The bioefficacy of zinc bacitracin in practical diets for broilers and laying hens

The Bioefficacy of Zinc Bacitracin in Practical Diets for Broilers and Laying Hens G. HUYGHEBAERT and G. DE GROOTE Rijksstation voor Kleinveeteelt, Burg. van Gansberghelaan 92, 9820 Merelbeke, Belgium

(Key words: zinc bacitracin, bioefficacy, metabolizable energy, broiler chick, laying hen) 1997 Poultry Science 76:849–856

The performance-enhancing effect of ZnB has been observed in different species of fowl, including turkeys, broiler breeders, laying hens, and broiler chickens (Moran and McGinnis, 1965; Tu ¨ ller, 1973; Rosen, 1980 a,b; Keppens et al., 1981; Foster and Stevenson, 1983; Choi and Ryu, 1987; Ma¨nner and Wang, 1991). The bioeffects appear to be influenced by dietary, environmental, and genetic factors (Nordskog and Johnson, 1953; Libby and Schaible, 1955; Carlson et al., 1956; Moran and McGinnis, 1965; Potter et al., 1977; Ma¨nner and Wang, 1991). Because of possible interactions between nutritional status and ZnB supplementation, the bioresponse (in terms of feed conversion rate or growth rate) to ZnB has not always been consistent (Moran and McGinnis, 1965; Bougon and L’Hospitalier, 1976; Foster and Stevenson, 1983). Further research of the bioeffectiveness of ZnB on metabolizability of nutrients (regarding amino acids and metabolizable energy) may clarify some of the dietary interactions. The current study was conducted to determine the response in dietary metabolizable energy and metabolizability of both fat and amino acids to graded supplementations of ZnB in practical broiler and in layer diets varying in nutrient density.

INTRODUCTION Intensive production of poultry has led to markedly increased output of meat and eggs. Further enhancements of animal performance in terms of growth rate, egg production, and feed conversion have been achieved with the widespread use of a number of products commonly referred to as feed additives, e.g., the gutactive antibiotic performance promoters. The antibiotic performance promoters, e.g., zinc bacitracin (ZnB),1 are largely unabsorbed from the intestine at the dietary concentration usually used. The main site of antibiotic activity is within the gastrointestinal tract, where ZnB acts to modify the intestinal flora as well as the gut wall structure (Elan et al., 1951; Visek, 1978; Stutz et al., 1983; Valfre, 1983; Armstrong, 1986; Free et al., 1986; Boorman, 1987; Bernsten, 1994). In this respect, the improved performance results from nutrient sparing by inhibition of fermentative losses and from enhanced metabolizability. Also, these additives appear to improve the postabsorptive metabolism in terms of egg quality, fertility, maternal carryover, and heat tolerance (Damron et al., 1991; Ma¨nner and Wang, 1991). Some of the mechanisms are not yet understood.

MATERIALS AND METHODS Received for publication July 17, 1996. Accepted for publication February 25, 1997. 1Albac (15% granulated) registered trade name of Alpharma, Oslo, Norway.

The bioefficacy of ZnB was determined by means of balance trials with both broiler chickens and laying hens. Day-old male broilers of a commercial broiler strain 849

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fluenced by nutrient density, age, and dietary ZnB level. No significant interactions between ZnB by nutrient density were found. Addition of ZnB resulted in a lower excreta:feed ratio and an improved N retention; there was a nearly linear relationship between these effects and dietary ZnB levels. Moreover, dietary MEn content was linearly enhanced by ZnB supplementation. As a consequence, the bioefficacy of ZnB can be expressed in terms of MEn units: the average MEn equivalency of ZnB was 2,080 and 1,184 Mcal/kg, for broiler chicks and laying hens, respectively.

