Lactation in the Dog: Milk Composition andIntake by Puppies

OLAV T. OFTEDAL1

Division of Nutritional Sciences, Cornell University,Ithaca, NY 14853

ABSTRACT The composition and intake of milk by mother-reared puppies wasstudied to compare protein and energy intakes of puppies with estimated requirements. Milk samples were obtained from five beagle bitches over the period of 7-37days postpartum. Dog milk contained on average 22.7% dry matter, 9.47% fat, 7.53%protein, 3.81% sugar and 146 kcal gross energy per 100 g. Protein comprised 31% ofmilk energy. Nonprotein nitrogen averaged 0.054%, equivalent to 4.4% of totalnitrogen. Milk intakes of puppies in the five litters were estimated from water kineticsfollowing administration of deuterium oxide (D2O). D2O dilution indicated that bodywater comprised 72-73% of puppy body weight, and fractional turnover rate of bodywater averaged 0.15-0.17% per day in weeks 3 and 4 postpartum. Milk intakes werecalculated as 160 ±5.4 g (mean ±SEM)at 19 days and 175 ±5.3 g at 26 days,equivalent to 17.0 and 14.6% of body weight, respectively. Daily milk yields of thebitches averaged 964 g at 19 days and 1054 g at 26 days. Dry matter intakes of thepuppies were equivalent to 3.9 and 3.3% of body weight at 19 and 26 days, respectively. Gross energy intakes averaged 223-224 kcal/kg"" per day, and protein intakesaveraged 0.33-0.36 g per gram body weight gain at these ages. Estimates of the energyrequirements of young puppies by the National Research Council appear to be toohigh. J. Nutr. 114: 803-812, 1984.

INDEXING KEY WORDS dogs •lactation •milk composition •milkyield •water kinetics

Milk composition and yield vary greatly age or less are reported to consume theamong diverse mammalian species (1-3). equivalent of 10-14% of body weight per dayEstimation of the nutrient requirements of (7, 9). The effects of maternal nutrition (9,both mother and suckling young requires 10), maternal size (2), breed (6), and litter sizequantitative information on lactation per- and mass on lactation performance in dogsformance. Dogs are known to produce a need clarification.rather concentrated milk containing 21-26% The following study was undertaken tototal solids, 8-12% fat and 7-10% protein measure milk composition and milk yield at(4-7), although lower levels of fat and protein peak lactation in well-nourished dogs of thehave recently been reported (8). Little in- beagle breed. This study is part of a largerformation is available on milk yields in dogs, project in which lactation performance isSeveral litters of various breeds have been compared among several species, and nutri-studied by weighing puppies before and after ent intakes of suckling young are related tosuckling (6, 7). A German shepherd was esti- body size, growth rates and estimated re-mated to produce 1.7 kg milk per day at the quirements (3, 11). Milk production waslactation peak at 3 weeks postpartum, where-aS bitches Of Smaller breeds produced leSS © 1984 American Institute of Nutrition. Received for publicationmilk but were only studied in the first 9 days 27,]une1983M

,„. ^ ... , . J - Current address: Department of Zoological Research, National(b). bUCkling pUppieS 4 WeekS Or Zoological Park, Smithsonian Institution, Washington, DC 20008.

803

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measured from the dilution and turnover ofdeuterium oxide (D2O) administered topuppies. Hydrogen isotopes have beenshown to yield valid estimates of milkproduction if corrections are made forchanges in body water pool size and forisotope recycling via maternal milk (11-15).Peak lactation was assumed to occur in week3 or 4 postpartum since puppies do notinitiate feeding on semisolid food until theemergence of deciduous dentition at 21 to 35days postpartum (16-18). Milk alone willsupport normal growth up to 4 weeks postpartum; thereafter, withholding of supplemental food may result in a reduced growthrate (19).

