Tropical animal feeding A manual for research workers
Chapter 5
Developing production systems for ruminants using tropical feed
resources requires an understanding of the relative roles and nutrient
needs of the two-compartment system represented by the symbiotic
relationship between rumen micro-organisms and the host animal. Fibrerich,
low-protein forages and crop residues are the most abundant and
appropriate feeds for ruminants in the tropics. Strategies to improve the
utilization of these feeds should aim: (i) to provide supplements to correct
the nutrient imbalances at the level of the microbes and the animal and;
(ii) to increase the availability of energy to rumen microbes by “highoffer”
(selective) feeding or chemical treatment (usually with urea).
The most limiting nutrients for rumen microbes are ammonia,
sulphur and phosphorus. For the animal, the needs for supplements are
determined by the rate of production (e.g., of work, of growth, of milk)
and reproduction, and mostly involve the supply of “by-pass” (or
“escape”) protein.
GENERAL CONSIDERATIONS
Introduction
In order to develop feeding systems, it is necessary to relate information
on the nutritional characteristics of feed resources to the requirements for
nutrients, depending on the purpose and rate of productivity of the animals
in question. In the industrialized countries, this information has been
incorporated in tables of “feeding standards” which interpret chemical
analyses of feed resources in terms of their capacity to supply the energy,
amino acids, vitamins and minerals required for the particular productive
purpose. These standards are steadily becoming more sophisticated with
the aim of improving their effectiveness in predicting rates of performance
of intensively-fed livestock and to derive least cost formulations.
Limitations to “conventional” feeding standards
The relevance of feeding standards for developing countries, particularly
those in the tropics, has been questioned from the socio-economic
(Jackson, 1980) and technical (Graham, 1983; Preston, 1983) viewpoints.
It has been apparent for many years that feeding standards based on
assigned nutritive values (e.g., net energy) are misleading when unconventional
feed resources are used (e.g., Preston, 1972; Leng and
Preston, 1976), since the levels of production achieved may be
considerably less than the level predicted. More importantly, this often led
to the rejection of many available feed resources which apparently were
too low in digestible energy to supply the energy needed for production.
It also encouraged researchers to copy feeding systems used in temperate
countries, which are relatively “predictable” but which require feed
resources that are unavailable and/or inappropriate on socio-economic
grounds in most developing countries.
An alternative approach
The justification for a new approach to the development of feeding
systems for ruminants, not based on conventional “feeding standards”, is
that:
-
The efficiency of the rumen ecosystem cannot be characterized by any
form, of feed analysis. -
Feed intake on some diets bears no relationship to digestibility and is
much more influenced by supplementation. -
Availability of amino acids cannot be inferred from the crude protein
content of the diet. -
The energy value of a diet, and the efficiency of its utilization, are
largely determined by the relative balances of glucogenic energy, long
chain fatty acids and essential amino acids absorbed by the animal.
In the early 1960’s, Professor Max Kleiber had expressed a similar
concern for these issues and stated (as quoted by Kronfeld, 1982)
“… metabolizable energy is not a homogeneous entity; instead it represents
an assembly of nutrients or metabolites each of which is used with a
specific efficiency for a particular purpose”. To this could be added that
the availability of these nutrients, and their interactions, affect the
efficiency of energy utilization.
The misconceptions inherent in any system based primarily on feed
analysis are that it is almost impossible to predict:
-
Whether the feed can support efficient rumen function.
-
The nature, amounts and the proportions of the end products of
fermentative digestion. -
The potential for rumen escape of nutrients and their digestibility in
the small intestine.
For these technical reasons, and also because of differing socio-economic
circumstances, it has been proposed that a more appropriate objective,
especially for developing countries, is to “match livestock production
systems with the resources available” (Preston and Leng, 1987).
This chapter sets out the guidelines for applying these concepts to the
development of feeding systems which aim to optimize the utilization of
locally available feed resources and to build on traditional practices.
