dietary energy sources for the american alligator, alligator mississippiensis (daudin)

Aquaculture, 89 ( 1990) 245-26 1 Elsevier Science Publishers B.V.. Amsterdam

245

Dietary energy sources for the American alligator, Alligator mississippiensis (Daudin )

Mark A. Staton=, Hardy M. Edwards, Jr. b, I. Lehr Brisbin, Jr.‘, Larry McNeased and Ted Joanend

“Mainland Holdings Pty. Ltd., Crocodile Farm, P. 0. Box 196, Lae (Papua New Guinea) bDepartment ofPoultry Science, University of Georgia, Athens, GA 30602 (U.S.A.)

Savannah River Ecology Laboratory, Drawer E, Aiken, SC29801 (U.S.A.) dLouisiana Department of Wildlife and Fisheries, Rockefeller Wildlife Refuge,

Grand Chenier, LA 70642 (U.S.A.)

(Accepted 25 January 1990)

ABSTRACT

Staton, M.A., Edwards, Jr., H.M., Brisbin, Jr, I.L., McNease, L. and Joanen, T., 1990. Dietary energy sources for the American alligator, Alligator mississippiensis (Daudin) . Aquaculture, 89: 245-26 1.

Three experiments were conducted to evaluate responses of alligators to dietary inclusion of fat, carbohydrate or vegetable protein. Dilution of a high-protein, low-fat, carbohydrate-free diet with graded levels of fat (3.6-16.6% dietary dry matter) resulted in significantly greater body weight gains and improved feed conversion. Percent carcass fat was greater with increased dietary fat. Digestibility of protein (87.3-89.2%) and energy (84.6-86.8%) decreased slightly with increases in caloric density of the diet. Digestible energy (DE) and a nitrogen-corrected digestible energy (DE,) increased with dietary gross energy. A second experiment compared two high-protein diets (meat and a purified diet) and the effects of supplementing these diets with glucose or extruded corn as 20% of dietary dry matter. The purified diet led to greater gains in body weight than the meat diet. Supplementation with corn did not significantly influence performance. Glucose supplementation of meat significantly improved body weight gains, but similar supplementation of the purified diet decreased performance. In a third experiment, alligators fed a carbohydrate-containing diet consumed significantly more feed and gained significantly more weight than those fed diets with protein as the corresponding energy source. Digestibility of protein, approximately 40% of which was isolated soybean protein, was very high (96.0-97.1%). Variation in energy digestibility (78.5-91.3Oh) was related to dietary fiber content. Liver lipogenesis from acetate was generally greater than from leucine and glucose. However, for alligators fed the carbohydrate-containing diet, liver lipogenesis from glucose and acetate was not significantly different. In several experiments, carcass and adipose tissue fatty acids were generally reflective of dietary treatment. However, steak acid levels were low and unaffected by diet.

INTRODUCTION

Leather products made from the hides of alligators (Alligator mississippiensis) and other crocodilians are highly valued. Farming has emerged as a major source of quality crocodilian hides because, on a worldwide basis, many

0044-8486/90/$03.50 0 1990 - Elsevier Science Publishers B.V.

246 MA. STATON ET AL.

natural populations are depleted and protected by law. In the southeastern United States, where over one hundred alligator farms are now established, alligator meat, sold primarily to seafood retailers and restaurants, also repre- sents a significant source of income to farms.

As carnivores, farmed alligators are fed a variety of inexpensive meats, such as livestock offal, fish scraps, and carcasses of wild animals. The preferred diet for alligators of all ages has been the ground carcass of nutria (Myocastov coypus), a fur-bearing rodent trapped in Louisiana (McNease and Joanen, 198 1) . The use of meat rations presents a number of problems to the alligator farming industry including availability, transportation, storage, handling and a lack of control over nutrition. The availability of practical feeds manufac- tured with conventional feedstuffs and feed-mill technology would offer many advantages to this industry. Feed formulation for alligators is complicated by a lack of information on nutrient requirements as well as several nutritional peculiarities which have been attributed to this species. These include a reported inability to digest plant proteins and carbohydrates other than glucose, or to grow maximally on diets containing modest amounts of fat (Coulson and Hernandez, 1983; Coulson et al., 1987). Since the bulk of energy sources in conventional feeds fall into these categories, such limitations represent re- strictions on feed formulation. Manufacturing options are also limited as carbohydrates are important in the manufacture of extruded pellets, a form of feed which appears well suited for alligator consumption.

