Influence of Plane of Nutrition on Body Composition, Organ Size and ...

ENERGY UTILIZATION DURING REALIMENTATION 25271800 h. Body weights were taken twiceweekly. Daily feed intake <strong>of</strong> individual animals was adjusted twice weekly based onactual body weights and targeted weightssuch that the planned gains or losses wereachieved. A premeasured amount <strong>of</strong> feed(Purina Rat Chow) was given to each animal at 1600 h each day and unconsumedfeed was removed at 0800 h the followingmorning. Weight <strong>of</strong> unconsumed feed wasrecorded daily. All animals had free accessto water at all times.During period 2, total excretion <strong>of</strong> urineand feces <strong>of</strong> 24 animals randomly chosenacross the eight treatment groups was collected for 3 d for determination <strong>of</strong> the metabolizableenergy content <strong>of</strong> the diet.Statistical analysis <strong>of</strong> the results did notreveal any significant differences due to sexor feeding level. Therefore a mean value <strong>of</strong>2.91 kcal/g was adopted as the metabolizableenergy content <strong>of</strong> the diet forcalculation <strong>of</strong> ME intakes.Data were analyzed by analysis <strong>of</strong>variance and regression procedures (22).The effect <strong>of</strong> sex on body composition traits<strong>of</strong> rats killed at the initiation <strong>of</strong> the studywas evaluated by one-way analysis <strong>of</strong>variance. The relationship between bodyprotein, body fat or energy content andbody weight was evaluated by use <strong>of</strong> amodel that included sex, body weight (BW,linear and quadratic) and all two-way interactions as independent variables. Nonsignificant independent variables were deletedstepwise in subsequent analyses.The effects <strong>of</strong> sex and treatment duringperiod 1 on food consumption, bodyweight, body composition, rate <strong>of</strong> gain andcomposition <strong>of</strong> gain were evaluated by twowayanalysis; the model included treatment, sex and the two-way interaction asindependent variables. Treatment groupmeans were compared by Student's f-test ifthe analysis <strong>of</strong> variance indicated significant treatment differences. The relationships between empty body weight, water,protein, fat, ash or energy and BW were determined by procedures like those describedabove.Data obtained during and at the end <strong>of</strong>period 2 were also analyzed by analysis <strong>of</strong>variance. The model includedsex and the two-way interactiontreatment,as independent variables. Student's i statistic was usedfor comparison <strong>of</strong> specific means (i.e, rats<strong>of</strong> the same sex and final weight group or <strong>of</strong>the same sex and rate <strong>of</strong> gain during period2) if analysis <strong>of</strong> variance indicated significant (P < 0.05) treatment or sex x treatment effects. Multiple linear regressionanalyses were used to describe relationships<strong>of</strong> visceral organ weights to empty bodyweight and average daily weight gain during period 2. Sex and its two-way interactions were included as independentvariables to test sex effects on these relationships. The model proposed by Koong andHill (22) was used to evaluate relationships<strong>of</strong> daily empty body energy gain (kilocalories)to average metabolic body size (kilograms empty body weight 0.75)and dailymetabolizable energy intake (kilocalories).Sex and previous nutritional treatment(treatment during period 1) as well as alltwo-way interactions were included in theinitial model to evaluate their effects onthese relationships. Estimates <strong>of</strong>maintenance [metabolizable energy required for zero body energy change,kcal/(kg°75-d)3 were calculated from theresulting equations after setting energychange to zero.RESULTSWeight and composition <strong>of</strong> male andfemale rats killed at the initiation <strong>of</strong> thestudy are presented in table 1. No significant differences (P > 0.10) between malesTABLE 1Least squares means for initial weight andcomposition <strong>of</strong> male and female ratsTrait Male FemaleRSD'No. animalsLiver <strong>of</strong>gEmptyweight,gBody body,gBodywater,gBodyfat,gBodyprotein,gBodyash,energy, kcal686.374.253.44.213.72.56120975.164.446.43.511.92.2910312.9712.078.'Calculatedas the square root <strong>of</strong> the error meansquare and based on 13 error degrees <strong>of</strong> freedom. Standard error may be calculated as RSD/VNjDownloaded from jn.nutrition.org by guest on August 9, 2014

2528 FERRELL AND KOONGand females were observed. Protein, fat andenergy contents <strong>of</strong> those animals were related to their body weight (g) by the following equations:1) Protein, g = -1.046 + 0.1718 x BWr2 = 0.99, n = 152) Fat, g = -0.552 + 0.0546 x BWr2 = 0.85, n = 153) Energy, kcal = -11.40 + 1.527 x BWr2 = 0.98, n = 15These equations were subsequently used topredict the initial protein, fat and energycontent from initial body weight for eachanimal killed at d 21. Weights <strong>of</strong> all rats(table 2) at the end <strong>of</strong> period 1 coincidedwith the experimental design except thatfemale rats <strong>of</strong> treatment H did not achievethe desired live weight, which resulted in atreatment x sex interaction. Similarly, females on the H treatment had lower emptybody weights and contained less water, protein, ash and energy than males on the Htreatment, whereas females and males on Mand L treatments did not differ with respectto these variables. Amount <strong>of</strong> empty bodyfat differed only due to treatment(P < 0.01). Amounts <strong>of</strong> all body components increased as nutrition level increased.However, when amounts <strong>of</strong> body components were expressed as percentages <strong>of</strong> empty body weight, treatment x sex was not asignificant (P > 0.05) source <strong>of</strong> variation.Percentage <strong>of</strong> water and ash decreased(P < 0.01), percentage <strong>of</strong> fat and concentration <strong>of</strong> energy increased (P < 0.01) andpercentage <strong>of</strong> protein did not change(P > 0.10) with increased plane <strong>of</strong> nutrition. Percentages <strong>of</strong> water or ash and concentration <strong>of</strong> energy did not differsignificantly (P > 0.05) between male andfemale rats, but female rats contained agreater (P < 0.05) percentage <strong>of</strong> fat (6.0 vs.4.6%) and a lower percentage <strong>of</strong> protein(21.5 vs. 22.1%) than male rats when evalTABLE 2uated across treatment groups.The treatment X sex interaction was alsoa significant (P < 0.05) source <strong>of</strong> variationin gain <strong>of</strong> body chemical components (except fat). Male rats assigned to the H treatment gained body tissues (except fat) moreLeast squares means for metabolizable energy (ME) intake, body composition and composition oj gain ojmale and female rats sacrificed at the end <strong>of</strong> period 1groupItemNo.animalsME <strong>of</strong>kcal/d1Final intake,g3Empty live weight,g3Emptybody weight,g3Emptybody water,g4Emptybody fat,g3Emptybody protein,g3Emptybody ash,kcaPEmpty body energy,g/d1Water body gain,Treatmentg/

