Effect of Dietary Protein Level and Length of Feeding on Performance and Abdominal Fat Content of Broiler Chickens1

Effect of Dietary Protein Level and Length of Feeding on Performance and Abdominal Fat Content of Broiler Chickens1 MARIO C. CABEL and PARK W. WALDROUP2 Department of Animal and Poultry Sciences, University of Arkansas, Fayetteville, Arkansas 72701 (Received for publication September 11, 1990) ABSTRACT Dietary CP levels influence carcass fat content of chickens but the length of time necessaiy to feed higher levels in order to increase carcass fat is not known. Studies were conducted to evaluate the effects of length of feeding of different levels of dietary CP on broiler performance and abdominal fat deposition. All diets were calculated to be isocaloric at 3,200 kcal ME/kg. Broilers fed 23% CP for 0 to 21 days, 20% CP for 21 to 42 days, and 18% CPfor 42 to 49 or 42 to 56 days served as the control group in each of two trials. In the first trial, broilers were fed constant levels of CP from day-old to market age with CP levels ranging from 15 to 27% (amino acids adjusted in proportion to CP level). Other broilers were fed 23% CP for 0 to 21 days and then fed either IS, 17,19,or21%CP to market age. In the second trial, broilers were fed constant levels of CP (18, 20, or 23%) from day-old to market age. Feeding CP levels lower than those fed the control group had a more pronounced effect on males than on females, with the primary effects being reduced body weight, poorer feed utilization, and increased carcass fat content. Feeding CP levels higher than those fed the control group generally did not increase filial body weight but generally improved feed utilization and decreased carcass fat content. The economics of improved feed utilization and reduced carcass fat must be considered in determining dietary CP levels to use at different ages for growing broilers. (Key words: broilers, abdominal fat, crude protein, body weight, feeding systems) 1991 Poultry Science 70:1550-1558

ments in body weight and feed efficiency have been obtained when the time of change was Protein and amino acid requirements of delayed (Yule, 1976; Proudfoot and Hulan, poultry vary with age, leading to a common 1980). More frequent dietary changes did not practice of decreasing levels of CP during the allow the use of reduced amino acid levels growing period. The National Research Coun(90% or less), and increased amino acid levels cil (NRC, 1984) suggests three levels of (110% of those suggested by Thomas et al, dietary CP (23, 20, and 18%) for three 1978) were of no significant benefit despite the growing periods: starter (0 to 21 days), grower modifications in feeding intervals (Smith and (21 to 42 days), and finisher (42 to 56 days), Waldroup, 1988). respectively. However, the optimum time to Narrowing the calorie:protein (C:P) ratio of make these dietary changes has not been clearly established and is dependent upon cost the diet has been shown consistently to factors and benefits derived from altering the enhance lean tissue accretion and reduce nutrient content of the feed (Pesti and Fletcher, abdominal fat deposition (Fisher, 1984). The C:P ratio may be altered by increasing the CP 1984). at constant energy or decreasing the energy at The change from a high-protein starter to constant CP with equal effectiveness. The low-protein finisher diet can be made as early latter approach can reduce performance when as 21 days of age without adversely affecting the nutrient content of the diets becomes body weight gains (Powell and Gehle, 1974). inadequate and the former can substantially Rousch (1983) suggested that the optimum increase the total cost of the feed. However, times to change from starter to finisher diets increasing the dietary CP level often improves for body weight and feed efficiency were 27 feed efficiency even in the absence of a growth and 24 days, respectively. However, improve- response (Jackson et al, 1982b; Pesti and Fletcher, 1984). In addition, less expensive protein sources have been utilized effectively 'Published with the approval of the director of the to increase the CP level of the diets for reducing abdominal fat deposition (Griffiths et Arkansas Agricultural Experiment Statioa al, 1977; Cabel et al, 1987, 1988). 2TO whom correspondence should be addressed. INTRODUCTION

1550

PROTEIN FEEDING SYSTEMS

Providing high levels of CP is a proven means of reducing abdominal fat content. However, little is known regarding the exact quantity of CP necessary or length of time elevated CP levels are needed to attain a reduction in abdominal fat content. The following trials were conducted to compare different CP feeding systems and to determine the effects of length of feeding of various CP levels on performance and abdominal fat content of broiler chickens. MATERIALS AND METHODS

