Download The effects of disease and climatic stress on the metabolisable

Metabolisable protein requirements of ruminants fed roughage based diets.
Leng, R.A.
Emeritus Professor, University of New England, Armidale, NSW 2351, Australia.
Correspondence address: PO Box 361,Coolum Beach, Qld,4561,Australia.
Summary
Metabolisable protein (MP) in ruminants fed forage based diets is regarded as a scarce resource
that is often precariously balanced in the available metabolisable energy (ME) to meet the
essential amino acid requirements of productive ruminants. MP is defined as the microbial
protein from the rumen that is digested and absorbed from the intestines plus any dietary
protein that becomes available for digestion. ME contains the energy of MP plus the energy
absorbed mainly as volatile fatty acids (VFA) plus smaller amounts of digestible fat and
carbohydrate of microbial or dietary origin.
Ruminants feeding on cellulosic crop residues and mature grasses are dependent on
microbial protein from the rumen as their major source of MP. These forages are often
deficient in crude protein and minerals. In the absence of supplemental sources of ammonia
and minerals, microbial growth efficiency is low; more of the fermentable feed is converted to
VFA with a concomitant reduction in microbial cell synthesis, reduced microbial pool size and
low feed intake. The major effect is that MP is a low proportion of total ME in ruminants given
these forages. The proportion of MP in the ME is increased by ensuring that the rumen fluid is
not deficient in essential microbial growth factors. The ratio may be further increased by
supplementation with bypass protein sources or manipulating the microbial ecosystem to
improve growth efficiency (elimination protozoa and addition of bentonite clays to the diet are
two methods that achieve this).
In all countries of SE Asia, large ruminants are fed for a major part of the feed year on
low protein, crop residues and wasteland grasses. Numerous studies have shown that on these
feeds the priorities are to create an optimum growth medium in the rumen and to optimize MP
by feeding supplemental bypass protein. A collation of data from different parts of the world,
from experiments to elucidate the benefits of supplementation of ruminants on these mature
forages has produced some significant relationships. The growth response in young cattle to
increased levels of dietary bypass protein was described by a logarithmic function. For applied
purposes the response may be separated into two linear relationships. It is suggested that the
initial response is a result of a better balance of substrates for growth with an improved
efficiency of utilization of the feed. This is followed by further linear increase in response to
the additional ME consumed at higher intakes of the protein supplement. The overall analysis
indicates that the ruminant on roughage based diets, as commonly used in SE Asian countries
are deficient in MP for optimum efficiency of forage utilization.
Traditionally, the requirements for essential amino acids for immune functions have
been considered as part of maintenance. Recent research indicates that there can be a high
demand for nutrients for the events that occur in immune activation. Pro-inflammatory
cytokines, produced during immune activation, appear to override hormonal control of nutrient
utilization or mobilize amino acids from skeletal muscle to meet the host’s defense processes.
Sheep with intestinal parasite infections appear to have a MP requirement that is up to 20-30%
higher then predicted standards. Similarly, infection or a simulated disease challenge appears
to increase the ruminant’s requirements for essential amino acids. Immune function is also
stimulated by numerous stressors including continuous and sub clinical exposure to disease,
poor animal management practices (e.g. tethering animals in pens on wet slippery floors) and
disturbances of homeostasis including danger signals provided by tissue damage from heat
stress and ingestion of toxic compounds. Many of these stressors are more prevalent in tropical
countries because of climatic factors (e.g. heat stress) and deficiencies of food quality and
quantity. It is proposed that the imposition of stressors either singly or in multiple events may
have significant effects on the optimum level of MP for productive ruminants in the tropics.
Whilst MP requirements for expression of immunity to a wide range of infections may all be of
the same order of magnitude, there is some evidence that underlying chronic stress can make
an animal more sensitive and responsive to additional stressors.
Heat stress is a significant factor in ruminant production in SE Asia and has generally
been accepted as a primary limitation to milk yield in cows. Heat stress appears to be able to
stimulate an immune-like response. One mechanism appears to be related to increased gut
permeability through damage from reduced blood flow to the gut (termed ischemia) when heat
stress induces a peripheral vasodilatation. Management stressors combined with disease and
parasite infections would be interactive with high environmental heat stress in immune
modulation of metabolism. The potential is that MP requirements of ruminants in SE Asia are
significantly higher than in temperate areas.
Bypass protein meals are scarce and an expensive component of a diet. However recent
studies appear to have identified a major source of bypass protein from cassava leaf meal
which has condensed tannin at optimal levels for protection of the protein from degradation in
the rumen.
Key words R.A. Leng, ruminant, metabolisable protein, forage diet, response relationships,
immune function.
Introduction
The development of feeding standards for ruminant livestock has been the impetus of much
past animal nutrition research. Requirements for energy and protein are relatively easily
established in monogastric animals. This is because the dietary digestible protein and energy
are closely similar to the chemical estimates from feed analysis. In ruminants, however,
absorbed nutrients bear little relation to the chemical composition of the feed; in the processes
of digestive fermentation in the rumen, organic matter is converted either to soluble nutrients
(largely volatile fatty acids or VFA) or the molecular building blocks for microbial cell
synthesis. Microbial protoplasm exiting the rumen and digested in the small intestine is
normally the largest source of essential amino acids in the forage-fed ruminant. The proportion
of dietary protein entering the intestines depends on its physical and chemical characteristics
and certain protein meals may contribute significant amounts of intestinally digestible protein
that escapes microbial degradation and augments the total essential amino acids absorbed.
The nutrient requirements for monogastric animals are more or less the same as the
standards established under laboratory conditions especially where there is excellent control of
disease and environment. However, the standards can be highly misleading for small scale
farmers who use locally available feed resources; have no controlled environment housing and
animals are subject to a variety of stressors. Over the last 10-20 years there has been an
increasing awareness that management, climate, disease incidence and the ingestion of toxic
chemicals may alter the requirements for specific nutrients by ruminants. These requirements
are often essential amino acids, glucose precursors or small molecules involved in protection of
signalling proteins and required for protection against invasion by foreign, non-self molecules
or to reduce inflammation once the stressor is eliminated. The discussion will be largely
targeted at the nutrients (mainly essential amino acids) required by non stressed ruminants or
when the immune system is triggered by a variety of stimuli.
Protein nutrition of ruminants
Proteins in pastures and forages are highly soluble and readily degraded in the rumen with little
dietary protein normally escaping to the small intestine. In general denatured plant proteins and
proteins with extensive disulphide bonding between amino acid chains are relatively stable in
the rumen and are only slowly fermented to ammonia and volatile fatty acids and thus have
variable escape properties. Soluble proteins may also be protected from microbial hydrolysis in
the rumen by chemicals that bind amino acids on the surface sites to which rumen microbes
would attach (Mangan, 1972). In practice soluble proteins may be protected from rumen
degradation by heat denaturation, or binding of specific sites with chemicals such as
formaldehyde, glucose and xylose or by condensed tannins present in foliages. The amount of
protein bypassing rumen fermentation from such sources depends then on their potential
degradability and residence time in the rumen. The amount of protein available from microbial
sources is directly related to feed intake, the efficiency of microbial growth in the rumen and
the biomass of protozoa which ingest and digest both bacteria (Coleman, 1975) and feed
particles (Ushida et al., 1984).