ENERGY UTILIZATION DURING REALIMENTATION 2529rapidly than did females, whereas gains <strong>of</strong>body tissues by male rats assigned to the Mor L treatments were less than or equal tothose <strong>of</strong> female rats assigned to those treatments.Relationships <strong>of</strong> amounts <strong>of</strong> body components to BW for rats killed at the end <strong>of</strong>period 1 were significantly affected(P < 0.05) by sex; thus regression equationshave been presented for each sex (table 3).These equations were subsequently used toestimate the weight <strong>of</strong> these componentsand energy content <strong>of</strong> rats at the initiation<strong>of</strong> period 2.The ME required for maintenance andefficiency <strong>of</strong> utilization <strong>of</strong> ME for body energy gain (EG, kilocalories per day) for allrats during period 1 were estimated by use<strong>of</strong> regression analyses. Initial analysesrevealed no significant differences betweensexes; thus data were pooled across sex togive the following result:4) EG = -47.0 x MBS + 0.53 x MEI(±5.7) (±0.03)r2 = 0.99, RSD = 1.06This equation yields an estimate <strong>of</strong> 88.5kcal/kg°75for the daily ME required formaintenance and an estimate <strong>of</strong> 0.53 for theefficiency <strong>of</strong> utilization <strong>of</strong> ME for body energy gain.Mean live weights <strong>of</strong> male and femalerats <strong>of</strong> the eight treatment groups at the end<strong>of</strong> period 2 are shown in table 4. Male rats<strong>of</strong> all treatments achieved the desiredweights during this period but female rats<strong>of</strong> treatments HH, HM and MH did not.Final weights <strong>of</strong> these groups were similarand indicated that females approachedmaturity at lower weights (and at weightsless than designed) than male rats. Thefemales <strong>of</strong> the other five treatments achievedthe weights specified by design.Mean final empty body weight andamounts <strong>of</strong> body water, fat, protein and energy <strong>of</strong> male and female rats <strong>of</strong> the eighttreatment groups are shown in table 4. It isobvious that weights <strong>of</strong> the body chemicalcomponents are highly correlated withempty body weight; however, one <strong>of</strong> the objectives <strong>of</strong> the study was to compare thebody composition <strong>of</strong> animals at the sameweight and age which had achieved thoseweights through different nutritional manipulations. Therefore, the comparisons <strong>of</strong>primary interest are between treatmentsHM and MH (group 1), among treatmentsHL, MM and LH (group 2) and betweentreatments ML and LM (group 3). In malerats, there were no significant differences(P > 0.05) in empty body or water weightsamong treatments within any given group.Protein weights or percentages were lower(P < 0.05) and fat weights (or, withingroup 2, percentages) were higher(P < 0.05) for male rats within a givengroup that were on higher planes <strong>of</strong> nutrition during period 2. No differences wereDownloaded from jn.nutrition.org by guest on August 9, 2014TABLE 3Relationships <strong>of</strong> empty body weight, weight <strong>of</strong> body water, protein, fat or ash and body energyto body weight (BW) <strong>of</strong> male and female rats killed at the end <strong>of</strong> period 1SexMaleFemaleN Component13kcal11Empty gWater, body,gProtein,gFat,gAsh,gEnergy,Empty gWater, body,gProtein,gFat,gAsh,gEnergy,kcaly=y=y=y=y=y=y--6.251--4.474--6.609-1.563--53.51--9.869=y --=y --2.776 13.86=y=y=y -30.93=-9.979-3.679-1.3761.089BWh0.775BWh0.289BWh0.103BWH0.0247BWi-2.010BWh1.010BWi-0.964BWh0.260BWh0.112BWi-0.0264BWh-Régressionh0.1323BW-0.000776BW"-0.000773BW2-0.000368BW2-0.000947BW2-0.00189BW2-0.000343BW2+0.001116BW2+0.00997BWr20.990.990.990.860.980.960.990.930.990.930.960.99RSD3.353.