Trial 1 A series of diets were formulated containing 15 to 27% CP in increments of 2%. All diets were calculated to be isocaloric (3,200 ME kcal/ kg). Within each CP level, minimum amino acid requirements were established based on the amino acid:CP ratios of the 23% CP starter diet suggested by the NRC (1984). Li addition, 20% CP grower and 18% CP finisher diets were formulated to meet the suggested CP and amino acid requirements for chicks from 21 to 42 days and 42 to 56 days (NRC, 1984). The composition of these diets and their calculated nutrient contents are shown in Table 1. Twelve different combinations of CP levels were evaluated to determine their effects on growth rate, feed utilization, and carcass composition. The standard three-protein system suggested by the NRC (1984) served as the control (23% CP for 0 to 21 days, 20% CP for 21 to 42 days, and 18% CP for 42 to 56 days). In 7 of the experimental treatments, CP was held constant at 15,17,19,21,23,25, or 27% for all three growing periods (one-protein system). In 4 of the experimental treatments, 23% CP was fed for 0 to 21 days and 15,17,19, or 21% CP was fed from 21 to 56 days (two-protein system). Each of the 12 resulting dietary treatments was fed to four replicate pens of males and four replicate pens of females with eight chicks per pen. Day-old Cobb 500 3 sexed broiler chicks were obtained from a local hatchery and randomly allocated among 96 pens in electrically heated battery brooders with raised wire floors. Chicks were brooded at 32.2 C for die first 7 days with the minimum temperature reduced 2.8 C/wk to 21.1 C. Experimental diets

'Cobb-Vantress, Inc., Siloam Springs, AR 72761.

1551

in mash form and tap water were provided for ad libitum consumption. Chicks were grown under continuous fluorescent lighting. Birds were group weighed by pen at 21 days of age and transferred to finisher batteries maintained in an environmentally controlled house. After the second weighing at 42 days of age, randomly selected birds from each pen (based on a priori selection of wing band numbers) were removed to reduce pen numbers to six chicks. Final body weights were obtained at 49 days of age for females and 56 days of age for males. Feed consumption by pen was obtained during each weighing period and adjusted for mortality by adding the weight of dead birds to the total pen weight. All the remaining birds were processed after the final weighing to determine abdominal fat content as described by Cabel et al. (1987). Trial 2 Diets were formulated to meet NRC (1984) suggested CP and amino acid recommendations for starter (23% CP, 0 to 21 days), grower (20% CP, 21 to 42 days), and finisher (18% CP, 42 to 49 days) periods. These were identified as "standard" (STD) protein formulas. A second series of diets were formulated to meet the same minimum amino acid recommendations but had no CP requirement. These were identified as "no protein minimum" (NPM) formulas. All diets were calculated to be isocaloric (3,200 ME kcal/ kg). Composition and calculated nutrient content of the diets are given in Table 1. Eight dietary treatments were compared. Within each protein system (STD or NPM) one group was fed the three-protein system (23% CP for 0 to 21 days, 20% CP for 21 to 42 days, and 18% CP for 42 to 49 days) suggested by NRC (1984). Three other groups within each protein system were fed constant levels of protein (18, 20, or 23%) throughout the growing period. Each dietary treatment was fed to six pens of chickens. Day-old Cobb 500 sexed broiler chicks were obtained from a local hatchery and randomly distributed among 48 floor pens (25 males and 25 females per pen; 9 birds per square meter) with pine wood shavings as litter. Pens were equipped identically with an automatic water fountain and two tube-type hanging feeders. Feeder flats and water jugs were used for supplemental feed and water during the first 7 days. Chicks were brooded at 32.2 C for the first

27%

25%

23%

21%

Trial 1 19%

17% 15%

Grower ]Finisher

ST

.93

23.00 22.83 1.36 .59

3,200.00

56.47 30.94 5.00 4.42 1.19 .95 .26 .50 .10 .17 .00 100.00

23%

^Supplied per kilogram of diet: iron, 100 mg; manganese, 100 mg; zinc, 100 mg; copper, 10 mg; iodine, 1 mg.

Supplied per kilogram of diet: vitamin A, 6,612 IU; vitamin D3, 2,204 ICU; vitamin E, 6.6 IU; menadione, 3.3 mg; ribofl choline, 495 mg; thiamine, 1.1 mg; pyridoxine, 1.1 mg; vitamin Bj2, .01 mg; biotin, .11 mg; folacin, .66 mg; ethoxyqu

3

Blended animal protein product (H. J. Baker & Bro., New York, NY 10017).