Metabolisable protein (MP) refers to the total protein that is digested in the intestines
from microbial cells and dietary protein exiting the rumen intact. The requirements for MP are
defined here as the digestible protein providing the essential amino acids for:
 Maintenance of homeostasis (including synthesis of hormone and immune signals and
to support the amino acid requirements to mount an immune response and to overcome
stress).
 Endogenous protein turnover (tissue, enzyme and protein turnover and replacement of
cells that die normally (termed apoptosis), or are sloughed from the tract in the
processes of digestion.
 Synthesis of new tissues in growth including requirements to meet gestational and
lactational needs.
 Replacement of damaged tissues.
This review will discuss newer concepts that suggest that the requirements for MP by
ruminants in many production systems are higher then accepted standards. These particularly
apply to animals subject to infestation with intestinal parasites (see Sykes and Greer, 2003) and
or in hot climates (see Leng, 2005). The review is restricted to ruminants fed forage based diets
as these represent the vast majority of domestic ruminants in developing countries. It is also
considered that in a fossil fuel deficient world that, with increasing cost of crop production and
a likely future scarcity of inexpensive grain, there will be an increasing dependency on
ruminant meat and milk for human consumption (Leng, 2004). This will necessarily have to be
produced more efficiently from forage or crop and agro-industrial by products. These are often
considered to be too poor in nutritional value for feeding and are therefore mostly wasted by
burning.
Principles for feeding ruminant livestock from available resources
Ruminants are capable of drawing their nutrients from a wide range of cellulose biomass. Their
fermentative digestive process sets certain limits on the efficiency with which the feed can be
utilised because of associated losses of feed energy as heat and methane in the rumen. In
addition the low digestibility of forages in general and specifically when these are crop
residues, imposes a ‘less then genetic potential’ ceiling on production.
Over the past 20 years, understanding of digestion and metabolism in ruminants has
greatly improved the feeding strategies for using crop residues such as straw (see Chenost and
Kayouli, 1997;Preston and Leng, 1986; Leng, 1990, Leng, 2004). The most important issue has
been recognition that the primary deficiency of ruminants fed crop residue is the availability of
MP (Leng, 2004). Improving MP by ensuring an adequate source of minerals and ammonia in
the rumen and providing a source of bypass protein was shown to allow straw (or straw with
enhanced digestibility after treatment with alkali) and other roughages to be used at much
higher efficiencies for production than was predicted from the content of metabolisable energy
or ME (Leng, 1990). Ruminant production from these feed resources is the key for meeting the
demand for large quantities of medium to high quality protein for human consumption, at
relatively low cost. This is not a new concept and the efficiency and level of ruminant
production that is achievable on such diets has been debated for a number of years (see Preston
and Leng, 1986). However, recent developments suggest that with more attention to
amelioration of disease, climatic and other stress factors, under real world conditions, the
requirements for MP may be decreased significantly. Conversely in adverse environments (e.g.
high disease incidence and management and environmental heat stress) the requirements by
ruminants for MP may be significantly increased.
Using crop residues for ruminant production
Crop residues, agro-industrial by products, and weeds/grasses from wasteland and fallow
cropping land, foliage of trees and shrubs and forage/tree crop foliage produced as an
intercrop, are the basal feed resources of ruminants in developing countries. Crop residues
such as straw are by far the greatest available biomass. Applying feeding standards based on
ME content results in straw accorded little nutritional value. Uninformed farmers regard it as a
poor feed because cattle generally loose weight when straw is fed without supplementation. In
1990, I challenged the description of crop residues as being of low quality and preferred to
term them “imbalanced forages” (Leng, 1990). The point is that with small additions of
nutrients to these forages, large responses in animal production can be achieved. The levels of
production achieved with appropriate low level supplementation are not predicted from the ME
content of the mixed diet.
It is necessary to point out at this point that ME is defined as the energy content of
absorbed nutrients less the energy lost in urine. Absorbed nutrients are largely the volatile fatty
acids produced in the rumen plus the microbial cell substances digested and absorbed from the
intestines. Depending on the efficiency of microbial growth in the rumen, which in turn
depends on the levels of ammonia and minerals in the rumen, the ratio of the energy in MP to
energy as volatile fatty acids may vary from below 1 to 4 to a theoretical maximum as high as
1 to 1 (see Preston and Leng, 1986) with a measured average of about 1.4 to 1 Mj/Mj. Based
on stoichiometry of rumen fermentation this represents a protein (from microbes available for
digestion) to energy (as VFA) ratio in the ME that may vary from 12 to as high as 47 g crude
protein per Mj. Feeding standards based on ME content of low digestibility forages do not
predict the production levels obtained in practice with cattle when both rumen nutrients and
low levels of bypass proteins are supplemented to low digestibility forage, particularly straws
(see Leng, 1990; Poppi and McClennan, 1995; Leng, 2004). One option is to move to systems
that depend on local research on ruminants fed locally available forages suitably supplemented
to ensure high microbial growth efficiency in the rumen and then fed increasing amounts of
locally available bypass protein sources.
Supplementation requirements for optimum use of low digestibility forages by ruminants
For efficient digestion of forage, rumen microbes require a culture medium that is balanced
with minerals and ammonia. Once these are provided, digestibility and intake (and therefore
production) are then limited by the structural nature of the plant fibre and the extent to which
this fibre is embedded in, or surrounded by lignin. Efficient methods are available for ensuring
that no deficiencies of minerals or ammonia occur in animals feeding on mature forage diets
(for example provision of multi-nutrient blocks; see IAEA, 1991). Supplementation of the
animal to ensure an efficient digestion of forage in the rumen improves 1) digestibility 2) feed
intake and 3) the flow of microbial cells and therefore protein to the intestines; when the rumen
medium is not deficient in essential microbial growth factors the relative availability of MP to
VFA energy is increased and more balanced to the animals requirements. This in turn increases
both efficiency of feed utilisation and level of productivity. This is the first step in improving
the quality of these forages for ruminant production (Leng, 1984).
Manipulation of the microbial ecosystem to increase the efficiency of net microbial
growth is an optional further strategy. For example defaunation removes the predatory effects
of protozoa that lower fluid phase bacterial biomass (Coleman, 1975) and reduce the amount of
microbial cells that flow to the intestines (Veira et al., 1983). Degradation of dietary particulate
protein is also reduced in the fauna-free rumen, increasing the dietary protein that becomes
available to the animal (Ushida et al., 1984). The fauna-free rumen also appears to have a
lower microbial maintenance energy requirement allowing for an increase in net bacterial
growth efficiency ( Mom-Seng et al., 2001) . Rumen manipulations to increase MP availability
include treatment to defaunate the rumen (Bird and Leng, 1978) or reduce the protozoan mix
and total biomass in the rumen by drenching with oil (Nhan et al., 2001). Providing a clay
mineral, bentonite has also increased the MP outflow from the rumen (Fenn and Leng, 1989;
Ivan et al., 1992). The third approach for improving MP availability is to supplement with a
source of bypass protein (often referred to as escape protein or rumen un-degradable protein)
(see Preston and Leng, 1986). In practice, bypass protein is sourced from oilseed meals, in
particular cottonseed meal (solvent extracted), hulled cotton cake (pressure extracted), copra
meal, gluten meal, soybean meal and fish and meat meals. Cassava hay, Lotus corniculatus hay
and a variety of tree foliages that contain condensed tannins may also be sources of bypass
protein particularly for animals in small farmer systems. Feeding animal protein to animals is
now banned in most countries of the world because of the possibility of contamination with the
prion responsible for ‘mad cow’ disease.