Here we report results of feeding trials conducted to further characterize these reported dietary limitations. Vegetable protein, carbohydrates and fat were included as protein and/or energy. In addition to production parame- ters, the influence of diet on carcass composition, digestibility of protein and energy, and lipogenesis from various energy substrates were considered. Ex- periment I evaluated the influence of increasing the fat content of the diet. Experiment II compared two high-protein diets and the effect of carbohydrate supplementation. Experiment III compared the responses to diets in which energy was presented as either carbohydrate, protein, or fat.

MATERIALS AND METHODS

Animals and housing Hatchling alligators were transported to facilities at Athens, Georgia, in 1986

and 1987 immediately after artificial incubation at Rockefeller Wildlife Ref- uge according to the methods of Joanen and McNease ( 1977 ) . Animals were fed vitamin-supplemented nutria meat (McNease and Joanen, 198 1) until the initiation of the experiments at l-3 months of age.

Experiments I and III were conducted for 20 and 12 weeks, respectively, in grow-out facilities consisting of rectangular, 0.6 m deep, covered, liberglass- lined tanks. Each tank provided a total living area of 0.9-1.1 m2 which was

DIETARY ENERGY SOURCES FOR ALLIGATOR MISSISSIPPIENSIS 247

partitioned into two identical pens. Six alligators in Experiment I, and five in Experiment III, were randomly assigned to each pen with the constraint that average body weight in each pen was statistically equal and within-pen variation in body weight was minimized. Four pens were then randomly assigned as replicates of each treatment. Living space was divided into dry and water (4-8 cm deep) in a 1: 2 ratio. Tank water was part of a heated (29-32°C) recirculating water system which cycled 20 of every 60-120 min. Housing conditions have been described in greater detail elsewhere (Staton et al., 1989a). At the beginning and end of these experiments, total length was measured ( + 0.5 mm) individually. Body weights were determined ( ? 0.5 g) col- lectively for each pen. Alligators were also weighed 5 weeks into Experiment I and 4 weeks into Experiment III, after which feed consumption was re- corded and feed conversion determined.

Experiment II lasted 11 weeks. Alligators were housed in 0.6 x 0.6 m plastic tanks filled to a water depth of 8 cm. Allocation of three animals to three replicates of each treatment group was performed as above. Water temperature was allowed to fluctuate with room temperature (29-32°C). A 10x20 cm plastic platform elevated above the water allowed animals to emerge from water freely and served as a feeding station. Body weights were determined ( +0.5 g) at the beginning and end of the feeding trial. In all experiments tanks were washed and refilled with clean warm (27-3 1 o C ) water after each feeding. Animals were maintained in darkness except during feeding and cleaning.

Diets Diets were either ground nutria meat supplemented with vitamins as rec-

ommended by McNease and Joanen ( 198 1) or dry semi-practical and purified diets. In Experiment I, fat levels were varied by diluting the dry basal ration (Table 1) with six graded levels of fat (diets l-7). This fat mixture was composed of menhaden fish oil (25%; Standard Products Co., Kilmar- nak, VA), and locally purchased lard (40%)) safflower oil ( 15%) and linseed oil (20% ) . This combination was used because its fatty acid composition generally resembled that of nutria fat in carbon chain length and degree of unsat- uration (Table 2 ) .

In Experiment II, a purified diet (Table 1) and nutria meat (approximately 30% dry matter) were supplemented with either extruded corn (Siler City Mills, Inc., Charlotte, NC) or glucose (Cerelose, Corn Products, Inc., Atlanta, GA) as 20% of dry matter. For the purified diet, glucose or corn were substituted for casein, gelatin, soybean protein, and amino acids in the pro- portions shown in Table 1.

In Experiment III, diet 1 was a high-protein, low-energy diet (Table 1) which contained 23% added fiber (Solka Floe, Brown Co., Berlin, NH). Three higher-energy diets were made by including isocaloric quantities of protein,

248 M.A. STATON ET AL.

TABLE 1

Composition of experimental diets

Ingredient (g/ 100 g diet) Basal I’ Purified II’ Basal III’

Blood meal Feather meal Gelatin Casein Isolated soybean protein Glycine DL-methionine Tryptophan Arginine HCI Fat mixture’ Limestone Defluorinated phosphate Potassium carbonate Sodium chloride Magnesium carbonate Trace mineral premix3 Selenium premix4 Vitamin premixS Chromic oxide Carboxymethyl cellulose Cellulose (Solka Floe)

