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Suslainable Forage-based Livesiock Systems in fhe Tropics
Chapter 5
Animal nutrition,production and reproduction
Nutritional needs are for energy, nitrogen (protein),essential minerals and vitamins which are met by theintake of various combinations of pastures, foragecrops, conserved forages (hay and silage) andconcentrates (grains and processed meals).
The feeding obiective is to supply the level andbalance of nutrients required to meet theproduction, reproductive and product quality targetsof the livestock enterprise as efficiently andeconomically as possible.
PrinciplesFeed intake, or the diet selected (amount andquality), is the key nutritional variable drivinganimal production and reproduction.
Feed ingested by ruminants must provide all thenutrients essential for fermentative digestion in therwnen, as well as providing the undegraded proteinsand starch which pass through (by-pass) the rumen tothe small intestine (Figure 3. 1).
Figure. 3. 1. Digestion of a basic diet supplementedwith by-pass nutrients (Preston L984).
DIGESTION
ABSORPTION
PRODUCTION
The intake of fermentable material in forage isenhanced by an increase, within limits, of the amountof escape or by-pass protein in the diet, and isdecreased when low quality forage (roughage)dominates the diet.
The efficiency of feed conversion increases, againwithin limits, as the amount of by-pass starchincreases.
Nutrients are required for 'maintenance'functions (thefactory overheads) and for'production' (thecommodities produced).
Energy intake required for resting or maintenancefunctions is proportional to metabolic body weight,which is a function of liveweight (LW) and expressedas LW o'7s ; grazing animals on average may require 30percent more maintentmce energy for walking thanstall-fed animals (Kleiber 1961).
Energy, protein and mineral requirements forproduction are additional to those required formaintenance; production is only possible aftermaintenance requirements are met.
Type, breed or genotype, class, size, physiologicalstate (eg. growing, gestating or lactating) and age ofthe animal all influence nutritional requirements for'maintenance' and'production' (Figure 3. 2).
( brte&, potozoa, fungi )Microbid protein
Page 27
Sustainable Forage-bos.d Liveslock Syslems in fhe Tropics
+Age
)
Figure. 3. 2. Generalised relationships betweenmaintenance energy needs and the liveweight, ageand pregnancy status of the animal (Morley L978).
Liveweight
_>Pregnancy
Liveweight gain depends mainly on theinterrelationship between energy and protein (aminoacids) delivered to the body tissues from the rumenand the small intestine up to the genetic limit forprotein synthesis; the limit is probably never reachedfor grazing animals.
>rboHn)8A0)lolsll(It
n)
d
t
+
A process approach toproduction from animalsat pastureThe process is illustrated by a simple model(Figure 3. 3). The model incorporates the primaryvariables of environment, plant growth, the amountand quality of pasture on offer, the diet selectionprocess which determines the amount and quality ofthe diet, and the animal metabolic processes whichdetermine the energy and nitrogen (N) balance of theanimal and its production.
Figure. 3. 3. A model relating pasture growth, dietselection, animal metabolism and animal production.The square boxes represent state variables, and theothers are rate variables.
Environnent
Pasture swardt.
spegles
&f-:PE=lcsf
Diet
Animalproduct
Page 28
Suslainable Forag.-bosed Liveslock Syslems in lhe Tropics
Feed intakeIn the paddock, many pasfure, animal and climaticvariables interact to modify the amount and quality ofintake. Important ones are the:
total amount of forage on offer, expressed as
total kg dry matter (DM) per ha, or the forageallowance available, expressed as kg of DM ororganic matter (OM) per animal or per unit ofanimal LW
feed quality or total dry matter digestibility ofthe pasture, expressed in terms of theproportion of the total DM or OM that theanimal can digest (% dry matter digestibility(DMD)) or 7o organic matter digestibility(oMD)
grazing time (hours per day)
amount and quality of feed supplements
harmful plant substances which depressintake
animal liveweight, age and physiologicalstate.