MATERIALS AND METHODS

Experimental animals. Five bitches \\erestudied at the long-established beagle research colony at Cornell University (20).The bitches were 1.5-3.7 years of age, hadpostpartum weights of 9.3-15.2 kg (mean= 12.7 kg) and were producing their first or

second litters. One week prior to the expected parturition date each dog wasremoved from the main colony to an isolatedwhelping room. They were individuallyhoused in 1.2- x 1.1-m cages (horizontaldimensions) with 0.7- x 0.7-m heatedwhelping boards, and were fed a commercial dry dog food (Wayne Dry Dog Food,Allied Mills, Inc., Chicago, IL) containingabout 26% crude protein, 9% fat, 4% crudefiber and 2.75 kcal metabolizable energy(ME) per gram.2 Water was provided adlibitum in elevated stainless-steel bowls thatwere too high for the puppies to drink from.At birth litter size ranged from 5 to 10 puppies; 5 to 7 puppies per litter survivedbeyond 1 week postpartum. The puppieswere first offered supplemental feed [drydog food (Wayne Dry Dog Food) soaked inevaporated milk and water] at 29-30 dayspostpartum. Puppies were weighed to thenearest gram at least three times per weekover the course of the study.

Milk sampling and analysis. Milk sampleswere collected at weekly intervals from 7-37days postpartum. Additional samples werealso taken during weeks 2 and 5 postpartum. Bitches were removed from theirlitters for 2-3 hours prior to milking. Oxy-

tocin (5 IU) was administered by intramuscular injection, and one or two teats evacuated as completely as possible by gentlemanual expression. An average of 21 ml( ±6.3 SD) was obtained in 10-15 minutes.Samples were frozen in sealed vials untilanalyzed.

Milk samples were thawed quickly, homogenized in a Potter-Elvehjem tissuegrinder and subsampled. The weekly samples were assayed in duplicate for majorconstituents. Total solids were determinedby oven drying, total nitrogen (TN) andnonprotein nitrogen (NPN) by a Kjeldahlprocedure, fat by the Roese-Gottlieb method(21) and sugar by the phenol-sulfuric acidcolorimetrie method, as previously described(21). Sample size did not permit NPN determination on eight samples. In such cases anadditional sample collected from the samebitch within 2 to 3 days was substituted.Both TN and NPN were measured on theseadditional samples. Protein was calculatedas 6.38 x (TPN - NPN). Gross energywas estimated from an equation developedby Perrin (22), as previously presented (21).

Milk intake estimation procedure. Milkintake was estimated from water kinetics of25 puppies in the five litters. D2O (99.8%purity) was administered by stomach tubeto puppies at 15-16 days and 22-23 dayspostpartum at a rate of 2.3 g/kg bodyweight. One young in each litter was notgiven D2O so that correction could be madefor isotope recycling. Two hours were allowed for isotope equilibration prior tocollection of about 2 ml of blood by jugularpuncture; in young puppies hydrogen isotopes equilibrate in 1.5 hours (23). Eachpuppy was bled at 2- to 3-day intervals suchthat four samples were collected duringeach weekly study period for determinationof water turnover. The second isotopeadministration immediately followed thefinal bleeding of the preceding period suchthat residual isotope levels could be measured. Blood water was isolated by heatdistillation and assayed for deuterium concentration by infrared spectrophotometry(24) using matched barium fluoride cells(0.11-mm path length) in a double beam

"Nutritional information supplied by manufacturer.

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grating infrared spectrophotometer (Model521, Perkin-Elmer Corp., Norwalk, CT).

Assayed deuterium levels were correctedfor body weight changes in computations offractional turnover rate (k) and body waterfraction (F)(ll). Isotope recycling via maternal ingestion of the excreta of sucklingyoung, followed by transfer of isotope inmilk water from mother to young (12),necessitated an additional correction. Theaccumulation of deuterium in an unin-jected, control puppy in each litter wasmonitored. On the assumption that theselevels are representative of recycled isotopein littermates, the deuterium levels in control puppies were subtracted from thedeuterium levels of their littermates prior toregression of corrected log D2O concentration against time after administration.Daily water loss, water gain and waterintake were computed as previously described (21). The proportions of milkconstituents catabolized to produce metabolic water were estimated by iterativecalculations detailed elsewhere (11). Statistical analyses were performed usingprograms of the Statistical Package for theSocial Sciences (SPSS) on a Honeywellcomputer at the Smithsonian Institution.Mean values are presented as mean ±SEMunless otherwise indicated.