The increased requirements for essential amino acids and therefore MP at critical stages
in the life of ruminants fed roughages is well established, however, bypass protein supplements
also contain fermentable nutrients(that increase both VFA and microbial cell production) and
the benefits of protein bypass are often blurred by the contribution of these to the nutrition of
the animal. Protein concentrates are usually an expensive component of a diet and it is
important to distinguish between these two roles. For example cereal grains supply a high level
of fermentable starch with low levels of bypass protein and materials such as cottonseed meal
combine a high level of bypass protein with a lower level of fibrous carbohydrates. Yet at low
level inclusion in a roughage diet the response of cattle to grain or cottonseed meal is the same
per unit of crude protein (Poppi and McClennan, 1995; see also Leng, 2003)
A protein supplement is usually expensive. Evaluation of a supplement to allow
economic decisions on how much to use is best addressed by regression analysis of intake and
response. Numerous experiments have been done in various areas of the world to evaluate the
response to increasing levels of MP on growth of both small and large ruminants given mature
forages from dry season pasture and crop residues. The results from studies with cattle are
reviewed below.
Benefits of providing protein supplements to cattle consuming ‘poor quality’ forage
Mature forages from grasses such as cereal and pastures have an ME content rarely more than
5 MJ ME /kg dry matter and are usually deficient in nitrogen and some minerals. Mineral and
or urea supplementation and alkali treatment with urea or ammonia to increase straw
digestibility and intake is a recommended procedure to improve production levels. The higher
digestibility of treated straw compared with non-treated straw often results in an increased live
weight gain of cattle of the order of 200- 300g/day (see Perdok and Leng, 1990; Finlayson et
al., 1994). The value of the increased production is, however, often less than the cost of alkali
treatment (see Chenost and Kayouli, 1997). With either treated or untreated forage,
supplementation with a bypass protein source increases the overall efficiency of use of
absorbed nutrients (Perdok and Leng 1990). The levels of production achieved when MP in the
ME is increased over an initial small range, is many fold that predicted from the ME content of
the feed (Leng, 1990). The laws of thermodynamics dictate that more efficient use of the ME
must result in lowered heat production in the animal. This is probably associated with a closer
balance of nutrients absorbed to those required (improved protein to energy) particularly where
total feed intake is not increased (see Perdok and Leng, 1990).
The points that are stressed here are that the ME system for predicting feed intake and
therefore feed quality and growth rates of ruminants has little relevance to the majority of
mature, low digestibility forages fed to ruminants. Therefore there is an urgent necessity to find
alternative approaches. The one suggested here is to establish relationships between increasing
intake of local sources of bypass protein and productivity in ruminants fed available forages.
The main reason for suggesting this arises from collation and analysis of a range of published
data sets where growth response in cattle fed mature forages to increasing intake of MP have
been established. These include the use of forages of differing digestibility and a range of
protein sources with variable content of bypass protein.
Supplementation strategies for young cattle on low quality forage
A number of researchers around the world have examined the benefits to ruminants fed forage,
of supplementation with bypass protein meals. In only some of the studies are response
relationships measured over sufficient range to detect trends. In others a treatment group, fed
the basal diet with adequate urea and minerals to balance the rumen and ensure no mineral
deficiencies has not been included. In both cases the material could not be included in the
analysis that follows. In examining the literature, research that satisfied the following criteria
were used; 1) the experimental animals were young; 2) the treatments included a control group
fed the forage with rumen nutrients (urea and minerals); 3) treatment groups of animals on the
same diet were given a number of levels of a recognised bypass protein. Research that met this
criteria included studies by Elliott and O’Donovan (1971),Creek et al., (1983), Saadulah
(1984), Wanapat et al., (1986), Perdok (1987), Zhang Weixan et al., (1994), Finlayson et al.,
(1994) and Dolberg and Finlayson (1995) ( see also Poppi and McLennan, 1995). The data
from these studies are shown in Figure 1. In order to remove some of the variability of weight
of animals used in different experiments and the differences of quality of protein meals, the
intake of supplement is expressed in, g crude protein intake per kg body weight per day
(gCP/kg LWt/d) and the response is calculated as the increase in live weight gain (kg/d) over
that of animals with no bypass protein in the diet (from Leng, 2004).
When these studies are combined, the relationship of bypass protein intake and growth
response in cattle on mature forage is described most accurately by a log linear relationship
indicating a diminishing response to increasing intake of crude protein (see Figure 1 ). For
practical application the response may be divided into an initial steep linear response followed
by a much lower response at higher intakes of supplement (Figure 2).These two response
relationships may be attributed to:


An initial effect of an increased MP supply and a more balanced array of nutrients for
efficient live weight gain (for instance the response in a 200kg steer is 1.2 g gain / 1g
cottonseed meal (42% CP) consumed). Total feed intake was either unaffected or
slightly increased over this initial supplement range (0-1g CP/kg LWt /day) and
therefore the improved growth was mainly attributable to increased feed conversion
efficiency.
At intakes above 1gCP/kg LWt/day there was a lower response per unit of supplement
(0.32 g gain per 1g cottonseed meal consumed). The total feed intake was increased but
at the higher rates of supplementation a substitution effect was apparent. The overall
greater availability of nutrients following the initial improved feed conversion
efficiency (an effect compatible with increased ME intake) can explain the increased
live weight gain with supplement levels above 1g CP/kg LWt/day.
Application of the results of feeding trials
Although there is a wealth of research demonstrating the benefits of these strategies of
supplementation of forage fed ruminants, the perception of low energy content of the forage
often leads to supplementation with starch-based concentrates that substitute the roughage.
Where grain is mixed with roughage the response to low inputs on growth rates of cattle (see
Poppi and McLennan, 1995) appear to be solely owing to the bypass protein in the grain (up to
1gCP/kg LWt/day) (Leng, 2004). The growth rates of cattle on molasses based diets with
restricted roughage intake (1% L Wt), fed urea and increasing levels of fish meal (see
Preston,1972) also appear to fit the same regressions (See Figure 3). Included in Figure 3 are
data from Ffoulkes and Preston (1977) where cattle on a molasses-based diet were fed fresh
cassava and sweet potato foliages with or without soybean meal. Again a case can be made that
the results are compatible with the data where roughage alone was fed to cattle. The major
conclusion is that cassava hay, which is high in crude protein, 22-25%CP in dry matter,
contains considerable proportion of its protein in the bypass form.
Conclusion
Under experimental conditions it appears that cattle fed basal roughage/sugar diets
supplemented to ensure optimal rumen conditions will support maintenance to moderate levels
of production depending on the digestibility of the feed. Improvements in feed conversion
efficiency are achieved by increasing the MP intake which follows a diminishing response
curve. Thus if the growth rate of cattle is established on the basal forage, recommendations can
be made for the requirements of extra MP to achieve a target growth rate and the economic
feasibility can also be assessed.
Increase in LWT [Kg/day]
1.2
1
0.8
0.6
y = 0.2339Ln(x) + 0.4621
0.4
2
R = 0.7942
0.2
0
0
1
2
3
4
5
6
7
Intake of protein meal [gCP/kgLWt/day]
Figure 1. The effects of increasing the intake of bypass protein in young cattle consuming a
‘poor quality forage’ that is adequately supplemented with N (to meet rumen ammonia
requirements)and minerals (Leng, 2004).