2

STD = formulated to meet the minimum amino acid and crude protein specifications; NPM = formulated to meet min specifications.

1

Ground yellow corn 74.84 43.38 49.68 55.97 62.26 68.56 81.11 66.40 73.55 Soybean meal (48% CP) 31.21 20.73 36.45 25.97 15.49 10.26 23.17 17.78 41.69 Propak (60% CP)2 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 Blended fat 6.74 4.62 2.50 1.44 5.68 3.56 .39 2.67 1.30 Ground limestone 1.11 1.13 1.03 1.05 1.08 1.16 1.02 .94 1.19 Dicalcium phosphate 1.08 1.11 1.12 1.09 1.10 1.13 .83 S6 1.09 .26 26 Salt 26 .26 26 26 26 .27 21 .50 Vitamin premix3 SO .50 .50 SO SO .50 .50 SO Trace mineral mix 4 .10 .10 .10 .10 .10 .10 .10 .10 .10 DL-methionine (98%) 22 .17 .14 .11 .04 .19 .09 .06 .00 Lysine HC1 (98%) .00 .00 .00 .00 .00 .00 .00 .00 .00 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 Calculated nutrient composition ME, kcaVkg 3,200.00 3,200.00 3,200.00 3,200.00 3,200.00 3,200.00 3,200.00 3,200.00 3,200.00 Crude protein (N x 6.25): Calculated 27.00 25.00 23.00 21.00 19.00 17.00 15.00 20.00 18.00 Analyzed 23.01 21.12 19.10 16.90 19.92 17.84 26.88 24.93 15.23 1.07 1.14 Lysine, % 1.67 1.52 1.37 1.22 .91 .78 .98 Methionine, % .64 .54 .43 .59 .49 .38 .42 .36 .69 Methionine + .77 .72 1.01 .93 .85 .69 .61 .63 cystine, % 1.09

Ingredients and calculated nutrient composition

TABLE 1. Composition of broiler diets with different protein levels

1553

PROTEIN FEEDING SYSTEMS

7 days with the minimum temperature reduced 2.8 C/wk to 21.1 C. Incandescent light bulbs were used to supplement natural daylight to provide 23 h of light daily. Experimental diets in mash form and tap water were provided for ad libitum intake. Weights by sex and feed consumption by pen were obtained at 21,42, and 49 days of age. Feed consumption was adjusted for mortality by adding the weight of dead birds to the total pen weight At 49 days, three randomly selected birds of each sex (weighing within one-half of a standard deviation of the average pen weight for that sex) were processed as previously described (Cabel et al., 1987) to determine dressing percentage and abdominal fat content Data for both trials were subjected to the analysis of variance using the General Linear Models procedure of the SAS institute (1985).

The model included dietary treatments as main effects with the residuals used as error term. Percentage data were transformed to arc sine for analysis. Significant differences between means were separated by repeated t tests using the Least Square Means procedure of the SAS Institute (1985). All observations of statistical significance were based on a probability of P<.05. RESULTS

Trial 1 The body weight of broilers was significantly influenced by dietary protein regimen (Table 2). At 21 days of age, male broilers fed diets containing less than 23% CP (amino acids reduced proportionally) weighed less than broilers fed 23% or more CP. Increasing CP above 23% resulted in no improvement in growth rate

TABLE 2. Average body weight gains, percentage abdominal fat, and dressing percentage of broilers fed diets with different levels of protein for various lengths of time (Trial 1)

Protein level (S, G, F) 1

Sex

0-to-21 day gain

21-to-42 day gain

42-to-49 or 42-to-56 day gain

Final2 weight

Abdominal fat pad

Dressing percentage frtr\

(%) 3

23, 20, 18 27, 27, 27 25, 25, 25 23, 23, 23 21, 21, 21 19, 19, 19 17, 17, 17 15, 15, 15 23, 21, 21 23, 19, 19 23, 17, 17 23, 15, 15 23, 20, 183 27, 27, 27 25, 25, 25 23, 23, 23 21, 21, 21 19, 19, 19 17, 17, 17 15, 15, 15 23, 21, 21 23, 19, 19 23, 17, 17 23, 15, 15 Pooled SEM

Male Male Male Male Male Male Male Male Male Male Male Male Female Female Female Female Female Female Female Female Female Female Female Female Male Female

