A two-pool model of tritiated water kinetics to predict body composition in unfasted lactating goats

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A two-pool model of tritiated water kinetics to predict bodycomposition in unfasted lactating goats

F. R. Dunshea, A. W. Bell, K. D. Chandler and T. E. Trigg

Animal Production / Volume 47 / Issue 03 / December 1988, pp 435 - 445DOI: 10.1017/S0003356100003585, Published online: 02 September 2010

Link to this article: http://journals.cambridge.org/abstract_S0003356100003585

How to cite this article:F. R. Dunshea, A. W. Bell, K. D. Chandler and T. E. Trigg (1988). A two-pool model of tritiated water kinetics topredict body composition in unfasted lactating goats. Animal Production, 47, pp 435-445 doi:10.1017/S0003356100003585

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Amm. Prod. I "Mix, 47: 435-445© I98H British Society of Animal Production

0003-3561/88/82730343502-00

A TWO-POOL MODEL OF TRITIATED WATER KINETICS TOPREDICT BODY COMPOSITION IN UNFASTED LACTATING GOATS

F. R. DuNSHEA'f, A. W. BELL1!, K. D. CHANDLER1 AND T. E. TRIGG2$

'School of Agriculture, La Trobe University, Bundoora, Victoria 3083, Australiaz Animal and Irrigated Pastures Research Institute, Kyabram, Victoria 3620, Australia

ABSTRACT

A two-pool model of tritiated water kinetics was investigated as a means of partitioning total body waterinto empty body water and gut water in 17 lactating goats. Empty body water, gut water and total bodywater were of a similar magnitude to, and highly correlated with, a rapidly equilibrating tritiated waterpool, a more slowly equilibrating pool and the sum of these two pools, respectively.

Empty body fat was poorly correlated with both live weight and empty body weight (R2 = 0-42 and0-51, respectively). However, there was a strong inverse relationship between the water and fat contentsof the empty body. Consequently, empty body fat was accurately predicted by a multiple regressionequation which included both empty body weight and empty body water as independent variables (R2 =0-97). Substitution of these variables with estimates derived from tritiated water kinetics still resulted ina high correlation (R2 = 0-88). Tritiated water kinetics offered little improvement over live weight alonein the prediction of empty body protein, empty body ash or fat-free empty body.

INTRODUCTION

THE close inverse relationship between bodywater and fat enables tritiated water (TOH)dilution to be used to predict total bodywater (TBW) and body composition in non-lactating, non-pregnant goats fasted for 48 h(Panaretto, 1963; Panaretto and Till, 1963).Fasting has been thought necessary todecrease the variable and sometimessubstantial contribution that gastrointestinaltract (GIT) water can make towards TBW.However, during ongoing trials on lactatinggoats fasting for such a period isunacceptable because milk production fallsdramatically after a 24 h fast, and takes anumber of days to return to pre-fasting levels(Massart-Leen and Peeters, 1985; Dahlborn,1987).

Typically, TBW has been treated as asingle pool with a slow turn-over andtherefore only one blood sample has been

Present addresses:+ Department of Animal Science, Cornell University,

Ithaca NY 14853, USA.; Peptide Technology, PO Box 444, Dee Why, NSW

2099, Australia.

thought necessary to determine the dilution ofTOH. This sample is generally taken afterabout 6 to 7 h post injection (Till andDownes, 1962; Panaretto and Till, 1963).However, equilibration of TOH is dependentupon water turn-over which is elevated duringlactation (MacFarlane and Howard, 1972),and varies with milk yield (Foot, Skedd andMcFarlane, 1979). Consequently, the use of asingle blood sample taken a fixed time afterTOH administration would be unsatisfactoryto measure TOH dilution in lactating animalsvarying widely in milk production and waterturn-over.