ABSTRACT Two balance trials were conducted to examine the response in metabolizable energy and metabolizability of both fat and amino acids to graded levels of zinc bacitracin (ZnB; Albac registered trade name of Alpharma, Oslo, Norway) in practical broiler and layer diets varying in their nutrient density. Broiler diets were supplemented with either 0, 20, or 50 mg ZnB/kg and layer diets were supplemented with either 0, 50, or 100 mg ZnB/kg. Each experimental diet was fed to five replicates of four broiler chicks each or nine replicates of individually housed laying hens, respectively. All balance parameters were significantly in-

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HUYGHEBAERT AND DE GROOTE TABLE 1. The composition and calculated nutrient content of the experimental basal diets Broiler chickens

Ingredients and contents

High density Low density

High density Low density (kg) . . . 451.80 100.00 2.90 227.30 . . . 47.60 8.80 51.70 14.00 44.70 36.20 3.40 1.40 . . . 0.019 . . . . . . 10.00 1,000.19

51.60 50.00 450.00 44.10 . . . 252.50 . . . 30.00 85.00 14.30 6.10 . . . 2.90 1.70 1.4 . . . 0.45 10.00 . . . 1,000.00

132.70 50.00 450.00 73.50 . . . 191.70 . . . 28.50 36.60 13.30 7.20 . . . 2.70 1.60 2.00 . . . 0.45 10.00 . . . 1,000.00

. . . 539.80 100.00 17.50 193.70 . . . 33.00 . . . 9.90 14.50 43.30 33.90 3.30 1.10 . . . 0.017 . . . . . . 10.00 1,000.17

3,110 . . . 0.90 0.45 11.02 1.90

2,960 . . . 0.90 0.43 7.25 1.81

. . . 2,900 3.88 0.36 7.77 1.55

. . . 2,715 3.64 0.34 4.07 1.46

21.54 1.27 0.89

20.23 1.21 0.85

16.37 0.85 0.69

15.88 0.77 0.65

1Provides in milligrams per kilogram of diet: vitamin A (retinylacetate), 4.5; cholecalciferol, 4.1; vitamin E (dl-a-tocopherylacetate), 13.5; cyanocobalamin, 0.01; riboflavin, 5.4; nicotinic acid, 40; menadione, 2.25; biotin, 0.15; choline, 371; pantothenic acid, 13.5; pyridoxine, 1.1; thiamin, 1.0; I, 2.1; Co, 1.4; Se, 0.43; Cu, 7.2; Mn, 86; Zn, 57; Fe, 68; Mg, 110; Monensin-sodium, 100. 2Provides in milligrams per kilogram of diet: vitamin A (retinylacetate), 4.2; cholecalciferol, 2.0; vitamin E (dl-a-tocopherylacetate), 10; cyanocobalamin, 0.01; riboflavin, 5.0; nicotinic acid, 20; menadione, 2.00; biotin, 0.12; choline, 620; pantothenic acid, 10; pyridoxine, 2.0; thiamine, 0.9; I, 1.15; Co, 1.2; Se, 0.40; Cu, 7.5; Mn, 120; Zn, 75; Fe, 60; Mg, 110.

(Ross) were obtained from a local hatchery. The broilers were housed on deep litter under normal environmental conditions for lighting (23 h/d), temperature, and ventilation and maintained on a well-balanced starter diet until 14 d of age. Prior to the start of the balance trial, 10% of the broilers (with too light or too heavy body weight) were removed to reduce variability in body weight. The medium-weight laying hens (ISA Brown2) were housed in battery cages (each cage unit has a surface 1,200 cm2). Prior to the start of the balance trial, 54 laying hens of about 40 wk of age were selected to improve uniformity of body weight (between 1.9 and 2.1 kg) and egg production (with a laying rate of at least 80%). Balance trials with broiler chickens and laying hens were conducted according to the European reference

2ISA-Brown, Company De Biest, Kruishoutem, 3Novo-Nordisk, Copenhagen, Denmark.

Belgium.

method (Bourdillon et al., 1990 a,b), which consists of 1) a 7-d period of adaptation to the respective experimental diets, and 2) the 4-d main balance period. During this main balance period, the birds were slightly restricted at a level of about 95% of ad libitum feeding in order to minimize ingredient selection in the feeders. Water was available for ad libitum consumption. Excreta were collected quantitatively on a daily basis and immediately stored at –18 C. Each dietary treatment was fed to five replicates of four broiler chickens per cage or to nine replicates of individually housed laying hens in cages. For each breed, two practical diets were formulated using linear programming procedures to obtain a low and high nutrient density, by modifying dietary fat level at least 3.70% (Table 1). For both diets, the practical ranges in nutrient density and dietary fat level overlapped. Both broiler diets were supplemented with a carbo(NSP) enzyme (Biofeed + CT3) to minimize the antinutritional effects of nonstarch polysaccharides (NSP) in wheat, and also contained the coccidiostat