Animal response to non-conventional feed resources
It is relevant to point out that the doubts concerning the usefulness of
feeding standards for ruminants in tropical countries surfaced during
development work in Cuba (Preston and Willis, 1974) in the 70’s when
livestock production systems were being established on non-conventional
feed resources (i.e., molasses-based diets). In these cases, although
nutrient requirements were satisfied according to traditional feeding
standards, the responses of the animals did not correspond to the predicted
levels of performance. This research demonstrated that small inputs of
“by-pass protein” (Peruvian fishmeal) dramatically increased growth rate
and feed efficiency of cattle (Figure 5.1). In contrast, this feeding system
was not able to support high levels of milk production (Figure 5.2),
presumably because of the greater demands in lactation for glucogenic
compounds and the relative deficiencies of these in the digestion
end-products on molasses-based diets, in turn caused by the lowpropionate,
high-butyrate fermentation in the rumen (Marty and Preston,
1970).
Figure 5.1. Effect of replacing urea with fish meal on performance of steers fed
a basal diet of molasses-urea (Source: Preston and Willis, 1974).
Figure 5.2. Replacing molasses with maize grain as basis of diet of dairy cows
increased rumen propionate, dry matter intake and milk yield (Clark et al.,
1972).
The high potential yield of animal products from a hectare of sugar
cane stimulated the subsequent research in Mexico, Mauritius and the
Dominican Republic that attempted to establish cattle production systems,
applying the principles developed for feeding molasses (both feed
resources had similar concentrations of soluble sugars) (see Preston and
Leng, 1978a,b). Research on the feeding value of derinded and chopped
sugar cane (Preston et al., 1976) demonstrated that:
-
Feed intake was low even though digestibility was high (60–70%)
-
The animals on this feed apparently needed glucose or glucose
precursors because all the sugars are fermented, rumen propionate
levels are no higher than observed on high-fibre diets, and the
presence of a dense population of ciliate protozoa (Valdez et al.,
1977) reduced the availability of microbial protein to the animal (Bird
and Leng, 1984).
The implication of these two findings is that rumen function did not
provide the required balance of nutrients for productive purposes (see
Leng and Preston, 1976). Recognition of the role of fermentable N and by-pass
protein in low-N diets led to research aimed at increasing productivity
of cattle and sheep on a range of high fibre and sugar-rich low-N feeds
(Leng et al., 1977; Preston and Leng, 1984, 1987). Prior to this work,
these feed resources were considered to have little value other than to
support maintenance and were universally referred to as “low quality”
fibrous feeds. This led to attempts to improve the digestibility of fibrous
feeds by, in particular, alkali treatment (Jackson, 1977,1978).
However, the value of alkali treatment was partially obscured by the
failure to recognize that the first limitation was not digestibility but the
imbalance of nutrients at the level of both the rumen and the whole animal
(Preston and Leng, 1987). Combining alkali treatment (ammonia) and
appropriate supplementation has finally led to a very extensive programme
of straw-based feeding systems being applied on farms in China (Dolberg
and Finlayson, 1995). The significance of this development is the
magnitude of the contribution of straw to the total dietary dry matter and
achievement of high rates of liveweight gain once thought to be the
prerogative of cereal grain feeding.
Nutritive value
In order that responses in animal productivity to supplements can be
predicted accurately on a particular diet, it is necessary to take account
of the constraints to metabolism. These relate specifically to the relative
amounts of amino acids, glucogenic energy, VFA energy and long chain
fatty acid energy in the end products of fermentative and intestinal
digestion, since this is what determines the animal’s productivity.
Productivity of ruminants is influenced primarily by feed intake which, in
turn, is determined by feed digestibility and the capacity of the diet to
supply the correct balance of nutrients required by animals in different
productive states. Therefore the two major variables that need to be
considered are:
The balance of nutrients required depends upon:
-
The amounts of dietary components unchanged by rumen
fermentation that are absorbed (amino acids, glucose and long chain
fatty acids). -
The rates of production of the end products of fermentative digestion
(which can be highly variable). -
The productive functions (pregnancy, lactation. growth, work,
maintenance, depletion or repletion of bodyweight). -
The environmental factors (disease, parasitism, temperature and
humidity, and other sources of stress).
