Experimental Data for Evaluating Broiler Models1

01W Applied Pouly Science, Inc

EXPERIMENTAL DATAFOR EVALUATING BROILER MODELS'

Primarv Audience: Nutritionists. Oualitv Control Personnel

important. Computer modelling DESCRIPTION OF PROBLEM I isincreasingly valuable in this remect. The use of a Doultrv

As the cost of research increases,the need to minimize the amount of research becomes

I

model, or specificaiy in this case a broil& simulation model, should reasonably predict

1 Presented at the 1994Poultry Science Association Informal Nutrition Conference Symposium: 2

Poultry Modelling - Theoretical and Practical Evaluations To whom correspondence should be addressed

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H. L. STILBORN~ Heartland Lysine, Inc., 8430 W.B y MawrAvenue, Suite 650, Chicago, IL 60631 Phone: (312) 380-7000 F M : (312) 380-7006 E.'E M O W , J R . Auburn University,Auburn, AL 36849-5416 R. M . GOUS University of Natal, Pietemaritzbutg South Africa 3200 M. D.HARRISON A. E. Skaley Manufacturing Co., Decatur, IL 62525

EVALUATING BROILER MODELS

380

wm= A where:

b (~(-B(f-t*)))

Wm = weight at which growth rate becomes zero (i.e.: mature weight) B = rate of maturing t* = time at which maximum growth rate occurs t = time

An advantage of the Gompertz equation is that the parameters can be interpreted in terms of the biology of the animal. The equation predicts that relative growth rate will decline linearly to zero as growth approaches maturity. If actual protein growth rate measurements are not available, measurements of broilers at two or more intervals during the growing period may help determine mature body weight and rate of maturing. This study utilized two strain-crosseswith the objective of comparing differences between them and providing necessary genetic information for modelling.

EXPERIMENTAL DESCRIPTION FOR MODEL EVALUATION Male and female broiler chicks came from two strain-crosses: Ross [8] male x Arbor Acres [9] female (RxAA) and Steggle [lo] male x Arbor Acres [9] female (SxAA). Chicks came from breeder flocks at approximately 38 wk of age and were randomized based upon strain-cross and sex (eight replicate pens) into floor pens containing fresh pine shavings in an open-sided house with thermostatically controlled curtains. Thirty-two pens with forty chickdpen were used, providing a stocking density of 0.104m2/bird. Each pen had one bell-type waterer and one cylinder feeder. All birds were vaccinated for Marek’s disease, Newcastle disease, and infectious bronchitis at 1 day of age. Vaccination for infectious bursal disease and fowl pox occurred at 14 days of age. The experiment started January 23, 1990 and ceased sixteen weeks later on May 29,1990.Temperature and humidity were monitored daily and summarized (Table 1). Diets based on corn and soybean meal were formulated to provide nutrient contents exceeding NRC [ll] recommendations for the starter, grower, and finisher phases (Table 2). To ensure nutrient adequacy, chicks received the starter feed from 0 to 4 wk instead of 0 to 3 wk. The same reasoning was used for the grower and finisher feeds by utilizing the 4 to 8 and 8 to 16-wk periods, respectively. Starter feed was pelleted and then crumbled. Grower and finisher feeds were fed in whole pellet form. Feed and water

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the broiler’s response to a test under defined conditions ( i e , temperature, stocking density). Testing a model validates its quality 111. When utilizing research data, researchers must have an accurate description of how the study was conducted, including daily temperature, relative humidity, stocking density, daily mortality, and dietary nutrient content (amino acids, crude protein, energy, and digestibility levels). Inaccurate information results in incorrect simulations. Determining growth curves for the different body components, corresponding to the genetic potential of various commercial broiler breeds, including sex, will provide the necessary information to compare performance of different breeds and strains. This information is valuable to modelling efforts. The information can be used to simulate the growth of broilers under varying feeding and environmental conditions [2]. Therefore the first requirement for modelling is breed description, characterized by mature body weight, mature fat content, mature feather content, and rate of maturing for weight, fat, and feathers. From this information, researchers can derive genetic parameters for each sex: mature protein, mature lipidprotein ratio, mature fat, mature water, mature ash, rate of body maturing, feather maturing rate, and fat deposition rate. Emmans and Fisher [3] assumed that the growing animal has an inherent potential growth rate which can be measured under an ideal (non-limiting) environment and that the animal will try to achieve its potential, thereby reaching maturity in the shortest possible time. Since the physical and chemical composition of the body changes with age during growth, a single function to describe the changes in a m position would not be sufficient [4]. Strict relationships between weights of the body components in potential growth [3,5,6] can be used to determine the body’s growth of water, ash, and lipid. This can be determined with the Gompertz [7] growth equation:

Poultry Modelling Symposium STILBORN et al.

381

TABLE 1. Environmental conditions of the broiler house during the studf

AGE (Wk)

MINIMUM

I

HIGH MAXIMUM

I MEAN(SD)B

LOW

MINIMUM TEMPERATUREIon

1 MAXIMUM I MEAN(SD)B

were provided ad libitum. Continuouslighting was utilized. At one day of age, forty chicks representing each strain-cross and sex were euthanized by C02, placed in gas-impermeable bags [12], then frozen (0°C) and held for later analysis. Wing-banding between 1 and 2 wk of age occurred in a random manner for future weighing and analyses. Broilers were weighed at 2, 4,6,8,12, and 16 wk of age by weighing each broiler within each pen and then being held in coops within the respective pen. A computer program sorted the birds in each pen from heaviest to lightest. Five broilers were selected so the distribution of body weights and their average paralleled that of the population within each pen. The birds were held for approximately two hours in the coops between initial weighing (full-fed basis) and the selection of sample birds. Sample birds (five per pen) were re-weighed after selection to obtain the fasted weight. Then each was euthanized by electrocution and frozen together in bags, respective of pen, for later analysis.The remaining birds were returned to the pen to continue growing until the next weighing period. Frozen birds were thawed in sequence from day 1 to 16 wk of age. Each bag (pen) was

completed before the subsequent age and pen were started. Day-old carcasses were handled differentlyfrom other ages. These tissues were pooled, based upon strain-cross and sex. Down was removed from the body, as was the residual yolk sac. Birds sampled at 2, 4, 6, 8, 12, and 16 wk were evaluated separately within the respective pen's population (designated 1, 2, 3,4, and 5 from heaviest to lightest, on a fullfed basis). Feathers from all sample birdslpedage period were pooled; then a random "grab" sample was taken from the composite after mixing in a bag. This sample of intact feathers was reduced to "bits and pieces" with scissors, then reduced further in a coffee bean mill to a fine powder by adding dry ice. This fine sample of feathers was used for proximate analysis. Defeathered carcasses were maintained on an individual basis. Each defeathered carcass was cut into pieces sufficiently small enough to pass through a meat grinder. The resulting mince was blended in a bowl using a Hobart mixer equipped with a bread dough paddle. A subsample, approximately 500 g, was then lyophilized to dry the mince and obtain an estimate of its moisture content. These lyophilized samples were further ho-

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SD = Standard Deviation

JAPR EVALUATING BROILER MODELS

382

INGREDIEm

STARTER

FINISHER 18-16 Wk)