Increase in LWt [Kg/day]
1.2
y=0.6102x
1
R2=0.6102
0.8
0.6
y = 0.1111x + 0.3524
R2 = 0.6495
0.4
0.2
0
0
1
2
3
4
5
6
7
Intake of protein meal [gCP/kgLWt/day]
Figure 2. The potential to describe the results as two distinct sets of data described by
independent linear regressions. These regressions are intended to provide prediction equations
relatively easily understood.
Increase in LWt gain (kg/day)
1.2
1
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
7
Intake of protein in supplement (g CP/kg LWt/day)
Forage diet
Molasses,restricted forage diet
Molasses cassava-sweet P
Log. (Forage diet)
Figure 3 . The response in cattle to supplementation with bypass protein appears to follow the
same pattern in cattle on sugar based diets (molasses) with restricted forage intake as that of
cattle on high roughage diets. The cattle growth data on molasses based diets is from Preston
(1972) and Foulkes( 1984) The data for cattle on molasses and fresh cassava or sweet potato
foliage is from Ffoulkes and Preston (1977).
Application of research results to commercial systems
There are, however, additional constraints to the productivity of ruminants that seem to affect
their MP requirements under commercial production conditions. Lower than anticipated
production is often found in practice where ruminants simply under-produce even on relatively
high quality forages. Lower production from a feed then anticipated, either from the feeding
standards or the response curves discussed here, is often seen in cattle under farm conditions.
There may be many reasons for this including, mismanagement, underfeeding or unavailability
of appropriate resources. However, it is clear that in practice, responses to supplementation are
affected by unknown factors that both lower the potential production of ruminants on roughage
based diets and lowers the response to supplementation.
Recent studies examining the effects of stressors on feed utilisation of animals are
providing some potential reasons for lack of replication on farm results from controlled feeding
trials. Central to this, is new research that is demonstrating the major role that immune
activation plays in nutrient utilisation, partitioning of nutrients and control of feed intake in
animals (Johnson, 1998; see Colditz, 2002; 2004). Alteration of immune function may occur in
response to a wide range of stress related conditions including management practices, adverse
climate (cold or heat stress), disease and parasitism and the ingestion of toxic chemicals. The
key link appears to be that between the immune system of the animal and increased nutrient
requirements particularly for essential amino acids. The present knowledge in this area is
briefly discussed below.
Immune system
It is generally accepted that malnutrition impairs immune function in humans (McWhirter and
Pennington, 1994) and this is also apparent in ruminants (Johnson,1998) and has been related
to the role of essential amino acids in the immune response (see Colditz, 2002). The converse
appears to be also true; the nutrient requirements for immune action may increase the animal’s
overall requirements for essential amino acids. MP is a scarce nutrient source in forage-fed
ruminants as indicated by the response curves shown in Figure 1. Improving MP availability
may counteract some of the consequences of diseases and stress that stimulate immune
function.
Immuno-nutrition is a growing area in humans but has been explored only to a limited
extent in domestic livestock. Domestic animals with clinical and subclinical infections eat less,
grow slowly, produce less milk and during pregnancy the size of the offspring may be
compromised. In all these situations stressed animals appear to convert feed to product
inefficiently relative to the efficiency achieved in the disease free-state. It is suggested that
reduced feed intake is the primary cause of the ill-thrift seen in sick animals (Johnson, 1998).
Recent research, however, indicates an increased requirement for essential amino acids for
immune function which could also be a major cause of low feed intake and ill thrift. (Lobley et
al., 2001; Colditz, 2002). Accompanying immune function stimulation is an increase in
synthesis of stress proteins such as acute-phase and heat shock proteins (HSPS). The latter are
produced in response to an increase in body temperature or a response to stress per se. It is
suggested here that the additional essential amino acid requirements for maintenance of
homeostasis when the animal is subject to stressors may have implications for feeding
standards and in particular the response of animals on low digestibility forages to
supplementation with bypass protein meals (or increased availability of MP).
Background to the nutritional requirements to mount an immune response in animals
The conventional description of the immune system is one that senses the presence of foreign
or non-self molecular structures which activate defence mechanisms that rid the body of these
molecular structures and prevent disruption of homeostasis. Once the foreign material has been
eliminated the immune system returns to a surveillance mode with only a small demand for
nutrients. Essentially two components are activated and include lymphocyte proliferation and
stimulation of acute-phase protein production in the liver (and other organs including gut and
mammary gland) and release into blood. In essence lymphocytes and cells of the immune
system produce soluble proteins called cytokines to convey information to other physiological
systems including liver and brain which orchestrate the animal’s response (see Colditz,
2002).The pro-inflammatory cytokines override hormonal control of nutrient utilisation
including,1) reduced fatty acid uptake by adipose tissue; 2) reduced tissue protein synthesis; 3)
enhanced skeletal muscle protein mobilisation; 4) increased glucose, fatty acid and acute phase
protein synthesis in the liver(see Colditz 2004). These changes in response to graded
production of cytokines may induce incremental decreases in productivity levels of ruminants
(Colditz, 2004), particularly where MP is limiting growth.
A new paradigm of immune reactions is proposed by Matzinger( 2005 ) which suggests
that activation of the immune system occurs whenever there are danger signals from invasive
organisms or toxic chemicals that may cause abnormal death of epithelial cells(lytic death)
and tissue damage. The danger theory is gaining credence particularly as the immune system is
seen as a sensory organ and regulator of metabolism (Husband, 1995; Moseley, 1997). This
more comprehensive mechanism invoking the immune response is compatible with the cascade
of events that may be involved in immune reactions that have the potential to alter the nutrient
requirements of animals. The requirements for essential nutrients in stressor-induced immune
reactions may explain a number of ill thrift syndromes in livestock (defined as lower then
expected productivity based on the standard nutrient requirements for livestock) under stressful
conditions (Colditz 2004; Leng, 2005; Cronje, 2005). The major consideration here is whether
immune stimulation effects the animal’s requirements for MP and consequently affects the
response relationships shown in Figure 1 there by altering supplement requirements and
production levels in ruminants.
Recent studies with both monogastric and ruminant animals suggest that the events that
lead to immune stimulation increase the requirements of the animal for a number of nutrients.
Grimble (2001) lists a number of immuno-modulatory nutrients with their roles. The nutrients
that appear to be required in larger amounts when the immune functions of the body are
stimulated include:
 Polyunsaturated fatty acids which act as anti inflammatory agents.
 The sulphur amino acids which enhance antioxidant status as they are required in
glutathione synthesis.
 Other essential amino acids which occur in immune proteins at higher concentrations
than in muscle protein.
 Glutamine which has several roles including as a nutrient for synthesis and a source of
energy for immune cells, improvement of gut barrier function and also as precursor of
glutathione and nucleic acid synthesis.
 Glucose, which is also a scarce nutrient in ruminants and which may be required for
synthesis of nucleic acids and glutamine required by proliferating lymphocytes.
 Arginine which is a substrate for nitrous oxide synthesis that is required to kill foreign
organisms.
 Nucleotides which are precursors for nucleic acid synthesis for DNA and RNA.
In the context of this review the potential increased overall requirements for essential amino
acids, glutamine and maybe glucose following immune stimulation appear to have the most
critical effect on production of ruminant animals that have limited availability of MP or a high
demand for these as in late pregnancy and early lactation. The metabolic consequences of an
immune stimulation may be subtle changes in feed intake, digestion metabolism and
partitioning of nutrients. Lower then accepted levels of production are associated with the
following conditions which all appear to involve immune system stimulation:
 Intestinal parasites (Sykes and Greer, 2003).