2530 FERRELL AND KOONGTABLE 4Live weight and weight <strong>of</strong> empty body components <strong>of</strong> male and female rats at the end <strong>of</strong> period 2componentSexMaleFemaleMaleMaleFemaleFemaleMaleMaleMaleFemaleFemaleFemaleMaleMaleFemaleFemaleBSDTreatment1HHHHHMMHHMMHEmpty body<strong>of</strong>liveanimals545544553443434349Finalweightg294198227232195200162165160163159153949498966.55Totalweight«265180211211178180150150143155147137878591866.1WaterK167.9115.0134.3'Treatments are designated by a two-letter code. The first letter indicates that the target gain during the initial21-d period was 105 (H), 40 (M) or - 25 (L) g. The second letter indicates that the target gain during the second21-d period was 105 (H), 40 (M) or -25 (L) g.observed for any body components <strong>of</strong>female rats <strong>of</strong> the HH, HM and MH treatments. This was apparently because theserats approached their mature size duringperiod 2; thus treatment effects were notmanifest. Empty body weights <strong>of</strong> femaleswithin group 2 (HL, MM, LH) or 3 (ML,LM) differed among treatments (P < 0.05),primarily reflecting differences in final liveweights <strong>of</strong> females within these groups.There were differences (P < 0.05) in watercontent among treatments within group 2or 3; however, these differences were directly related to differences in empty bodyweight. When amounts <strong>of</strong> water were expressed as percentages <strong>of</strong> empty bodyweight, no differences (P > 0.05) were evident (data not presented). Protein and fatcontents (or percentages) <strong>of</strong> female rats <strong>of</strong>groups 2 and 3 paralleled these variables inmale rats. Protein content was lower(P < 0.05) and fat content was higher(P < 0.05) for rats on higher planes <strong>of</strong>nutrition during period 2 than for rats onlower planes <strong>of</strong> nutrition.Live weight or body component gainswere calculated as the difference betweenamounts <strong>of</strong> these components at the beginning and end <strong>of</strong> period 2 and are presentedin table 5. The treatment x sex interactionwas a significant source <strong>of</strong> variation (P 0.05). The lower (P < 0.05) ME intake and the resulting lower (P < 0.05)gains <strong>of</strong> HH females compared to HH maleswere major contributors to this result. Inboth sexes, gains <strong>of</strong> all body components increased as ME intake during period 2 increased. Live weight or empty body weightgains <strong>of</strong> male rats assigned to the H treatment during period 2 (HH, MH and LH)were similar. Within this group, proteingain as a proportion <strong>of</strong> live weight gain waslower (P < 0.05) for those that had been onthe lower planes <strong>of</strong> nutrition previouslyDownloaded from jn.nutrition.org by guest on August 9, 2014

ENERGY UTILIZATION DURING REALIMENTATION 2531MetabolizableTABLE 5(ME) intakes, live weight gain and gain <strong>of</strong> empty body components<strong>of</strong> male and female rats during period 2weightbodySexMaleFemaleMaleMaleFemaleFern intakekcal/d55.141.838.942.941.743.424.827.730.728.129.931.614.717.816.119.51.58Livegaing/d4.641.941.754.871.643.47-1.191.864.800.031.484.55-1.861.79-1.251.860.51Emptyweightg/d4.362.581.994.432.413.66-0.761.704.341.262.044.39-1.611.70-0.451.980.41Watergaing/d2.471.151.022.540.921.71-0.521.052.780.500.852.60-1.131.12-0.691.1gaing/d0.670.230.290.800.240.56-0.310.130.62-0.190.160.63-0.270gaing/d1.020.390.560.870.430.680.000.380.870.210.380EnergyaleMaleMaleMaleFemaleFernaleFemaleMaleMaleFemaleFemaleRSDTreatment1HHHHHMMHHMMHHLMMLHHLMMLHMLLMMLLMME'Treatments are designated by a two-letter code. The first letter indicates that the target gain during the initial21-d period was 105 (H), 40 (M) or —25 (L) g. The second letter indicates that the target gain during the second21-d period was 105 (H), 40 (M) or -25 (L) g.