " also make it possible to assess the digestibility of grazing forage, provided that the nature and composition of the ingested forage is known. The use of artificial "markers " is only possible where the exact quantity of intake can be ascertained, although the simultaneous use of two types of "markers" and the measurement of faecal nitrogen make it possible to counteract most of the drawbacks which occur where one "marker " only is used, and thus to obtain sufficiently precise results.
The following is an example that will illustrate this method and will demonstrate the procedure in the case of sheep on rangeland.
Given: a sheep consuming forage containing 12.5 percent lignin and 7.8 percent nitrogenous matter (as a percentage of dry matter); it is given and ingests 10 gr of chromium oxide. The faeces examined (sample taken from the rectum) contains (as a percentage of dry matter):
1.20 percent chromium oxide,
21.20 percent lignin,
5.30 percent nitrogenous matter.
Faecal excretion is calculated in the following ratio:
g (round figure)
The quantity of dry matter consumed is calculated by means of the following equation:
c) Several other methods may provide valuable assistance in the assessment of the level of ingestion. Among others, there are:
- use of animals with a hole cut in the rumen,
- digestibility "in vitro " on two occasions.
These methods are well known and I shall do no more than name them.
We are thus not entirely without the means of evaluating quantities of forage ingested by livestock. Unfortunately, most of these methods make use of laboratory techniques and are not amenable to use by rangeland experts on site; this explains the adoption of the average base value mentioned above. It would appear, however, that any use of this value in rangeland studies, whatever the quality of the pasture, the season, or the vegetative stage of development of the plants, is mistaken; and that it is necessary to moderate it with respect to digestibility, or at least to the estimated energy value of the forage.
Assessment of energy value of forage
The problem of the evaluation of the energy value of forage is no easier to solve than the preceding problems, but it has been the subject of many more important studies the world over. The studies relating to tropical forage vegetation and in particular to African forage vegetation which have been carried out now for almost half a century have been of interest to only a small number of researchers, and the accumulated results do not amount to much when compared to the infinite variety of species of forage vegetation and association of vegetation encountered in natural rangeland.
Whichever form of energy is considered (digestive energy, metabolizable energy, or net energy), and whichever mode of expression (TDN, calories, or nutritive units), the energy value of forage vegetation and nutriments in general is directly related to the manner in which the organic constituents are digested by the animal; that is to say, the digestibility of organic nutriments. Measurement of the digestibility of nutriments forms the basis of all assessments of energy, but digestibility is not a precise or static phenomenon; it is, in fact, influenced by a variety of factors, which I will not enlarge on but will only mention:
- "Internal " rectors relating to the animals in question: type, race, age, individuality, and physiological and pathological states.
- "External " factors relating to the nutriments consumed individually or as part of a ration: their structure and composition and the volume of the ration; and also environmental conditions, which are important because frequently excessive in tropical countries.
Study of these factors demonstrates that the complexity of this problem and the considerable variations which are concerned in digestibility thus make it impossible to confer an absolute value. It also demonstrates the necessity of increasing measurements and the impossibility of blindly applying results obtained to conditions that are not comparable.
Nevertheless, these concepts remain essential nutritional factors in the assessment of energy values of nutriments.
We must state clearly that all the methods make it possible to measure the apparent digestibility of nutriments only, and not the actual digestibility.
1. The "reference " method is, according to all evidence, the measurement " in vivo " of digestibility factors of different organic nutriments of foodstuffs. It is unfortunately impossible to apply successfully on rangeland, because a minimum of fixed equipment is required and tests are necessarily long and expensive. The experiment may be carried out with sheep: numerous research workers have, in fact, discovered that the results obtained can be directly transferred to cattle; however, although this alternative makes it possible to reduce the amounts of forage required in the measurements, these remain considerable, given that such tests must be made on several animals. The very large number of possible associations of plant species on natural rangeland necessitates a large number of experiments (a minimum of one for each vegetative stage of each combination) and a countless number of chemical analyses.
2. The use of " markers " makes it possible to study digestibility in animals while grazing. If we consider the above example we may see that digestibility factors may be determined in the following equation:
given that for nitrogenous matter
and for dry matter
This method makes it possible to avoid a certain number of the inconvenient aspects of the preceding method. However, it requires a great deal of time and considerable logistic laboratory support.