RESULTS

Milk composition. Over the period of 7 to37 days postpartum (table 1) there were nosignificant differences in total solids, fat,protein or gross energy content amongsampling times [P > 0.05, analysis of vari

ance (ANOVA)]. Sugar content did differamong sampling times (P < 0.05, ANOVA),the mean value rising from 3.47% at 7 to 9days postpartum to 4.13% at 29 to 30 dayspostpartum. The mean values for all sampling times were: 22.7 ±0.41% total solids,9.47 ±0.386% fat, 7.53 ±0.123% protein,3.81 ±0.079% sugar and 146 ±3.6 kcalgross energy per 100 g. If converted to a drymatter basis dog milk was found to contain41.4 ±0.87% fat, 33.4 ±0.60% protein,17.0 ±0.049% sugar and 641 ±3.9 kcal/100 g dry matter. Fat, protein and sugarprovided 58.7 ±0.86%, 30.5 ±0.66% and10.5 ±0.33% of total gross energy, respectively. There were no significant differencesin milk composition among the five bitches,whether compared on a whole-milk, drymatter or gross energy basis (P > 0.05,ANOVA). NPN ranged from 0.045 to 0.068%(mean = 0.054 ±0.0012%), equivalent to3.4-5.2% of total nitrogen. NPN did notdiffer significantly among bitches or sampling times (P > 0.05, ANOVA).

Water and milk intakes. Water and milkintakes were calculated for the midpoints ofeach study period, i.e., for 19 and 26 dayspostpartum (table 2). Growth rates and estimated body weights at these ages derivefrom regressions of puppy weight on postnatal age These regressions were highlylinear (mean r2 = 0.990). Regressions of the

logarithm of corrected deuterium concentration on time after isotope administrationwere also highly linear (mean r2 = 0.996).Deuterium content in body water of unin-jected control puppies reached levels equivalent to 11.5 ±0.45% and 10.6 ±0.30% ofthe levels in injected littermates by the final

TABLE 1

Composition of dog milk1'*

Time of milk sampling, days postpartum

Constituent3Total

solids,%Fat,%Protein,%Sugar,%Gross

energy,kcal/100g7-923.5

±1.31*10.90±1.370*7.17±0.092'3.47±0.087*155

±12.3*15-1622.9

±1.41'9.67±1.171*7.59±0.275*3.61±0.111*b147

±11.9*22-2322.1

±0.51*8.73±0.230*7.72±0.390*4.04±Q.nS**141

±3.4*29-3022.3

±0.64*8.89±0.410*7.49±0.303*4.13±0.149C142

±4.5*36-3722.6

±0.72*9.16±0.482*7.70±0.281*3.81±o.ise'1*144

±5.0*Total22.7

±0.419.47±0.3867.53±0.1233.81±0.079146

±3.6

'Mean ±SEM; n = 5 for each time period; n - 25 for total. "Means with the same superscript in a row do not differ bymore than the shortest significant range at the 0.05 significance level (Duncan's multiple-range test). 'Percentage values

represent gram per 100 g.

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blood sampling in the first and second studyperiods, respectively. Correction for isotoperecycling was therefore warranted.

Fractional turnover rate of body water (k)declined from 0.168 ±0.0036 per day forthe first study period to 0.152 ±0.0034 perday for the second (P < 0.001, paired i-test).By contrast body water fraction (F) did notdiffer significantly (P > 0.05, paired f-test)between the two periods (table 2). Calculated water losses, water gains and waterintakes at 19 and 26 days postpartum arepresented in table 2. Water intake wasequivalent to 15.1 and 13.9% of body weightat 19 and 26 days, respectively.

Water intake derives from both preformedmilk water and metabolic water from thecatabolism of milk solids. Body compositiondata presented by Sheng and Huggins (25)indicate weight gain comprises 12% proteinand 13% fat in the period of 16-33 dayspostpartum. On the basis of these values,combined with milk composition, growthrate and water intake data, one can calculate that the amounts of fat and proteincatabolized are equivalent to 69 and 72% ofingested fat and 63 and 67% of ingestedprotein at 19 and 26 days, respectively. Itwas assumed that 100% of ingested sugarwas catabolj^ed. Ingestion of 100 g milk willthen yield 77.3 g preformed and 11.7g metabolic water at 19 days and 77.3 g preformedand 11.7 g metabolic water at 26 days.Milk intake was estimated as water intakex 1.129at 19 days and water intake x 1.124at 26 days (table 3). Individual milk intakesof puppies ranged from 126 to 239 g/day at19 days and from 134 to 229 g/day at 26 days.