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Sorghum Yellow corn Wheat Heated full fat soybeans Soybean meal (44% CP) Soybean meal (48% CP) Alfalfa meal (16% CP) Meat meal (48% CP) Animal renderers fat Dicalcium phosphate Limestone-grounded Limestone-pelleted Salt-NaCl DL-methionine L-lysine·HCl Carophyll red Biofeed + T Premix-broilers1 Premix-layers2 Total Calculated nutrient content MEn-broilers, kcal/kg MEn-layers, kcal/kg Calcium, % Available phosphorus, % Crude fat, % Linoleic acid, % Analyzed content CP, % Lysine, % TSAA, %

Laying hens

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BIOEFFICACY OF ZINC BACITRACIN

TABLE 2. The influence of the addition of zinc bacitracin (ZnB) on E:F ratio,1 N retention, fat metabolizability, and metabolizable energy of high and low density diets for broiler chickens, including statistical analysis Type of diet2

ZnB

High density High density High density Low density Low density Low density

(mg/kg) 0 20 50 0 20 50 Pooled SE

ME

N retention

MEn

Fat metabolizability

0.302 0.289 0.274 0.287 0.276 0.263 0.001

3,155 3,208 3,270 3,044 3,090 3,156 9

(kcal/kg) 154 162 165 149 150 155 1

3,001 3,046 3,105 2,895 2,940 3,001 8

(%) 69.1 71.6 73.6 68.4 70.5 73.2 0.4

0.288a 0.275b

3,111a 3,097b

161a 151b

3,050a 2,946b

71.4a 70.7a

0.294a 0.282b 0.268a

3,100c 3,149b 3,213a

152c 156b 160a

2,948c 2,993b 3,053a

68.7c 71.1b 73.4a

18.3*** 25.1*** 0.1

44.1*** 14.6*** 0.1

47.7*** 15.7*** 2.5

39.1*** 13.0*** 0.1

0.7 9.5*** 0.1

in a column with no common superscript differ significantly (P ≤ 0.05). ratio = amount of freeze-dried excreta per kilogram of feed intake. 2Averages per dietary treatment are based on five replicates of four birds each. 3The analyzed ZnB concentration was 0, 21, and 49 mg/kg for the high density diets and 0, 19, and 52 mg/kg for the low density diets, respectively. ***P < 0.001. a–cMeans 1E:F

Monensin-sodium4 (100 mg/kg). Supplementation of ZnB was at 0, 20, or 50 mg and 0, 50, or 100 mg/kg diet for broiler and layer diets, respectively. The maximum ZnB levels correspond with the respective dosages allowed by the Feedstuff Codex.5 Analyses of all diets for ZnB included at these concentrations indicated recoveries of about 95 to 100% (see footnotes, Tables 2 and 3). Samples of feeds and freeze-dried excreta were analyzed for gross energy (GE: adiabatic bomb calorimetry), N (macro-Kjeldahl: protein = N × 6.25), fat (Soxhlet procedure based on petroleum ether extraction with a HCl pretreatment for the excreta only), and amino acids (ion-exchange chromatography with a peroxidation pretreatment for the sulfur amino acids in feed samples) (EU reference methods). Excreta were analyzed on a replicate basis for GE, N, and fat but not for amino acids [on pooled excreta samples, pooling was done according to the excreta:feed (E:F) ratio of each replicate]. As a consequence, no statistical analysis can be done for the parameter amino acid metabolizability. The MEn contents of the experimental diets were calculated from their respective E:F ratios as well as the corresponding GE contents, followed by a correction for N retention to zero, by using an energy equivalent of 8.22 kcal/g N retained. The MEn values were, however, 4Elancoban 200 (20% activity); registered trade name of Elanco, Benelux. 5Ministry of Agriculture, Brussels, Belgium.