.600* .603* .590* 585*° 556°* .548* .493* .437° 594* .606" .579* 586*° 542* 544* .546" .535*°* 523 a b c .490°** .466* .409" .527*°* 534 a b c 539*° .488** .011 .017

bcd

1.158 1504 a b c 1.246" 1.218"° 1504 a b c 1.138°** 1.089** 1.034e 1.195*1* 1^00*°° 1.195ab* 1.121** .969 1.044 1.063 .998 1.015 .989 .954 .962 1.029 .959 .954 .990 .031 .034

.827 .774 .826 .711 .796 .843 .790 .773 .816 .719 .770 .802 .296** 59711* •278** .334*° .253* .315*** .374* .338*° •276°= 314 abc 585°* •279°* .055 .023

2585* 2580* 2.663* 2514* b 2.556* 2.529*b 2.373°° 2544* 2.605* 2525* b 2544" 2509* 1.807 1.884 1.886 1.866 1.791 1.795 1.793 1.709 1.832 1.807 1.778 1.757 .057 .040

3.375** 2.644e 2.979** 2.970*8 3500* 3.377** 4.843* 5.124" 3.381** 3.761* 4.265° 5.190* 4.4321* 3.718e 3.754** 4.260°** 3.701e 4.330°* 4.767*° 5.160*

66.45 66.01 6654 6551 66.10 65.79 66.10 65.40 65.81 6656 66.18 66.17 65.41 64.93 65.39 65.04 64.99 65.44 64.88 64.42

4.079*** 4.098*08 4.4901* 5534* .195 .196

64.88 64.76 6556 65.66 59 59

•^Within columns and sex, means with no common superscripts differ significantly (P<05). 'S, G, F, = starter (0 to 21 days), grower (21 to 42 days), and finisher (42 to 56 days). 2 49 days for females and 56 days for males. Considered as the control group.

1554

CABEL AND WALDROUP

of male broilers. A similar response was observed among female broilers. From 21 to 42 days of age, there seemed to be some compensatory gain among male broilers that had been given CP levels lower than 23% during the first 21 days (Table 2). Gains of birds given 17,19, or 21% CP during this time did not differ significantly from those of male broilers given the control diet series. Male broilers that were given 25% CP during this time gained significantly more weight than did those fed the control diet series, but those given 27% CP did not differ from those given the control diet series. Among males given 23% CP during the first 21 days and then subjected to a reduction in CP, only those reduced to 15% CP exhibited a significant reduction in weight gain. There were no differences among weight gains of females during this period of time. From 42 to 49 days of age, female broilers seemed to make their compensatory gains (Table 2). Among broilers that had been fed constant CP levels ranging from 19 to 27%, there were no significant differences in gain as compared with the birds that had been fed the control series. Female broilers that had been fed 17% CP constantly gained significantly more during this time than those fed the control series, and those mat had been fed 15% CP constantly had gains that did not differ from those fed the control series. During the period of 42 to 56 days of age, there were no differences in body weight gains among male broilers. At 49 days of age, there were no differences in body weight among female broilers fed any of the dietary treatments (Table 2). At 56 days of age, however, significant differences existed in body weights among male broilers. When males were fed constant CP levels ranging from 19 to 27%, there were no differences in body weights compared with those fed the control diet series. Male broilers fed constant 15 or 17% CP diets weighed significantly less at 56 days of age than those fed the control diet series. When broilers were fed 23% CP for 0 to 21 days and then given CP levels as low as 15% during the period 21 to 56 days, there were no significant differences in 56-day weight as compared with those fed me control diet series. There were no significant differences in carcass dressing percentage among male or female broilers fed the different experimental diets (Table 2). However, significant differences in abdominal fat content were observed among birds fed the different dietary treatments. When