30.75 19.00 2.00

40.50 -

0.20 -

0.30

2.00 -

1.40 0.50 -

0.20 0.05 1.00 0.10 2.00 -

- 2.00

44.15 33.00

2.00 0.30 0.20

2.00 33.40 25.20

2.00 0.15 0.40

9.50 0.50 2.50 1.40 0.75 0.35 0.20 0.05 1.00 0.10 2.00 -

-

5.00 1.50 1.50 1.40 0.75 0.35 0.20 0.05 1.00 0.10 2.00

23.00

‘Number indicates the experiment in which the diet was used. ‘40% lard, 25% fish oil, 20% linseed oil, and 15% safflower oil. 3Provided the following in mg/kg of diet: Mn, 240; Zn, 200; Fe, 120; Cu, 20; I, 4.2; Ca, 300-360. “Provided 0.1 mg Se/kg of diet. ‘Provided the following per kg of diet: vitamin A, 18 000 IU; vitamin D, 2000 IU; vitamin E, 150 IU; menadione sodium bisulfite, 25 mg; thiamine, 15 mg; riboflavin, 15 mg; pyridoxine, 25 mg; vitamin Bi2, 0.042 mg; niacin, 200 mg; pantothenic acid, 50 mg; folic acid, 4.0 mg; biotin, 1.0 mg; choline, 1500 mg; inositol, 50 mg; para-amino-benzoic acid, 50 mg; ascorbic acid, 450 mg; ethoxyquin, 150

mg.

carbohydrate or fat at the expense of added dietary Iiber. In diet 2, protein (identical composition to protein in the basal diet, Table 1) was included as 18.6% of diet. In diet 3, a carbohydrate source (high solubility dextrin, Sigma Chemical Co., St. Louis, MO) was included as 23% of diet. Diet 4 contained 9.9% added fat (mixture described above), Gross energy values for protein, carbohydrate and fat of 5.4,4.1 and 9.3 kcal/g, respectively (Sturkie, 1986), were used in making these substitutions. The added fiber contents of diets l- 4 were 23,4.4,0 and 13.1%, respectively.

In all experiments, animals were fed three times weekly. In Experiment I, alligators were adapted from meat to experimental diets during the first 5 weeks. From weeks 6-20 of Experiment I and for all of Experiments II and III, animals were fed to satiation by providing more feed than would be consumed at each feeding. Dry diets were fed to alligators by adding water (40- 45% by weight) to form a moistened cake of a consistency which could be


TABLE 2

Major fatty acids in nutria carcass fat, added fat, and basal diet fat of Experiment I

Fatty acid Nutria’ Added fat’ Basal diet Experiment I

14:03

16:0 16:l 16:2 17:o 18:0 18:l 18:2 18:3 20:4 20:s 22:5 22~6

< 2.8

24.0 6.8 1.0 1.0 8.1

18.4 18.7 14.8 2.2 0.2 1.1 1.1

% of fatty acids 3.4

17.0 4.7 0.8 0.9 9.2

25.9 19.9 12.6

3.3 0.5 1.8

, 7.4

28.8 7.1 1.3 1.2 9.8

32.9 9.7 1.5 0.4

‘Staton et al. (1989b). ‘40% lard, 25% fish oil, 20% linseed oil, and 15% safflower oil; used in Experiments I, II and III. 3Carbon chain length : number of double bonds.

chopped into bite-sized pieces. Feed was placed in dry areas of the tanks for a minimum of 1.5 h. Dry matter consumption was calculated by subtracting the weight of feed uneaten from that offered, and adjusting for moisture.

Feed and fecal analysis Chromic oxide was included at 0.1% as a dietary marker. Digestibility of

protein and energy was determined with a standard equation (see NRC, 1983, p. 40):

Percent nutrient digestibility

=100-100x (’ c O 00 r2 3 in feed) (% nutrient in feces)

(% Cr2 O3 in feces) ’ (% nutrient in feed)

Alligators deposit feces as semi-dry pellets, usually in water. Intact fecal pellets were removed within l-2 hours after tank cleaning in order to minimize contamination and leaching. Feces were freeze-dried prior to analysis. Feed and fecal chromic oxide was determined by the method of Brisson ( 1956). Dietary and fecal crude protein were determined using the Kjeldahl nitrogen method (AOAC, 1970). Gross energy was measured in an adiabatic bomb calorimeter (Parr Equipment Co., Moline, IL). All determinations were made in duplicate.


Digestible protein (DP) and digestible energy (DE) were calculated by multiplying the analyzed values of dietary crude protein and gross energy by their respective digestive coefficients. As dietary protein was included at high levels and served as an energy source for alligators, a nitrogen-corrected DE (DE,) was calculated to reflect a zero nitrogen balance:

DE, = DE - 8.22 x g N retained per g of diet

where:

Cr, O3 in diet g N retained per g of diet = N per g of diet-N per g of feces X Cr o

2 3 in feces

This nitrogen correction is taken from a standard poultry equation (Scott et al., 1982, p. 537) for metabolizable energy. The nitrogen correction constant of 8.22 (the gross energy of uric acid) almost certainly leads to an underesti- mate of the DE, of these diets for alligators since their nitrogenous excretory products include urea and ammonia as well as uric acid (Coulson and Her- nandez, 1983).