 Disease organisms (Johnson, 1998).
 Poor hygiene (Williams et al., 1997; Klasing, 1988; Lipperhride et al., 2000).
 Psychological stress in calves restrained on slippery floors (Alsemgeest et al., 2000);
 General stress effects ( Elasser et al., 2000).
 Ingestion of mycotoxins (Oliver, 2005; Litherland et al., 2004).
 Heat stress (Hahn, 1999; Cronje, 2005) including heat stress enhanced by the
consumption of mycotoxins produced by endophytic fungi found in some grasses
(Leng, 2005).
The question then arises: “Is there a common underlying cause for these ill thrifts?”
Brief description of the nutrients involved in immune response
Probably the most comprehensive study of the immune reaction in domestic livestock relates to
parasitism (see numerous reviews in Knox et al., 2003). Immune activation impacts on
behaviour, metabolism and an instant increase in demand for essential amino acids for
synthesis of a range of defence molecules that provides the acquired immunity and for
proliferation of lymphocytes. For rapid cell growth (or replacement of damaged cells) (e.g. in
lymphocytes, gut epithelial or even wool follicle) there is a high demand for glutamine that
appears to provide for purine and pyrimidine in nucleic acid synthesis and also an obligate fuel
for these proliferative tissues (see Colditz, 2002). The immune proteins appear to have higher
concentrations of some essential amino acids than for example muscle tissue (Colditz, 2002).
Where demand for immune function is high, mobilisation of amino acids from skeletal muscle
is often high to meet these demands (Lobley et al., 2001). The extra requirements for essential
amino acids and glutamine may substantially increase the animal’s total requirements for
amino acids over short periods of hours or over longer periods where the effects are more
protracted. Lobley et al., (2001) have demonstrated a marked mobilisation of muscle protein in
sheep subject to endotoxin challenge and a marked reduction in some essential amino acids in
blood. Increased requirements for MP in parasitised animals have been demonstrated in young
growing sheep (van Houtert et al., 1995) or pregnant and lactating ewes (Donaldson et al.,
1998; see Houdijk et al., 2001).The effects of augmenting MP availability by supplementing
with a bypass protein in parasitised or non-parasitised sheep are shown in Figure 4. The poor
live weight gain in parasitised lambs is returned to the live weight gain of non parasitised
lambs on the same diet when MP is increased by roughly 30% more than standard
requirements (van Houtert et al., 1995 see Figure 4). The underlying reasons for the apparent
increase in essential amino acid requirements appear to be:
 Damage to gut tissues, particularly the breakdown of the tight cell junctions in the
intestinal epithelium by macrophage activity(protease) resulting in cell death(Houdijk
et al., 2001; Stear et al., 2003 ) and ingression of endotoxins, derived from Gram
negative rumen microbes that stimulate the immune response.
 White cell proliferation and acute phase protein synthesis (Colditz, 2002).
 Repair of damaged intestinal tissue (Stear et al., 2003).
 Increased leakage of proteins into the intestine that are fermented in the lower colon
rather then being digested and reabsorbed. This is less of a problem when the species of
worms reside only in the abomasum(Stear et al., 2003) but a major factor with multispecies infections of both the abomasum and upper small intestine including the ileum(
Steel et al., 1982)
 With Haemonchus contortus infestations, blood loss appears to be a major drain
stimulating synthesis of plasma protein and blood cells (Rowe et al., 1988). H contortus
could also stimulate the release of cytokine and the immune response through damage
to abomasal tissue.
 Elevated production of mucin by the globlet cells in the gut wall (Stear et al., 2003).
A major function of intestinal epithelial cells is to provide a physical barrier between the highly
immuno-reactive subepithelial tissue and the contents of the intestinal lumen. The barrier
function of the intestinal mucosa is maintained by the tight junction complex joining adjacent
epithelial cells (Denker et al., 1998). Disruption of the intestinal epithelial tight junction
complexes results in a ‘leaky gut’ with an increased intestinal permeability. Increased porosity
of the gut barrier appears to be pivotal effect and may be the link with other stressors that may
also stimulate the immune system.
100g fish meal
100g FM infected
0g FM
0g FM infected
16
14
Liveweight (kg)
12
10
8
6
4
2
0
-50
0
50
100
150
Days from commencement of infection
Figure 4. The benefits of increased MP(fish meal, FM) supplement on growth in lambs
infected with Trichostrongylus colubrisformis and fed oaten chaff balanced at the rumen level
with urea and minerals (van Houtert et al., 1995).Upper data are from lambs with and without
infection fed 100g fish meal showing no effects on live weight of infection; the lower data
group refers to lambs with no supplement and indicates that live weight change is effected by
infection in the absence of fish meal supplementation.
Heat stress and lowered productivity in ruminants
Ruminants are highly sensitive to heat. Cattle control body temperature when heat load is high
by sweating but their capacity to sweat per unit of body surface is much below that of humans
and most other animals (see Blaxter, 1962). Cattle can be heat stressed at relatively low
temperature and /or humidity. Bos Indicus cattle are apparently more resistant to heat stress
partly because of their smaller body weight and therefore greater relative surface area and also
their greater capacity to sweat (Frisch and Vercoe, 1979). Buffalo on the other hand are prone
to heat stress because of the lack of sweat glands and must be able to wallow in mud and water
to control body temperature. Climate induced heat stress is consistently a major constraint to
ruminant productivity in the SE Asian area.
Heat stress in dairy cows can be measured as the thermal humidity index (THI) which
considers both environmental temperature and humidity (see Johnson et al., 1961). A THI of
72 is regarded as critical in reducing feed intake in dairy animals. Table 1 illustrates the
potential extremes of heat stress that cattle may be subjected to in SE Asia. Body temperature
is not constant but has a circadian rhythm. In heat-stressed animals, rectal body temperature
rises and the circadian rhythm usually has a higher minimum and maximum value. It is well
established that heat stress has a considerable impact on the productivity and welfare of
livestock (see Cronje, 2005 for review) and periods of elevated environmental temperatures are
associated with an immediate reduction in feed intake, but a reduction of milk yield occurs
some 3-4 days later (Maust et al., 1972; Hahn, 1999). Depending on the level of heat stress, as
a single event or the repeated impost of heat stress, the losses of production can vary from
small to substantial and under excessive heat stress the animal may die.
The present concept of metabolic damage induced by heat stress is that death occurs
from multi-organ dysfunction through the reperfusion injury that occurs following ischemia of
these organs because of the initial diversion of blood flow to the periphery (Jessen, 2001).
During heat load, when body temperature rises, blood flow rate to the gut may be reduced by
40-50%.The barrier functions of the gut are believed to be disrupted or damaged by lack of
oxygen allowing leakage of proteins into the lumen and ingress of endotoxins and
lipopolysaccharide with stimulation of the immune response (Jessen, 2001;Gisolfi and Mora,
2000). Endotoxins appear in blood with as little as 1oC rise in core temperature in the rabbit
(Butkow et al., 1984) which suggests that damage to the gut lining occurs at a relatively mild
degree of heat load. Cronje (2005) has recently reviewed the literature on heat stress in
ruminants and has firmly argued for the application of a new paradigm in humans and domestic
animals “that places damage to the tissues of the gut as the pivot through which the adverse
effects of heat load are promulgated” The cascade of events as summarised by Cronje (2005)
are:
 Heat load directs blood to the skin at the expense of blood flow to the gut.