Pasture on offer or forage allowance
The amount of forage available to each animaldetermines the ease with which the animal can obtainthe highest quality and most digestible diet possiblefrom the feed on offer. Intake is maximised when theforage allowance exceeds about 50 g of DM per kg LWper day (Figure 3. 4). Under light stocking rate thisequates to a green pasture yield above about 1000 and2500 kg DM per ha for temperate and tropicalpasfures respectively.
Figure 3. 4. Forage allowance and its effect on intakeby calves, beef cattle and dairy cows (Emst et a1.1980).
Forage quality and digestibility
Digestibility is determined by the energy and proteincontent of the forage.
These are related to the ratio of plant cell componentsin the form of 'cell contents'(high energy and protein)and cell walls (fibre and silica). The less digestible cellwall component increases as plants age (Figures 3. 5and 3. 6).
Figure. 3.5. Changes in the chemical composition ofgrasses as they age (Osboume L980).
Figure 3. 6. Digestibility of temperate and tropicalgrasses, and of grain and crop by-products in thetropics (McDowell 1985).
Gro:sr Croin brprcductrMoircCotlonscd mcolSorghunCitru pulpPrcnul rrccninorCollmsd coltiWhcol bton
A{olorrgRko bron
Moiac lloYcr
Whel:towRica rtaow
Sugorconc lopr
Sugorcone bcgorcPconut hullr
Ricc hullrCollcc hullr
Cdl{0 conhnk
300
850Eo
3.0
TGmp.?ot
,ar^fl.,J-
-'l
H'*"t[-lE
=JCDg
=ocD
ll'jGt
5ocD6or
30 50
Forage Allowance (g
70
OM/kg Lw/d)
Page 29
Sustainable Foroge-bos.d Livestock Sysfems in the Tropics
Tropical forage plants have overall higher crude fibreand silica contents than temperate species andtherefore have lower digestibility. Mature grasses are
generally less digestible than mature legumes, anddifferences between tropical and temperate legumesare less than those between tropical and temperategrasses (Figures 3. 6 and 3.7).
Figure 3. 7. Frequency distributions showingdifferences in digestibility between tropical C-) andtemperate (-) grasses and legumes (Wilson andMinson 1980).
Dry matter digestibilitY (%)
Intake of forage is highly correlated with thedigestibility of the diet selected (Figure 3. 8).
Figure 3.8. Relationship between intake of tropicalforage and digestibility of the diet selected (Minson1ee0).
Digestibility ot Organic Matter
The lower digestibility of tropical compared withtemperate forages results in lower rates of intakes byanimals grazrng tropical forages (Figure 3. 9).
Figure 3. 9. Relation between meanvoluntary dailyintake (g per unit of metabolic weight) and dry matterdigestibility for a wide range of tropical (-) andtemperate (--) grasses (Minson 1980).
50 54 58 62 66 70 74
Dry matter digeslibility (%)
Overall digestibility of green pasture in the paddockdecreases as the amount of the green pasture on offerincreases, although the digestibility of the dietincreases due to the greater opportunity for dietselection (Figure 3. 10).
Figure 3. 10. Digestibility of green pasture on offerand the diet of sheep in relation to the amount ofavailable green pasture (Hamilton et a\.1973).
r500 2500
Green herbage available (kg/ha)
Precision in selecting a diet differs between animaltypes due to bite differences; for example, the 'sweepof the tongue'by cattle compared with'nibbling'bysheep; some production benefits may be achieved bygrazingcattle and sheep together as they selectdifferent, though overlapping, ranges in diet.
o3rr 60o)
o
iu..g
650c6)ls
x
-o
l,o!o
c
3io=oCD!
ot6E8oc!6olt
500
LEGUMESGRASSES
II
IIIII
Page 30
Suslainable Forage-based Livestock Systems in fhe Tropics
(!]JgE
oEi:CDcN(!
a
.=EooJ6c
o(t(lolJ-
o(,Gcc
=r28o!!ol
o24I6
sizoaE
ct;t16
High moisture content of forage lowers intake(Figures 3.1.1), but energy and protein supplementsenhance it. Legumes generally provide forageprotein at higher levels than grasses (except when thegrasses are young and leafy), and therefore generallyimprove intake.