A number of workers have taken bloodsamples over a period of hours or days andfitted single-order exponential decay curves tothe TOH specific radioactivity (SRA) ordeuterium oxide (D2O) enrichment curves(Robelin, 1977; Chilliard, Robelin andRemond, 1984; Brown and Taylor, 1986).While this eliminates much of the variabilityin water turn-over between animals andprovides reasonably accurate estimates ofTBW, there still exists an unknown andvariable amount of water in the GIT.Estimates of GIT volume have been obtained

435

436 DUNSHEA, BELL, CHANDLER AND TRIGG

by using soluble markers such as Cr ethylenediamine tetra-acetic acid (EDTA) (Trigg andTopps, 1981) or by attempts to relate rumenor GIT contents to food intake (Cowan,Robinson, McHattie and Fraser, 1980;Chilliard et al., 1984), with variable success.Rumen emptying has been used directly tomeasure rumen volume (Chilliard et al., 1984)but this requires surgical preparation and withrepeated use, must interfere with digestion inongoing studies.

More recently, attention has been directedto using models of water kinetics whichinclude two or more water pools in efforts toseparate GIT water from empty body water(EBW) in ruminants (Byers, 1979; Ferrell andJenkins, 1984; Odwongo, Conrad andStaubus, 1984; Arnold, Hentges and Trenkle,1985; Faichney and Boston, 1985; Odwongo,Conrad, Staubus and Harrison, 1985). Thework reported in this paper is part of aseries of experiments investigating the amountof fat mobilized by' dairy goats during earlylactation. Tritiated water kinetics wereinvestigated as a means of measuring changesin body composition in well fed, lactating,goats.

MATERIAL AND METHODS

Animals and feedingSeventeen lactating Saanen goats aged 1 to

5 years and weighing 31 to 63 kg werehoused indoors in thermoneutral conditionsfor at least 2 months before parturition. Kidswere removed immediately after parturitionand the does were then machine-milked.Thereafter, goats were milked twice daily atabout 08.00 and 16.00 h. Eight of the goatswere surgically prepared with a rumencannula (2-6 cm internal diameter, StarrRubber, Australia) at least 1 month beforeparturition (Hecker, 1969). Ten goats(including four with rumen cannulae) receivedad libitum a diet of 650 g lucerne chaff, 250 goaten grain, 100 g lupins per kg and wereslaughtered at 77 days post partum. Tritiatedwater kinetics were determined in these goatsat 10, 38 and 76 days of lactation. Theremaining seven goats (four with rumencannulae) received ad libitum either lucernechaff or 600 g lucerne chaff, 400 g oaten

grain per kg diet and were slaughtered at 28to 56 days post partum. Food was offeredtwice daily except for the 4 days prior toTOH injection and until slaughter 24 h later.During this time the goats were fed every 2 hvia a belt-driven automatic feeding system.Clean water was available at all times. Atleast 24 h before TOH injection apolyethylene catheter (0-80 mm internaldiameter, 1-20 mm external diameter, DuralPlastics, Australia) was inserted 20 cm into anexternal jugular vein. Catheters were keptpatent by daily flushing with sterile saline(9 g/1) containing heparin (500 k units per 1)and were sealed with stainless steel pins.

TOH injectionTritiated water (5 Ci/1, Amersham

International, Bucks., England) was diluted insterile saline to a final activity of about30 mCi/1 and stored at 4°C. Tritiated waterwas injected immediately after the goats hadbeen milked (16.00 h) and weighed. Goatsgenerally urinated before milking. If not,urination was induced by placing a hand overthe muzzle to inhibit breathing (Corbett,Farrell, Leng, McClymont and Young, 1971).After taking a blood sample to determineresidual or background TOH SRA, a singleinjection of TOH (10 \iCilkg) followedimmediately by 20 ml sterile saline wasadministered via the jugular catheter. Totaldose of TOH was determined by differencein the weight of the disposable syringesbefore and after injection. Blood samples(4 ml) containing Na2EDTA as ananticoagulant (1 g/1) were taken via thejugular catheter at 15, 20, 25, 30, 37, 45, 55,80, 120, 180, 300, 480, 720, 1080 and 1440min post injection. Plasma TOH SRA wasdetermined by liquid scintillation counting ofduplicate 0-5 ml samples of water obtainedafter lyophilization of plasma.