 Reduced visceral blood supply reduces oxygen availability in one of the most
metabolically demanding organs (see MacRae et al., 1997) in the body resulting in cell
damage.
 Death of cells lining the gut stimulates an immune response and in addition the barrier
functions of the gut wall become more porous allowing uptake of endotoxins which
also results in further stimulation of immune function.
 Endotoxin stimulates the production of nitrous oxide that initiates events that cause
resumption of blood flow away from the periphery to the splanchnic bed (known as
reperfusion).
 In extreme heat stress, vasodilation of peripheral blood vessels followed by reperfusion
of blood vessel feeding the splanchnic bed results in marked drop in blood pressure,
increased heart rate and collapse and death.
Table 1. THI values, as an index of potential climatic conditions leading to heat stress. THI
were calculated from dry (Td oC ) and wet bulb temperature (Tw oC) using the following
equation THI = Td +O.36Tw+41.2( after Johnson, 1991).
Country
Months
Average
Index of heat
THI
THI in hot
severity
>72(a)
months(b)
(axb)
United Kingdom (London)
0
Thailand (Bangkok)
12
75.7
908
Malaysia (Kula Lumpur)
12
78.7
944
Bangladesh (Dhaka)
10
75.8
758
South Vietnam (Can Tho)
12
79
948
In sub clinical heat stress, the body responds to damage to critical protein molecules by
producing heat shock proteins(HSPS) that stabilise protein structure and removes the effects of
denaturation and reduce the effects of inflammatory cytokines (see Cronje, 2005). The
requirements for essential amino acids are thus raised in heat stressed animals by the immune
system demand including the extra need for heat shock protein synthesis and repair of damaged
epithelial cells in the portal drained viscera.
Immune stimulation in stressed animals
The proposition that stress impacts on neuroendocrine, autonomic and central nervous system
and may influence immune functioning has now become widely accepted (Anisman, 2002;
Colditz, 2004). Stimulation of immune response by management practices that cause
discomfiture to the animal appears to be a real possibility and there is at least one paper
supporting this where calves on wet slippery floors were much less productive than more
comfortable housed animals. In response to acute stressors, numerous biological processes are
activated. Hypothalamic pituitary adrenal activity is increased with resultant increases in amine
turnover which if protracted leads to increased requirements for specific nutrients including
essential amino acids (Elasser et al., 2000).Little research is available on the effects of stresses
on nutrient requirements but it appears to be a fact of life that the outcomes from feeding
animals can be highly variable, depending on management. It is hypothesised here that the link
is potentially through the immune response activation where the animal is discomforted by any
stressors. This is supported by the recent publication of Colditz (2004).
Conclusions
New insights into the maintenance of homeostasis in animals throw light on the potential for
environmental and managerial factors impinging on the nutrient requirements of livestock. The
major control mechanism is possibly through the immune system and the production and
release of pro-inflammatory cytokines that signal changes in the animal by orchestrating the
provision of essential amino acids and other compounds (e.g. glutamine) for acute phase
protein synthesis, lymphocyte proliferation and synthesis of defence molecules including heat
shock proteins. The overall essential amino acid and glutamine requirements of the immune
system appear to be high. In addition with many of the stresses that cause immune stimulation
there is an associated tissue damage that needs to be repaired. Abnormal cell death (lytic death
as opposed to normal cell death and turnover (known as apoptosis) may also provide the signal
to the immune system to respond. Where cell damage occurs in the gut, breakdown of the gut
barriers may also occur and this may be associated with uptake of antigens such as
lipopolysaccharides from Gram negative microbes which also invoke an immune response.
Thus in adverse climatic conditions or where toxic elements are contained in the feed or
disease and parasitism are endemic, the MP requirements of ruminants may be substantially
increased to meet the competing needs of production and protection of the animal. Houdijk et
al., (2001) concluded that MP requirements for the expression of immunity to a wide range of
infections may all be of the same order of magnitude, despite the different underlying immune
mechanisms. If stress is also considered to induce immune function to a similar extent as
disease and parasitism, this then indicates that in the more stressful tropical environments, that
it is likely that standards for MP requirements may be considerably higher then those predicted
for ruminants in temperate countries. The highly stressful conditions under which ruminants
are held in the tropics have major repercussions on their life-time production. It is well known
that periods of essential amino acid deficiencies in early life reduce the potential production
when the animal is mature (see Preston and Leng, 1986). It has been shown that in growing
sheep, protein supplementation enhances the expression of immunity, resulting in decreased
parasite eggs in faeces and increased nematode expulsion (see Houdijk et al., 2001). Studies in
the tropics have shown that tannin-protected proteins also have a similar effect on faecal egg
count in goats (Thi Mui Nguyen et al., 2005). This may be more aligned with the bypass
protein content of the supplement then the effects of tannins on parasites per se
MP availability to ruminants is set by the nutrient balances in the feed. The lowest
availability will occur on low digestibility roughages, particularly those deficient in essential
nutrients required by the rumen micro-biota. Under well-managed conditions, increasing MP
on such diets, via correction of any rumen nutrient deficiencies and supplementation with
bypass protein is effective in increasing the growth of young animals. The level of response
will depend on the partitioning of the absorbed amino acids between growth and the
requirements for protein synthesis during immune stimulation. The animal’s response to
bypass protein will depend on the level of stress incurred. It is also proposed here that under
the prevailing climatic factors associated with the tropics, those temperate country standards
for the requirements of MP and ME may often be misleading. The optimum requirements for
MP may be increased by multiple factors including feed deficiencies, management stressors,
presence of toxic materials in feed, enduring or periodic heat stress and the incidence of
disease and parasitism. Energy requirements of ruminants in the tropics for maintenance of
body temperature are much lower (or absent) than for similar animals in cool environments,
suggesting they need a higher ratio of essential amino acids to volatile fatty acids in the ME.
Increased requirements for amino acids for immune functions could further increase the overall
requirement for MP relative to energy. This may require the animal to uncouple ATP
production from mitochondrial respiration thereby dissipating energy as heat (see Ketelaars
and Tolkamp, 1996). This would add to the heat load and detrimental effects on ruminants
already precariously balanced for control of body temperature.
Research is needed to elucidate the metabolic costs of these stressors on the
requirements for essential amino acids in growth and lactation under tropical conditions.
Stress imposed by climate and management practices in the tropics may be a primary factor
that leads to low efficiencies within the livestock industries. This can be researched initially by
comparison of MP requirements for young growing animals on roughage diets under worst
case scenario production systems and best practice management.
Bypass protein meals are scarce and relatively expensive in most countries in the tropics.
Recent research have shown that hay made from cassava foliage has a high content of bypass
protein presumably because of its tannin-content (3-4 mg/kg DM) (Ffoulkes and Preston, 1977;
Wanapat et al., 1986). The potentially very high yields per hectare [see Wanapat et al., 2003)
of cassava foliage suggests that this may be a practical and valuable source of bypass protein
for ruminant production in SE Asia in the future. Response curves to feeding cassava foliage to
cattle fed local forages and crop residues need to be established and a market created for the
sale of foliage meals.