GROWER 14-8 Wkl

( 0 4 Wkl

I

%

Corn

48.79

52.14

6650

saybean meal

3950

3150

26.25

750

550

4.00

Poultry fat

1.75

150

1.25

DLMethionine

0.25

0.15

0.05

Limestone

1.25

1.25

1.10

CoccidiostatA

0.11

0.11

-

to 100%

OtherB

ANALYZED NUTRIENT CONTENT

A"Coban,"8.8% (40 g/lb) monensin sodium premix. Elanco Products Co.,Indianapolis, IN 48285. %otal of 0.85 as contributed by 0.35% salt and 0.5070 vitamin-mineral premix. Vitamin-mineral remix provided the following per kg of complete feed vitamin A, 8OOO I U cholecalciferol, 2000 1U;vitamin E, 8 1 6 menadione, 2 mg; riboflavin, 6 mg; antothenicacid, 13 m , niacin, 36mg; choline,500 mg; folic acid, 05 mg; thiamine, 0.5 mg; pyridoxine, 2.2 mg; biotin, 0 . b mg; ethoxyquin, 1 2 f m g manganese, 65 mg; iodine, 1mg; iron, 55 mg; copper, 6 mg; zinc, 55 mg. c A M E , w a s measured using adult Single Comb White Leghorn roosters that were allowed 2 hr feeding periods p e ~ day and excreta completely collected by cup [13]. DBased on the fractional contributions of each feedstuff as measured on samples obtained at time of feed mixing.

'Amino acid analysis conducted by Heartland Lysine, Inc., Chicago, IL 60631.

.

. ..

o acid level as determined utilizing the Heartland Lysine, Inc., 1992 True D i m Poultw based on the amino acid contributions from corn, soybean meal, a n d

D

L

w

Kjeldahl nitrogen. Petroleum ether was the mogenized in a coffee bean grinder. Dry ice solvent used in a Goldfish apparatus to estiwas added to freeze the fat and facilitate mate fat (triglyceride and cholesterol ester) blending. All samples were relyophilized to content. Ash determination was based on inremove moisture condensate. cineration at 600°C for 24 hr. Values for CP, Crude protein (CP), ether extract (EE), and ash measurements were performed on all ' EE, and ash were expressed on a dry matter (DM) basis. samples. Crude protein was determined from ~

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Dicalcium phosphate

Poultry Modelling Symposium STILBORN et al.

383

Data were analyzed by analysis of variance as a factorial arrangement of strain-cross and sex [14]. Analysis was performed using the General Linear Models (GLM) procedure.

ers at all age periods except day 1, at which time no difference was detected between sexes. In research concurrent with this study, Moran [Is]reported RxAA broilers exhibiting heavier live weights compared to the SxAA broilers throughout the study. The females were consistently lighter than the males. Comparing Ross and Hybro genotypes, Hyankova et al. [161 reported that body weight at day 42 depended on genotype and sex. Feed intake (Table 4) by broilers per age period was greater for the RxAA strain-cross consistently through all age periods except for the last, where no strain-cross or sex effect

RESULTSAND DISCUSSION

TABLE 3. Live weights of two straincrosses of broiler chickens and both sexesA k

I

I

SEX

STRAIN-CROSS

AGE (Days) 1

RXAA SXAA

28

42

Male

37.3

321

Female

37.9 35.8

306

1077 950

2113 1818

295

976

2008

864

1683 15.7

Male Female

Pooled SEM (2&lf)

I

Strain Cross Sex

14

355 0.50

I

278 3.1

1

6.4

*

I

*

56

8

NS

112

103.7

8

1

84

*

8

8

8

8

AAll values are the average of eight replicate pens startingwith forty chicks per pen. BNS = P > .OS; = P < .OS.No interactionsoccurred between strain-cross and sex (P > .OS).

Pooled SEM (28df)

7.3

8

Strain Cross Sex

7.8

20.0

70.9

82.9

8

a

IBNS = P > .OS;* = P < .OS.No interactionsoccurred between strain-cross and sex (P > .OS).

146.3 NS

8

NS

I

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Live weights (Table 3) were influenced independently by the main effects of straincross and sex at all ages except day 1where only a strain-cross effect was observed. The S x A A broilers exhibited consistently lower body weights (average of male and female) across all age periods compared to the RxAA birds. Female broilers, regardless of strain-cross, weighed less than male broil-

JAPR EVALUATING BROILER MODELS

384 was detected. Male broilers consistently consumed more feed per period until day 84. During the initial 14 days, the feed efficiency (Table 5) of male broilers was better than that of females, especially for the SXAA male. Cumulative feed efficiency during subsequent ages was influenced by both straincross and sex. The SxAA broilers utilized feed more efficiently as also observed in other research [151. Male broilers, regardless of strain-cross, converted feed more efficiently

NS

Interaction

lcNS = P>.OS;

* e

SCX

e

1

= P<.OS.

