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The Response of Chicks to Several Antibiotics in Different Diets and Environments*
The Response of Chicks to Several Antibiotics in Different Diets and Environments* J. B I E L Y AND B. E.
MARCH
Department of Poultry Science, The University of British Columbia, Vancouver, Canada (Received for publication November 10, 1958)
Penicillin and chlortetracycline have been used extensively in diets fed in our laboratory. T h e following experiments were conducted to determine if a greater
or more consistent growth response might be obtained with an antibiotic which had not previously been used in the laboratory. Accordingly the effect of oleandomycin has been compared with t h a t of chlortetracycline, penicillin and oxytetracycline. T h e relative growth responses were studied with chicks fed two different diets in Experiment 1 and with chicks reared in two different environments in Experiment 2. EXPERIMENTAL Experiment 1. T h e antibiotics were tested as supplements to the two basal diets shown in Table 1. Diet 2 was formulated to be deficient in vitamins of the Bcomplex in order t h a t there should be the greatest likelihood of obtaining a response to the antibiotics. T h e protein content of both diets was calculated to be 22 percent. T h e following amounts of each antibiotic were added per lb. of diet: oleandomycin phosphate 5 mgm., procaine penicillin G 2.5 mgm., chlortetracycline HC110 mgm., and oxytetracycline 10 mgm. T h e levels of the last three antibiotics were chosen to be somewhat in excess of the amounts usually employed when these antibiotics are fed as growth stimulants. Since oleandomycin has not been widely tested the level was chosen arbitrarily.
Each experimental diet was fed to duplicate lots of 18 male New Hampshire day-old chicks. The chicks were reared in * Acknowledgment. Supported by grants from the b a t t e r y brooders with wire screen floors Department of Agriculture (under the sponsorship and had free access to feed and water. T h e of the Bacteriology Division, Science Service, Ottawa, Canada) and Chas. Pfizer and Co., Inc., chick room had been in continuous use. T h e batteries had been cleaned since the Terre Haute, Indiana, U. S. A. 771
Downloaded from http://ps.oxfordjournals.org/ at National Institute of Education Library, Serials Unit on May 20, 2015
A N T I B I O T I C S appear to exert their - * * • greatest effect with chicks reared in an " o l d " environment according to data reported by Coates et al. (1951), Hill et al. (1953), Lillie et al. (1953), Morrison et al. (1954) and others. Nevertheless Waibel et al. (1954) and Libby and Schaible (1955) noted t h a t , with the continued feeding of antibiotics in a given environment, the response obtained gradually decreased. This would indicate that, in an " o l d " environment, chicks do not grow a t a maximum rate (1) because of the presence of bacteria detrimental to the well-being of the chick or (2) because bacteria which might be beneficial to the chick are suppressed in a highly infected environment. I t has been noted t h a t the response to antibiotics in our laboratory was variable. At first it was surmised t h a t the differences in response to antibiotics reflected small variations in the vitamin content of the diets fed. In view of increasing evidence t h a t the degree of response to dietary antibiotics is related to the sensitivity of the bacteria to the antibiotics used, it appeared t h a t changes of bacterial flora which predominate in our chick rooms might also be a factor in the variable response to antibiotics.
772
J. BIELY AND B. E. MARCH TABLE 1.—Composition of basal diets Ingredient
— 56.05 23.54 2.5 2.0 4.26 4.0 1.0 1.0 1.5 0.5 1.23 1.67
— — —
0.10 0.10 0.21 0.05 1.125 0.20 0.327 1.044
—
lb. lb. lb. lb. lb. lb. lb. lb. lb. lb. lb. lb. lb.
44.5 23.0 24.0
— — —
5.0
— — —
0.5
lb. lb. lb. (44% protein)
lb. (herring)
lb.
— —
lb. 1.0 lb. 2.0 0.0133 lb. lb. lb. lb. lb. mgm. gm. gm. gm.
2,270 I.U./lb. 680 I.C.U./lb. 0.01251b. 0.0125 1b.
— — — — — — — —
0.027 gm. 2,000 I.U./lb. 120 I C . U . / l b .