Figure 3. 11. Effect of dry matter content of sorghumsilage on voluntary intake by lactating cows (-) andbeef steers (--) (Minson 1990).
300 400
Sitage DM (s/kg)
Grazing time
Animals have to graze for longer periods to achievedaily feed intake requirements when the amount offorage on offer and intakes are low (Figures 3. 12 and3. 13).
Figure 3. 12. Relationship between graztngtime frommoming to aftemoon milking and pasture yield onoffer in winter in south-east Queensland (Cowan andO'Grady 1976).
Figure 3. 13. Relationships between graztngtime bysheep, rate of for4ge intake, and forage yield (Alldenand Whittaker 1970).
1000 2000 3000 4000
Forage yietd (kg DM/ha)
Heat stress from daily maximum temperafures above250C decreases grazing time of dairy cattle betweenmoming and aftemoon milking, a constraint prevalentfor a substantial period between 5-6 months of theyear in tropical dairying regions of northem Australia(Figure 3. 14).
Figure 3. 14. Association between the proportion ofdaily grazing occurring between moming andafternoon milking and daily maximum temperature(Cowan et al.1993).
20 25 30
Oaily maximum temperature (oCl
qg€ ,ooild'Fl
(5
oc!^=@cc
EOo;9_-c'i;E
.;E'ng6o
Pasture yicfd (kg DM/ha)
Page 31
9us*ainable Foroge-based Livesfock Syslems in lhe Ttopics
Heat stress is also a factor for beef cattle and sheeP
grazhgin the hot and arid inland regions.Besides the effect on grazing behaviour, extreme lowand high temperatures can directly cause lamb deaththrough exposure, especially in open grasslands.
Harmful substances
Some pasture plants and weeds may containsubstances that suppress intake; these substances fallinto three categories:r inorganic compounds and minerals eg. nitrate,
copper and selenium. orgtmic compounds eg. various tannins,
alkaloids (mimosine), alicyclics (saponins) andglycosides
. fungal/microbialtoxins(mycotoxins).
Hungry or stressed animals can die when gorging ona range of plant species that are either intrinsicallytoxic or are toxic at certain times under certainconditions.
Feed supplements
Energy, protein and mineral supplements in the formof silage, hay, licks, inorganic minerals, grain or mealconcentrates may enhance or decrease (substitute forforage) forage intake, depending on the quality of thebase diet (Figures 3. 15 and 3. 15).
Figure 3. 1"5. Voluntary intake of signal grass with (-)and without (--) a protein supplement of 4 g/kgLW 0'75 soybean meal (Minson 1990).
substitutaon
forage+
supplement
enhancement
forage only
'10 15
Crude protein in forage (%)
Figure 3. L6. Effect of concentrate supplement onintakes of high, medium and low quality forages(Braster 1983).
Forage intake Total intake
\
0s1005100s10concentrate suPplied (g OM/kg LW)
FORAGE QUALITY
- High -.-'- Medium Low
Total intake and digestible organic matter intake isnormally enhanced by supplementation (Figure 3.1'6),
at least until deficiencies of dietary components are
overcome. Further increments in the level ofsupplementationbeyond this point leads to anaccelerating substitution of the base diet by thesupplement (Figure 3. 1.7).
Figure 3. 17. Silage intake in relation to concentrateallowance (after @stergard L980).
ot=oogt
=J_.9
=o€t,
=Jg1s=oo10
^70n\oCDvct)
-9 oo(6
.=Fa!c6>50
'' r: r: :: a: :: :: :::
:: :: ! a,1..:.::::::!a'.. t::::!,
:':::::::J
'
Digestible organic
'tfgt*
..f
Page 32
Suslainable Forage-based Livestock Systems in lhe. Tropics
Animal production andproduction efficiencyThe approximate nature of relationships betweendaily animal production and feed intake, feed qualityand animal physiological status are surunarised inFigure 3. 18.