male broilers were fed constant levels of CP during the growing period, there was a reduction in abdominal fat pad as the dietary protein level increased. In comparison with those fed the control diet series, however, significant differences were observed only with the highest level of CP (27%). Broilers fed 15 or 17% CP had significantly more abdominal fat than those fed the control diet series. When male broilers were fed 23% CP for 0 to 21 days and then were given CP levels as low as 15% during the periods of 21 to 56 days, abdominal fat content was significantly increased in broilers fed 15 or 17% CP as compared with those fed the control diet series. When female broilers were fed constant levels of CP during the growing period, those fed diets containing 21, 25, or 27% CP had significantly less abdominal fat than those fed the control diet series (Table 2). Female broilers fed a constant level of 17 or 19% CP did not differ in abdominal fat content from those fed the control diet series, but those fed a constant level of 15% had significantly higher abdominal fat content than those fed the control diet series. When female broilers were fed 23% CP for 0 to 21 days and then were given CP levels as low as 15% during the period of 21 to 49 days, abdominal fat content was significantly increased in broilers fed 15% CP as compared with those fed the control diet series. Feed efficiency by broilers was significantly influenced by dietary CP regimen (Table 3). Male broilers fed diets containing 15 or 17% CP from 0 to 21 days (amino acids reduced in proportion to CP) had significantly poorer feed efficiency than tiiose fed the control diet series. Increasing the CP level in excess of 23% had no significant effect on feed efficiency by male broilers. A similar response was observed among female broilers. During the grower period, feed efficiency was consistently better for male broilers fed diets with CP levels greater than those fed the control diet series, but differences were statistically significant only for those fed the diet with 27% CP (Table 3). Feed efficiency of broilers fed diets with CP levels that were lower than those of the control diet series was consistently poorer, but differences were statistically significant only for males that had been fed 23% CP for 0 to 21 days followed by 15% CP for 21 to 42 days. There were no significant differences in the 21 to 42 day feed efficiency among female broilers fed the different dietary protein regimens.

1555

PROTEIN FEEDING SYSTEMS

From 42 to 56 days, there were no significant differences in feed efficiency among male broilers fed the different dietary CP regimens (Table 3). Although there were significant differences in feed efficiencyfrom42 to 49 days among the females fed the different dietary CP regimens, only one group (those fed 17% CP constantly) differed significantly when compared with those fed the control diet series. Trial 2 Body weight of broilers were significantly influenced by the CP level of the diet and length of feeding (Table 4). In birds fed the STD series, CP levels of 20% or less significantly reduced weight gains of both males and females at 21 days but not in me grower or finisher periods, as

compared with mose fed the control CP series (23,20, and 18%). Final body weights of males or females fed a constant 20% CP level did not differ significantly from those counterparts fed the control CP series. Males fed 23% CP continuously gained significantly more from 21 to 42 days and had significantly greater 49-day body weights than males fed the control protein series. Final body weights for both sexes were significantly reduced when 18% CP diets were fed continuously. Birds fed NPM-formulated diets tended to weigh less than their STDfed counterparts, but differences in final weights were significant only for males fed 18 or 20% CP on a constant basis or for females fed 18% CP on a constant basis. There were no significant differences in dressing percentage among males fed the

TABLE 3. Cumulative feed efficiency (gain:feed) of broilers fed diets with different levels of protein for various lengths of time (Trial 1) Protein level (S, G, F) 1 (96) 23, 20, 183 27, 27, 27 25, 25, 25 23, 23, 23 21, 21, 21 19, 19, 19 17, 17, 17 15, 15, 15 23, 21, 21 23, 19, 19 23, 17, 17 23, 15, 15 23, 20, 183 27, 27, 27 25, 25, 25 23, 23, 23 21, 21, 21 19, 19, 19 17, 17, 17 15, 15, 15 23, 21, 21 23, 19, 19 23, 17, 17 23, 15, 15 Pooled SEM a-e

Sex

0 to 21 days

21 to 42 days

.694 abc .704*° .679abc .639°* .640c .652bc .579** .546e

All** .517* .504*° .488"° .511*° Aid* .446 cde .435** .494°° 478 abc .477bc .422e .462 .465 .481 .455 .466 .456 .447 .459 .473 .456 .437 .444 .014 .012

42 to 49 or 42 to 56 days

0 to 49 or 0 to 56 days2

flrr-Virl Male Male Male Male Male Male Male Male Male Male Male Male Female Female Female Female Female Female Female Female Female Female Female Female Male Female

1 .m ** .715* .673*bc .682*bc .720* .651*° .675*° .678*° .721* 637 abc

1 .m* .561° .678*° .670*° .693* .669*b .031 .074

.377 .381 .359 .325 .365 .388 .368 .377 .366 .309 .323 .352 gOgbcd 317 bcd 582°* .376*b .271* .348 bc .440* .347bc .294°* .332 bc * 308 bcd M2** .029 .025

47(jabc .503* .485*° .465°°* .483*° AlJ* .442*e .435e .485*° Ml** A56** M2** ASG** .496* .486 abc .488*" .482*bc .473°°* ASl*** .457* 483 abc 47g abcd .469 b0 * .466°* .009 .008

Witbin columns and sexes, means with no common superscripts differ significantly (P<05). S, G, F = starter (0 to 21 days), grower (21 to 42 days), finisher (42 to 56 days). 49 days for females and 56 days for males. Considered as the control group.