TABLE 2

Body water turnover in suckling puppies

Time after parturition, day

Measure 19 26

Body wt, gWt gain, g/dayFractional

turnover, (k)Wt fraction,1 (F)

Water loss, g/dayWater gain, g/dayWater intake, g/day

942 ±25.436.8±1.28

1199±33.336.8±1.28

0.168±0.00360.152±0.00340.726±0.00680.716±0.0084115±4.2 130±4.126.6±0.80 26.1•0.68

142±4.8 156±4.7

'Grams body water per gram body weight.

Milk intakes of puppies were comparedamong litters and between the two postnatal ages by two-way ANOVA. Whetherexpressed as a daily amount, as a percentageof body weight per day, or per gram bodyweight gain, mükintake was significantlyinfluenced by both litter and age effects(table 3). Although the absolute amount ofmilk consumed per day at 26 days (175±5.3 g) was greater (P < 0.01) than that at19 days (160 ±5.4 g), this amount represented a smaller percentage of body weight(14.6% at 26 days vs. 17.0% at 19 days, P< 0.001). Since the same estimate of growthrate was used for both age categories, milkintake per gram body weight gain was ofcourse greater at 26 days (table 3). Themean milk intakes in four litters were relatively similar (147-156 g/day at 19 days;159-178 g/day at 26 days), but the puppies inthe litter of bitch BR 82 consumed appreciably more milk on average (203 g/day at19 days and 221 g/day at 26 days). Puppiesin the larger litters tended to be smaller andgrow at a reduced rate but did not appear toingest substantially less milk than the puppies in smaller litters at these ages (table 3).

Total milk output of lactating bitches(milk intake per puppy x litter size) wasestimated as 964 ±57.6 g/day (n = 5) at 19days and 1054 ±57.7 g/day at 26 days(table 3). These estimates are not significantly different (P > 0.05, paired i-test).These yields correspond to 7.6% of maternalweight or 143 g/kg°75at 19 days and 8.3%or 157 g/kg°7Sat 26 days.

Nutrient intakes. By combining meanmilk composition (table 1) and mean milkintake (table 3) data, the intake of variousconstituents can be calculated. At 19 dayssuckling puppies ingested 36.3 g dry matter(3.86% of body weight), 15.2 g fat, 12.0 gprotein, 6.1 g sugar and 234 kcal (224kcal/kg0 7S).At 26 days intakes were 39.7 g

dry matter (3.31% of body weight), 16.6 gfat, 13.2 g protein, 6.7 g sugar and 256 kcal(223 kcal/kg075). For each gram of body

weight gain, puppies ingested 0.33 g proteinand 6.4 kcal at 19 days and 0.36 g proteinand 7.0 kcal at 26 days.

DISCUSSION

Milk composition. Milk collected frombeagle bitches contained on average 22.7%

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TABLE 3Milk intakes of suckling puppies'

BR82BR50CA46CD 18CA45All litters

BR82BR50CA46CD 18CA45All litters

55677

Milk intake of puppies

BitchdesignationLitter sizeDaily intakeAs ÕÕwtPer

gramwt gainMilk

outputof bitch

203152154156147160

±5.221166178165159175

±519

dayspostpartum18.714.816.717.317.34

17.0 ±0.3026

ilnu\postpartum15.712.815.114.414.83

14.6 ±0.22

4.514.024.134.544.55

4.37 ±0.074

4.914.394.804.824.92

4.79 ±0.071

Analysis of variance

1015760924

10921029

964 ±57.6

1105830

106811551113

1054 ±57.7

Litter effectF(4,44)

Age effectF(l,44)P

< 0.001

P < 0.01P

< 0.001

P < 0.001P

< 0.001

P < 0.001—

'Milk intakes are litter means. Values after ± are SEM.