References
Alsemgeest, S.P.M., I.E. Lambooy, H.K. Wierenga, S.J. Dieleman, B. Meerkerk, A.M.Ederen,
T.A. Niewold and E.D. Van, 1995. Influence of physical stress on the plasma concentrations
of amyloid-A (SAA) and hepatoglobulin (HP) in calves. Vet.Quaterly 17: 9-12.
Anisman, H., 2002. Stress, immunity, cytokines and depression. Acta, Neurop. 14(6), 251-261.
Bird, S. and R.A. Leng, 1978. The effects of defaunation of the rumen on the growth of cattle
on low protein high-energy diets. Br. J. Nutr. 40, 163-167.
Blaxter, K.L.,1962. The energy metabolism of ruminants. HUTCHINSON . London UK.pp194
Butkow, N., D. Mitchell, H.Laburn and E. Kenedi, 1984. Heat stroke and endotoxaemia in
rabbits . In: Thermal Physiology, . pp 511–514. J.R.S. Hales, RAVEN PRESS, New York.
Chenost, M. and C. Kayouli, 1997. Roughage utilisation in warm climates. FAO Animal
Production and Health Paper 135, FAO, Rome. pp226.
Colditz, I.G., 2002. Effects of the immune system on metabolism: implications for production
and disease resistance in livestock. Live. Prod. Res. 75, 257-268.
Colditz,I.G., 2004. Some mechanisms regulating nutrient utilisation in livestock during
immune activation:an over view. Aust.J.exptl.Agric. 44(5), 453-457.
Coleman, G.S.1975., Interrelationships between rumen ciliate protozoa and bacteria. In:
Digestion and Metabolism in the Ruminant, I.W. McDonald and A.C.I.Warner,
UNIVERSITY OF NEW ENGLAND. Armidale Australia, pp149-164
Creek, M. J., T.J.Barker and W.A. Hargus,1983. An evaluation of the use of anhydrous
ammonia to treat rice straw. UNDP/FAO Beef Industry Development Project.
EGY/82/007.Field Document No.8 FAO Rome.
Cronje, P.B., 2005. Heat stress in livestock-the role of the gut in its aetiology and a potential
role for betaine in its alleviation In ‘Recent Advances in Animal Nutrition in Australia’
UNIVERSITY OF NEW ENGLAND. Armidale Australia (in press).
Denker,B.M. and S.K. Nigam, 1998. Molecular structure and assembly of tight junction. Am.J.
Physiol. 274, F1-F9.
Donalson, J., M. van Houtert and A.Sykes, 1997. The effect of protein supply on the
periparturient parasite status of the mature ewe. Proc.N.Z.Soc.Anim.Prod. 57, 186-189.
Dolberg, F. and P. Finlayson, 1995. Treated straw for beef production in China. World Anim.
Rev. 82, 14-24.
Elasser, T.H., KC Klasing, N Flipov and F Thompson, 2000. The Metabolic consequences of
stress: Targets for stress and priorities of nutrient use. In ‘The Biology of Animal Stress’, G P
Moberg and J A Mench, pp77-110. CAB INTERNATIONAL.Wallingford.
Elliott, R. C.and M.W.O’Donovan, 1971. In ‘Report of the Henderson Research Station.,1971.
Harare Zimbabwe.
Ffoulkes, D.,1984.Utilisation of diets based on molasses for feeding ruminants for survival.
PhD thesis, University of New England, Armidale, NSW, Australia, Division of Animal
Science.
Ffoulkes, D. and T.R. Preston, 1977. Cassava or sweet potato forage as combined sources of
protein and roughage in molasses based diets: Effect of supplementation with soybean meal.
Trop.Anim.Prod. 3,186-192.
Fenn, P. and R.A. Leng, 1989. Wool growth and sulphur amino acid entry rate in sheep fed
roughage based diets supplemented with bentonite and sulphur amino acids. Aust. J. Agric.
Res. 40, 889-896.
Finlayson, P. Zhang Weixian, Chuan Xue, and F. Dolberg, 1994. Economic aspects of utilising
fibrous crop resdidues for beef production in China. LRRD 6(3)
http://www.cipav.org.co/lrrd/lrrd6/3/4.htm
Frisch, J.E. and J.E. Vercoe,1979. Adaptive and productive features of cattle growth in the
tropics; their relevance to buffalo production. Trop.Anim.Prod. 4, 214-228
Gisolfi, C.V. and F. Mora, 2000. The hot brain: survival, temperature, and the human body.
THE MIT PRESS, London UK. pp286
Grimble,R.F.,2001. Nutritional modulation of immune function. Proc. Nut.Soc. 60, 389-397
Hahn, G.L., 1999. Dynamic responses of cattle to thermal heat loads. J.Anim.Sci (Supplement
2) 77,10-20.
Houdijk, J.G.M., N.S. Jessop, and I. Kyriazakis, 2001. Nutrient partitioning between
reproductive and immune functions in animals. Proc.Nut.Soc. 60, 515-525.
Husband, A.J.,1995. The immune system and integrated homeostasis. Immune Cell Biol. 73,
377-382.
IAEA, 1991. Proceedings of international Symposium on Nuclear Related Technologies in
Animal Production and Health . IAEA Vienna 1991.
Ivan, M, de S. Dayrell, S. Mahadevan and M. Hidiroglou,1992. Effects of bentonite on wool
growth and nitrogen metabolism in fauna free and faunated sheep. J.Anim.Sci 70, 3194-3202.
Jessen, C., 2001. Temperature regulation in humans and other mammals. SPRINGERVERLAG New York.pp193.
Johnson, R.W., 1998. Immune and endocrine regulation of food intake in sick animals.
Domestic Animal Endocrin. 15(5) 309-319
Johnson, H.D., H.H. Kibler, A.C,Ragsdale, I.L. Bewry and M.D. Shanklin, 1961. Role of heat
tolerance and production level in response of lactating Holsteins to various temperature –
humidity conditions. J.Dairy Sci. 44, 1191-1198.
Johnson, H.D., 1991. The lactating cow in the various ecosystems: environmental effects on its
production . In Feeding dairy cows in the tropics. FAO Animal Production and Health Paper
86, A Speedy and R Sansoucy pp 9-21
Ketelaars, J.J. and B.J. Tolkamp, 1996. Oxygen efficiency and the control of energy flow in
animals and humans J.Anim.Sci 74, 3036-3051.
Klasing, K.C.,1998. Nutritional aspects of leucocytic cytokines. J.Nutr. 118, 1436-1446.
Knox, M.R., J.W. Steel, C.A. Anderson and L.L. Muir (editors ).2003 Nutrition –Parasite
Interactions in sheep Aust.J.exptl.Agric. 43(12), 1383 -1488.
Leng, R.A., 1984. The potential of solidified molasses-based blocks for the correction of multinutritional deficiencies in buffaloes and other ruminants fed low-quality agro-industrial by
products. In The Use of Nuclear Techniques to Improve Domestic Buffalo Production in Asia.
IAEA Vienna 1984 STI/PUB/684
Leng, R.A.,1990. Factors effecting the utilisation of “poor quality” forages by ruminants
particularly under tropical conditions. Nut.Res.Rev. 3, 277-303.
Leng, R.A., 2003. Drought and dry season feeding strategies for cattle sheep and goats.
PENAMBUL BOOKS, Coolum Beach, Qld, 4573 Australia pp 271.