X

1556

CABEL AND WALDROUP

various dietary treatments (Table 4). Females fed NPM-formulated diets with 18 or 20% CP had significantly lower dressing percentage in comparison with those fed STD-formulated diets of similar CP level. Abdominal fat content of male and female broilers was reduced by constant feeding of 23% CP, as compared with those fed the control CP series, but differences were not statistically significant. Constant feeding of 18% CP resulted in an increased abdominal fat content regardless of whether diets were formulated with or without a minimum CP. These differences reached statistical significance for males fed both STD and NPMformulated diets and for females fed NPMformulated diets. Feed efficiency of broilers was significantly affected by CP feeding systems (Table 5). At 21 days, broilers fed diets with 18 or 20% CP had significantly poorer feed efficiency than those fed 23% CP. During the periods of 21 to 42 days and 42 to 49 days, however, feed efficiency by these groups was significantly better than that of broilers fed the control CP series. At the conclusion of the study at 49 days, overall feed efficiency was significantly better for those fed

constant 18 or 20% CP STD diet, as compared with those fed the control CP series, and equal to that of broilers fed the control CP series when given diets with NPM formulation. Constant feeding of 23% CP throughout the study resulted in significantly improved feed efficiency, as compared with feed efficiency of those fed the control CP series, regardless of the type of formulation used. DISCUSSION

Although weight gains and feed efficiency seemed to be directly proportional to dietary CP level during the starter period, benefits of continuous feeding on overall performance were limited to improved feed efficiency at the highest CP level for both trials. Jackson et al. (1982b) and Pesti and Fletcher (1984) also observed improved feed efficiency with higher CP levels in the absence of a growth response. The cost of continuously feeding high CP levels, however, may outweigh the improvement in feed efficiency. Jackson et al. (1982b) reported higher costs and lower returns when dietary CP levels greater than 20% were used.

TABLE 4. Average body weight gains, percentage abdominal fat, and dressing percentage of broilers fed diets with different levels of protein for various lengths of time (Trial 2) Protein level (S, G, F) 1

Sex

0-to-21 day gain

21-to-42 day gain

42-to-49 day gain

49-day weight

n—\

(%) 23, 20, 18 23, 23, 23 20, 20, 20 18, 18, 18 23, 20, 18 2 23, 23, 23 2 20, 20, 20 2 18, 18, 18 2 23, 20, 18 23, 23, 23 20, 20, 20 18, 18, 18 23, 20, 18 2 23, 23, 23 2 20, 20, 20 2 18, 18, 18 2 Pooled SEM a

Male Male Male Male Male Male Male Male Female Female Female Female Female Female Female Female Male Female

.649* .645* .593° .493 e .619" .640*" .563 d .477° .594* .585* •555"° .473 d .576* .575*" .532c .449° .008 .010

1.226"° 1.351a 1291ab

1.166° 1.192c 1.294s 1.225"° 1.091d 1.016 1.023 1.059 .992 1.027 1.040 1.017 .945 .024 .023

Dressing percentage

Abdominal fat pad tr*\

.389 .406 .460 .402 .374 .502 .436 .381 .314 .357 .308 .342 .302 .307 .325 .301 .032 .032

2.264"° 2.402" 23448b 2.061 d 2.185° 2.436a 2.224° 1.94915 1.924a 1.965" 1.922" 1.807b 1.906"" 1.922" 1.874ab 1.695° .037 .038

cd

3.014 2.323d 3.485°° 4.038"" 3.365"° 2572 d 2.860°d 4590" 3.827" 3.430b 3.809" 4.115"" 4.717" 3.853b 3.635b 4.701* 259 265

6227 61.79 63.09 62.73 62.96 62.39 62.55 62.80 62.91""° 63.04""° 6456"" 64.47a 62.82°° 63.14""° 61.95° 62.03° .36 .58

*Within columns and sexes, means with no common superscripts differ significantly (P<05). S, G, F = starter (0 to 21 days), grower (21 to 42 days), finisher (42 to 46 days). 'Diets formulated to meet minimum amino acid requirements at each protein level but not the minimum protein