Rumen CrEDTA spaceOn some occasions when TOH kinetics

were determined in the animals with rumencannulae, rumen volume was estimated usingCrEDTA as a non-radioactive tracer for thesolute phase of the rumen digesta (Downesand McDonald, 1964). Immediately after

PREDICTION OF BODY COMPOSITION IN GOATS 437

TOH injection, 50 ml of a stock solution of then ashed in a muffle furnace at 600°C forCrEDTA (68 g/1) was administered via the 12 h. Fat was determined in six x 10 grumen cannula. Rumen fluid samples (20 ml) samples of freeze-dried, minced tissue usingwere taken via the cannula at 1, 2, 3, 4, 6, the chloroform : methanol : water extraction8, 12 and 24 h thereafter. These samples method of Aitkinson, Fowler, Garton andwere strained through muslin, centrifuged at Lough (1972). As the nitrogen concentration5000 g for 10 min and the supernatant was of tissue protein is variable and difficult tostored at -15CC until analysis. Chromium determine (Bird, Flinn, Cayley and Watson,concentration was determined using atomic 1982; Benedict, 1987), tissue protein wasabsorption spectrophotometry at 425-4 nm. calculated by difference i.e. protein = DM —Standards were prepared by diluting the (fat + ash).CrEDTA stock with rumen fluid obtainedfrom an uninfused goat on a similar diet.

Calculations and statisticsPreliminary curve-peeling analyses as

Slaughter and body composition described by Shipley and Clark (1972) showedImmediately after the last blood sample, that the relationship between plasma TOH

the goats were milked and induced to SRA and time over the period 15 min tourinate. Before slaughter the goats were 24 h post injection was best described by aweighed, shorn and then reweighed. Any second-order semilog function which implieddifference in live weight (LWT) between the the presence of two pools. The model usedtime of infusion and slaughter was assumed was a two-pool open system in steady stateto have the same composition as GIT as used by Byers (1979) to describe D2Ocontents at slaughter. A lethal dose of kinetics in non-lactating cattle. In this modelsodium pentobarbitone was given water enters into GIT water (GW, pool B)intravenously and the animal was bled out. and leaves from empty body water (EBW,Blood was weighed then discarded and pool A) as either urine, milk or insensibleassumed to contain 819 g water, 1 g fat, loss. There is also a small loss from GW via8-3 g ash and 171 g protein per kg (Brown faeces. The TOH SRA data were fitted to aand Taylor, 1986). Hooves and GIT contents second-order exponential decay function usingwere removed, weighed and the latter mixed the maximum likelihood program (MLP;before a subsample was taken for dry matter Ross, 1980). Tritiated water pools were(DM) determination. The body to be calculated by dividing the TOH dose rate byanalysed, which consisted of the whole body the zero time intercepts of the decayless hair, hooves and GIT contents, was function.stored at -15°C in plastic bags until body A single-order exponential equation wascomposition was determined. The empty body used to describe the relationship betweenwas defined as this frozen tissue plus blood, rumen fluid CrEDTA concentration and time.Losses during manipulative stages were Linear regressions were applied to theassumed to be comprised entirely of water relationship between loge transformed rumenand accounted for proportionately 0-0118 (s.e. fluid chromium concentrations and time.0-001) of LWT. The frozen body was Rumen CrEDTA space was calculated byweighed, cut into small pieces with a dividing the CrEDTA dose by thebandsaw and minced through four concentration at zero time obtained byprogressively smaller endplates. The final extrapolation.endplate had 5-mm holes. After each pass All regression analyses were performedthrough the mincer the tissue was thoroughly using Minitab version 5-1 (Ryan, Joiner andmixed by hand. The DM content of the Ryan, 1985). The models used for predictionminced tissue and GIT contents were of body composition were linear anddetermined by oven drying at 105°C for 48 h allometric. Stepwise regression analyses wereof triplicate 80 g or 300 g subsamples, used and only predictors that contributedrespectively. The dry tissue samples were significantly (P < 0-05) were retained.


RESULTS

ProductionMilk production over the 24 h between

TOH administration and slaughter rangedfrom 0-72 to 3-40 kg/day, averaging 1-72 (s.e.0-16) kg/day. Mean DM intake (DMI) overthe same period was 1-65 (s.e. 0-10) kg/day.LWT at TOH administration and slaughterwere not different, being 46-7 (s.e. 2-2) and46-9 (s.e. 2-2) kg, respectively.

Total body waterMean TBW was 29-9 (s.e. 1-5) kg and

accounted for proportionately 0-641 (s.e.0-009) of LWT. Total body water wascorrelated with LWT with residual coefficientsof variation (RCV) of less than or equal to0-06 (Table 1, equations 1 and 2). There wasan even higher correlation between TBW andthe sum of the two TOH pools (T) regardlessof whether the linear (equation 3) orallometric (equation 4) model was used. Thelinear regression equation overestimated TBWby proportionately 0-11.