not corrected for endogenous secretions or metabolic losses. The MEn equivalencies of ZnB (as a supplement) were calculated as the difference between the treatment means. The metabolizability of both fat and amino acids for the experimental diets was calculated from their respective E:F ratios as well as their corresponding fat and amino acid contents. Metabolizability results were not corrected for endogenous secretions or metabolic losses or the urinary contribution (Terpstra, 1975). Data were analyzed by ANOVA using Statgraphics (Statistical Graphics, 1991). Data within each breed were analyzed as a two-way factorial arrangement of treatments (2 × 3) in a completely randomized design. The model included density, level of ZnB, and the interaction of density and level. When significant differences among treatment means were found, means were compared using LSD multiple range test (Statistical Graphics, 1991). Statements of statistical probability were based on P ≤ 0.05. The effects of ZnB on main balance parameters were examined by linear regression analysis, with five dependent values (replicates) for each supplement of ZnB (Statgraphics, 1991; Snedecor and Cochran, 1989). Furthermore, exponential curves for MEn were fitted to ZnB supplementation by means of nonlinear regression using the NLIN procedure of SAS (SAS Institute, 1988): MEn = a + b (1 – e–cx) where MEn = dietary MEn content (Megacalories per kilogram); a = intercept (at x = 0); a + b = asymptote

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Statistical analysis Nutrient density High Low ZnB supplement3 0 mg/kg 20 mg/kg 50 mg/kg Analysis of variance Nutrient density (n = 2) ZnB supplement (n = 3) Interaction

E:F ratio

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HUYGHEBAERT AND DE GROOTE TABLE 3. The influence of the addition of zinc bacitracin (ZnB) on E:F ratio,1 N retention, fat metabolizability, and metabolizable energy of high and low density diets for laying hens, including statistical analysis

Type of diet2 High density Low density

ZnB (mg/kg) 0 50 100 0 50 100 Pooled SE

N retention

MEn

Fat metabolizability

0.308 0.284 0.265 0.301 0.280 0.268 0.002

2,875 2,958 3,012 2,664 2,744 2,794 6

(kcal/kg) 85 93 100 78 86 94 1

2,790 2,865 2,912 2,586 2,658 2,700 6

(%) 83.5 86.1 87.3 76.7 78.3 79.8 0.2

0.286a 0.283a

2,948a 2,734b

93a 86b

2,855a 2,648b

85.6a 78.2b

0.304a 0.282b 0.267c

2,770c 2,851b 2,903a

82c 90b 97a

2,688c 2,761b 2,806a

80.1c 81.1b 83.6a

0.7 40.3*** 0.7

302.2*** 39.8*** 0.1

5.1* 9.1*** 0.1

342.3*** 37.9*** 0.1

360.1*** 26.8*** 0.6

in a column with no common superscript differ significantly (P ≤ 0.05). ratio = amount of freeze dried excreta per kiligram of feed intake. 2Averages per dietary treatment are based on nine replicates of individually housed birds. 3The analyzed ZnB concentration was 0, 49 and 94 mg/kg for the high density diets and 0, 48, and 104 mg/kg for the low density diets. *P < 0.05. ***P < 0.001. a–cMeans 1E:F

(about the maximum dietary MEn content); c = curvature steepness coefficient (which describes the shape of the curve); and x = dietary ZnB level (milligrams per kilogram).

RESULTS AND DISCUSSION The main balance parameters were influenced significantly by both nutrient density and ZnB level without significant interactions (Tables 2 and 3). Only fat metabolizability (broilers) and E:F ratio (layers) were not significantly affected by nutrient density. Differences in feed composition was the main reason for not taking “type of bird (age)” as a factor in a multifactorial analysis of variance. The effects on E:F ratio and N retention were significantly influenced to a variable degree by nutrient density, ZnB supplementation, and age of the bird. The E:F ratio was similar for both broiler chickens and laying hens, tended to decrease at lower nutrient density (significantly for broiler chickens only), and decreased significantly at higher ZnB supplementation. Therefore, the E:F reduction was linearly related to dietary ZnB levels (Table 4). The N retention was clearly higher in broiler chickens than in laying hens and increased significantly at both higher nutrient density and dietary ZnB levels. Nitrogen retention (as a percentage of N

intake) is higher in young fast-growing broiler chickens than in high-producing laying hens; in this study the average percentage N retention (at 0 mg ZnB/kg) for broiler chickens and laying hens were 55.1 and 38.6%, respectively. Both figures correspond to an energy equivalency of 151 and 82 kcal/kg, respectively (Tables 2 and 3). Nitrogen retention and dietary ZnB level were linearly related to each other (Table 4). This enhanced N retention can be explained by an improvement of both metabolizability and utilization (related to energy retention) of the N compounds. The average amino acid metabolizability improved with about 2.1% (from 1.4 to 3.3%) ZnB supplementation (Table 5). However, only the minor part of the N retention accounts for the improved N metabolizability. Therefore, the source of this improved N utilization could not be determined with this experimental technique. The improved N metabolism confirms the nutrient-sparing effects of performance promoters (Armstrong, 1986; Boorman, 1987). Both the reduced excreta production (E:F ratio) and improved N excretion may offer poultry producers an additional financial return, which is of more interest in those areas with high animal density and manure surplus. Fat metabolizability was lower in young broilers than in adult laying hens. This finding is due mainly to agerelated differences in bile and lipase secretion into the small intestine (Ketels, 1994). Fat metabolizability was