Figure 3. 18. Generalised responses in daily liveweight gain, milk production and wool growth to some feedand animal physiological variables (Morley 1978).
LIVEWEIGHTGAINS(ks/ksLIVEWEIGHT/OAY)
MILKPRODUCTION(kg/DAY)
wooLGROWTH(s/DAY)
Production is determined mainly by intake ofdigestible dry matter and crude protein providedminerals are not limiting and there are no harmfulsubstances in the feed.
Production efficiency is related to animal age and theassociated changes in the composition of weight gain,the level of production, the extent to which the diet ismetabolised in the body (eg. metabolisable energy(ME) component), rumen volatile fatty acid (VFA)pattem, and mineral interactions:
. production efficiency falls as the compositionof weight gain is increasingly dominated by fatdeposition as animals gain in body weight withage, and as the proportion of nutrient intakeused for protein synthesis falls (Figure 3. 19)
Figure 3. 19. Liveweight and the composition oIweight gain in cattle (Minson 1990).
100 200 300 400
Empty Liveweight (kg)
o!
ECD
.g6oEIo
=oc.9=6oo,Eoo
INTAKE DIGESTIBILITY FATNESS
STAGE IACTATIONlNTAKE DIGESTIBILITY
.X, PROTEININTAKE DIGESTIBILITY
Pog. 33
Sustainable Forog.-bos.d Liyeslock Syslems in lhe Tropics
r utilisation of energy in the diet for productionbecomes more efficient as the metabolisableenergy status of the diet increases; it is alsoinfluenced by rumen VFA composition (Figures3.20,3.21 and 3.22).
Figure 3. 20. Effect of maize grain (high ME) in a haydiet on the efficiency of metabolisable energy use formaintenance and fatteni.g i. cattle (Minson 1990).
o.2 0.4 0.6 0.4
Proportion of Maize in Diet
Figure 3. 21. Relationship between metabolisableenergy intake and energy retention in sheep fedspring or autumn herbage (Corbett ef a\.1966).
Figure 3. 22. Relationship between the proportion ofproprionic acid in the rumen VFAs and the efficiencywith which metabolisable energy consumed abovemaintenance is used for tissue synthesis or kf (%)(Preston and Leng 1987).
ca
6n
&
20
20 3() .lO 50 60* P.ogidt. ir YFA
r utilisation of protein in the diet forproduction becomes more efficient as theprotein concentration of the diet increases andas the proportion of by-pass protein increases(Figure 3.23)
Figure 3. 23. Relation between non-ammonia crudeprotein absorbed in the small intestine and crudeprotein concentration in dried and fresh forage(Minson 1990).
120 1c0 ?00
Crudr Protrin ln Forr0r (g/l!)
tropical grass and legume forages provide lessby-pass protein than temperate forages andanimals require energy and/or proteinsupplementation for high levels of production(Figure 3. 24)
ooo
Slartc.
g
-oo
al al-
uJ o.7
ocs 0.6d
:: o.5o
oEo3 0.4
ul
o.3
!'loaIc-i t00ao;.L? ooo6
Sustainable Forage-based Live.stock Syslems in the Tropics
Thoglcal8trt8
ol{6l
c
oa0tfiodcoa)
trq0
Figure 3. 24. Predicted relationship between intestinalcrude protein supply and crude protein content ofcondensed tannin-free forages consumed in the freshstate. Levels of dry matter digestibility (% DMD) andthe likely range in values for tropical and temperategrasses and legumes are indicated (Poppi andMclennan 1994).