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Strain Cross

than did the females. Selection of male broilers for feed efficiency rather than feed intake will lead to lower feed intakes by these birds [17]. The percent yield for breast fdets and tenders (Table 6) was initially influenced by the strain-cross and sex interaction at day 14, thereafter periodically influenced by the main effects. The percent tender yield was influenced by the interaction of the main effects at day 42. RxAA broilers yielded more breast

I N S / NS I N S 1

*

I N S ] NS

*

I N S ] NS

I

NS

I

NS

I

Poultry Modelling Symposium STILBORN et al.

385

TABLE 7. Whole carcass (feather free) and crude protein contents of two straincrosses of broiler chickens and both sexes SIIWN-CROSS

SEX

lA RXAA SXAA

I

14B

I

AGE (Days)

BB

I

42B

I

56B

I

84B

I

112B

Male

34

299

1002

1953

3028

4655

5266

Female

32

289

856

1601

2386

3679

4421

31

271

902

1858

2854

4428

5033

31

256

789

1545

2299

3483

4187

Male Female

2.7

9.7

11.6

19.8

44.2

55.6

Pooled SEM

N A ~

Strain Cross

NA

Sex

NA

Interaction

NA

NS

NS

NS

Male

595

48.8

48.7

45.2

Female

57.1

47.6

46.7

42.4

39.8

34.6

30.6

Male

60.6

50.0

50.6

47.2

46.1

45.1

47.1

Female

62.7

49.0

49.1

44.6

42.0

38.3

34.5

RxAA SXAA

Pooled SEM

N A ~

Strain Crws

NA

Sex

NA

-=

*

*

0.35

*

0.69

0.38

43.7

0.51

*

NS

NS

42.4

445

0.65

0.69

*

DNS = P > .OS; = P .OS. No interaction occurred between strainaoss and sex (P > .OS) for carcass crude protein (%DM) content.

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nal fat. This difference may account for the discrepancy. While examining yield only at day 56, Broadbent el al. (181 reported that female broilers yield more total breast meat than males when expressed as percent of eviscerated carcass. With the exception of day 56, no interaction between strain-cross and sex was observed for feather-free whole carcass weights of broilers (Table 7). The RxAA broilers were heavier than the S x A A ones. Male broilers exhibited significantlyheavier weights than females, thus following live weights. Chemical composition of the carcass (Tables 7 and 8) indicated the CP and ash contents on a dry matter basis were consistently higher for the SxAA strain-cross during the growth periods studied. The SxAA broilers contained less carcass fat as denoted by the lower EE values at all ages. Female

meat by day 28 than the SxAA broilers; however, by day 42, strain-cross did not influence fillet yield. RxAA broilers had greater proportions of fillets and tenders yielded at day 56; however, females contained the greater proportion of tenders yielded. With the exception of fillet yield at day 112, female broilers consistently yielded greater proportions of breast meat at days 84 and 112. Utilizing the same strain-crosses, Moran [lq reported females exhibited more breast fillet and tender yield at 42 and 56 days of age, however in the current study this trend was not as clear, except for tender yield at day 56. The data provided by Moran [15]expressed breast meat yield as a percent of total parts of a chilled, eviscerated carcass (without abdominal fat). In the current study, breast meat yield was expressed as percent of live weight, which would include viscera and abdomi-

JAPR EVALUATING BROILER MODELS

386

TABLE 8. Whole carcass (feather free) ether extract and ash contents of two straincrosses of broiler chickens and both sexes

WN-CROSS

SEX lA

RxAA SXAA

I

14B

1

AGE (Days)

BB

1

42'

I

56'

I

84'

I

112'