(HH>MH = LH). Percentages <strong>of</strong> emptybody weight gain as protein were 23.4, 19.6and 20.0 for HH, MH and LH male rats,respectively. This pattern was also evidentfor male rats assigned to the M treatmentduring period 2. Percentages <strong>of</strong> empty bodyweight gain as protein were 28.1, 22.4 and21.2 for HM, MM and LM male rats. Theseresults suggest that protein gain duringperiod 2 was influenced by protein mass atthe beginning <strong>of</strong> the period. Similar patterns were not observed when similar comparisons were made with female ratsassigned to the H or M treatments duringperiod 2. No consistent patterns were apparent in males or females for the proportion <strong>of</strong> fat in live weight gain with respectto previous nutritional treatment.This experiment was designed in such away that there were three groups <strong>of</strong> ratswith the same age but different bodyweights at the beginning <strong>of</strong> period 2 resulting from different levels <strong>of</strong> food intake during period 1. The effect <strong>of</strong> previousnutritional treatment on the efficiency <strong>of</strong>utilization <strong>of</strong> ME for maintenance or gainwas evaluated by combining treatments according to the level <strong>of</strong> nutrition duringperiod 1 (HH + HM + HL; MH + MM + ML;LH + LM). No evidence <strong>of</strong> significant sex effects was observed in the initial analyses;thus male and female rats were both includedin the final regression analyses (table 6). Efficiencies <strong>of</strong> utilization <strong>of</strong> ME for gain (b2)were lower (P < 0.05) for rats <strong>of</strong> the Htreatment during period 1 than for those <strong>of</strong>the M or L treatment. Although not statistically compared, maintenance (bilb2) appeared to decrease substantially as previouslevel <strong>of</strong> nutrition decreased. Estimates <strong>of</strong>the metabolizable energy required for energy equilibrium <strong>of</strong> rats from the H, M and Lprevious treatment groups were 114, 107and 86 kcal/(kg°7s-d), respectively.Weights <strong>of</strong> internal organs obtained atthe end <strong>of</strong> period 2 are shown in table 7. Itis not surprising that organ weights wererelated to body weight. However, whenDownloaded from jn.nutrition.org by guest on August 9, 2014

2532 FERRELL AND KOONGTABLE 6Evaluation <strong>of</strong> metabolizable energy utilization by rats as influenced by previous nutritional manipulationPrevioustreatment1HMLNo.coefficients2fc,<strong>of</strong>animals272612Regression bt-62.10.093-70.8 ±13.9 0.544 ±0.065-51.9 ±9.5 0.658 ±±21.10.603 ±0.125r'0.750.890.97BSD3.722.551.45'H, M and L indicate groups <strong>of</strong> rats that were fed to gain 105, 40, or -25 g during the initial 21-d period. 2Model was Y = b¡X¡ + b2X2,where Ywas daily empty body energy gain (kcal/d), Xi was average metabolic body size (kg075)and X¡metabolizable energy intake (MEI, kcal/d).comparisons were made within groups <strong>of</strong>animals with similar final weights, animalson higher nutritional levels during period 2had heavier liver and gut weights than thoseon lower nutritional levels (LH > MM = HL;LM>ML; P < 0.05). This relationship wasalso true for kidney weights <strong>of</strong> the lowweight groups (LM>ML; ? < 0.05). Heartweight did not differ among treatmentsTABLE 7within weight groups. Further analysis(table 8) showed that average daily gainduring the period immediately beforeslaughter significantly influenced relationships between liver or gut weight and emptybody weight; however, rate <strong>of</strong> gain did notsignificantly influence relationships between heart or kidney weight and emptybody weight.Effect <strong>of</strong> nutritional treatment and sei on organ weights <strong>of</strong> rats<strong>of</strong>animals5455445534434343Liverg15.4s9.6"10.259.7s11.Treatment1HHHMMHHLMMLHMLLMBSDSexMaleFemaleMaleFemaleMaleFemaleMaleFemaleMaleFemaleMaleFemaleMaleFemaleMaleFemaleNo.Ie9.6'7.1'7.357.2"7.4"7.477.8"3.5«3.9'4.674.870.88Heartg0.9450.6850.77s0.7350.76"0.695Downloaded from jn.nutrition.org by guest on August 9, 2014'Treatments are designated by a two-letter code. The first letter indicates that the target gain during the initial21-d period was 105 (H), 40 (M) or —25 (L) g. The second letter indicates that the target gain during the second21-d period was 105 (H), 40 (M) or - 25 (L) g. Sex, treatment and sex x treatment were significant (P < 0.01)sources <strong>of</strong> variation in weights <strong>of</strong> all organs. 2"4Meanswithin sex and within groups <strong>of</strong> animals <strong>of</strong> similar weight(i.e., HM, MH; HL, MM, LH; ML, LM) differ significantly (P < 0.05). '"'Means within sex and within groups<strong>of</strong> animals gaining at similar rates during period 2 (i.e., HH, MH, LH; HM, MM, LM; HL, ML) differ significantly (P < 0.05).