1

1557

PROTEIN FEEDING SYSTEMS

Diets with CP levels below 19%, however, significantly reduced feed efficiency. Female gains were less during the starter period but tended to be higher during the finisher period and male gains were significantly worse during die latter period. Comparable overall performance was obtained at lower grower and finisher CP levels when adequate CP level (23%) was provided for the first 21 days. Protein and amino acid requirements are generally considered to decrease with increasing age, which may explain why CP level affects performance less during the grower and finisher periods. The comparable performance of birds on the two-protein system, regardless of CP level during me grower and finisher phase, can also be attributed to the reduced CP requirement at later ages. Jackson et al. (1982b) reported a significant CP by sex interaction and concluded that there were sex differences in CP requirements. Similar observations were also reported by Moran (1973). The present results can also be interpreted as showing differences in the CP requirement between the sexes. However, this difference may actually imply disparity in the degree of response to nutrient density rather than differences in nutrient requirement Waldroup et al. (1991) concluded that male and female broilers have the same nutritional requirements for CP but not to the same ages. The availability of synthetic amino acids has allowed the formulation of low-protein diets that meet amino acid requirements.

Stilborn and Waldroup (1988) fed amino acidsupplemented isocaloric diets from 21 to 42 days of age and found comparable 42-day body weights and feed efficiency with dietary CP levels as low as 14%. Results from die present study showed reduced performance in birds fed the 18% CP diet from 1 day of age. The reduction in performance, however, does not necessarily imply mat a level greater than 18% CP is needed to maintain acceptable performance. It does imply the need to maintain the minimum amino acid content of the diet at the required level, regardless of CP level. The importance of maintaining the nonessential nitrogen content of the diet at low CP levels should not be overlooked, especially in relation to its effect on carcass composition. Yoshida and Morimoto (1970) and Jackson et al. (1982a) reported a curvilinear decline in carcass fat with increasing dietary CP that plateaued at about 28% CP despite significantly increased CP intake at high dietary CP levels. Results of the present study are in agreement with the above findings; broilers fed higher CP levels deposited significantly less abdominal fat and those fed lower CP levels deposited significantly more fat. Because the degree of fatness in broilers can be rapidly and reversibly affected by the C:P ratio of the diet (Yoshida and Morimoto, 1970) and abdominal fat deposition occurs at a much faster rate than total fat during the finishing period (Griffiths et al, 1977), the proper approach in reducing abdominal fat through high dietary CP may be

TABLE 5. Cumulative feed efficiency (gain:feed) of broilers fed diets with different levels of protein for various lengths of time (Trial 2) Protein level (S, G, F) 1

0 to 21 days

21 to 42 days

42 to 49 days

0 to 49 days

(kg:kg) <*) 3 3 9 cd 23, 20, 18 .735" .444" .477° a^a&b .731*0 23, 23, 23 .488"1 .509* a*7aab 20, 20, 20 .501*° .693' .487*1 .389*0 18, 18, 18 .627* .493 b .491* 2 b d 23, 20, 18 .715 .454'icd .323 .473' 23, 23, 23 2 .727*° .397* 513* .486*1 20, 20, 20 2 .364°° .678' .477"1 Mi* d 2 b bc .634 18, 18, 18 .367 .475° .467 Pooled SEM .006 .005 .007 .009 a-dWithin column comparisons, means with no common superscripts differ significantly (P<05). X S, G, F = starter (0 to 21 days), grower (21 to 42 days), finisher (42 to 56 days). 2 Diets formulated to meet minimum amino acid requirements at each protein level but not the mirnmnm protein specification.