Male

22.2

31.1

32.3

38.1

41.9

46.1

42.7

Female

20.6

32.4

34.9

40.6

46.1

54.8

58.4

Male

16.4

29.6

28.9

34.8

37.9

42.0

39.4

Female

15.8

30.6

32.2

37.8

43.3

49.7

52.8

N A ~

Strain Cross

NA

Sex

NA

051

0.66

8

0.64 8

0.79

0.95

8

1.15 8

*

Male

75

7.3

7.8

7.2

7.1

6.8

7.4

Female

65

7.2

7.5

7.6

6.6

5A

5.1

Male

65

75

8.2

7.9

7.8

7.2

8.2

Female

8.0

7.4

7.9

75

7.0

6.3

5.9

Pooled SEM

NAc

0.08

0.11

0.09

0.10

0.13

0.14

Strain Cross

NA

NS

*

8

8

8

Sex

NA

NS

8

8

RxAA SXAA

*

*

DNS = P > .OR = P < .OS. No interactions occurred between strain-scross and sex (P > .OS) for carcass ether extract or ash (%DM) contents.

broilers exhibited lower carcass CP and higher E E values than did males during the entire study, as expected. Carcass ash content for female broilers generally was lower than that of male broilers after day 14. Even though the EE values reflect dry matter basis, the percent difference in carcass fat content between males and females increases with age, regardless of strain-cross. Selection of male broilers for feed efficiency rather than feed intake [17] resulted in less fat, a higher percent water content in whole carcasses, and numerically more protein. Female broilers contained greater percent fat content in eviscerated carcasses than did male broilers at 56 days of age [18]. Protein and ash were similar between both sexes at this same age. However, these values were expressed on an "as is" and not on a dry matter basis.

The SxAA broilers contained less abdominal fat at 42 and 56 days of age [15], a finding which is similar to the trend observed with the carcass fat data presented, since abdominal fat is highly correlated with carcass fat. This data also rationalizes the advantage in feed conversion. Females contained more abdominal fat at days 42 and 56 than did their male counterparts [15]. Only strain-cross and sex independently influenced feather yield (Table 9). Only at day 28 was a strain-cross effect observed where the RxAA broilers contained a higher gram content of feathers. From day 28 to day 112, males consistently contained more grams of feathers than did females, reflecting the greater surface area found for males vs. females. The CP, EE, and ash composition of the feathers (Tables 9 and 10) exhibited varying

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Pooled SEM

Poultry Modelling Symposium STILBORN et al.

387

TABLE 9. Feather yield and crude protein contents of two straincrossesof broiler chickens and both sexes

WN-CROSS

SEX lA

RXAA SXAA

I

14B

1

AGE (Days)

BB

1

56’

I

84’

1

112’

1.2

9

52

107

163

257

277

Female

1.2

9

48

88

139

195

221

Male

0.8

9

47

100

166

259

283

9

45

91

144

211

216

Female

1.0

0.24

0.9

Strain Cross

NA

NS

*

Sex

NA

NS

*

2.1 NS 8

2.6

.

NS

4.1 NS

4A NS 8

Male

91.1

87.6

89.8

90.7

89.4

91.9

90.4

Female

90.6

87.3

89.7

90.4

89.3

89.6

88.9

Male

89.5

88.1

90.8

90.3

90.2

895

89.8

88.7

88.3

90.3

90.6

90.0

89.2

89.6

Female Pooled SEM

N A ~

Strain Cross

NA

8

Sex

NA

NS

Interaction

NA

NS

0.28

0.29

0.42

NS

NS

NS

NS

NS

NS

NS

NS

0.38

0.22

0.28

8

NS

*

8

8

%slues represent an average of five birds from each of eight replicate pens, per strain-cross and sex. ‘NA = Not applicable. DNS = P > .OS;

= P < .OS. No interactions occurred between strain-cross and sex (P > .OS) for feather yield (gbird).