. Tcmporrta lrars| & lorumo 80% DMD
150
r Tloplcal
I lcrunc
5O!T DMD
TbmDonto lcgumo
TloDlcrl loSuno
Tloplcal Srass TbnD€rrt! traal0Lo 50 100 150 200 250 300 350
s CPlkg DM
dietary crude protein requirements for weightgain are influenced by the level of rumendegradability of the protein source; high andefficient daily liveweight gain is difficult toachieve when protein degradability is greaterthan about 707o.
Animal production innorthern Australia:an introductionBeef production
Cycles of liveweight gain (LWG) in the surnmer wetseason and loss in the winter dry season are typical forbeef cattle fed mainly from pastures; compensatoryweight gain early in the wet season also typicallyoccurs after significant weight loss in the dry season(Figure 3. 25).
Figure 3.25. Typical pattern of LWG by beef steersgrazing average quality black speargrass pasture.
Liveweight gains and losses on these native pasturesare closely related to the nitrogen concentration ofpasture on offer and to the nitrogen concentration ofthe dieU a threshold of about 1.0 % N is required inthe diet for transition from weight loss to weight gain(Figure 3. 26).
600
b0l(ll.fl +oo
Bo
A
200
12345Age inyars
Page 35
Suslainable Forage-bos.d Livesfock Systems in lhe Tropics
.9E0.oEc'; o.ooLo)
U.oc,9b 0.o-o(L
Figure 3. 26. Pasture quality attributes and LWG ofsteers grazing black spear grass pastures: (a) Relationbetween the proportions of green herbage in the dietand in the pasture; (b) Relation between the nitrogencontent of green herbage and its age; (c) Relationbetween LWG and diet nitrogen (Hendrickson et aI.
1ee2).
.25 .5 .75 1.0
Proportion ol green in pasture
This explains the close relationship existing betweenLWG from native pastures and the length of thegrowing season in the seasonally, dry tropics wherethe transition between seasons and between green anddry feed is most abrupt (Figure 3.2n.
Figure 3. 27. Relationship between annual LWG andlength of growing season in the seasonally dry tropics(Gillard 1979).
t0 20 30 40
Length of grou/ing season (weeks)
Deficiencies of essential minerals can have largeimpacts on production (Figure 3.28).
Figure 3. 28. Influence of diet phosphorus on dailysteer LWG in autumn ( Mclean et a\.1990).
e 180dc)
-gbc,Eda0
il:S rzo
'0)
cl3trtr
60
be
cocoocoo)o=cCooo
(UEoS +so-cDo=J
150
3.0
2.4
1.8
1.2
0.6
2A 40 60 80 100 r20
Green age (days)
.8 1.0 1.2 1.4 t.6Diet N (%)
(UE\
=:Jo)tz(')
oo)C(U
!()!
.9)o3o)
J
20
l5
l0
5
0
-i0
Page 36
.6
Sustainable Forog.-based Livestock Systems in the Tropics
The nutritional limitations of native pastures can besummarised as energy and N deficits in the dryseason, and P and N deficits in the wet season.
Most improved tropical grass/legume pastures areunable to support the very high liveweight gainsrequired to achieve 2year turn-off of finished cattle at>300 kg, as demanded by the present premiummarkets for beef (see Chapter 5). To meet suchmarkets, energy and/or protein supplements may berequired on many of these pastures, even during theirpeak growth period.
Dairy production
Herd averages of milk production components percow per lactation in northem Australia can rangefrom 2 500 to 10 000 L milk, 70 to 320 kg protein and100 to 350 kg butterfat. However, milk yields inpractice rarely exceed 7000L.
The higher levels of milk production require highlevels of nutrition year-round; these can be achievedthrough a combination of tropical pastures (summer),irrigated temperate pastures (winter), nitrogen-fertilised tropical/temperate grasses and forage crops,and supplements (hay, silage and grain) fed over mostof the year.
The pattem of change in feed intake, liveweight andmilk yield of cows between calvings is shown inFigure 3. 29.