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CABEL AND WALDROUP

to increase dietary CP during the growing or finishing periods. Salmon et al. (1983) and Pesti and Fletcher (1984) obtained reduced abdominal fat in birds fed high CP grower or finisher diets. Cabel et al. (1987, 1988) indicated that abdominal fat can be reduced by feeding high dietary CP levels 1 to 2 wk before slaughter using feather meal as a nonspecific nitrogen source. In the present study, continuous feeding of high levels of dietary CP, as compared with a normal reduction in CP over time, resulted primarily in improvements in feed efficiency and a reduction in abdominal fat deposition with a possible concomitant increase in total meat yield. However, this benefit from feeding high dietary CP levels, including improved feed efficiency, can be obtained by increasing the dietary CP level of the grower or finisher diets. It seems that there are no advantages in feeding high dietary CP levels throughout the growing period. The two-protein system was comparable with the conventional three-protein system in terms of performance and abdominal fat content. Moran (1979) suggested that chickens have the ability to correct earlier protein deficiencies through a phenomenon called compensatory growth. Compensatory response to early protein deficiency was also noted by Pesti and Fletcher (1984). The present results indicate the existence of growth compensation during the finisher phase, particularly in females fed lower CP levels. The degree of restriction in male birds fed diets with 15 and 17% CP may have been too severe, as the birds were unable to attain comparable market weights despite greater gains during the grower and finisher periods. In addition, the grower and finisher diets may have been inadequate to allow the rate of compensatory growth needed to overcome the early depression. REFERENCES Cabel, M. C , T. L. Goodwin, and P. W. Waldroup, 1987. Reduction in abdominal fat content of broiler chickens by the addition of feather meal to finisher diets. Poultry Sci. 66:1644-1651. Cabel, M. C , T. L. Goodwin, and P. W. Waldroup, 1988. Feather meal as a non-specific nitrogen source for reducing abdominal fat in broilers during the finisher period. Poultry Sci. 67:300-306. Fisher, C , 1984. Fat deposition in broilers. Pages 437-470 in: Fats in Animal Nutrition. J. D. Wiseman, ed. Butterworths, London, England. Griffiths, L., S. Leeson, and J. D. Summers, 1977. Fat

deposition in broilers: Effect of dietary energy to protein balance, and early life caloric restriction on productive performance and abdominal fat pad size. Poultry Sci. 56:638-646. Jackson, S., J. D. Summers, and S. Leeson, 1982a. Effect of dietary protein and energy on broiler carcass composition and efficiency of nutrient utilization. Poultry Sci. 61:2224-2231. Jackson, S., J. D. Summers, and S. Leeson, 1982b. Effect of dietary protein and energy on broiler performance and production costs. Poultry Sci. 61:2232-2240. Moran, E. T., Jr., 1973. Protein needs of the male and female broiler chicken. Pages 19-24 in: Proceedings of the Maryland Nutrition Conference, University of Maryland, College Park, MD. Moran, E. T., Jr., 1979. Carcass quality changes with the broiler chicken after dietary protein restriction during the growing phase and finishing period compensatory growth. Poultry Sci. 58:1257-1270. National Research Council, 1984. Nutrient Requirements of Poultry. 8th rev. ed. National Academy Press, Washington, DC. Pesti, G. M., and D. L. Fletcher, 1984. The response of male broiler chickens to diets with various protein contents during the grower and finisher phases. Br. Poult Sci. 25:415^t23. Powell, T. S., and M. H. Gehle, 1974. Effect of different feeding regimes on performance of color sexed broilers, sexes reared separate. Nutr. Rep. Int 10: 333-337. Proudfoot, F. G., and H. W. Hulan, 1980. Performance of chicken broilers changed from starter to finisher diets at different ages. Can. J. Anim. Sci. 60:799-801. Rousch, W. B., 1983. An investigation of protein levels for broiler starter and finisher rations and the time of ration change by response surface methodology. Poultry Sci. 62:110-116. Salmon, R. E„ H. L. Classen, and R. K. McMillan, 1983. Effect of starter and finisher protein on performance, carcass grade, and meat yield of broilers. Poultry Sci. 62:837-«45. SAS Institute, 1985. SAS® User's Guide: Statistics. 1985 ed. SAS Institute, Inc., Cary, NC. Smith, N. K., Jr., and P. W. Waldroup, 1988. Can amino acid content of broiler diets be reduced by more frequent changes in diet composition related to age of broiler chickens. Nutr. Rep. Int 37:431-438. Stilbom, H. L., and P. W. Waldroup, 1988. Minimum levels of dietary protein for growing broilers. Poultry Sci. 67(Suppl. l):36.(Abstr.) Thomas, O. P., P. V. Twining, E. H. Bossard, and J. L. Nicholson, 1978. Updated amino acid requirements of broilers. Pages 107-111 in: Proceedings of the Maryland Nutrition Conference, University of Maryland, College Park, MD. Waldroup, P. W., N. M. Tidwell, and A. L. Izat, 1991. The effects of energy and amino acid levels on performance and carcass quality of male and female broilers fed separately. Poultry Sci. 69:1513-1521. Yoshida, M., and H. Morimoto, 1970. Interrelationship between dietary protein level and carcass composition of chicks. Agric. Biol. Chem. 34:414-422. Yule, W. J., 1976. Optimum change to finisher diet and most efficient marketing age of sex separated broUers. Nutr. Rep. Int. 14:121-124.