ENERGY UTILIZATION DURING REALIMENTATION 2533DependentvariableHeart,gLiver,0.029-0.64 ±gKidney,0.370.08 ±gGut,0.063.77 ±gIntercept0.147 ±0.30Empty'Model was Y = b„+ b,EBWTABLESRegression <strong>of</strong> organ weights on body weight and rate <strong>of</strong> gain <strong>of</strong> rats1average daily gain. All weights are in grams.bodyweight0.00310.00020.053 ±0.0030.0087 ±0.00040.033 ±±0.002Averagedailygain-0.00750.00460.206 ±0.059-0.002 ±0.0090.285 ±±0.048r20.840.930.920.91BSD0.650.840.130.68+ b2ADG, where Y was organ weight, EBW empty body weight and ADGDISCUSSIONNumerous data are available from studies in which the influence <strong>of</strong> plane <strong>of</strong> nutrition on body composition and composition<strong>of</strong> gain has been evaluated. Results fromthese studies have not been consistent. Asnoted by Black (23), data are available toshow, for example, that body fat at a givenweight increases, is not affected or decreasesin response to increased energy intake.Black concluded that most <strong>of</strong> the conflictingreports could be reconciled if the interactions among plane <strong>of</strong> nutrition, chemicalcomposition <strong>of</strong> the diet, frequency <strong>of</strong>feeding and stage <strong>of</strong> maturity were considered. If other factors are constant,response to nutritional restriction is dependent on the degree <strong>of</strong> restriction, the duration <strong>of</strong> the restriction and the time afterrealimentation at which the response ismeasured.In contrast to studies reported previously,the design <strong>of</strong> this study allows direct comparison <strong>of</strong> the composition <strong>of</strong> body weightchange <strong>of</strong> animals differing in weight, but<strong>of</strong> the same age and gaining body weight atthe same rate (eg., HM, MM and LMgroups). Comparison <strong>of</strong> this type (table 5)suggested that as previous plane <strong>of</strong> nutritiondecreased, rate <strong>of</strong> protein gain decreasedbut rate <strong>of</strong> fat gain changed little. Thisdesign also allowed a direct comparison <strong>of</strong>the body composition <strong>of</strong> animals <strong>of</strong> thesame age and weight that had achieved thatweight as a result <strong>of</strong> differing growth patterns (e.g., HL, MM and LH groups, table4). Body protein content at the end <strong>of</strong>period 2 decreased and body fat content increased as plane <strong>of</strong> nutrition decreased during period 1. These results show that in thisexperiment both composition <strong>of</strong> gain duringperiod 2 and body composition at the end <strong>of</strong>period 2 were altered in response to plane <strong>of</strong>nutrition during period 1.Webster (24) concluded that any improvement in food conversion efficiencyduring compensatory growth could bealmost entirely attributed to differences inthe energy content <strong>of</strong> the gain. In the current study, energy content <strong>of</strong> the live weightgain decreased as previous level <strong>of</strong> nutritiondecreased (e.g., in males HH + HM = 3.00vs. MH + MM = 2.52 vs. LH + LM = 2.32kcal/g), which tends to add credence to thatconclusion. Also, when animals <strong>of</strong> thesegroups were compared, body protein followed a similar pattern. Commensuratewith the decrease in body protein, weobserved a decrease in the energy requirement for energy stasis. These findings areconsistent with previous reports (25-27)suggesting that basal heat production ormaintenance requirements were associatedwith body lean or body protein mass. Also,previous reports (28, 29) have shown thatprotein gain is less efficient energeticallythan fat gain. Thus, the differences in energetic efficiency <strong>of</strong> body energy gain maybe attributed to differences in the composition <strong>of</strong> gain.Other data suggest that increased efficiency during compensatory growth cannotbe attributed solely to body compositionand composition <strong>of</strong> gain (30-31). Thosedata demonstrate that maintenance requirements <strong>of</strong> steers decreased in responseDownloaded from jn.nutrition.org by guest on August 9, 2014