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Statistical analysis Nutrient density High Low ZnB supplement3 0 mg/kg 50 mg/kg 100 mg/kg Analysis of variance Nutrient density (n = 2) ZnB supplement (n = 3) Interaction

ME

E:F ratio

0.301 – 0.00054 × ZnB 3,158 + 2.28 × ZnB 155 + 0.216 × ZnB 2,932 + 2.06 × ZnB 69.4 + 0.088 × ZnB 0.287 – 0.00049 × ZnB 3,045 + 0.08 × ZnB 148 + 0.133 × ZnB 2,897 + 2.10 × ZnB 68.4 + 0.097 × ZnB 0.71 0.55 0.58 0.51 0.37 0.63 0.55 0.42 0.53 0.51

0.0075 45 4.0 45 2.5 0.0082 45 3.4 43 2.1

0.00007 0.0015 0.00092 0.00296 0.016 0.00042 0.00163 0.0088 0.00199 0.0027

Model 0.649 0.782 0.093 0.897 0.621 0.863 0.963 0.535 0.923 0.873

Lack-of-fit = = = = = = = = = =

0.307 – 0.00043 × ZnB 2,880 + 1.37 × ZnB 85 + 0.150 × ZnB 2,794 + 1.22 × ZnB 83.7 + 0.038 × ZnB 0.299 – 0.00033 × ZnB 2,670 + 1.30 × ZnB 79 + 0.151 × ZnB 2,591 + 1.15 × ZnB 76.7 + 0.031 × ZnB

Regression equation 0.64 0.64 0.32 0.62 0.62 0.60 0.59 0.24 0.58 0.42

R2

Lysine Threonine Methionine Cystine Methionine + cystine Tryptophan Alanine Arginine Aspartic acid Glutamic acid Glycine Histidine Isoleucine Leucine Phenylalanine Serine Tyrosine Valine Average

84.9 78.7 89.8 76.2 84.1 83.6 79.4 90.1 82.0 90.1 63.1 86.1 84.1 85.0 86.8 82.1 84.8 82.5 83.0

20 mg ZnB/ kg diet

High density diet

83.9 78.7 89.7 77.4 84.6 83.6 79.6 90.7 82.8 90.5 62.8 85.8 84.3 85.4 87.0 82.8 85.1 82.1 83.2

50 mg ZnB/ kg diet 83.3 77.2 88.4 75.6 83.5 81.8 76.9 89.3 80.6 88.9 65.6 84.2 82.0 83.6 85.2 81.4 84.2 79.9 81.8

0 mg ZnB/ kg diet

Broiler chickens

84.4 78.1 89.4 74.9 83.4 82.1 78.6 86.5 81.7 89.5 67.5 84.3 83.4 84.6 86.1 81.8 85.3 81.5 82.4

20 mg ZnB/ kg diet

Low density diet

84.7 79.8 89.9 77.7 84.9 83.0 79.6 86.7 83.1 90.1 66.0 85.6 84.2 85.3 86.7 83.5 85.9 82.1 83.3

50 mg ZnB/ kg diet (%) 79.8 78.3 89.2 78.2 85.1 80.2 80.2 88.8 83.6 89.3 52.7 86.4 84.4 86.8 86.1 83.1 86.4 82.0 82.3

0 mg ZnB/ kg diet

acid metabolizability is based on one analysis each of the diet and its corresponding pooled excreta sample.