total solids, 9.5% fat, 7.5% protein and3.8% sugar. Mean values from prior studieson dog milk are tabulated for comparison(table 4). This list includes 19th centuryresults of questionable analytical accuracyas well as studies involving only a few samples from one or two dogs. Samples collectedvery early or late in lactation have beenexcluded as not representative of establishedlactation. Despite variation in sampling andanalytical procedures, most reports fallwithin the ranges of 21-26% total solids,8-12% fat, 7-10% protein and 3-4% sugar(table 4). The results reported herein areconsistent with these values. By contrast therecent data of Lönnerdal and colleagues (8)indicate much lower fat (4.8%) and protein(5.2%) levels in beagle milk collected 11-40days postpartum. This discrepancy maystem from inappropriate application ofrapid spectrophotometric methods. Colordevelopment in the sulfuric acid-phosphoricacid-vanillin reaction employed in the

determination of fat depends on the degreeof unsaturation of the lipids (34). Binding ofCoomassie brilliant blue G250 dye to protein is likewise a function of the amino acidcomposition of the protein (35, 36). Thesemethods are valid only if standardized to theparticular mix of lipid and protein constituents found in dog milk. It appears that thiswas not done

The variation among the remainingstudies may be a function of sampling oranalytical bias or may represent real differences among dogs. Dog breeds vary tremendously in body size and conformation,but no correlation to the gross compositionof milk could be determined by Russe (6)who studied breeds ranging in size fromdachshunds to Saint Bernards. Beagle milkcollected by Luick and colleagues (5) contained more total solids, fat and protein, butless sugar (table 4) than was found in thepresent study. By contrast the recent data ofMundt and colleagues (7) are very similar to

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TABLE 4

Published data on the composition of dog milk at midlactation

SourceFemalesmilked

Days afterbirth

No. ofsamples

Totalsolids Fat Protein Sugar

Ssubotin 1866(26)Tolmatscheff1867(27)Abderhalden1898(28)Abderhalden1899(29)Dijkstra

1910(30)Grimmer1915(31)Daggs

1931(10)Deniges1935(32)Anderson

et al. 1940(4)Luicket al. 1960(5)Russe1961(6)Laueret al. 1969(33)Mundtet al. 1981(7)Lönnerdal

et al. 1981(8)Presentstudy312111321372716512-38?355-1110-146-205-2621-35?18-3015-365-35307-2811-407-3712'2913222'8122-4'3032-63'218-28'62-71'2520.8————20.823.924.222.626.022.026.422.0—22.78.811.811.511.66.58.512.410.68.312.311.112.39.74.89.58.8s8.227.027.3s6.837.2'8.039.337.559.8*6.958.2*7.3"5.2"7.5'2.73.23.33.12.6—3.22.73.73.33.32.93.64.53.8

'Early and/or late lactation samples excluded. 2Protein determined by precipitation and weighing. 'Assaymethod not specified. 4Protein calculated as protein nitrogen (Kjeldahl) x 6.38. 'Protein calculated as totalnitrogen (Kjeldahl) x 6.38. 'Protein measured by dye-binding procedure.

the present results although four breeds aswell as mongrels were included. Breed differences in the composition of dog milkmust be minor if they exist at all.

An elevation in total solids and protein atboth the beginning and the end of lactationhas been noted in prior reports (4, 6, 31). Inthe present study there were no substantialchanges in milk composition from 7 to 37days postpartum, a week after supplementalfeeding of puppies commenced. Markedcompositional changes are apparently associated with mammary involution at about39—49days (4, 6). Weaning is of courseinfluenced by feeding and managementpractices, and perhaps by breed as well (6).Lonnerdal et al. (8) have reported a rise infat and protein contents of beagle milk from0 to 40 days, and a subsequent decline in fatcontent. These trends are at odds with otherfindings and could reflect sensitivity of thespectrophotometric assay procedures toqualitative as well as quantitative changesin milk constituents. Canine caseins andwhey proteins certainly differ in amino acidcomposition (37) such that varying proportions during lactation (6) will affect proteincontent as measured by dye-binding.

The NPN content of dog milk averaged0.054%. The data of Grimmer (31) andRusse (6) indicate mean NPN values of 0.068and 0.112%, respectively. In the presentstudy NPN accounted for only 4.4% of totalnitrogen, as compared to 5.7 (31) and 9.3%(6). Protein estimates based on total nitrogen (e.g., see refs. 4, 5, 7, 33) will overestimate true protein by an amount equal toNPN x 6.38, i.e., by about 0.3 to 0.7percentage points.