Leng, R.A.,2004. Requirements for protein meals for ruminant meat production in developing
countries. In Protein sources for the animal industries. FAO Animal Production and Health
Proceedings 1, FAO Rome pp 225-254
Leng, R.A., 2005. Production Research Priorities- Endophyte Ill thrift. In Perennial Ryegrass
Toxicosis in Australia. K.F.M. Reed, S.W. Page and I.J. Lean) MEAT AND LIVESTOCK
AUSTRALIA LIMITED, North Sydney, Australia, 2005 [in press]. pp200
Litherland, A.J., D.L. Layton, C.J. Boom, T.L. Knight, M. Hyslop, M.G. Lambert and T.L.
Cook, 2004. Ill thrift in young growing cattle and sheep Proc.N.Z.Soc.Anim.Prod. 64, 197202.
Lipperheide, C., M. Rabe, S. Knura, and B. Petersen,2002. Effects of farm hygiene on blood
chemical variables in fattening pigs. Tierartzliche Umschau 55, 30-36.
Lobley, GE, S.O. Hoskin and C.J. McNeil, 2001.Glutamine in animal science and production.
J.Nutr.. 131, 2525S-2531S.
Mangan,J.L.,1972. Quantitative studies on nitrogen metabolism in the bovine rumen. The rate
of proteolysis of casein and ovalbumin and the release and metabolism of free amino acids. Br.
J. Nutr. 27, 261-266
Matzinger, P., 2005. The real function of the immune system or tolerance and the four D’s
(danger, death, destruction and distress). http://cmmg.biosci.wayne.edu/asg/polly.html sighted
9/03/2005.
Maust, T.L., R.E. McDowell and N.W. Hoven,1972. Effect of summer weather on
performance of Holstein cows in three stages of lactation. J.Dairy Sci. 55, 1133-1139.
MacRae, J.C., L.A. Bruce, D.S. Brown and A.G. Calder, 1997. Amino acid use by the
gastrointestinal tract of sheep given lucerne forage. Am. J. Physiol. 273, G1200-G1207.
McWhirter, J.P. and C.R. Pennington,1994. Incidence and recognition of malnutrition in
hospital . Br. Med. J. 308, 945-948.
Mom Seng, T.R.Preston , R.A.Leng and U ter Meulen, 2001.Effect of a single drench of
cooking oil on the rumen ecosystem and performance of young local yellow cattle fed rice
straw and cassava foliage. LRRD 13(4) http://www.cipav.org.co/lrrd13/4/seng134.htm
Moseley, P., 1997. Heat shock proteins and heat adaptation of the whole organism.
J.Appl.Physiol. 83, 1413-1417.
Nguyen Thi Hong Nhan, Nguyen Van Hon, N.T. Ngu ,T.R. Preston and R.A. Leng,
2001.Practical application of defaunation of cattle on farms in Vietnam: Response of young
cattle fed rice straw and grass to a single drench of ground nut oil. Asian-Aust. J. Anim. Sc.
14(4), 485.
Oliver, J.W., 2005. Pathophysiologic response to endophyte toxins. In Neotyphodium in Cool
Season Grasses. C.A., Roberts, C.P. West and D.E. Spiers) BLACKWELL PUBLISHING
pp291-304.
Perdok, H.B.,1987. Ammoniated rice straw as a feed for growing cattle. PhD Thesis,
University of New England, Armidale, Australia
Perdok, H.B.and R.A. Leng,1990. Effect of supplementation with protein meal on the growth
of cattle given a basal diet of untreated or ammoniated rice straw. Asian –Aust. J.Anim. Prod.
3, 269-279.
Poppi, D.P.and S. J. McLennan,1995. Protein and energy utilisation by ruminants at pasture.
J.Anim.Sci 73, 278-290.
Preston, T. R.,1972. Molasses as a feed for cattle. In World Review of Nutrition and Dietetics
17,KARGER, Basle, Switzerland pp280-311
Preston, T.R. and R.A. Leng, 1986. Matching Livestock Systems to Available Resources in the
Tropics and Sub Tropics, PENAMBUL BOOKS ,Armidale, Australia. pp245.
Rowe, J. B., J.V.Nolan, G. de Chaneet, E.Teleni, and P. Holmes, 1988. The effect of
haemonchosis and blood loss into the abomasums on digestion in sheep. Br..J. Nutr. 59,125139.
.
Saadulah, M., 1984., Studies on the utilisation of rice straw by cattle. PhD Thesis, Royal
Veterinary University. Copenhagen, Denmark.
Stear, M.J., S.C. Bishop, N.G. Henderson and I. Scott, 2003. A key mechanism of
pathogenesis in sheep infected with the nematode Teladorsagia circumcincta Animal Health
Res. Rev. 4(1), 45-52.
Steel,J.W., W.O. Jones and L.E. Symons, 1982. Effects of a concurrent infection of
Trichostrongylus colubriformis on the productivity and physiology and metabolic response of
lambs infected with Ostertagia circumcincta .Aust.J Agric.Res. 33, 131-140.
Sykes, A.R. and A.W. Greer, 2003. Effects of parasitism on the nutrient economy of sheep :an
overview. A. J. Expt. Agric. 43(12), 1393-1398.
Thi Miu Nguyen, Dinh Van Binh and E.R. Orskov, 2005. Effect of foliages containing
condensed tannins and on gastrointestinal parasites. Anim.Feed Sci. Techn. 121(1-2), 77-87.
Ushida K., J. P. Jouuany, B. Lassalas and P. Thivend, 1984. Protozoal contribution to nitrogen
digestion in sheep. Can. J.Anim.Sci. 64, (suppl.),20-21.
Van Houtert, M.F.J., I.A. Barger, J.W. Steel, R.G. Windon and D.L. Emery,1995. Effects of
dietary protein on responses of young sheep to infection with Trichostrongulus colubriformis .
Vet. Parasitol. 56, 163-180.
Veira, D.M., M.Ivan and P.Y. Jui, 1983. Rumen ciliate protozoa: Effects on digestion in the
stomach of sheep. J Dairy Sci. 66, 1015-1022.
Wanapat, M., 2003. Manipulation of cassava cultivation and utilization to improve protein to
energy biomass for livestock feeding in the tropics. Asian-Austral J. Anim.Sc. 16(3): 463-472.
Wannapat, M., S. Duangchan, S. Pongpairote, T. Anakewit and P. Tongpanung ,1986.Effects
of various levels of concentrates fed with urea-treated rice straw for pure bred American
Brahman yearling cattle. In “Ruminant Feeding Systems Utilizing Fibrous Agricultural
Residues, R M Dixon IDP. Canberra. Pages 149-157.
Wanapat, M., A. Petlem and O. Pimpa, 2000. Supplementation of cassava hay to replace
concentrate use in lactating Holstein Friesian crossbreds. Asian-Aust. J. Anim Sc. 13(5):600604.
Williams,N.H., T.S.Stahly and D.R. Zimmerman, 1997. Effect of chronic immune system
activation on body nitrogen retention, partial efficiency of lysine utilization and lysine needs of
pigs. J.Anim.Sci 75, 2472-2480.
Zhang Weixian,G.U. Chuan Xue, Frands Dolberg, and Peter M Finlayson, 1994.
Supplementation of ammoniated wheat straw with hulled cottonseed cake. LRRD 6(1)
http://www.cipav.org.co/lrrd6/1/chinal.htm/chuan1.htm sighted 1-6-05.