Figure 3. 29. Pattems of feed intake,liveweight andmilk yield over the lactation cycle (Broster L983).
Examples of the relationships between milkproduction components and some forage and feedattributes are given in Figures 3. 30 and 3. 31..
Figure 3. 30. Relation between fat corrected daily milkyield and pasture on offer for cows receiving either 0,
2, 4 or 6 kg of concentrate supplement each day(Cowan et al. t977).
Concrnrmr/cow/dev (k0)
06 .4 a2 AO
oooooo6o
234P.nur on ollI (lg GOM/h. x rmo)
Figure 3. 31. Dry matter intake and milk responses toincreasing crude protein in the diet of dairy cows(Oldham 1980).
__ DM INTAKE
- MILK YIELo
510!
'oo
;8!:oG
1",lrtt\lr -9t;l.elro tt:1,.
=
to t2
ozo crude
ta 16
protein in diet
Page 37
Sustainable Forage-based Livestock Syslems in the Tropics
Daily milk yields are depressed in summer whendaily maximum temperatures exceed 250C, probablydue to a combination of heat stress and reducedgraztng time and intake (Figures 3.14 and 3. 32).
Figure 3. 32. Association between daily milk yield ofdairy cows and daily maximum temperature duringsuruner (from data of Cowan et a\.1993).
24 26 28 30Daily maximum t€,mperature ( oC)
In summary, the major limitations to milk productionin the subtropical dairy environment of northemAustralia are the supply of energy and forage protein,and heat stress.
Wool production
Most sheep in northern Australia are extensivelymanaged for wool production in harsh, semi-arid andarid environments. Lamb finishing is limited and,where it is practised, is confined to the morefavourable eastern margins of the Queensland sheeparea, often in association with cropping lands.
Average annual greasy fleece weights produced inQueensland range from 3.8 to 4.3 kg per sheep, withclean wool growth rates being between 0.4 to L.5 mgper cm2per day. Most wool is between 20 to24micron, fine to medium wool.
Typical seasonal pattems of wool growth in north-westem Queensland on mitchell grass pastures arecharacterised by peaks of growth within the mid-suruner to late-autumn period (Figure 3. 33).
Figure 3. 33. Seasonal pattems of wool growth onmitchell grass pastures (Lorimer 198L).
0
1970 197 2 1 973
This period coincides with pasture growth fromseasonal rain. However, close relationships betweenwool growth, forage dry matter digestibility andforage protein content (see Figure 3. L8) have still to beunequivocally demonstrated in this environmen! thisis probably because nutritious ephemeral plants thatare commonly selected in the diet represent only asmall proportion of the forage on offer (Loriner L978,
1e81).
In summary, the major constraints to wool productionare the extremely variable quality of forage within andbetween seasons.
Animal reproduction
The period from late pregnancy to peak lactation isnutritionally demanding (Figure 3. 34).
Figure 3. 34. Effect of physiological state on potentialretention of nitrogen in relation to digestible organicmatter intake (Arskov L970).
Early Larc Early Peak Lrte\---rJPngnancy Lactrtion
LJJ UF4n0rr...r'. 03'o! ^rrEa5 o
uJnz-:
22
(t€\B8zoEJ
=l8
E0o6oo
Icil1'
2o
Euly Lrter-.\-
Grolth
Poge 3B
Maintcnanct
Sustainable Forage-based Livestock Syslems in t\e. Tropics
Aspects of animal reproductive performemceinfluenced by nutrition are the LWG of the dam,pregnancy rate, progeny birthweight, post-birthanoestrus interval, calf and lamb mortality, and milkproduction.
Some of the reproductive responses to nutrition areillustrated by those measured with beef cows andcalves grazed on temperate pastures (Figures 3. 35
and 3.36).
Figure 3. 35. Influence of pre-calving pastureallowance in the last 8 weeks of pregnancy on dailycow liveweight change, condition score, post-calvinganoestrus interval and caU birthweight (Nicol andNicoll1987).