In 100 g milk, 1.88 ±0.062 g of solidswas not accounted for by the summation offat, protein and sugar. Part of the residual isdue to ash and part to NPN constituents. Ifpublished data on the ash content of dogmilk in the period of 5 to 35 days postpartum(4, 7, 10, 28, 30-33) are considered collectively, a mean ash content of 1.15% can becalculated (n = 57). Assuming the NPNconstituents of dog milk to be similar inproportion to those of cow's milk (38), NPN

X 5.34 = 0.29% gives an approximation ofthe combined weight of NPN constituents(11). The remaining 0.44 g ( = 1.88 - 1.15- 0.29) represents minor organic andinorganic compounds not included in thevarious analytical fractions (38) as well as

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analytical error. On this basis it wouldappear that any such error was small.

Water and milk intakes. Suckling beaglepuppies were estimated to consume 142 gwater, equivalent to 160 g milk, at 19 dayspostpartum, and 156 g water, equivalent to175 g milk, at 26 days postpartum (table 2).These values are only as accurate as theestimates of body water fraction (F) andwater turnover rate (fc)on which the calculations are based. Isotope dilution procedures have been reported by Sheng andHuggins (23, 25) to overestimate body watercontent in growing beagles when comparedto values obtained by direct dessication.Dilution of tritiated water indicated bodywater percentages of 75-92% at 8-21 days(23), whereas direct dessication values were65-74% at the same sampling times (25).These results have been contested with allegations that the analytical proceduresmust have been in error (39). Neither themean values obtained in the present study(72-73%) nor the values reported by Romsosand colleagues (9) for 4-week-old puppiesadministered tritiated water (74-76%) appear to be greatly in excess of expectedvalues. Widdowson (40) reported that3-week-old puppies contain 68% water. Itappears that isotope dilution may overestimate body water in puppies by a fewpercentage points but not to the degreereported by Sheng and Huggins (23).

The decline in water turnover rate from0.168 in the first week of study to 0.152 inthe second may reflect in part that puppieswere not allowed supplemental water orfeed prior to 29 to 30 days postpartum.Maternal régurgitationof feed to puppieswas not observed; disgorging of food hasbeen reported as early as 21-24 days postpartum by Martins (41). Any ingestion ofwater from sources other than mother's milk

would lead to overestimation of milk intake.Fractional turnover rates of 0.168 and 0.152correspond to body water half-lives of 4.1and 4.6 days. Four-week-old beagle puppiessuckling bitches fed canned, semipurifieddiets of high or low carbohydrate contenthad somewhat longer half-lives of 5.0 and6.8 days, respectively (9). These data werenot corrected for either isotope recycling orchanging size of the body water pool, however. Baverstock and Green (12) calculate

that at least 47% of the isotope lost by dingopuppies is ingested by the mother; some ofthis is recycled to the young in milk. Isotoperecycling was not measured directly in thepresent study although accumulation ofD2O in uninjected control puppies indicatedthat it did occur.

The milk intakes determined herein aremuch higher than the 81 ±13 ml/day and59 ±13 ml/day reported by Romsos et al.(9) for beagle puppies suckling bitches fedtwo semipurified diets. Aside from themethodological problems mentioned above,these puppies exhibited abnormally lowgrowth rates, gaining on average only 13and 11 g/day in the two groups. Normalgrowth rates for beagle puppies are 35-40g/day (17, 19, 42, 43). Records for theCornell dog colony for 1975-1976 indicateaverage growth rates of 34.9 ±0.99 g/dayfor puppies in litters of five (n = 25 puppies), 29.9 ±1.23 g/day for puppies inlitters of six (n = 30) and 27.2 ±1.08 g/dayfor puppies in litters of seven (n = 35). Thegrowth rates observed in the present study(36.8 ±1.28 g/day) for puppies sucklingbitches fed a commercial dry dog food aresomewhat above the colony norms, butwithin the normal range for beagle puppies.The semipurified diets used by Romsos et al.(9) apparently did not support normal milkyields.