2534 FERRELL AND KOONGto nutritional restrictions. In the study <strong>of</strong>Foot and Tulloh (30), weights <strong>of</strong> the emptybody, fat, water and protein remained constant during the 120-d period <strong>of</strong> weight stasis, but feed intake decreased from 82 to 56g/kg n' 7^ . Their data suggested that thedecreased maintenance was associated withdecreased mass <strong>of</strong> internal organs, especially the liver. Ledger and Sayers (31) similarlynoted that feed required to maintain liveweight <strong>of</strong> steers fed to maintain live weightdecreased with time. Their data indicatedthat weights <strong>of</strong> internal organs and emptydigestive tract decreased in response tomaintenance feeding.The data reported in tables 7 and 8 suggest that weights <strong>of</strong> the internal organs wereinfluenced by nutritional treatment andthat weights <strong>of</strong> the liver and gut werepositively associated with rate <strong>of</strong> gain priorto slaughter, as well as with body weight.That weights <strong>of</strong> internal organs are influenced by plane <strong>of</strong> nutritionnoted in several species (30-35).has beenThere arealso data suggesting that metabolic rate perunit weight as well as weight may change inresponse to changes in plane <strong>of</strong> nutrition orphysiological state (36). Since these tissueshave a high rate <strong>of</strong> energy expenditurerelative to their size, changes in size andmetabolic rate <strong>of</strong> the tissues may have asubstantial impact on maintenance requirements and, during the realimentationperiod, on efficiency <strong>of</strong> body weight gain.The data presented here suggest that thegreater efficiency <strong>of</strong> growth <strong>of</strong>ten observedduring realimentation after a period <strong>of</strong>nutritional restriction results, in part, froma decrease in the maintenance requirements. This decrease appears to result fromalterations in proportion <strong>of</strong> high energy expending internal organs as well as fromalterations in body composition. Also,alterations in composition <strong>of</strong> body tissuegains (e.g., greater proportion <strong>of</strong> water)should be considered a component <strong>of</strong> thegreater efficiencies observed duringrealimentation.LITERATURE CITED1. Armsby, H. P. (1917) The <strong>Nutrition</strong> <strong>of</strong> FarmAnimals, Macmillan, New York.2. ARC (1980) The Nutrient Requirements <strong>of</strong>Ruminant Livestock, Agricultural Research Council, Commonwealth Agricultural Bureaux, Slough,England.3. L<strong>of</strong>green, G. P. & Garrett, W. N. (1968) Asystem for expressing net energy requirements andfeed values for growing and finishing beef cattle.J. Anim. Sci. 27, 793-806.4. Brody, S. (1945) Bioenergetics and Growth,Reinhold, New York.5. Kleiber, M. (1961) The Fire <strong>of</strong> Life, Wiley, NewYork.6. Blaxter, K. L., Clapperton, J. L. & Wainman,F. W. (1966) Utilization <strong>of</strong> the energy and protein <strong>of</strong> the same diet by cattle <strong>of</strong> different ages. J.Agrie. Sci. Cambridge 67, 67-75.7. Byers, F. M. (1982) Patterns and energetic efficiency <strong>of</strong> tissue growth in beef cattle <strong>of</strong> fourbreeds. 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