The availability of nutrients from a diet is highly dependant on:
-
The microbial ecosystem in the rumen which influences the
availability of microbial protein, VFA energy and glucogenic energy. -
The chemical composition and physical form of the diet which
influence the amounts of protein, starch and long chain fatty acids
which escape the rumen fermentation.
At the present time, it is not possible to predict the nutrients required by
ruminant livestock and to match these with nutrients available from
digestion, because of the many interactions between the animal, its rumen
microbial ecosystem and the diet. The most widely available low-cost
feeds for ruminants in the majority of developing countries are usually
native pastures, crop residues and to a lesser extent agro-industrial
by-products. The expensive, and often unavailable (or exported), feeds
are the protein meals, derived from oilseed residues and the processing of
animals, fish and cereal grains.
Generally, energy (the basic feed resource) and fermentable nitrogen
(urea) are relatively inexpensive ingredients, while the sources of amino
acids and glucogenic compounds (the protein meals, cereal grains and
cereal by-products) are very expensive. Since it is fermentation of
carbohydrate which provides the energy for microbial growth, and as the
feed is often low in digestibility, it is generally desirable to supply
fermentable energy on an ad libitum basis. The basal diet should not
therefore be restricted.
As a rule of thumb, 3 g of fermentable N per 100 g of fermentable
organic matter are required to meet the needs for efficient microbial
growth. It is not always necessary to provide this amount since some feed
protein will be fermented to ammonia and some urea-N may enter the
rumen in saliva. These processes reduce the amount of non-protein
nitrogen needed. In addition there is evidence that the rumen microbes
need small amounts of amino acids and other nutrients for efficient
microbial growth. The potential of the diet to satisfy the requirements of
the animal for amino acids, glucogenic precursors and long chain fatty
acids depends on the pattern of fermentation and on the dietary protein,
lipids (or their constituent fatty acids) and starch that escape fermentation
and are digested in the intestines.
The extent to which the protein in a supplement escapes the rumen is
partly a function of its rate of degradation (solubility) in the rumen. It is
likely to be influenced greatly by the rate of flow of fluid and small
particles out of the rumen. This latter characteristic will be influenced by
processing of the feed (by physical or chemical means), the presence of
some green forage, the amount of protein reaching the duodenum and
external factors such as temperature and exercise/work.
The same factors will influence the supply of glucose and glucogenic
precursors in terms of the likely by-pass of starch to the duodenum.
However, the nature of rumen fermentation will have a major influence in
terms of the supply of propionic acid for glucose synthesis.
RELATING NUTRIENT SUPPLY TO PRODUCTIVE STATE
Introduction
There is insufficient information available to permit the precise
quantification of the proportions of the different nutrients required for
different productive states. Nevertheless, an approximation of the needs
of animals can be attempted. The suggested scheme attaches relative
priorities to the groups of nutrients according to the physiological and
biochemical processes underlying the expression of the particular
productive state (see Figure 5.3).
The groups of nutrients to be varied for different productive states are:
Figure 5.3. Metabolic substrates and productive function (Source: Preston and
Leng, 1987).
The sources of these nutrients are summarized in Figure 5.4. VFA energy
arises from the rumen fermentation of all types of organic matter
principally carbohydrates. The principal way of increasing VFA energy
in a particular feed is to increase intake (e.g., by selection through high
offer level), to increase the rumen degradability (urea supplement), to
supplement with by-pass protein or to treat with alkai (ammoniation).
Figure 5.4. Sources of nutrients for metabolism (Source: Preston and Leng,
1987).
Manipulation of the rumen to provide extra protein and glucogenic
precursors is still at the experimental stage. Dietary supplementation is the
most obvious way of manipulating the supply of absorbed amino acids,
glucose and glucose precursors.
Most supplements are expensive and their use in ruminant nutrition
competes with monogastric animal and human nutrition. If the primary
feed resource is a product of low nutritive value which would have been
wasted if it were not fed to ruminants, it can be argued that the ruminant
uses these concentrate supplements more efficiently than monogastric
animals. For this reason, the term “catalytic” supplement has been used
to describe these effects (Preston and Leng, 1987). Sucked milk, given in
small amounts (<2 litres daily) as a supplement for calves given a straw-or
molasses-based diet, is a good example of a “catalytic” supplement.