Figure 3. 36. L:rfluence of post-calving (calving tojoining) pasture allowance on post-calving anoestrusinterval and pregnancy rate for low (L) and high (H)pre-calving pasture allowances (Nicol and Nicoll1e87).
2345Pasture allowance (kg DM/100 kg LWd)
Phosphorus deficiency may suppress feed intake byup to 50% and therefore can limit reproduction; lowovarian activity, conception rates, calving rates andbranding rates are known to be associated with P
deficiency.
Pregnancy and reproductive rates usually exceed 85%
with high quality feed systems but may be as low as
30% with extensively managed animals on low qualityforages.
G- 1006\
;Ee0oC
880o)IL70
3 1oooo^P9soo-qo{1'iQaoE58'- 70(L
34[Calf I ztbirthweight 311 <(ks)
I
281
90F \Poslcalving t D-----..-anoestrus I -----.-
intervat t0 I --------.=-1_
(days) l-
501
2rt
Condition I ,_--------;-score o I
t
_,[ )-
0.6 r[.
cowtivaueight | .
-
change O.2 I .-(ks/day) I /-orL /
2345Pasture allowance (kg DM/100 kg LWd)
Page. 39
Susfainable Forage-bos.d Liveslock Systems in lhe Tropics
References Ostergaard, V. (1980). In "Factors Influencing Fertilityrn the Post-partum Cow". Ed. H. Karg and E.
Broster, W.H. (1993). In "Feeding and Breeding of Shallenberger. Martinus Nijhoff:London.Dairy Cows". Proc. Sydney Unio. DairyHusbandry Foundation. Poppi,.D.P., and Mclennan, S.R. (1995). l. Anim. Sci.
Corbett, J.L. et aL (1966). Animal ProductionS,LS-27. 73'278-290'
Preston, T.R., and Leng, R.A. (1987). In "MatchingCowan, R.T., and O'Grady, P. (1976).Trop. Grassldsl0, Ruminant Production Systems with Available
21'3-21,8. Resources in the Tropics and Subtropics".Penambul Books:Armidale.
Cowan, R.T. ef al. (1977). Aust.l. Exp. Agric. Anim.Husb.17,37T379.
Cowan, R.T. ef aL (1993). Trop. Grasslds27,150-'1.6L.
Gillard, P. (1979). Aust.I. Exp. Agric. Anim. Husb.19,325.
Hamilton, B.A. et aI. (1977). Aust. l. agric. Res.24,271,-277.
Hendrickson,R. et al. (7992). Proc. Aust. Soc. Anim.Prod. 14,20L-208.
Kleiber, M. (1961). "The Fire of Life". New York:John Wiley & Sons.
Lorimer, M.S. (1978). Trop. Grasslds 12,97-108.
Lorimer, M.S. (1981). Trop. Grasslds 15,183-192.
McDowell, R.E. (1985). Proceedings RegionalWorkshop on Livestock ProductionManagement. Manila:Asian DevelopmentBank. pp. 37-66.
Mclean, R.W. et al. (1990). Trop. Grasslds.24,197-208.
Minson, D.J. (1990). In "Forages in RuminantNutrition". Academic Press:New York.
Morely, F.W. (1978). In "Measurement of GrasslandVegetation and Animal Production", Ed.L.'t Mannetje. CAB:Slough.
Nicol, A. M. and Nicoll, G. B (1987). 'Pastures for beefcattle' pp 119-132.In 'Livestock Feeding onPasture'. Pub N2 Editby A. M. Nicol. Soc.
Anim. Prod. Occass. Pub. No.2.
Oldham, J.D. (1980). In "Recent Advances in AnimalNutrition". Ed. W.A. Haresign. Butterworths:London. pp. 33-65.
Osboume, D.F. (1980). In "Grass, its Production andUtilisation", Ed. W. Holmes, Oxford:Blackwell.pp.70-124.
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