3. Nutritionists have for some time sought to link digestibility to certain chemical content constituents of the nutriment and thus to assess their nutritive value on the basis of the results of simple laboratory analyses. Initial attempts were concerned with untreated cellulose (in doses according to the Weende method), and regression equations were established which make it possible to calculate directly the net energy value of forage, based on its cellulose and ash content. In this way the so-called "?" tables were drawn up at the end of the studies at Dijsktra, which were and are still widely used throughout the world, and which we still use in the evaluation of the food value of forage in the absence of any other sound, viable, practical method.
Nevertheless, it has been amply demonstrated that Weende's cellulose was not a well-defined chemical entity, and that it was not a valid standard in the measurement of digestibility; in other words, if a correlation between digestibility and cellulose does exist, it is not sufficiently close to provide acceptable results.
Researchers, such as Jarrige in France and Van Soest in the U.S.A., have attempted to define precisely the nature of the different glucides which make up untreated cellulose and the non-nitrogenous extract. They have established techniques which make it possible to calculate doses of these different carbohydrates, and have discovered, by means of simultaneous " in vivo " experiments, fairly close correlations between certain of them and digestibility, making possible an estimate of digestibility by means of regression equations.
Thus did Jarrige demonstrate, after a study carried out on about a hundred forage species, that ligno-cellulose was the best standard in the measurement of digestibility; and he provided the following regression equation:
Digestibility of organic matter y = 0.913 x + 110.7 = 2.98 where x is the faecal ligno-cellulose, expressed as a percentage of the organic matter of the faeces.
As for Van Soest, he established a series of techniques which make it possible to distinguish cellular content and walls (NDF), ligno-cellulose (ADF), hemicellulose, and lignin of the forage species. Also, he considers that the S constituent, which represents cellular content, has 98 to 100 percent digestibility; but that a considerable amount of similar products of endogenous origin (mucus, salts, bile waste...) are found in the faeces, and thus are inevitable factors resulting from the digestive process. In respect of sheep, this loss represents in the region of 12.9 percent in dry weight of the food consumed, which must be taken from the digestible part. In respect of cattle, metabolic loss (M) may be estimated by means of the following equation: M = 36.57 - 0.275 d, where d is the estimate of actual digestibility (cf. following example).
In addition, Van Soest concludes from his studies that the digestibility of
cell walls (NDF) may be negatively correlated to log X, where X is the percentage
of lignin in the ADF (X=), ADF
ADF corresponding to the cellular content of the food + lignin + silica; (thus) X is the degree of lignification of the NDF, and the more lignified the cell walls are, the less digestible they are. Moreover, silica reduces digestibility; and Van Soest recommends the application of correction from the point when the amount of silica exceeds 2 percent.
Van Soest's equation for overall digestibility is:
Digestibility of dry matter as a percentage = 0.98 S + NDF (1.473 - 0.789 log X) - M - Silica correction where S, NDF and M are expressed as a percentage of dry matter.
Example: Given forage vegetation with the following composition in dry matter:
Cellular content S
32.32 percent DM
Cellular membrane NDF
68.68 percent DM
45.5 percent DM
6.7 percent DM
4.3 percent DM
Digestible cellular content = = + 31.4
Ratio of digestibility of membranes (1) = 1.473 - 0.789 (log. 14.7) = 0.55 (i.e. 55 percent).
Silica correction (2):
Estimate of actual digestibility:
Metabolic loss, sheep
Metabolic loss, cattle:
M = 36.57 - (0.275 55.9)
Apparent digestibility DM:
(1) A table makes it possible to assess digestibility of membranes from X, and thus avoid calculation.
(2) The following may be deduced as a function of S content:
- 3 percent for 1 percent of S in respect of
- 4.4 percent for 1 percent of S in respect of
Other relationships based on a similar concept have been formulated by various authors.