It is mandatory that research should produce response relationships to
distinguish economic from biological optima. As a rule of thumb, the role
of the supplement ceases to be “catalytic” when it exceeds about 30% of
the diet dry matter. Beyond this point it assumes a major role and
substitution occurs. The productive functions and the need for
supplementary nutrients are discussed in order of the least to the most
demanding.
Work
Work requires ATP (adenosine triphosphate) generated from the oxidation
of long-chain fatty acids, with obligatory requirements for glucogenie
compounds and for amino acids (to repair the wear and tear of tissues and
replace protein secretions) (see Leng, 1985). The working animal can
often obtain sufficient nutrients from a nitrogen-deficient diet so long as
it balances the protein:energy ratio needed for tissue turnover by “burning”
off acetate which is in excess of requirements. However, body weight loss
may restrict the period of work. If the work period is to be prolonged and
weight loss is to be minimized, then the nutrients available must be
balanced so as to satisfy the needs of the working animal. The
digestibility and the intake of the basal diet may also have to be increased
by supplementing with urea to correcta deficiency of fermentable nitrogen
in the rumen. This may be the only manipulation necessary, but
supplements rich in fat and by-pass protein could be beneficial particularly
where the animal is in a productive state (e.g., pregnant or lactating). If
weight loss continues because work is prolonged, it may be necessary to
increase the degradability of the basal diet, for instance by ammoniation
(urea treatment).
The mature, unproductive ruminant does not appear to require
nutrients over and above those provided by an efficient fermentative
digestion. Since the heavily working animal uses largely long chain fatty
acids and glucose (Pethick and Lindsay, 1982; Leng, 1985), the
supplements used should contain or provide these substrates. This is
particularly important in the case of long chain fatty acids, since their
absorption and use for fat deposition or mobilization and for work will be
much more efficient and will require less glucose oxidation than fat
synthesis from acetate and subsequent utilization in muscle metabolism.
Maintenance
Maintenance alone obviously requires less energy expenditure than work
so there is a proportionately higher demand for amino acids (relative to
energy) than in the working animal. This will always be provided by a
rumen system which is adequate in fermentable nitrogen. Animals in
negative energy balance for an extended period on low-nitrogen
roughage-based diets extract more digestible energy from the basal diet
when this is supplemented with fermentable nitrogen (see Table 5.1).
| Hay intake (kg DM/d) | Live weight change (kg/d) | Birth weight of calf (kg) | |
|---|---|---|---|
| Spear grass | 4.2 | -0.81 | 22 |
| Spear grass+urea+S | 6.2 | -0.31 | 31 |
| Spear grass+urea/S+ | |||
| by-pass protein* | 8.1 | +0.75 | 32 |
Growth
Growing animals have a very high requirement for amino acids for tissue
synthesis and glucose for oxidation in specific tissues (e.g., brain). In
addition, considerable amounts of glucose must be oxidized to provide the
NADPH required to synthesise fat from acetate. It is imperative to
recognize that high growth rates cannot be supported on the products of
fermentative digestion and that by-pass protein supplements are essential
to take advantage of the VFA energy absorbed.
Many factors influence the level of protein supplementation to be
used. Response relationships must be established which relate protein
supply to animal productivity for each basal (carbohydrate) resource and
for the available protein sources. The response pattern will vary according
to the nature of the basal diet and the particular protein supplement. Data
taken from Bangladesh and Cuba demonstrate this rationale.
Cattle on ammoniated (urea-treated) rice straw, when supplemented
with only 50 g/d fish meal, increased their liveweight gain threefold
(Figure 5.5). On a molasses-based diet of higher energetic potential, 450
g/d of fishmeal were needed to raise liveweight gain from 300 to 900
g/day (Figure 5.1).
Figure 5.5. Adding small amounts of a by-pass protein (fish meal) to a basal
diet of ammoniated (urea ensiling) rice straw dramatically increases gain in live
and carcass weight (Source: Saadullah, 1984).