— —
* A trace-mineral mix containing 6% manganese, 0.12% iodine, 2.0% iron, 0.2% copper, 0.006% zinc, 0.02% cobalt, 27% calcium.
previous test but no attempt was made at special disinfection. The birds from the preceding experiment were in an adjoining room. The chicks were weighed individually at 31 days of age. Experiments 2A and 2B. The basal diet had the same composition as Diet 2 in the first experiment. The antibiotics used as supplements were oleandomycin, penicillin and chlortetracycline at the same levels as in the previous experiment. In each experiment the diets were fed to duplicate lots of 20 male New Hampshire day-old chicks. The chicks used in both experiments were of the same hatch and came from the same dams as did the chicks in Experiment 1. The chicks were weighed individually at 35 days of age. The chicks in Experiment 2A were kept in battery brooders in the same room in which Experiment 1 had been conducted.
The experiment was started two days after the completion of Experiment 1. When Experiment 1 was terminated the birds were moved into an adjoining room and the brooders cleaned. The chicks in Experiment 2B were reared in battery brooders in another building. Sanitary practice in this second building during preceding months had not been so strict as in the room used in Experiments 1 and 2A. At the conclusion of the experiment air samples were taken in the two chick rooms used. Plate counts were made for total aerobes using tryptone blood agar base with added yeast extract (2.5 gm. per liter) as medium. The plates were incubated at 41°C. for 24 hours. RESULTS
The average weights of the chicks in both experiments are given in Table 2. In
Downloaded from http://ps.oxfordjournals.org/ at National Institute of Education Library, Serials Unit on May 20, 2015
Ground wheat Ground yellow corn Soybean oil meal (51% protein) Corn gluten meal Alfalfa meal (17% protein) Stabilized animal fat Fish meal (menhaden) Distillers' dried solubles Dried whey (50% delactose) Dried brewers' yeast Iodized salt Calcium carbonate Dicalcium phosphate Limestone Bonemeal Manganese sulphate Delamix* DL-methionine Choline chloride (25%) My vamix (20,0001.U. vitamin E/lb.) Vitamin B,2 Riboflavin Calcium pantothenate Niacin Menadione Vitamin A Vitamin D 3 Butylated hydroxytoluene Nicarbazine
Diet 2/100 lb.
Diet 1/100 lb.
773
ANTIBIOTIC RESPONSE
TABLE 2.—Average weights of chicks Antibiotic supplement None Oleandomycin Penicillin Chlortetracycline Oxytetracycline
None Oleandomycin Penicillin Chlortetracycline
grams grams 411 337 483 (+18%)* 395 (+17%)* 457 (+11%) 347 ( + 3%) 426 ( + 4 % ) 357 ( + 6%) 422 ( + 3%) 354 ( + 5 % ) Experiment 2 A Average weight of chicks at 35 days of age grams 383 512 (+34%) * 437 (+14%) 441 (+15%)
Experiment 2B Average weight of chicks at 35 days of age
grams 436 529 (+21%) * 453 ( + 4%) 480 (+10%)
* Figures in parentheses indicate increment over the average weight of chicks fed the basal diet.
Experiment 1 the two basal diets fed promoted different rates of growth. Diet 2, which was formulated to be deficient in B-complex vitamins, resulted in slower growth and some curled-toe paralysis. There was also some curled-toe paralysis in the chicks fed Diet 1. Oleandomycin stimulated the rate of growth of chicks fed both basal diets by 17-18 percent. Penicillin stimulated growth of the chicks fed Diet 1 but did not affect the growth of chicks fed Diet 2. Chlortetracycline had no appreciable effect on growth rate of chicks fed either diet. Curled-toe paralysis occurred in the chicks fed the antibiotics as well as those fed the basal diets. The rate of growth was much poorer in the chicks fed the basal diet in Experiment 2A than in 2B. In both Experiments 2A and 2B there was a marked response to oleandomycin. Curled-toe paralysis was again evident in all lots of chicks whether or not an antibiotic was fed. When the basal diet was supplemented with oleandomycin the rates of growth were similar in the chicks fed the diet in the two experiments. Penicillin and chlor-
DISCUSSION
It will be seen from Table 2 that in every instance where antibiotics were used they stimulated growth although the increase in growth rate was slight in some cases. A consistently greater response was obtained to oleandomycin than to penicillin, chlortetracycline or oxytetracycline. In Experiment 1, oleandomycin resulted in a 17-18 percent increase in the average weights of chicks fed either Diet 1 or Diet 2. It was anticipated that each of the antibiotics would stimulate growth when added to Diet 2. In spite of the deficiency in B-complex vitamins, supplementation of the diet with any of the antibiotics other than oleandomycin improved growth only slightly. Penicillin produced a greater response when used with Diet 1 than with Diet 2, suggesting that in this instance its effect resulted from suppression of bacteria which produce a mild toxemia rather than from an increase in bacteria capable of synthesizing vitamins. In Experiments 2A and 2B the chicks under the different environmental conditions responded differently to the basal diet. The chicks fed the basal diet in the room with the higher bacteria count grew faster than those in the room with the lower count. On the basis of the response obtained to antibiotic supplementation of
Downloaded from http://ps.oxfordjournals.org/ at National Institute of Education Library, Serials Unit on May 20, 2015
None Oleandomycin Penicillin Chlortetracycline
Experiment 1 Average weight of chicks at 31 days of age • Diet 1 Diet 2
tetracycline increased growth of chicks in Experiment 2A to the same rate as the chicks fed the basal diet in Experiment 2B. In Experiment 2B the chicks receiving chlortetracycline grew at a rate intermediate between those on the basal diet and those receiving oleandomycin in the diet. The response to penicillin in the chicks in this experiment was negligible. The count of total aerobes showed a count of 885 and 3,355 bacteria per cubic foot for the chicks' rooms used in Experiments 2A and 2B respectively.