Tissue analysis and radiotracer studies Carcass and tissue lipids were extracted using the method of Folch et al.

( 1957). Fatty acids from the Folch extract were methylated by refluxing in 5% sulfuric acid in methanol for 2.5 h. Following extraction with petroleum ether, fatty acid composition was determined by gas-liquid chromatography according to column conditions and methods of identification described by Nugara and Edwards ( 1970).

Eight weeks into Experiment III, the alligator weighing closest to the average for each replication of each treatment was injected intraperitoneally with 0.5 &i l-carbon-l 4-acetate about 2 h after a feeding. Six h later, the animal was sacrificed by cervical dislocation and frozen immediately in liquid air. Later the animal was thawed and the liver and a l-2 g sample of adipose tissue from the ventral base of the tail were removed. Lipids were extracted as above, and carbon- 14 activity was counted on a Tri-Carb liquid scintilla- tion spectrometer (Packard Instrument Co., Downers Grove, IL). Upon ter- mination of the 12-week feeding period, the procedure was repeated with the foilowing exceptions. Three alligators were randomly selected from each replicate from diets 1 and 3. One animal from each tank received 0.5 #Zi of either 1 -carbon-14-acetate, 2-carbon- 14-leucine, or U-carbon-14-glucose. Nine h later animals were sacrificed. Livers were immediately excised and frozen in liquid air. Carbon- 14 activity in liver lipids and adipose tissue was considered to be an indicator of lipogenesis from acetate, leucine or glucose (cf. Marion and Edwards, 1963).


Statistical analysis Data were analyzed by analysis of variance according to simple linear

regression and the general linear model (GLM) procedures of SAS Institute ( 1985 ). When treatment differences were linear combinations of dietary variables (Experiment I), regression techniques and contrast procedures for custom hypothesis testing were employed. Results from Experiment II and, in Experiment III, the influence of two diets on lipogenesis from three energy substrates, were analyzed as 2 x 3 factorial experiments.

RESULTS

Experiment I Body weight gains of alligators varied significantly with diet (Table 3 ) . Body

weight gains (Y) were related to dry matter intake (X) by the equation Y= -24.6+ 1.37x (r 2 = 0.95; P-C 0.00 1). Alligators fed formulated diets ate more, gained significantly more weight, and were longer than did those fed nutria meat. Increased dietary fat led to greater body weight gains and significantly improved feed efficiency. As dietary fat increased, percent carcass fat increased and percent carcass protein decreased. Improved feed efficiency was thus partially or totally due to higher carcass fat deposition. It is important to note, however, that actual protein deposition also increased (Table 3).

The fatty acid profile of alligator carcass lipids varied with diet (Table 4). Percentages of most ( 10 of 12) fatty acids were linearly related (P-C 0.05 ) to dietary fat content. The relative abundance of 14 : 0, 18 : 2, 18 : 3, 20 : 5, 22 : 5 and 22 : 6 increased with fat intake while that of 16 : 0, 16 : 1, 17 : 0, 18 : 1, and 20 : 4 decreased. These changes generally made carcass fat resemble dietary fat in composition (Table 2). A marked exception to this trend is the uni- formity of 18 : 0 levels across dietary treatment. This fatty acid was present in carcass lipids at levels only one-half that in added dietary fat.

Protein and energy digestibility decreased slightly with increases in caloric density of the diet (Table 5). Alligators digested 87.3-89.2% of dietary protein and 84.6-86.8% of gross energy. Digestible energy and DE, increased from 4321 to 4638 and 3425 to 3863 k&/kg, respectively.

Experiment II Body weight gains averaged 230 g. Alligators fed the purified diet averaged

245 g body weight gain, significantly more than the 2 14 g of those fed pure or carbohydrate-supplemented nutria meat (Table 6 ) . Overall, the supplementation with glucose or corn did not significantly affect body weight gains. Al- ligators fed diets containing 20% dry matter in the form of glucose or extruded corn averaged weight gains equivalent to those fed the unsupplemented diets. However, the diet-supplementation interaction was significant. Sup-

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TABLE 4

Experiment I. Major fatty acids in carcass fat of alligators in relation to dietary treatment

Diet: Nutria 1 2 3 4 5 6 7 Standard P> F2 % Dietary fat’: 13.0 3.6 5.8 7.9 10.1 12.3 14.4 16.6 error