main effects plus interactions between the two main effects over time. During the first 28 days, only strain-cross influenced feather CP content such that the feathers of SxAA broilers contained higher CP levels. However, during the last two sampling times (days 84 and 112), the RxAA male feathers contained the highest dry matter CP content. The SxAA female exhibited the lowest feather CP content at day 84, but by day 112 the RxAA female feathers contained the lowest protein levels. The RxAA broilers’ feathers contained less fat at day 14. But, by day 28, the interaction of main effects significantly affected feather EE content. The fat content of feathers was higher for female broilers from day 56 to day 112, consistent with what was observed with carcass fat content. The feathers of SxAA birds contained a lower EE content at 56 and 84 days of age. An interaction between the main

effects was evident at day 84 such that the RxAA female feathers contained the highest fat content, followed by the SXAA female, while the males exhibited the lowest fat content. In general, the EE content of the feathers wasvariable. Ash content on a dry matter basis was variable during the first 14 days and then again by the end of the study. At day 14, ash content was highest for the S x A A female and lowest for the RxAA female while for the male broilers, ash values fell in between these. The same interaction was evident again by day 112. The RxAA broilers had higher feather ash content than did the S x A A broilers, and the SxAA female exhibited the lowest ash content. During days 28 and 42, feather ash content was not influenced by strain-crossor sex. At day56, the females’ feathers contained less ash than did their male counterparts’, but the opposite was observed by day 84, especially for the

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N A ~

SXAA

42’

Male

Pooled SEM

RXAA

I

JAPR EVALUATING BROILER MODELS

388

SEX

WN-CROSS

AGE (Day) lA

RXAA SXAA

I

14'

I

2SB

I

42'

I

56'

1

&4'

I

112'

1.9

2.2

1.6

1.5

1.4

1.4

1.3

Female

1.8

2.2

1.7

1.6

1.6

1.8

2.1

Male

1.7

2.5

1.7

1.5

1.4

1.4

1.3

Female

1.7

2.4

1.6

1.5

1.7

1.6

2.0

NAc

0.03

0.03

0.04

0.03

0.04

0.15

.

NS

NS

8

NS

Pooled SEM

IPROBABILITIES~

I

Sex

NA NA

NS

8

NS

Interaction

NA

NS

8

NS

NS

Male

1.0

2.0

1.4

1.2

1.2

Female

0.7

1.9

1.4

1.1

1.1

1.0

1.0

Male

1.0

2.0

1.4

1.1

1.2

0.9

0.9

Female

1.0

2.2

1.4

1.1

1.0

1.1

0.7

0.04

0.03

0.02

0.03

0.02

0.04

NS

NS

NS *

NS

Strain Cross

RXAA SXAA

Pooled SEM

N A ~

Strain Cross

NA

Sex

NA

Interaction

NA

I

NS *

NS

NS

.

NS

NS

NS

*

8

8

NS

1.0

v a l u e s represent an average of five birds from each of eight replicate pens, per strain-cross and sex.

I

1.0

8

NS

I

'NA = Not applicable.

DNS = P>.05:

=

Pe.05.

SxAA broilers. The RxAA broilers exhibited higher feather ash content than did S x A A broilers at day 56.

The Gompertz equation [7lwas fitted to the values on Tables 3, 6, and 9 to determine the genetic parameters for live weight, feathers, and breast meat. Both the estimated mature live weight and breast meat yields were consistently lower for the SxAA strain-cross (Table 11).The SxAA males were estimated to yield greater feather contents than the other broilers at maturity. The maturing rate for live weight (h) of the RxAA male broiler was

slightly greater than the S x A A male. The & values for females were lower than those for males. The rate of maturing for breast fillets (Bfil) was consistently higher in males than in females. However, for tenders the rate of maturing (Bten) values were similar, despite differences in mature tender yield. Research by Gous et al. [2] also reported differences in mature body weight and B d values between various commercial broiler strains and sexes. The females had lower and mature live weights than the males.

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Male

Poultry Modelling Symposium STILBORN ef al.