Reproduction
Improvements in fertility brought about through nutrition are usually
attributed to increased energy intake. There is, however, information to
show that the supply of glucogenic precursors relative to total energy is an
important feature of the improved energy status which results in increased
fertility.
Conception and puberty
Recent studies have demonstrated that even when the protein supply is
adequate, the “quality” of the energy can also be a limiting factor. At the
same metabolizable energy intake (the basal diet was low-N Coastal
Bermuda grass pasture), puberty was reached at lower liveweights when
glucose availability in the animal was enhanced by supplementation with
monensin (Table 5.2). This is not a recommended practice but serves to
demonstrate the concept. There are, of course, ways of increasing the
glucogenic potential of the absorbed nutrients without recourse to
chemical additives (e.g., by the use of by-pass protein).
| Control | Monensin | |
|---|---|---|
| Liveweight, kg | ||
| Initial | 219 | 219 |
| Final | ||
| 313 319 | ||
| Feed intake, kg/d | 8.0 | 7.7 |
| Rumen VFA, molar % | ||
| Acetic | 74 | 69 |
| Propionic | 19 | 26 |
| Butyric | 6 | 3 |
| Fertility (% cycling) | 58 | 92 |
The effects of by-pass protein on conception rates of cows grazing
sub-tropical pasture during the dry season are shown in Table 5.3. A
supplement providing fermentable energy (molasses) was much less
effective, confirming the report of Moseley et al. (1982) that it is the
“quality” of the energy (i.e., energy in the form of glucogenic compounds)
which is the critical issue.
| No | Molasses | Cottonseed | |
|---|---|---|---|
| suppl | meal | ||
| Liveweight (kg) | 302 | 332 | 343 |
| Pregnancy (%) | 10 | 20 | 60 |
Growth of the foetus
The growth of the conceptus has little effect on the protein and energy
demand of ruminants until the last third of gestation when most of the
foetal tissues are deposited. Because of the time course of growth of the
conceptus which increases the daily need for nutrient to only a small
extent, it appears that rumen function even on diets of low digestibility can
support the birth of a viable offspring of normal weight. This was shown
in studies in which urea was included in the drinking water of ewes on
nitrogen deficient pasture (Table 5.4).
| Pasture | Pasture + urea | |
|---|---|---|
| Ewes lambed | 20 | 20 |
| N intake (g/d) | 8 | 15 |
| Ewe liveweight change (kg) | -12 | -8 |
| Lambs surviving | 12 | 16 |
| Lamb birthweight (kg) | 2.9 | 3.2 |
| Lamb growth (g/d) | 35 | 81 |
Increases in calf birth weight were recorded when pregnant cattle,
given a basal diet of hay of low digestibility (45%), were supplemented
with urea. However, to prevent bodyweight loss and/or promote weight
gain of the dam through pregnancy, it was necessary to provide additional
by-pass protein (Table 5.1).
It appears that urea supplementation enhances milk production to a
level that ensures survival of the offspring. But to allow the young animal
to grow, milk yield must be further stimulated by feeding a by-pass protein
meal.
Male reproduction
Male reproduction has been enhanced under grazing conditions by
supplementary feeding. Lindsay et al. (1982) showed that bulls could be
maintained in good condition on poorly digestible, low-nitrogen spear
grass pasture by providing 1 kg daily of a protein supplement (Table 5.5).
| Control | By-pass protein | |
|---|---|---|
| Initial weight (kg) | 433 | 433 |
| Liveweight change (kg) | -40 | + 14 |
| Roughage intake (kg/d) | 5.55 | 7.74 |
| Change in scrotal | ||
| circumference (mm) | -20 | +0.7 |
More importantly, the circumference of the scrotum decreased consider-ably
when no supplement was fed; and it is known that a bull with a lower
scrotal circumference is less fertile and has a lower libido (Blockey, 1980).