Fatty acid 14:03 16:0 16:l 16:2 17:o 18:0 18:l 18:2 18:3 20:4 20:s 22:s 22:6

1.8 2.1 20.2 20.3

6.6 11.2 1.2 1.2 1.0 1.2 6.4 4.9

23.0 37.4 17.8 10.9 10.5 2.8 3.1 1.5 0.5 0.3 1.2 0.4 1.3 0.6

-% of fatty acids 2.1 2.1 2.2 2.5 2.5 2.3 0.10

19.2 19.4 18.4 18.0 17.3 17.5 0.27 9.3 8.9 8.1 8.2 7.4 6.8 0.46 1.0 1.0 0.9 0.9 0.9 1.0 0.10 1.1 0.8 0.9 0.9 0.8 0.9 0.09 4.8 5.0 4.9 4.6 4.8 5.0 0.19

36.3 35.8 34.3 33.4 32.5 32.4 0.87 13.2 14.3 15.4 16.5 17.8 17.5 0.71 4.7 5.6 6.5 7.1 7.8 7.9 0.40 1.5 1.3 1.1 1.1 1.1 1.1 0.15 1.0 1.4 1.8 2.1 2.1 2.1 0.15 0.8 0.6 0.7 0.6 0.7 0.8 0.07 1.0 1.1 1.4 1.4 1.5 1.8 0.11

0.007 0.001 0.001 0.062 0.003 0.767 0.001 0.001 0.001 0.020 0.001 0.018 0.001

‘Calculated analysis based on total lipid determinations of dietary ingredients. *For diets 1-7 only. Simple linear regression analysis of fatty acid composition as a function of dietary fat level. 3Carbon chain length: number of double bonds.

plementation of nutria meat with glucose significantly increased performance while gains with corn supplementation did not significantly differ from unsupplemented nutria meat. The carbohydrate-free purified diet resulted in gains which were statistically equivalent to those obtained with corn supplementation. Glucose supplementation of the purified diet resulted in significantly lower body weight gains.

Experiment III Alligators fed diets with carbohydrate as the supplementary energy source

(diet 3 ) were longer and gained more weight than those fed other diets (Table 7 ) . When protein was the additional energy source (diet- 2 ), animals were longer and heavier than those fed either the energy-deficient basal diet 1 or the fat-supplemented diet 4. These results are in close agreement with consumption patterns. Feed conversion for the basal diet was less efficient than for the other three diets, apparently due to the high levels of presumably un- digestible fiber in this diet.

Alligators digested protein efficiently (96.0-97.1% ) from all diets (Table 8). Digestibility of dietary gross energy ranged from 78.5 to 91.3% for diets 1, 3, and 4. Much of this variability was due to the gross energy of cellulose. When dietary gross energy was adjusted downward by excluding the energy of added cellulose, digestibility coeffkients were considerably higher, in closer

254. M.A. STATON ET AL.

TABLE 5

Experiment I. Digestibility of protein and energy and digestible energy values (with and without nitrogen correction) for carbohydrate-free diets

.Diet Calculated Analyzed Digestibility’ Digestible energy’

% total % crude I crude gross energy protein energy DE,, DE lipid protein protein @Cal/kg)

1 3.6 73.7 76.9 4985 89.2 86.8 3425 4327 2 5.8 72.0 76.1 5025 88.8 86.0 3432 4321 3 7.9 70.3 73.7 5116 88.9 86.0 3540 4401 4 10.1 68.5 71.5 5232 88.6 86.0 3665 4498 5 12.3 66.8 70.0 5289 88.4 86.0 3733 4547 6 14.4 65.1 69.7 5375 87.7 84.4 3734 4539 7 16.6 63.4 67.4 5482 87.3 84.6 3863 4638 Standard error 0.48 0.62 69 93 Linear regression analysis* Independent variable: Dietary protein 0.001 0.004 0.001 0.001

0.474 0.368 0.936 0.892 Dietary gross energy 0.001 0.001 0.001 0.001

0.523 0.427 0.919 0.872 Dietary fat 0.001 0.001 0.001 0.001

0.524 0.443 0.986 0.851

‘Calculations based on analyzed values. ‘Values reported are P> F and rz using simple linear regression analysis.

TABLE 6

Experiment II. Alligator body weight gains (g) in response to supplementation of two high-protein diets with glucose and extruded corn as 20% of dry matter

Supplement Nutria meat Purified diet

None 182a’ 273” Glucose 236b 213b Corn 229ab 24gab Mean 214 245 Standard error 15.7 Analysis of variance (P> F)

Diet 0.006 Supplementation 0.440 Diet x supplementation 0.001

‘Data are analyzed as a 2 x 3 factorial experiment. Within-diet (column) means were separated with Duncan’s new multiple range test. Column means sharing the same superscript were not significantly different (PcO.05). The smallest alligator in this experiment died due to undetermined causes. As replicate groups contained only three animals, data for the tank containing this animal were excluded.