389

TABLE 11. Estimates of mature body weight, mature feather content, mature breast fillet, and tender contents, and rates of maturing for these parameters

SIXAINCROSS

I

SEX

I

BODYWEIGHT WeightA

I

I

FEATHERS WeightA

kg

RxAA SxAA

I

I BREASTFILLER3 IBREASTTENDERS Weight*

kg

I

BfiF

WeightA

kg

I

B., B

kg

Male

6.11

0.0382

0.297

0.0414

0.699

0.0382

0.221

Female

5.14

0.0361

0.230

0.0407

0.613

0.0339

0.17

0.0356

Male

5.86

0.0375

0.304

0.0430

0.669

0.0372

0.213

0.0350

Female

4.71

0.0367

0.230

0.0466

0559

0.0350

0.183

0.0349

0.0356

1. Broiler strain-crosseswith different selection pressures on the male sire performed differently when given the same feeds. 2. Carcass composition of the two strain-crosses and sexes is distinctly different. Female broilers exhibit lower carcass protein and higher carcass fat content as they approach maturity when compared to their male counterparts. 3. Supplying separate feather and carcass yields combined with composition will permit nutrient requirements for production of these two components to be calculated. 4. Growth curves for live weight and feather yield may vary between strain-crosses and sexes.

REFERENCES AND NOTES 1. Ivey, FJ. and H.B. Harlow, 1991. Value of a growth model for broilers as a decision making tool. Proc. N.F.I.A. Nutrition Institute. 2. Gous, RM., C.E HancocL, and G.D. Bradfield, 1992. A characterization of the potential wth rate of six breeds of commercial broiler. Pa es B e 9 2 in: Proc. 19th World Poultry Assn. Meeting, bol. 11, Amsterdam, The Netherlands. 3. Emmans, G.C. and C. Fisher, 1986. Problems in nutritional the0 Pages 9-39 in: Nutrient Requirements of Poultry and atrittonal Research. C. Fisher and K.N. Boorman, eds. Poultry Sci. Symposium 19. Butterworths, London, UK.

4. Gous, RM., 1991. Simulation models as a means of predicting the nutrient and environmental re uirements of broilers of the future. Pages 83-103 in:%roc. 18th Carolina Poultry Nutr. Conf., Charlotte, NC. 5. Emmans, G. C., 1988. Genetic corn nents of potential and actual growth. Pages 153-lG in: Animal Breeding Opportunities. Occ. Publ. No. 12, Br. SOC. h i m . Prod. 6. Emmans, G.C., 1989. The wth of turkeys. Pages 135-166 in: Recent Advances in c r k e y Science. C. Nixey and T.C.Grey, eds. Poultry Sci. Symposium 21. Butterworths, London, UK.

7. Gomperlz, B., 1852. On the nature of the function expressive of the law of human mortality and on a new method of determining the value of life contingencies. Pages513-585 in: PhilosophicalTranslationsof the Royal Society.

8. Ross Poultry Breeders, Inc., Elkmont, AL 35620. 9. Arbor Acres Farm, Inc., Glastonbury, (JT 06033. 10. Australian Poultry Ltd., Beresfield, New South Wales, 2322, Australia. 11. Nalional Research Council, 1984. Nutrient Requirements of Poultry. Nutrient Requirementsof Domestic Animals. 8th Rev. Ed. Natl. Acad. Press, Washington, D C. 12. Cryovac, W.R Grace and Co.,Duncan, SC. 13. Revington, W.H., N. Acar, and ET. Moran, Jr., 1991. Research Note: Cup versus tray excreta collections in metabolizable energy assays.Poultry Sci. 701265-1268. 14. SAS Inslitute, 1988. SASlSTAT User's Guide, Release 6.03 Edition. SAS Institute, Inc., Cary, NC. 15. Moran, Jr., ET., 1994. Response of broiler strains differing in body fat to inadequate methionine: Live performance and processing yields. Poultry Sci. 73:11161126.

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CONCLUSIONS AND APPLICATIONS

390 16. Hynnkova, L,Z Soukupova, J. Wolf, and V. Jakubec, 1992. Effects of genotype, nutrition, sex and their interactions on carcass composition of broilers. Zivocisna Vyroba 37(8):683-692.

17.4.14 RAE and DJ. FcureU, 197. A comparison of the energy and nitrogen-metabolism of broilers se-

EVALUATING BROILER MODELS lected for increased growth rate, food consumption, and conversion of food to gain. Br. Poultry S i . 18411426. 18. Broadbent, LA,BJ. Wilsonand C. Fisher, 1981. The composition of the broiler chicken at 56 days of a e. and chemical composition.

8,

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