This shows quite clearly that protein nutrition influences male fertility. As
with female fertility there appears to be evidence for beneficial responses
to manipulating propionate production in the rumen. At the same feed
intake, bulls reached puberty earlier and, at puberty, had a greater scrotal
circumference and larger testicles (Neuendorff et al., 1982)
Milk production
The major constraint to milk production on diets based on crop residues
and agro-industrial by-products appears to be the availability of
glucogenic compounds to provide the glucose for lactose synthesis and for
oxidation to provide the NADPH for synthesis of fatty acids (e.g., Figure
5.2).
There is good evidence that, in large ruminants, about 50% of the fatty
acids of milk arise from dietary fat. A dietary source of lipid can thus
reduce considerably and imbalance caused by relative deficiencies of
glucogenic energy and amino acids in the end products of rumen digestion.
For many feeding systems in the tropics the level of fat in the diet could be
a primary constraint to milk production. This could be particularly
important in diets based on molasses or sugar-cane. Supplementation of
lactating animals, particularly on diets based on tropical pastures, crop
residues and sugar-rich agro-industrial by-products, should aim to correct
the imbalances of nutrients for milk production. By-pass protein usually
increases feed intake and as a consequence promotes milk production.
But to balance energy quality, fat must be mobilized and glucose diverted
from oxidation and tissue synthesis to lactose production. In these
circumstances, animals tend to lose body weight (Orskov et al., 1977).
Dietary fat may reduce this effect. Adding a source of by-pass starch in
such a diet balances the ratio of glucogenic precursors to protein and
energy and will tend to prevent body fat mobilization. The points to be
stressed are that:
-
By-pass protein because of its effects on feed intake almost always
stimulates milk production and depending on the imbalance in
nutrients (fermentation pattern) may cause animals to mobilize body
reserves. This may be prevented by the use of high-fat, high-protein
meals that supply both protein and long chain fatty acids for digestion
post ruminally. -
By-pass starch or manipulation of the rumen to give higher
propionate production, because it balances nutrients for milk
production, may prevent mobilization of body reserves without large
effects on feed intake and therefore on milk production. But because
it balances the nutrients for milk production, efficiency of energy
utilization is increased and body weight is often increased.
Wool and hair production
The effect of nutrition on wool production appears to be dependent almost
entirely on the quantity, and quality, of the balance of amino acids
absorbed. Therefore, feed intake is the primary limitation to wool or fibre
growth although at any one feed intake, wool growth can be stimulated by
altering the balance of protein relative to energy in the products of
fermentative digestion (e.g., removing protozoa from the rumen). Thus on
diets that require fermentative digestion, including those based on sugars
or fibre, a by-pass protein supplement will increase wool growth (Table
5.6).
GUIDELINES FOR DEVELOPING FEEDING STRATEGIES FOR
RUMINANTS
Introduction
When fibre-rich crop residues and by-products are the primary feed
resource for ruminants, feeding strategies must be based on a clear
understanding of the relative roles and nutritional needs of rumen micro-organisms
and of the host animal (see above).
| Basal | By-pass protein | By-pass protein + By-pass starch | ||||
|---|---|---|---|---|---|---|
| G | S | G | S | G | S | |
| Daily liveweight gain (g) | 32 | 45 | 68 | 107 | 81 | 119 |
| Patch weight at 105 days (mg/cm^2/d) | 0.54 | 0.74 | 0.82 | 1.27 | 0.76 | 1.11 |
| Feed intake (g/d) | 465 | 538 | 604 | 755 | 664 | 736 |
| Feed conversion (DM) | 14.8 | 11.9 | 8.9 | 7.0 | 8.2 | 6.2 |
| Rumen fluid half life (hr) | 16.1 | 14.1 | 8.6 | 9.0 | 12.1 | 12.7 |
* Wool or hair clipped from a 10 cm^2 mid-side patch
The new approach identifies high fibre forages as the most important
category of tropical feeds and emphasizes that these are imbalanced
sources of nutrients for both rumen micro-organisms and the animal. The
recent advances in understanding of rumen function and the role of “by-pass”
or “escape” nutrients has revealed important ways forward for
improving productivity of ruminants in the tropics. These concepts have
been tested and applied on a wide scale in many tropical countries and can
be summarized as follows:
The research which led to the new concepts of “balancing nutrients”
has shown that provided the protein to energy ratio in absorbed nutrients
is high, productive efficiency can be up to tenfold that predicted from
traditional feed evaluation methods (Leng, 1990). The breakthrough came
when the ruminant animal was treated as a two compartment system
(Figure 5.6) (Leng and Preston, 1976; Preston and Leng, 1987) in which
there is:
-
A microbial fermentative digestion system that functions efficiently
when there is a balanced supply of microbial nutrients within an
appropriate ecosystem.