Empty body waterEBW averaged 22-0 (s.e. 1-1) kg which

was proportionately 0-594 (s.e. 0-01) of emptybody weight (EWT). EBW was highlycorrelated with pool A with both the linearand allometric models having RCV of about 004(equations 5 and 6). Pool A overestimated EBWby proportionately 0-07.

Gut fill and waterGut fill (GF) averaged 9-31 (s.e. 0-54) kg

which accounted for proportionately 0-196(s.e. 0-002) of LWT (range 0-146 to 0-259).GF was relatively poorly correlated withLWT (equation 11), due to GF being avariable proportion of LWT.

A relatively strong correlation wasestablished between GF and pool B (equation12). This relationship was improved if DMIor water turn-over (WTO) were included asindependent variables (equations 14 and 16,RCV = 0-078 and 0-088, respectively). GFwas related to both DMI and WTO withsimilar precision (equations 13 and 15).

TABLE 1Regressions of total body water (TBW), empty body water (EBW), gut fill (GF) and gut water(GW) on live weight (LWT), tritiated water pool A (A), tritiated water pool B (B), the sum oftritiated water pools A and B (T), empty body weight (EWT), dry-matter intake (DMI) and

water turn-over (WTO) for 17 lactating goats

ResidualComponent Equation

no.

TBW (kg) .

EBW (kg) .

GF (kg) •

GW (kg)

1234

: 56789

10

11121314151617

18

aaaa

aaaaaa

aaaaaaa

a

Model

+ bx LWTLWT61

+ />, TTh<

+ b{ LWT+ LWT*1

+ b, EWTEWT*1

+ b{ A

+ b, LWT+ b, B+ bx DMI+ b,B + b2 DMI+ b, WTO+ b, B + b2 WTO+ b, DMI + b2 WTO

+ b, B

a

0-3030-6530-3001-00

-0-4820-4621-660-763

-0-1010-895

0-9063-063-180-9824-872-222-22

2-51

b,

0-6310-9930-8920-971

0-4801000-5870-9300-9291-01

0-1790-6673-710-5311-070-5193-98

0-575

s.e. b2 s.e.

0-08470-07150-03760-0418

0-03180-07020-04750-08010-04640-0465

0-04180-09030-9320-0698 2-03 0-4790-2380-0842 0-534 0-1430-946 0-125 0-0372

0-0733

s.d. (kg)

•79•68•02•03

•17•23•50•50

0-900-83

1-541-071-600-731-560-821-65

0-86

cv0-0600-0560-0340-035

0-0530-0560-0680-0680-0410-038

0-1650-1150-1720-0780-1680-0880-177

0-109

R-

0-9180-9280-9740-973

0-9380-9320-8990-9000-9640-969

0-5490-7840-5140-9050-5410-8820-519

0-804


However, inclusion of both of these variablesas independent predictors of GF did notresult in any improvement over the equationsusing the single predictors (equation 17),suggesting a high degree of multicolinearity(see section on water turn-over).

Gut water (GW) averaged 7-89 (s.e. 0-46)kg which accounted for proportionately 0-849(s.e. 0-007) of GF. GW was closely relatedto pool B (R2 = 0-80, RCV = 0-109,equation 18).

Empty body weightMean EWT was 37-2 (s.e. 1-9) kg,

representing 0-791 (s.e. 0-008) of LWT. EWTwas closely correlated with LWT (Table 2,equation 19). The best predictors of EWTwere found to be LWT minus the estimatesof GF from equations 12 and 14 (equations20 and 21).

Empty body fatThe average empty body fat (EBF) content

of the goats was 8-24 (s.e. 0-73) kg. Due tothe large range in EBF (0-076 to 0-26 ofLWT) and in GF, EBF was poorly correlatedwith LWT (Table 3, equation 22, RCV =0-288).

There was a strong inverse correlationbetween the percentages of fat and water inthe EB (% fat = 103-8 - 1-38 % water, R2

= 0-928, residual s.d. = 1-6 %). However,when pool A was introduced into theequation in place of EBW the resultantequation was a poor predictor of thepercentage of fat in the EB.