774
J. BIELY AND B. E. MARCH
Although the total number of air-borne bacteria in the room used in Experiment 2A was relatively low, the population must have contained greater numbers of bacteria directly or indirectly deleterious to the chick than did the chick room in Experiment 2B. The average weights of the chicks in the different lots indicate that bacteria responsible for the difference in the growth rates of chicks fed the basal diet in the two environments were sensitive to penicillin and chlortetracycline. It would appear, however, that both environments contained bacteria which were
resistant to these antibiotics. Suppression of these bacteria by oleandomycin, therefore, may have been responsible for the stimulus to growth obtained with this antibiotic. SUMMARY
The growth response of chicks to oleandomycin, chlortetracycline, penicillin and oxytetracycline as dietary supplements at levels of 5, 10, 2.5, and 10 mgm. of the respective compounds per pound of diet were compared. The experiments included two dietary and two environmental treatments. At the levels employed of the antibiotics tested, oleandomycin promoted faster growth than did the other antibiotics, irrespective of the diet or environment. REFERENCES Coates, M. E., C. D. Dickinson, G. F. Harrison, S. K. Kon, S. H. Cummins and W. F. Cuthbertson, 1951. Mode of action of antibiotics in stimulating growth of chicks. Nature, 168: 332-339. Hill, D. C , H. D. Branion, S. J. Slinger and G. W. Anderson, 1953. Influence of environment on the growth response of chicks to penicillin. Poultry Sci. 32: 462^-66. Libby, D. A., and P. J. Schaible, 1955. Observations on growth responses to antibiotics and arsonic acids in poultry feeds. Science, 121: 733-734. Lillie, R. J., J. R. Sizemore and H. R. Bird, 1953. Environment and stimulation of growth of chicks by antibiotics. Poultry Sci. 32: 466-475. Morrison, A. B., W. G. Hunsaker and J. R. Aitken, 1954. Influence of environment on the response of chicks to growth stimulants. Poultry Sci. 33: 491-494. Waibel, P. E., O. J. Abbot, C. A. Baumann and H. R. Bird, 1954. Disappearance of the growth response of chicks to dietary antibiotics in an old environment. Poultry Sci. 33: 1141-1146.
SEPTEMBER 10-12. SOUTHWESTERN ASSOCIATION CONVENTION, HOT SPRINGS, ARKANSAS SEPTEMBER 25-27. NATIONAL POULTRY, BUTTER AND EGG ASSOCIATION CONVENTION AND TRADE SHOW, HOTEL SHERMAN, CHICAGO
Downloaded from http://ps.oxfordjournals.org/ at National Institute of Education Library, Serials Unit on May 20, 2015
the diet fed it may be concluded that the factor which depressed growth in Experiment 2A was associated with the microflora in the room and not with some management factor. Penicillin or chlortetracycline stimulated growth of the chicks in Experiment 2A so that at 5 weeks of age they were similar in weight to the chicks fed the unsupplemented diet in Experiment 2B. Penicillin scarcely affected the growth rate of chicks in Experiment 2B. Chlortetracycline caused some increase in growth rate so that the chicks fed the diet supplemented with this antibiotic were heavier than those fed a similar diet in Experiment 2A. Oleandomycin markedly stimulated growth of the chicks in both environments, but to a greater extent in Experiment 2A. In both experiments, as in Experiment 1, the response was greater to oleandomycin than to either penicillin or chlortetracycline. The average weights at 5 weeks of the chicks fed the diet supplemented with oleandomycin were similar in the two environments.