and where:
-
The animal relies on the products of the microbial system and those
digestible feed components that escape the rumen fermentation.
Figure 5.6. Nutritional strategy for feeding the ruminant (Source: Preston and
Leng, 1987)
The results of applying these concepts have substantiated the hypothesis
that ruminants in the tropics fed on fibrous crop residues and dry pastures
were not deficient in energy per se but were inefficiently utilizing the feed
that was available. Therefore, when nutrients were more closely balanced
there were substantial gains in productivity.
The rumen microbial system alters the nutrients finally made available
to the animal converting fibrous carbohydrate, sugars and starches and
soluble protein to microbial cells, short chain organic acids and waste
products in the form of methane, carbon dioxide and heat. The critical
issue for the animal is the ratio of protein (from microbial and dietary
origin) to energy yielding substrates (the P/E ratio expressed as g protein/
MJ of energy from volatile fatty acids available for metabolism), since this
determines efficiency and level of productivity (Preston and Leng, 1987).
However, even when the rumen system is optimized by providing an array
of essential nutrients for microbes, the P/E ratio is usually still inadequate
to support optimum efficiency of utilization of the basal feed resource.
The demonstration in Cuba (Preston et al., 1967) that flame-dried fish
meal (a protein known to escape the rumen fermentation) dramatically
increased rate and efficiency of liveweight gain on highly digestible but
low protein diets (molasses and urea), led to the broader understanding of
the critical role of: (i) supplying nutrients to the rumen microbial ecosystem,
and (ii) of protein supplements to the animal, as factors
determining rate and efficiency of ruminant production from forage-based
diets. This in turn led to the introduction of the concept of “by-pass
protein” (Leng et al., 1977).
Another important step in the understanding of tropical ruminant
nutrition has been to appreciate that, when animals are in an environment
where the temperature is less than their body temperature, they will
oxidize acetogenic substrate to maintain body temperature. This results
in an increase in the effective P/E ratio in the metabolites available for
production. Conversely, when environmental temperatures exceed body
temperature the resultant heat stress causes a rise in basal metabolic rate
and the catabolism of protein. In practice, this means that the requirement
for protein (amino acids) per unit energy substrate will generally be greater
for ruminants in tropical environments than for those in temperate
environments. In summary the major features of the new approach are:
-
In the tropics there is a greater response by ruminants to
supplementation strategies as compared with responses in temperate
countries. -
Feed evaluation standards developed in temperate countries have little
application in the tropics and have been positively detrimental to
development of sustainable livestock production systems in those
regions.
The proposed strategy considers the ruminant animal as composed of two
subsystems:
Feeding the rumen microbes
-
The first need is for ammonia (>200 mg/litre of rumen fluid to
maximize intake as well as digestibility) (Figure 5.7), most
conveniently ensured by free access to multinutritional blocks based
on urea-molasses. -
Macro and micro-minerals (P, S and Co are most important but will
be supplied usually by other dietary components (e.g., in multinutritional
block, in green forage and/or by-pass protein supplement) -
Other micro-nutrients (amino acids, peptides, branched chain acids)
will rarely be deficient as these arise from lysis of microbes and are
supplied by other dietary components as in the case of minerals). -
An optimum ecosystem to promote rapid colonization of basic
substrate. A small quantity of highly digestible green forage (about 2
kg fresh matter/100 kg liveweight is usually sufficient) is the best way
of safeguarding this parameter (Figure 5.8). -
Maximum rate of intake of fermentable carbohydrate. Usually the
most appropriate way will be by ensuring free choice selection of the
basal feed which in the case of a fibrous crop residue means, wherever
possible, offering more than 50% in excess of needs (Owen 1994;
Figure 5.9). In general, the less digestible the basal feed, the higher
degree of offer is required (e.g., at least twice the expected intake in
the case of residual pressed sugar cane stalk (Figure 5.9)). The other
approach is to treat with ammonia (by urea-hydrolysis) (see Chapter
7).