An extensive range of independentvariables was investigated to determine thebest predictors of EBF. Equations 28 and 29,which included pool A and estimates of EWTfrom equations 20 and 21, respectively, were

TABLE 2Regressions of empty body weight (EWT) on live weight (LWT) andestimated empty body weight (eEWT) obtained by subtracting estimates of

gut fill from LWT for 17 lactating goats

ResidualComponent Equation Model

no. a

EWT (kg)19 a + b, LWT -2-3220 a + b, eEWTlt 1-3921 a + b, eEWT2:f -0-279

0-8420-9520-996

s.e.0-04210-04020-0387

s.d. (kg)

1-551-361-01

CV

0-0420-0350-027

R2

0-9640-9750-985

t eEWTl = LWT - (3-06 + 0-667 B) from (equation 12).t eEWT2 = LWT - (0-982 + 0-531 B + 2-03 DMI) from (equation 14).

TABLE 3Regressions of empty body fat (EBF) on live weight (LWT), empty body weight (EWT),tritiated water pool A (A), the sum of tritiated water pools A and B (T) and estimated empty

body weight (eEWTl and eEWT2) for 17 lactating goats

ResidualComponent

EBF (kg) .

Equationno.

2223242526272829

a -a -a -a -a -a -a -a -

Model

f- bx

(-ft,f- b,t-b,H 6,h b,Yb,Y b,

LWTEWTEWT + b2

LWT + b2

LWT + b2

EWT + b,eEWTlt +eEWT2± +

EBWTAAb2 Ab, A

a

-1-75-2-24

0-016-0-932-1-34

0-1742-02

-0-198

b,0-2130-2811-020-8090-6010-7821-010-866

s.e.

0-06430-06660-07020-1090-1800-1180-1160-0971

b2

-1-35-0-867-0-782-0-882-1-33-1-01

s.e.

0-1220-1500-3430-1930-1960-158

s.d. (kg)

2-372-110-571-332-091-381-111-09

CV

0-2880-2560-0690-1610-2540-1680-1350-132

R-

0-4220-5430-9690-8300-5790-8170-8820-886

t eEWTl = LWT - (3-06 + 0-667 B) from (equation 12).$ eEWT2 = LWT - (0-982 + 0-531 B + 2-03 DMI) from (equation 14).


those for which the best correlations withEBF were obtained (RCV of 0135 and 0132respectively)

Empty body proteinEmpty body protein was correlated with

LWT (Table 4, equation 30, RCV = 0-130)and with pool A and T (equations 32 and33, RCV = 0-128 and 0-114, respectively).Inclusion of both LWT and T as independentvariables did not significantly improve thecorrelation (equation 34).

Empty body ashEmpty body ash could be predicted by

LWT (equation 35, RCV = 0-082). Thecorrelation was marginally improved if anallometric model was used (equation 36, RCV= 0-079).

Fat-free empty bodyWater, ash and protein comprised 0-761

(s.e. 0-0036), 0-188 (s.e. 0-0041) and 0-051(s.e. 0-0012) of the fat-free empty body(FFEB) respectively. The FFEB wascorrelated with LWT and with pool A(equations 37 and 39, RCV = 0-059 and0-051, respectively). The best predictiveequation included both LWT and pool A asindependent variables (equation 41, RCV =0-045).

Rumen volumeRumen volume was correlated with rumen

CrEDTA space (R2 = 0-63, RCV = 0-15,Table 5, equation 42), but the correlationbetween rumen water and CrEDTA spacewas higher (R2 = 0-72, RCV = 0-126,

TABLE 4Regressions of empty body protein (EBP), empty body ash (EBA) and fat-free empty body(FFEB) on live weight (LWT), tritiated water pool A (A) and the sum of tritiated water pools

A and B (T) for 17 lactating goats

Component Equation Model

EBP (kg)

EBA (kg)

FFEB (kg)