MARCH
Department of Poultry Science, The University of British Columbia, Vancouver, Canada (Received for publication November 10, 1958)
Penicillin and chlortetracycline have been used extensively in diets fed in our laboratory. T h e following experiments were conducted to determine if a greater
or more consistent growth response might be obtained with an antibiotic which had not previously been used in the laboratory. Accordingly the effect of oleandomycin has been compared with t h a t of chlortetracycline, penicillin and oxytetracycline. T h e relative growth responses were studied with chicks fed two different diets in Experiment 1 and with chicks reared in two different environments in Experiment 2. EXPERIMENTAL Experiment 1. T h e antibiotics were tested as supplements to the two basal diets shown in Table 1. Diet 2 was formulated to be deficient in vitamins of the Bcomplex in order t h a t there should be the greatest likelihood of obtaining a response to the antibiotics. T h e protein content of both diets was calculated to be 22 percent. T h e following amounts of each antibiotic were added per lb. of diet: oleandomycin phosphate 5 mgm., procaine penicillin G 2.5 mgm., chlortetracycline HC110 mgm., and oxytetracycline 10 mgm. T h e levels of the last three antibiotics were chosen to be somewhat in excess of the amounts usually employed when these antibiotics are fed as growth stimulants. Since oleandomycin has not been widely tested the level was chosen arbitrarily.
Each experimental diet was fed to duplicate lots of 18 male New Hampshire day-old chicks. The chicks were reared in * Acknowledgment. Supported by grants from the b a t t e r y brooders with wire screen floors Department of Agriculture (under the sponsorship and had free access to feed and water. T h e of the Bacteriology Division, Science Service, Ottawa, Canada) and Chas. Pfizer and Co., Inc., chick room had been in continuous use. T h e batteries had been cleaned since the Terre Haute, Indiana, U. S. A. 771
Downloaded from http://ps.oxfordjournals.org/ at National Institute of Education Library, Serials Unit on May 20, 2015
A N T I B I O T I C S appear to exert their - * * • greatest effect with chicks reared in an " o l d " environment according to data reported by Coates et al. (1951), Hill et al. (1953), Lillie et al. (1953), Morrison et al. (1954) and others. Nevertheless Waibel et al. (1954) and Libby and Schaible (1955) noted t h a t , with the continued feeding of antibiotics in a given environment, the response obtained gradually decreased. This would indicate that, in an " o l d " environment, chicks do not grow a t a maximum rate (1) because of the presence of bacteria detrimental to the well-being of the chick or (2) because bacteria which might be beneficial to the chick are suppressed in a highly infected environment. I t has been noted t h a t the response to antibiotics in our laboratory was variable. At first it was surmised t h a t the differences in response to antibiotics reflected small variations in the vitamin content of the diets fed. In view of increasing evidence t h a t the degree of response to dietary antibiotics is related to the sensitivity of the bacteria to the antibiotics used, it appeared t h a t changes of bacterial flora which predominate in our chick rooms might also be a factor in the variable response to antibiotics.
772
J. BIELY AND B. E. MARCH TABLE 1.—Composition of basal diets Ingredient
— 56.05 23.54 2.5 2.0 4.26 4.0 1.0 1.0 1.5 0.5 1.23 1.67
— — —
0.10 0.10 0.21 0.05 1.125 0.20 0.327 1.044
—
lb. lb. lb. lb. lb. lb. lb. lb. lb. lb. lb. lb. lb.
44.5 23.0 24.0
— — —
5.0
— — —
0.5
lb. lb. lb. (44% protein)
lb. (herring)
lb.
— —
lb. 1.0 lb. 2.0 0.0133 lb. lb. lb. lb. lb. mgm. gm. gm. gm.
2,270 I.U./lb. 680 I.C.U./lb. 0.01251b. 0.0125 1b.
— — — — — — — —
0.027 gm. 2,000 I.U./lb. 120 I C . U . / l b .
— —
* A trace-mineral mix containing 6% manganese, 0.12% iodine, 2.0% iron, 0.2% copper, 0.006% zinc, 0.02% cobalt, 27% calcium.
previous test but no attempt was made at special disinfection. The birds from the preceding experiment were in an adjoining room. The chicks were weighed individually at 31 days of age. Experiments 2A and 2B. The basal diet had the same composition as Diet 2 in the first experiment. The antibiotics used as supplements were oleandomycin, penicillin and chlortetracycline at the same levels as in the previous experiment. In each experiment the diets were fed to duplicate lots of 20 male New Hampshire day-old chicks. The chicks used in both experiments were of the same hatch and came from the same dams as did the chicks in Experiment 1. The chicks were weighed individually at 35 days of age. The chicks in Experiment 2A were kept in battery brooders in the same room in which Experiment 1 had been conducted.