Figure 5.7. The optimum rumen ammonia concentration to optimize both fibre
digestibility and intake is about 200 mg/litre (Source: Perdok, 1987).
Figure 5.8. Adding a small amount of Leucaena hay to a maize stover diet
increases rate of maize stover digestion in cattle (Source: Kabatange and
Shayo, 1991).
Figure 5.9. Effect of level of offer on intake of residual pressed cane stalk
(Owen, 1994).
Feeding the animal
The aim is to increase the protein/energy (P/E) in the nutrients absorbed
for metabolism by:
The amounts of supplement to be provided will be dictated by the
marginal value of animal product added per unit of additional supplement.
This in turn will be determined by the shape of the response curve between
output and input. Examples of such relationships are given for sugar cane
in Mexico (Figure 5.10) and wheat straw in China (Figure 5.11).
Supplying foliages with natural protection as a function of the protein
(many tropical tree foliages contain phenolic and other substances that
react with the protein during chewing, thus protecting it from rumen
fermentation) usually will be the most economical way.
Results are given in Figure 5.12 for the effects on milk production in
goats of supplementing a basal diet of King grass with the foliage of
Erythrina poeppigiana. Milk yield was a direct function of the amount
of legume foliage added.
Figure 5.10. Fattening cattle with sugar cane; the effect of by-pass nutrients
present in rice polishings (Source: Preston et al., 1976).
Figure 5.11. Response curve to cottonseed cake of steers fattened on a basal
diet of ammoniated wheat straw at two locations in China (Source: Dolberg and
Finlayson, 1995).
Figure 5.12. Milk production of goats fed King grass: effect of giving Erythrina
tree foliage (Source: Esnaola and Rios, 1990).
Controlling (reducing the numbers and/or activity of) the rumen
protozoa will increase the flow of protein to the small intestine, and thus
increase the P/E ratio and hence the productive parameters (Preston and
Leng, 1987). This has been conclusively demonstrated in experiments
where protozoal populations have been eliminated by detergents (e.g., see
Figure 5.13).
Figure 5.13. Effect of defaunation on growth of lambs fed straw, sugar, urea and
cottonseed meal (BP protein) (Source: Navas, 1991).
Many tropical tree and shrub foliages contain secondary plant
compounds that naturally inhibit protozoal activity. However, although
reductions in rumen protozoal numbers have been obtained by
supplementing the animal with foliages from trees such as Enterolobium
cyclocarpum (Khang et al., 1994) or with seeds rich in saponins from the
tree Sapindus saponaria (Diaz et al., 1993), it has not yet proved feasible
to translate these effects into practical production systems.
For some production traits (e.g., growth and milk production) it will
be advantageous to supply by-pass oil since this can be incorporated
synthesizing fat from acetate and glucose. On high-fibre diets, such as
crop residues and pasture, “un-protected” lipid added above 5% of the diet
dry matter will depress fibre digestion. This negative effect can be
avoided by “protecting” the lipids with calcium salts to form insoluble
soaps (Palmquist and Jenkins, 1982; Palmquist, 1984).
There may be other indirect benefits from use of oil. Thus, Rodriguez
and Cuellar (1994, unpublished data) mixed 6% crude palm oil and 2%
calcium hydroxide with the leaves of the legume tree Erythrina fusca and
found that the intake of leaves was increased. Supplementing crossbred
(F1 Holstein x Zebu) cows (basal diet was grazing on African Star grass
pasture) with 6 kg/day of this mixture (6% oil, 2% calcium hydroxide and
92% leaves) supported the same milk production as 4 kg daily of
concentrates.
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