3031323334

3536

3738394041

a + b, LWTa LWT*1

a + b, Aa + bj Ta + 6, LWT + b2 T

fl + b, LWTa LWT*1

a + b, LWTa LWT61

a + b, Aa A">a + ft, LWT + 6, A

-0-2900-115

-0-140-0-323-0-424

0-2040-053

-0-5700-6310-0341-21

-0-952

0-1231-000-2360-1750-026

0-0270-859

0-6300-9941-211-000-262

0-01920-1730-03600-02210-0053

0-00330-0996

0-04860-06070-08410-07010-122

0-142

0-74C

0-0730

0-233

Residual

s.d. (kg)

0-710-760-700-630-65

0-120-121-721-851-491-411-31

CV

0-1300-1390-1280-1140-119

0-0820-080

0-0590-0640-0510-0480-045

R-

0-7330-6950-7400-7860-790

0-8200-832

0-9240-9100-9430-9460-959

TABLE 5Regressions of rumen fill (RF), rumen water (RW), gut fill (GF)and gut water (GW) on rumen CrEDTA space (C) for eight

lactating goats

Component

RF (kg)RW (kg)GF (kg)GW (kg)

Equationno.

42434445

aaaa

Model

+ * ,C+ b, C+ * ,C+ b, C

a

2-772-255-214-39

b,0-3720-3310-5150-443

0000

s.e.

•117•0839•164•123

Residual

s.d. (kg)

0-820-591-150-86

CV

0-1500-1260-1510-113

R2

0-6270-7220-6220-685


equation 43). When the regression lines wereconstrained through zero, rumen CrEDTAspace overestimated rumen volume and rumenwater by proportionately 0-29 and 0-39,.respectively. The regressions of GF and GWon rumen CrEDTA space had RCV of 0151and 0-113, respectively (equations 44 and 45).

Rumen CrEDTA space and TOH kineticswere estimated concurrently a total of 13times in four of the fistulated goats duringtheir first lactation as well in all eight of thefistulated goats prior to slaughter. There wasa significant correlation between these twoindices of GF (Rumen CrEDTA space (1) =- 4-83 + 1-16 pool B (kg), R2 = 0-66,residual s.d. = 1-74 1, no. = 21).

Water turn-overWater turn-over (WTO) was positively

related to both milk yield (MY) (WTO(I/day) = 1-69 + 1-41 MY (kg/day), R2 =0-37, residual s.d. = 1-26 I/day) and DMI(WTO (I/day) = - 0-95 + 3-10 DMI(kg/day), R2 = 0-75, residual s.d. = 0-781I/day). Mean WTO and half life were 4-17(s.e. 0-37) I/day and 4-12 (s.e. 0-37) days,respectively.

DISCUSSION

The present findings confirm that a two-pool model of TOH kinetics can be used topredict body composition in lactating goats.The D2O enrichment curve of plasma waterbetween 20 min and 12 h after a bolus ofD,O was administered to young lambs wasfound by Robelin (1977) to be best describedby a second-order exponential decay curve.Byers (1979) proposed that the two waterpools generated from such a modelapproximated EBW and GW in cattle. Sincethen, other researchers have attempted to fitkinetic models to D2O and TOH kinetics inruminants (Odwongo et al, 1984 and 1985;Ferrell and Jenkins, 1984; Faichney andBoston, 1985; Martin and Ehle, 1986).Generally, if blood sampling did notcommence until 15 min or longer afteradministration of the tracer, a second-orderexponential model provided the best fit tothe data (Byers, 1979; Ferrell and Jenkins,1984; Odwongo et al, 1985; Martin andEhle, 1986). In contrast, when sampling was

commenced earlier three or more pools wereidentified (Odwongo et al, 1984 and 1985;Faichney and Boston, 1985).

Byers (1979) suggested that the rapiddecrease in blood D2O levels during the first20 min after injection of a bolus of D2O wasdue to equilibration within the first pool(putatively EBW). No attempt was made toinclude these early data in any curve-peelinganalysis. Odwongo et al. (1984 and 1985)found that a three-pool model best fit theD2O enrichment curve when sampling began5 min after injection. However, when thesefirst data points were excluded from theanalysis, a two-pool model was the best fitfor the D2O curve after 25 min (Odwongo etal, 1985). Faichney and Boston (1985) foundthat a four-pool model best described TOHkinetics in mature wethers. The half-lives ofwater molecules in the first two pools suggestthat sampling in this experiment wascommenced before equilibration within theEBW. These authors suggested that the firsttwo pools may comprise the free water andmacromolecule-bound water components ofEBW. They differentiated GW into rumenwater and post-rumen GIT water, althoughthe latter was grossly overestimated.