Lofgreen, Garret and Harris, in the U.S.A., have recently published a method for evaluating the nutritional value of foodstuffs based on the assessment of TDN and the use of regressive equations established after the statistical analysis of the results of experiments. TDN's are usually calculated on the basis of the analysis of the composition of foodstuffs and digestibility ratios, but where these are unknown, equations are used which permit evaluation based solely on data from conventional bromatological analysis. Different equations exist, according to the species of animal and the nature of the foodstuff studied. Thus for standing forage vegetation and rangeland pasture (foodstuff of class 2), the equation for cattle is as follows:
TDN as a percentage = 54.572 + 6.769 X - 51.083 YX + 1.851 Z - 0.334 P - 0.049 X2 + 3.384 Y2 - 0.086 XZ + 0.687 YZ + 0.922 YP - 0.112 Y2P.
X = Weende's untreated cellulose
Y = ether extract
Z = nitrogen-free extract
P = crude protein (N X 6.25).
On the basis of TDNs the different energy values of foodstuffs may be calculated:
Metabolizable energy ME = TDN 3.62 Calories/kg DM;
Net energy for maintenance NEm = 77/F Mcal/kg DM;
Net energy for gain NEg = 2.54 - 0.0314 F Mcal/kg DM;
Net energy for milk production NEm = Mcal/kg DM where TDN is expressed in g/kg DM and F is the result of the equation: log F = 2.2577 - 0.2213 ME (ME as Mcal/kg DM).
All relationships thus established were the result of work carried out on the forage vegetation of temperate regions. Major work is necessary in order to judge the validity of these formulae in respect of tropical forage vegetation. Also, the techniques for the measurement of levels involved in the identification of membranous constituents are usually long and delicate and are not particularly amenable to series-type analyses. Nevertheless the method is of interest and may prove very useful in future studies of tropical forage vegetation.
4. Lastly, it is necessary to note methods concerning digestibility "in vitro". Given the problems posed by " in vivo " methods and the difficulties involved in carrying them out, numerous attempts have been made to assess the digestibility of foodstuffs by reproducing in the laboratory the processes which take place in the digestive tracts of ruminants.
The method most often used is the well-known technique of Tilley and Terry.
The results obtained in temperate regions may be fairly closely correlated to those obtained by the "in vivo" method, although experiments carried out in tropical countries demonstrate the necessity for considerable preparatory work and numerous comparative studies.
The method using experiments in two parts also makes it possible to gather data concerning the quantities of foodstuffs that the animal is able to consume.
5. " In vitro " techniques for the measurement of digestibility require fistulated animals as producers of rumen fluid. Jarrige and Thivend have recently established a method which makes it possible to dispense with this, by making use of a " cellulase ", fungal in origin, which digests certain constituents of the foodstuffs (cellulose, hemiceliulose, proteins, and starch) and in so doing behaves in almost the same way as the rumen fluid.
The technique is simple, and its results may be reproduced; the results appear to be as precise as those of the preceding method.
Difficulty arises as to whether it is now possible to find " cellulase " which is fixed and constant.
Assessment of nitrogen value
The assessment of digestible nitrogenous matter does not pose such arduous problems as those just mentioned. It would now appear to be firmly established that digestible nitrogenous matter content is directly linked to the total nitrogenous matter content, and that it is possible to predict, with sufficient accuracy DNM based on the TNM by means of the following formulae, established by Jarrige and Demarquilly:
In respect of green, graminaceae forage vegetation:
DNM percentage = 0.929 TNM - 3.52
or more simply,
DNM percentage = TNM - 4.5
and in respect of stored forage
DNM percentage = TNM - 5
These formulae, valid for temperate forage vegetation, are also applicable to tropical forage vegetation which, for comparable amounts of protein appears to produce the same values of digestibility as those of temperate zones. The only difference occurs in respect of nitrogenous matter content; temperate forage vegetation has very rarely, if ever, such low levels of NM as has completely developed tropical forage vegetation.
Examination of formulae reveals that when the amounts of TNM are lower than 4.5 percent, the nitrogen value of the forage vegetation is nil, and digestibility of proteins even becomes negative, in the sense that excretion of metabolical nitrogen in the faeces is greater than the amount of nitrogen ingested.
The evaluation of the nutritive value of natural rangeland still remains, when account is taken or available methods, fairly crude and not devoid of possibly very important shortcomings.
In addition, it must also be recognised that although methods exist which produce more precise and exact results, such methods are difficult to put into use " on site " over large areas of rangeland. In fact, they require specialist personnel to carry out the essential experiments, and necessitate a great deal of time and the logistic support of either a livestock research station or a well-equipped analysis laboratory.