83.5 77.3 88.7 73.3 82.3 82.2 77.8 89.7 81.1 89.6 60.5 84.9 83.4 84.5 86.2 81.3 84.6 81.1 81.8

Amino acid

1Amino

0.014 44 9.1 40 1.2 0.011 46 11 41 1.6

SEE 0.00001 0.00001 0.0018 0.00001 0.00001 0.00001 0.00001 0.0101 0.00001 0.00026

Model

79.3 78.4 89.7 79.8 85.5 87.2 80.5 84.9 83.5 89.7 58.4 86.7 83.8 87.4 88.7 83.6 87.8 80.8 83.1

82.3 81.3 90.1 80.1 86.3 81.9 83.6 90.4 85.2 90.6 59.3 87.7 86.2 88.5 88.0 85.1 88.3 84.5 84.4

81.1 80.4 90.8 79.9 86.7 81.8 85.2 89.7 85.2 90.5 63.6 87.4 86.4 88.2 87.5 84.4 87.5 84.8 84.5

76.5 76.2 87.3 79.5 84.0 86.1 78.7 84.1 81.3 88.2 46.4 84.9 81.7 85.8 87.1 81.9 86.3 78.2 80.0

50 mg ZnB/ kg diet

100 mg ZnB/ kg diet

50 mg ZnB/ kg diet

0 mg ZnB/ kg diet

Low density diet

80.5 79.8 90.8 82.8 87.0 89.0 82.8 85.8 84.9 90.2 53.9 87.3 84.9 88.3 89.6 84.6 88.3 82.4 84.1

100 mg ZnB/ kg diet

0.703 0.450 0.897 0.428 0.207 0.344 0.439 0.959 0.403 0.967

Lack-of-fit

P

High density diet

Laying hens

TABLE 5. The effect of zinc bacitracin (ZnB) supplementation (milligrams per kilogram of diet) of broiler and layer diets on amino acid metabolizability1

0 mg ZnB/ kg diet

2The

1E:F

= = = = = = = = = =

SEE

P2

Laying hens

= excreta to feed; N ret = N retention, Fat met = fat metabolizability. total Sum of Squares (SS) is split into 1) SS-model and 2) SS-residual; the latter being in turn split into SS-lack-of-fit and SS-pure error.

E:F ratio ME, kcal/kg N ret, kcal/kg MEn, kcal/kg Fat met, % E:F ratio ME, kcal/kg N ret, kcal/kg MEn, kcal/kg Fat met, %

High

Regression equation

R2

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Low

Balance parameter1

Nutrient density

Broiler chickens

TABLE 4. Linear regression analysis for the main parameters on milligrams of zinc bacitracin (ZnB) supplementation per kilogram of diet

BIOEFFICACY OF ZINC BACITRACIN

853

854

HUYGHEBAERT AND DE GROOTE TABLE 6. Regression of dietary MEn content on zinc bacitracin (ZnB) supplementation by means of linear or exponential modelling

Type of bird

Nutrient density

Broilers

High Low

Laying hens

High Low

1Values

Linear model MEn = (0.51 MEn = (0.53 MEn = (0.62 MEn = (0.58

2,932 – 45) 2,897 – 43) 2,794 – 40) 2,591 – 41)

Exponential model + 2.06 × ZnB + 2.10 × ZnB + 1.22 × ZnB + 1.15 × ZnB

MEn = (0.51 MEn = (0.53 MEn = (0.63 MEn = (0.59

3,001 – 47) 2,895 – 45) 2,790 – 41) 2,586 – 42)

+ 0,592 (1 – e–(0.0039

× ZnB))

+ 0,734 (1 – e–(0.0031

× ZnB))

+ 0,205 (1 – e–(0.0090

× ZnB))

+ 0,176 (1 – e–(0.0106

× ZnB))

within brackets are R2 and SEE, respectively.