In the present studies, blood samplingroutinely commenced 15 min after injectionand continued for 24 h. This sampling regimewas chosen because preliminary studiesshowed that goats had relatively highfractional water turn-over rates during earlylactation. Thus, equilibration of TOH withinboth pool A and pool B was complete within15 to 80 min, respectively, compared with 20min and 120 min for non-lactating, non-pregnant cattle (Byers, 1979).

The two-pool model of TOH kinetics inlactating goats accurately predicted the twomajor pools of water in the whole body,EBW and GW. Byers (1979) was able topredict EBW and GW in non-lactating, non-pregnant cattle with a similar degree ofaccuracy. Other attempts to correlate GWwith any of the pools generated during thefitting of models to D2O enrichment curves inruminants have been less successful, forvarious reasons. Ferrell and Jenkins (1984)stanchioned their cattle overnight withoutfood or water before infusing D2O. These


cattle were then provided with food and major reasons for the lower correlationwater and were not slaughtered until 3 days between GF and CrEDTA space thanlater, during which time increases in GF between rumen volume and CrEDTA space iswould have occurred. The cattle studied by that the post-ruminal GIT contents constituteOdwongo et al. (1984) were not slaughtered a large and variable proportion of GF.until 11 to 15 days after D2O infusion, generally having a DM content greater thansufficient time for substantial changes in GW that of the rumen. The overestimation ofand GF to occur. Nevertheless, Odwongo et rumen water is consistent with previousal. (1984) found GW to be linearly related to findings that this technique overestimatedWTO with an R2 of 0-85, although inclusion rumen water in sheep (Poppi, Minson andof data from both calves and mature cattle Ternouth, 1981) and in steers (Dixon andwould ensure high correlations when Milligan, 1985). It has been suggested thatdeveloping regression equations. The CrEDTA may bind to digesta (Warner, 1969;correlation between GF and WTO was not as Dixon and Milligan, 1985) and under somehigh in the goats in the present study (R2 = circumstances is absorbed from the rumen0-54) and no relationship was found between (Dobson, Sellars and Gatewood, 1976;GF and WTO in the goats of Brown and Faichney and Gherardi, 1985) both of whichTaylor (1986). Prediction of GF from WTO would result in overestimation of rumenis based on the relationship between DMI volume. This suggests that the use ofand water intake which were found to be CrEDTA dilution to estimate rumen water issignificantly correlated in the present study. of little benefit when determining body

Arnold et al. (1985) compared body composition in unfasted, lactating goats,composition estimated using the prediction EBF was poorly related to LWT with theequations from the one-pool (Robelin, 1982) correlation between EBF and EWT beingand two-pool (Byers, 1979) models of D2O only slightly better. The best predictivekinetics with chemically determined body equation for the estimation of EBF was thatcomposition. They concluded that the two- including EWT and EBW. However, it is notpool model did not offer any improvement possible directly to determine these physicalover the one-pool model in predicting body parameters in vivo and therefore it wascomposition. This is not surprising, since the necessary to include the best estimates ofone-pool model should be satisfactory under these two parameters into the equation. EWTthe conditions reported by Arnold et al. was accurately predicted by LWT minus GF(1985) where GW and GF were small and estimated from either pool B (equation 20)relatively constant proportions of LWT (0-067 or pool B and DMI (equation 21). EBW was(s.e. 0-003) and 0-087 (s.e. 0-004), closely correlated with pool A (equation 5).respectively). Similarly, Brown and Taylor Inclusion of eEWT2 (estimated EWT from(1986) were able accurately to predict EBF equation 36) and pool A in the predictiveusing a one-pool model of D2O kinetics in equation for EBF resulted in an R2 of 0-89pregnant and lactating goats when GF was a and RCV of 0-132. Brown and Taylor (1986)relatively small proportion of LWT (0-107 obtained similar precision with the one-pool(s.e. 0-005)). However, in situations where model to estimate TBW and EBF from D:OGW and GF are larger proportions of LWT, kinetics in 10 lactating and five non-lactatingthe one-pool model would be less adequate, dairy goats. However, as stated earlier, GFFor example, in the present work the RCV was small and relatively constant in theirof the prediction of EBF using the two-pool group of goats, a condition favourable tomodel was lower than that of the one-pool using the one-pool model.model of TOH kinetics (0-132 and 0-161, Foot et al. (1979) analysed therespectively). relationships between TOH space and EBF