The experiment was started two days after the completion of Experiment 1. When Experiment 1 was terminated the birds were moved into an adjoining room and the brooders cleaned. The chicks in Experiment 2B were reared in battery brooders in another building. Sanitary practice in this second building during preceding months had not been so strict as in the room used in Experiments 1 and 2A. At the conclusion of the experiment air samples were taken in the two chick rooms used. Plate counts were made for total aerobes using tryptone blood agar base with added yeast extract (2.5 gm. per liter) as medium. The plates were incubated at 41°C. for 24 hours. RESULTS
The average weights of the chicks in both experiments are given in Table 2. In
Downloaded from http://ps.oxfordjournals.org/ at National Institute of Education Library, Serials Unit on May 20, 2015
Ground wheat Ground yellow corn Soybean oil meal (51% protein) Corn gluten meal Alfalfa meal (17% protein) Stabilized animal fat Fish meal (menhaden) Distillers' dried solubles Dried whey (50% delactose) Dried brewers' yeast Iodized salt Calcium carbonate Dicalcium phosphate Limestone Bonemeal Manganese sulphate Delamix* DL-methionine Choline chloride (25%) My vamix (20,0001.U. vitamin E/lb.) Vitamin B,2 Riboflavin Calcium pantothenate Niacin Menadione Vitamin A Vitamin D 3 Butylated hydroxytoluene Nicarbazine
Diet 2/100 lb.
Diet 1/100 lb.
773
ANTIBIOTIC RESPONSE
TABLE 2.—Average weights of chicks Antibiotic supplement None Oleandomycin Penicillin Chlortetracycline Oxytetracycline
None Oleandomycin Penicillin Chlortetracycline
grams grams 411 337 483 (+18%)* 395 (+17%)* 457 (+11%) 347 ( + 3%) 426 ( + 4 % ) 357 ( + 6%) 422 ( + 3%) 354 ( + 5 % ) Experiment 2 A Average weight of chicks at 35 days of age grams 383 512 (+34%) * 437 (+14%) 441 (+15%)
Experiment 2B Average weight of chicks at 35 days of age
grams 436 529 (+21%) * 453 ( + 4%) 480 (+10%)
* Figures in parentheses indicate increment over the average weight of chicks fed the basal diet.
Experiment 1 the two basal diets fed promoted different rates of growth. Diet 2, which was formulated to be deficient in B-complex vitamins, resulted in slower growth and some curled-toe paralysis. There was also some curled-toe paralysis in the chicks fed Diet 1. Oleandomycin stimulated the rate of growth of chicks fed both basal diets by 17-18 percent. Penicillin stimulated growth of the chicks fed Diet 1 but did not affect the growth of chicks fed Diet 2. Chlortetracycline had no appreciable effect on growth rate of chicks fed either diet. Curled-toe paralysis occurred in the chicks fed the antibiotics as well as those fed the basal diets. The rate of growth was much poorer in the chicks fed the basal diet in Experiment 2A than in 2B. In both Experiments 2A and 2B there was a marked response to oleandomycin. Curled-toe paralysis was again evident in all lots of chicks whether or not an antibiotic was fed. When the basal diet was supplemented with oleandomycin the rates of growth were similar in the chicks fed the diet in the two experiments. Penicillin and chlor-
DISCUSSION
It will be seen from Table 2 that in every instance where antibiotics were used they stimulated growth although the increase in growth rate was slight in some cases. A consistently greater response was obtained to oleandomycin than to penicillin, chlortetracycline or oxytetracycline. In Experiment 1, oleandomycin resulted in a 17-18 percent increase in the average weights of chicks fed either Diet 1 or Diet 2. It was anticipated that each of the antibiotics would stimulate growth when added to Diet 2. In spite of the deficiency in B-complex vitamins, supplementation of the diet with any of the antibiotics other than oleandomycin improved growth only slightly. Penicillin produced a greater response when used with Diet 1 than with Diet 2, suggesting that in this instance its effect resulted from suppression of bacteria which produce a mild toxemia rather than from an increase in bacteria capable of synthesizing vitamins. In Experiments 2A and 2B the chicks under the different environmental conditions responded differently to the basal diet. The chicks fed the basal diet in the room with the higher bacteria count grew faster than those in the room with the lower count. On the basis of the response obtained to antibiotic supplementation of
Downloaded from http://ps.oxfordjournals.org/ at National Institute of Education Library, Serials Unit on May 20, 2015
None Oleandomycin Penicillin Chlortetracycline
Experiment 1 Average weight of chicks at 31 days of age • Diet 1 Diet 2
tetracycline increased growth of chicks in Experiment 2A to the same rate as the chicks fed the basal diet in Experiment 2B. In Experiment 2B the chicks receiving chlortetracycline grew at a rate intermediate between those on the basal diet and those receiving oleandomycin in the diet. The response to penicillin in the chicks in this experiment was negligible. The count of total aerobes showed a count of 885 and 3,355 bacteria per cubic foot for the chicks' rooms used in Experiments 2A and 2B respectively.