TABLE 7

Experiment III. Influence of energy source on alligator body weight gain, total length gain, dry matter consumption,

and feed efficiency

Diet Dietary treatment

protein energy % supplement’

Weeks 1-12 Weeks 5-12

energy cellulose total length body weight consumption feed : gain source % % gain (cm) gain (g) (gDM) (g DM/g)

1 56.1 None 0 23 1282 144” 88= 1.25” 2 71.0 Protein3 18.6 4.4 17b 23Sb 12Ob 0.79s 3 55.5 Carbohydrate4 23 0 19’ 300’ 161’ 0.83s 4 55.5 Fats 9.9 13.1 13” 151’ 77” 0.99b Standard error 0.1 8.5 9.8 0.08

‘Addition of energy in the form of protein, carbohydrate, or fat was made at the expense of cellulose (Solka Floe). *Means separated using Duncan’s new multiple range test following establishment of differences with analysis of variance (PcO.02). Column means sharing the same superscript are not significantly different. 31dentical in composition to protein of basal diet. 4High-solubility (80%) dextrin. 5Mixture composed of 40% lard, 25% fish oil, 20% linseed oil, and 15% safflower oil.

TABLE 8

Experiment III. The influence of energy source on digestibility of protein and energy and on digestible energy

Diet Dietary

energy supplement

Analyzed % Gross Digestibility* Digestible energy’ energy as

gross energy crude cellulose’ protein energy non-cellulose DE DE. (kcal/kg) protein energy

%

1 None 4772 56.1 19.3 96.1=* 78.5” 97.28 37451 3036” 2 Protein” 4946 71.0 0 97.1s 91.3b 94.7s 4513b 3606b 3 Carbohydrate4 4685 55.5 3.6 96.8” NA’ NA NA NA 4 Fat6 5199 55.5 10.1 96.0” 83.6’ 91.9” 4296’ 3595s Standard error’ 0.2 0.6 0.7 30 29

‘Calculated using analyzed values except for cellulose, which was assumed to contain 4 kcal/g. *Means separated with Duncan’s new multiple range test following establishment of mean differences with analysis of variance (PC 0.01). Column means sharing the same superscript are not significantly different. 31dentical in composition to protein of basal diet. 4High-solubility (80%) dextrin. ‘Fecal samples from this diet were too small to obtain energy values from them. 6Mixture of 40% lard, 25% fish oil, 20% linseed oil, and 15% safllower oil. ‘Pooled standard error of the mean.

agreement numerically (9 1.9-97.2%), but still significantly different. Diges- tible energy values varied considerably, with major differences between diets attributable to the fiber and crude protein content. DE, for the protein-supplemented diet 2 was approximately 900 kcal/kg lower than the non-adjusted DE. However, the corresponding difference was only 700 k&/kg in the diet containing fat as the added energy source.

Eight weeks into Experiment III, alligators fed carbohydrate (diet 3) had

256 M.A. STATON ET At.

TABLE.9

Experiment III. The influence of dietary energy source on liver weights and lipids, and lipogenesis in adipose tissue and liver in alligators after 8 weeks

Diet Supplementary’ N Liver Lipogenesis energy source Weight % of % total cpm’/g adipose cpm’/g liver cpm’/liver

g body lipids tissue lipid lipid weight

1 None 4 4.64a3 2.11” 2.26 0.02 40.6” 4.0” 2 Protein4 4 5.29” 2.01” 2.39 0.03 3.0b 0.4b 3 Carbohydrate’ 4 7.91b 2.50” 2.31 0.05 10.5b 1.9 4 Fat6 4 5.10” 2.14” 2.60 0 10.7b 1.4” Standard error 0.27 0.08 0.17 0.03 7.8 1.0

‘The basal diet contained 23Oh added fiber. To it was added energy (940 kcal/kg) in the form of protein, carbohydrate or fat. ZCounts per minute x 1000 derived from intraperitoneally injected C-14-acetate. ‘Means separated with Duncan’s new multiple range test following establishment of difference with analysis of variance (P< 0.05 ) . Column means sharing the same superscript are not significantly different. 41dentical composition to protein in basal diet. 5High-solubility (80%) dextrin. 6Mixture of 40% lard, 25 fish oil, 20% linseed oil, and 152 safflower oil.

heavier livers, expressed both in absolute weights and as percentages of body weight (Table 9). Liver lipids averaged 2.39% of wet weight and did not differ significantly among treatment groups. Lipid carbon- 14 activity in adipose tissue from the base of the tail was only slightly greater than background. This indicates that major lipogenic activity does not occur in this storage fat depot in alligators. Alligators fed the energy-deficient basal diet displayed greater liver lipid carbon- 14 activity than did those fed higher energy diets, which did not differ significantly. The total amount of liver lipid synthesized by animals fed the protein-supplemented diet was significantly lower than by animals fed the other diets (Table 9).