Estimation of rumen contents and GF with in two groups of lactating sheep. In the firstthe widely used CrEDTA technique showed group of 10 Scottish Blackface sheep, whichrather poor correlations over the limited were given a restricted ration to minimizerange and number of animals examined. The variations in GF, EBF was very highly


correlated with LWT and TOH space (R2 =0-99, RCV = 0-094) and reasonably wellcorrelated with LWT alone (R2 = 0-81). Thesecond group included 25 lactating BorderLeicester x Blackface ewes that were eithergrazing pasture or receiving a basic roughagediet. In these ewes EBF was poorlycorrelated with LWT (R2 = 0-45) and thecorrelation between EBF and LWT and TOHspace was not as high as in the first group(R2 = 0-87, RCV = 0-306). It appears thatincluding TOH space as a second independentpredictor with LWT improves the correlation,but the final correlation is very muchdependent upon the relationship with LWTalone. A similar pattern is evident inrelationships between EBF, LWT and D2Ospace in non-lactating cattle. For example,Wright and Russel (1984) found EBF inmature cows to be related to LWT alonewith an R2 of 0-91. Inclusion of D2O spaceincreased the R2 to 0-99. On the other hand,Ferrell and Jenkins (1984) found a poorcorrelation between EBF and EWT (R2 =0-49). When D2O pool A was introduced asa second independent variable the resultantregression equation had an R2 of 0-86.

The best single predictors for empty bodyprotein were either LWT or T. The lack of alarge improvement afforded by the inclusionof the water pools with LWT in theregressions has been observed before inlactating sheep (Foot et al., 1979) and innon-lactating cattle (Bird et al, 1982; Wrightand Russel, 1984), and sheep (Donnelly andFreer, 1974; Trigg, Domingo and Topps,1978). This is probably due in part to therelatively strong multicolinearity betweenTBW and LWT.

The biological half-life of water determinedin the lactating goats in this series ofexperiments was similar to values for lactatingewes (Foot et al, 1979) but lower than forlactating dairy cattle (Odwongo et al., 1984and 1985; Woodford, Murphy and Davis,1984; Rogers, Odwongo and Conrad, 1985).As would be expected WTO was related toboth milk production and DMI. The largevariability in observed WTO ratesdemonstrates that relying on a single bloodsample taken a fixed time after TOHadministration can introduce substantial errors

in estimation of TOH dilution. For example,determination of TOH dilution in a plasmasample taken 6 h after TOH administration inthe goat with the highest turn-over ratewould have overestimated TBW byproportionately 0-26, whereas a similardetermination in the goat with the lowestWTO would have overestimated TBW byonly 0-11. On the other hand, extrapolationof the slower decaying exponential equationto time zero resulted in overestimations ofTBW of 0-12 and 0-11 respectively, which isthe same degree of overestimation found inequation 3.

For a TOH dilution technique to be ofbenefit in estimating body composition itmust provide a significant improvement inprediction over LWT alone. The workreported here confirms that this is the case inlactating goats. The benefits of a two-poolmodel of TOH kinetics rather than taking asingle sample at a fixed time afteradministration of the TOH are two-fold.Firstly, it permits accurate estimation of GFand GW, so that there is no need to depriveanimals of food or water for any period. Thisis a decided advantage in studies on lactatinganimals. Additional information includes EWTand EBW, neither of which can be obtainedby other indirect techniques. Secondly, theuse of a kinetic model eliminates theproblems involving variability in WTO, whichare associated with variations in milk yieldand DMI in lactating animals. It is concludedthat a two-pool model of TOH kinetics is auseful tool for studying changes in EBFcontent during early lactation in goats.

ACKNOWLEDGEMENTS

The authors wish to thank Miss R. Fitzpatrick andMiss R. Vavala for care of the animals. F.R.D. wassupported by a grant from the Australian Dairy ResearchCommittee.

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(Received 11 March 1988—Accepted 1 May 1988)

A two-pool model of tritiated water kinetics to predict body composition in unfasted lactating goats

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