biological efficacy of ZnB in terms of MEn units across the whole supplementation range. Based on the linear regression analysis and averaged for both nutrient densities, the MEn equivalency of ZnB was 76% (respectively 2,080 and 1,184 Mcal/kg) higher in broiler chickens than in laying hens. This difference in bioefficacy may be due to 1) age- or sex-related differences in digestion capacity (endogenous enzyme secretion; Ketels, 1994), 2) age- or sex-related differences in microbial flora in the gastrointestinal tract, and 3) differences in dietary composition (with e.g., wheat vs yellow corn as their main cereal component). In this respect, Moran and McGinnis (1965) and, more recently, Annison and Choct (1993) demonstrated that antibioticinduced responses were dependent on the NSP content in the cereal. The apparent higher tolerance to NSP of older birds may be due to the presence of a more developed or stable microflora. Choi and Ryu (1987) also determined a nutritional value for ZnB from differences in growth rate and feed conversion rate in broilers fed corn-soybean meal diets. They indicated a 145 mg ZnB supplement could save 15 g protein and 0.105 Mcal of ME. Based on these effects, but not considering any modifications in body composition or compensatory growth, effects on metabolizable energy equivalency of ZnB can be estimated as 726 Mcal/kg. Although there was no significant interaction, the MEn equivalency of ZnB was slightly affected by nutrient density with differences of about 2% in broiler chickens and 6% in laying hens. Similarly, Foster and Stevenson (1983) and Choi and Ryu (1987) did not observe any interaction between plane of nutrition and concentration of ZnB. The total increase in MEn can be explained by improvements in metabolizability of feed ingredients. The maximal MEn increases (averaged for both densities) for both broiler chickens and laying hens were 105 and 118 kcal/kg, respectively (Tables 2 and 3). The corresponding increases in protein metabolizabilities were, respectively, 1.45 and 2.70% (Table 6), which corresponded to about 13 and 14 kcal MEn/kg; the improved protein metabolizability accounted only for about 12% of the total MEn increase. The respective

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clearly influenced by nutrient density in laying hens, but not in broiler chickens. This age- or sex-related effect of nutrient density might be due to the relatively higher proportion and variation as well of “encapsulated or not added” fat in the layer than in the broiler diets. Thereby, fat metabolizability is lower as “encapsulated in feedstuffs” than as “supplemented”. Furthermore, fat metabolizability increased significantly at higher dietary ZnB concentrations, whereby both are linearly related with each other (Table 4). The MEn content of the diets clearly depended on nutrient density (type of bird) and ZnB supplementation. The determined MEn values were nearly identical to their respective MEn values, as calculated by means of least cost diet formulation (Table 1). Because of the increased N retention, the response to graded levels of ZnB is slightly more pronounced in terms of units of metabolizable energy than of MEn units (Tables 2 and 3). As a consequence, the energy equivalency of ZnB is higher in terms of metabolizable energy units than of MEn units. However, only the MEn responses are discussed because the least-cost diet formulation is based on MEn values. The dietary MEn values were linearly related to dietary ZnB levels (Table 4). Exponential modeling did not improve the goodness of fit (Table 6). Some reports mentioned responses of birds to graded levels of ZnB. Dose-responses of weight gain were linear (with ZnB range from 0 to 150 mg/kg) or linearly related to the log of the doses of ZnB (Daghighian and Waibel, 1982; Choi and Ryu, 1987). Rosen (1980, and personal communication) derived from a comprehensive “broiler” data bank that body weight gain and feed conversion rate responded quadratically to graded levels of ZnB (up to 275 mg/kg). The estimated biological optima for broilers were, respectively, 110 and 130 mg/kg for weight gain and feed conversion rate. For layers, however, the databank gave rise to a more linear dose-response in terms of egg mass production (0 to 100 mg ZnB/kg). Exponential models are not useful to calculate the biological efficacy of ZnB because of the “inverse” interrelationship between the curvature steepness coefficient and asymptote. On the other hand, the slope of the linear regression can be considered as a measure of the

BIOEFFICACY OF ZINC BACITRACIN

ACKNOWLEDGMENTS This study was financially supported in part by Alpharma, Oslo, Norway. The skilfull assistance of Carine Saelens and Pierre De Corte is gratefully acknowledged.

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increases in fat metabolizabilities were 4.7 and 3.5% (Tables 2 and 3), which corresponded to approximately 40 and 19 kcal MEn/kg; the improved fat metabolizability accounted for 39 and 16% of the total MEn increase. The contribution in the total MEn increase is higher from fat than from protein, which in turn appeared more pronounced in young broilers. The contribution of the fat ME in the total MEn (as a percentage) was slightly improved in broilers (from 20.0 to 20.6%) and in layers (from 15.9 to 16.6%) after ZnB supplementation. The response in fat metabolizability was not influenced by either age or dietary fat level. For practical application, the bioefficacy values of ZnB (2,080 and 1,184 Mcal/kg, respectively, for broilers and layers) can be introduced in least-cost diet formulation in which its MEn equivalency is applied to the commercial premix ALBAC, which is considered as a new feedstuff characterized by a very high MEn content. However, further research is needed in order to determine the way in which the N-sparing effect or N (amino acids) equivalency can be applied to the leastcost diet formulation.

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