The milk intake of 26-day-old puppieswas equivalent to 14.6% body weight, adecline from the 17.0% of body weightconsumed at 19 days. Mundt and colleagues(7) reported that milk intakes of puppies inthree litters of various breeds averaged10.0-13.5% of body weight in the first 4weeks postpartum. These data derive fromweights taken before and after puppies wereallowed to suckle. The experimental regimen of separation and periodic, controlledaccess of young to mother may cause areduction in secretion rates, may interferewith normal maternal nursing behavior, ormay result in the accumulation of amountsof milk that young are unable to consume inrelatively short suckling bouts (3). Henceweight differential procedures tend tounderestimate milk consumption rates.Mundt et al. (7) note that puppies consumedmore than 95% of daily milk intake in fouror five of the six suckling bouts permitted

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per day; i.e., one or two suckling bouts perday were relatively unsuccessful. Thesepuppies reportedly consumed 1.7-3.0 g milkper gram body weight gain in weeks 3 and4 (7). By contrast puppies in the presentstudy were calculated to consume 4.4 g milkper gram gain during week 3 and 4.8 g milkper gram gain during week 4. On this basisit appears that the weight differential procedure underestimated milk intake by aboutone-third.

Peak milk yields of beagle bitches appearto be about 1 kg/day in week 4 postpartum.Milk yield is undoubtedly influenced bybody size (2, 3). Using a weight differentialprocedure, Russe (6) estimated that at 6days postpartum a German shepherd produced 915 g as compared to 102 g and 184g for two dachshunds. Only the Germanshepherd was studied throughout lactation;peak yield (about 1.7 kg/day) was observedat 22-27 days (6). Assuming a body weightof 30 kg, milk production of this dog wasabout 130 g/kg075 as compared to 157g/kg°75for beagle bitches at 26 days post

partum in the present study. Bias associatedwith the weight differential method mayexplain some or all of this 20% difference

Nutrient intakes in relation to estimatedrequirements. The Subcommittee on DogNutrition of the National Research Council(NRC) (44), following Payne (45), listed thedaily ME requirement of 3- and 6-week-oldpuppies as 274 kcal/kg0'75. In the present

study puppies ingested only 244 kcal grossenergy per kilogram0 75at 26 days. If conver

sion factors of 4.0 kcal ME per gram proteinor sugar and 9.0 kcal ME per gram fat areadopted, dog milk can be calculated tocontain 131 kcal ME/100 g. The ME intakeswould then be 219 kcal/kg"7S at 19 days and200 kcal/kg0 7Sat 26 days. It appears that the

National Research Council (44) overestimated the metabolizable energy needs ofyoung puppies by 25-35%.

Recently Mundt et al. (7) estimated thedaily maintenance requirements of puppiesas 72 kcal gross energy per kilogram0 75from

a regression of gross energy intake of suckling puppies on growth rate. As the milkintakes of the puppies were probably underestimated by a substantial amount (seeabove), this estimate cannot be consideredreliable

Weaned puppies have been estimated bythe Subcommittee on Dog Nutrition, NRC(44) to require 22% protein in the drymatter of a diet containing 3.5-4.0 kcal MEper gram dry matter. Dog milk containsabout 131 kcal ME/100 g or 5.77 kcal MEper gram dry matter, a value about 50%above the NRC diet. If dog milk is to meetNRC requirements it should contain 1.5x 22 = 33% protein on a dry matter basis.The protein content of dog milk was indeedfound to be 33.4% of dry matter.

Puppies ingested 0.33 g protein per grambody weight gain at 19 days and 0.36 gprotein per gram body weight gain at 26days. Payne (45) assumed weight gain inpuppies to contain 17% protein, whereascarcass analyses by Sheng and Huggins (25)indicate protein content to remain at about10-12% of body weight from birth to 6weeks postpartum. At peak lactation puppies apparently incorporate only one-thirdto one-half of ingested protein into tissue

ACKNOWLEDGMENTS

I would like to thank Dr. Richard G.Warner for authorizing use of the dogcolony at Cornell University, Dan Shattuckand staff of the colony for care of the animals and assistance with milk and bloodcollection procedures, and Dr. CharlesRoberts and David Dance of the Smithsonian Institution for performing statisticalanalyses. Milk analyses were conducted inthe laboratory of Dr. John Sherbon. Drs.Maiden C. Nesheim and Harold E Hintzoffered constructive criticism and support.

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