Twelve weeks into this experiment, livers of carbohydrate-fed (diet 3 ) animals, expressed in absolute weight (9.74 g) or as percent of body weight (2.49%), were heavier (PC 0.00 1) than those of alligators fed diet 1 (5.20 g, and 2.02%, respectively). At 12 weeks, but not at 8 weeks, liver lipid content of animals fed carbohydrate (3.08% total lipids) was significantly greater (P< 0.002) than that for diet 1 (2.30%). Carbon-l 4 activity was influenced by both lipogenic substrate and dietary treatment (Table 10). Acetate was more readily converted to fat than were glucose or leucine. The diet by substrate interaction was significant for total liver lipogenesis and lipid-specific carbon- 14 activity. Of interest here is that for alligators fed the carbohydrate- containing diet, there were no significant differences in either total Liver or lipid-specific activities from acetate vs. glucose. This contrasts with the sig-


TABLE 10

Experiment III. The influence of diet on lipogenesis from C-14 labeled lipogenic substrates

Diet

1 3

Supplementary’ energy source

None Carbohydrate Average Standard error P>F

Diet Substrate Diet x substrate

None Carbohydrate Average Standard error P>F

Diet Substrate Diet x substrate

Lipogenic substrate

acetate glucose

< cpm’/g liver lipid 365.7a3 29.1b

37.5” 20.0ab 201.6” 25.2b

29.2

0.001 0.00 1 0.001

< cpm2/liver 34.1” 3.9b 12.3” 6.7” 23.2” 5.lb

3.6

0.041 0.001 0.003

leucine

, 24.0b 12.lb 18.1b

, 2.9b 3.4” 3.2b

‘The basal diet contained 23% added fiber, representing no energy supplementation. In the carbohydrate-supplemented ration, this fiber was replaced by high-solubility (80%) dextrin. ‘Counts per minute x 1000 derived from intraperitoneally injected C-l 4-labeled lipogenic substrates. 3Within diet means separated with Duncan’s new multiple range test. Row means sharing the same superscript are not significantly different.

TABLE 11

Experiment III. Influence of dietary energy source on the average composition of fat from tail depot adipose tissue of alligators

Diet Supplementary energy source

Fatty acid’

16:O 16:l 18:0 18:l 18:2 18:3

I None 19.3=* 8.1” 4.3” 37.0ab 15.2= 4.8” 2 Protein 19.1” 7.9a 4.3” 35.gbC 16.1a 5.5” 3 Carbohydrate 18.8” 8.3” 4.1” 38.8” 13.5b 4.6” 4 Fat 17.0b 6.5b 4.1” 33.3” 19.0” 7.gb Standard error 0.40 0.16 0.06 0.85 0.57 0.31

‘Carbon chain length : number of double bonds. ‘Means were separated with Duncan’s new multiple range test following establishment of differences with analysis of variance (PC 0.05). Column means sharing the same superscript are not significantly different.


niticantly greater lipogenesis from acetate than from glucose in alligators fed the energy-delicient basal diet.

Significant differences in the fatty acid composition of tail depot fat of alligators fed the four diets (Table 11) generally reflected the influence of added dietary fat. Compared with the results of other high-energy diets, carbohydrate-fed (diet 3) animals had significantly greater levels of 18 : 1 and lower levels of 18 : 2. As in Experiment I, 18 : 0 levels were lower than and unaffected by dietary treatment.

DISCUSSION

For the high-protein diets used in this study, replacement of dietary protein with carbohydrate or fat usually resulted in enhanced performance or, at least, growth responses equivalent to those obtained with higher-protein diets. There were, however, two exceptions. Although glucose is known to be absorbed well by alligators (Coulson and Hernandez, 1983), its substitution in high- protein diets in the present experiments yielded variable responses. A growth depression was observed when glucose was added to the purified diet, whereas improved body weight gains were seen when glucose was added to the nutria meat diet. This inhibitory effect is similar to that seen with carnivorous fish. For example, Hilton and Atkinson ( 1982 ) reported decreases in rainbow trout growth rate in proportion to increasing levels of dietary glucose. Such delete- rious effects of dietary glucose in carnivorous fish may be related to insuffi- cient or delayed secretion of insulin and a resultant inability to control blood glucose levels (Feruichi and Yone, 198 1, 1982; Palmer and Ryman, 1972; Wilson and Poe, 1987). Similarly, alligators may be physiologically un- equipped to cope with large influxes of exogenous glucose. The growth pro- motion resulting from glucose supplementation of the nutria meat diet may be explained by the fact that glucose tended to readily wash from the meat when dislodged or carried into the water. Thus, glucose intake of alligators fed glucose-supplemented nutria meat was less than for those fed the glucose- supplemented purified diet and may have been within a range tolerated by alligators.