774
J. BIELY AND B. E. MARCH
Although the total number of air-borne bacteria in the room used in Experiment 2A was relatively low, the population must have contained greater numbers of bacteria directly or indirectly deleterious to the chick than did the chick room in Experiment 2B. The average weights of the chicks in the different lots indicate that bacteria responsible for the difference in the growth rates of chicks fed the basal diet in the two environments were sensitive to penicillin and chlortetracycline. It would appear, however, that both environments contained bacteria which were
resistant to these antibiotics. Suppression of these bacteria by oleandomycin, therefore, may have been responsible for the stimulus to growth obtained with this antibiotic. SUMMARY
The growth response of chicks to oleandomycin, chlortetracycline, penicillin and oxytetracycline as dietary supplements at levels of 5, 10, 2.5, and 10 mgm. of the respective compounds per pound of diet were compared. The experiments included two dietary and two environmental treatments. At the levels employed of the antibiotics tested, oleandomycin promoted faster growth than did the other antibiotics, irrespective of the diet or environment. REFERENCES Coates, M. E., C. D. Dickinson, G. F. Harrison, S. K. Kon, S. H. Cummins and W. F. Cuthbertson, 1951. Mode of action of antibiotics in stimulating growth of chicks. Nature, 168: 332-339. Hill, D. C , H. D. Branion, S. J. Slinger and G. W. Anderson, 1953. Influence of environment on the growth response of chicks to penicillin. Poultry Sci. 32: 462^-66. Libby, D. A., and P. J. Schaible, 1955. Observations on growth responses to antibiotics and arsonic acids in poultry feeds. Science, 121: 733-734. Lillie, R. J., J. R. Sizemore and H. R. Bird, 1953. Environment and stimulation of growth of chicks by antibiotics. Poultry Sci. 32: 466-475. Morrison, A. B., W. G. Hunsaker and J. R. Aitken, 1954. Influence of environment on the response of chicks to growth stimulants. Poultry Sci. 33: 491-494. Waibel, P. E., O. J. Abbot, C. A. Baumann and H. R. Bird, 1954. Disappearance of the growth response of chicks to dietary antibiotics in an old environment. Poultry Sci. 33: 1141-1146.
SEPTEMBER 10-12. SOUTHWESTERN ASSOCIATION CONVENTION, HOT SPRINGS, ARKANSAS SEPTEMBER 25-27. NATIONAL POULTRY, BUTTER AND EGG ASSOCIATION CONVENTION AND TRADE SHOW, HOTEL SHERMAN, CHICAGO
Downloaded from http://ps.oxfordjournals.org/ at National Institute of Education Library, Serials Unit on May 20, 2015
the diet fed it may be concluded that the factor which depressed growth in Experiment 2A was associated with the microflora in the room and not with some management factor. Penicillin or chlortetracycline stimulated growth of the chicks in Experiment 2A so that at 5 weeks of age they were similar in weight to the chicks fed the unsupplemented diet in Experiment 2B. Penicillin scarcely affected the growth rate of chicks in Experiment 2B. Chlortetracycline caused some increase in growth rate so that the chicks fed the diet supplemented with this antibiotic were heavier than those fed a similar diet in Experiment 2A. Oleandomycin markedly stimulated growth of the chicks in both environments, but to a greater extent in Experiment 2A. In both experiments, as in Experiment 1, the response was greater to oleandomycin than to either penicillin or chlortetracycline. The average weights at 5 weeks of the chicks fed the diet supplemented with oleandomycin were similar in the two environments.