In Experiment I, replacement of protein with dietary fat led to generally increased consumption, significantly increased weight gains, and improved feed conversion. In contrast, fat supplementation (diet 4) of the low-energy basal ration (diet 1) in Experiment III did not enhance performance in terms of consumption or growth. However, it was apparent that some, if not most, dietary fat was absorbed as feed conversion of diet 4 was superior to that of diet 1, and fatty acid composition of adipose tissue of alligators fed diet 4 resembled the added dietary fat. The reason for poor results of these diets was that consumption was low, relative to the protein- and carbohydrate-supplemented diets 2 and 3. Both diets 1 and 4 were characterized by high levels of


added fiber, which may reduce gastric emptying, intestinal digestion, and movement of nutrients to absorptive mucosal surfaces (Leeds, 1982). In fish, excessive fiber is known to inhibit feed intake (NRC, 1983 ) . Leary and Lovell ( 1975) found that catfish (Ictalurus punctatus) consuming cellulose as 8% or more of the diet exhibited reduced feed intake and growth. Thus, the high fiber content of diets 1 and 4, approximately 25% and 15%, respectively, may have caused the poor performance of alligators fed these two rations.

In Experiment III, diets 2 and 3 contained less than 8% fiber, and it would appear that differences in performance between diets 2 and 3 were only slightly, if at all, influenced by fiber content. Obviously, carbohydrate was well utilized. If dextrin were not digestible, performance should have been similar to that supported by the high-fiber basal ration. Furthermore, the observed alterations of liver energy metabolism in response to dietary carbohydrate would not be anticipated if this carbohydrate were not utilized by alligators.

Fat was synthesized in the liver but was not synthesized in significant quantities in adipose tissue from the tail. In adipose tissue, relatively low levels of stearic acid ( 18 : 0) and high level of oleic acid ( 18 : 1) were found. This sug- gests that stearic acid, whether originating from the diet or from synthesis within alligator tissues, was actively desaturated to oleic acid. Oleic acid may have an energy storage function in alligator adipose tissue.

Experiments II and III show that protein of vegetable origin is digestible by alligators. Isolated soybean protein contributed over 40% of dietary protein. Digestibility of protein from these diets (Experiment III) was very high (96.0- 97.1%). Furthermore, feed conversion was comparable to that for diets containing only animal protein (Experiment I). Plant proteins should not, as a class, be dismissed as nutrients of little or no feeding value for alligators. Fur- ther research is required to determine the importance of processing in making plant proteins available to alligators.

The concept of DE, was introduced as an alternative to DE for high-protein diets. This calculated DE, should be a better indicator of the dietary energy available to a crocodilian since a large portion of the dietary energy is protein. This calculation is practical for. alligators as nitrogenous wastes are deposited independent of or are easily separable from fecal material. Metabolizable energy determinations would be much more difficult on a routine basis as urinary material varies from liquid to the semi-solid paste of uric acid. Although calculated DE, values suffer from a lack of estimates of urinary and endoge- nous energy losses, the error involved was probably small compared with the nitrogen correction required to adjust DE to zero nitrogen balance (resulting in the DE, estimate). The accuracy of DE, would be enhanced by determinations of the gross energy of alligator urinary nitrogenous wastes. In Experi- ment I, the nitrogen correction of the DE, calculation represented 14- 18% of gross energy and 16-20% of DE. This resulted from the very high levels of


protein fed to alligators and the high digestibility of dietary protein. As ex- pected, the nitrogen correction of DE, decreases as the protein content of the diet decreased.

Alligators readily consumed diets containing fat, carbohydrates, and plant proteins. Resulting growth performances indicated efficient utilization of these dietary components, as did data on feed conversion, digestibility, carcass composition, and lipogenesis. The feeding value of these energy and protein sources is influenced by their composition, degree of processing, and other dietary components. Further research is required to make knowledgeable, routine incorporation of these nutrients into practical alligator feed possible.

ACKNOWLEDGEMENTS

This work was supported by a grant from the Louisiana Wildlife and Fish- eries Department, by State and Hatch funds allocated to the Georgia Agricul- tural Experiment Stations of the University of Georgia, and a contract (DE-ACOg-76SROO-8 19) between the United States Department of Energy and the University of Georgia.

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dietary energy sources for the american alligator, alligator mississippiensis (daudin)

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