Effects of added lard fed to broiler chickens during the starter phase. 2. Serum lipids

Effects of Added Lard Fed to Broiler Chickens During the Starter Phase. 2. Serum Lipids1,2 E. DAVID PEEBLES,*,3 J. D. CHEANEY,* J. D. BRAKE,*,4 CAROLYN R. BOYLE,† MICKEY A. LATOUR,*,5 and C. D. McDANIEL* *Poultry Science Department and †College of Veterinary Medicine, Mississippi State University, Mississippi State, Mississippi 39762 and high density lipoprotein cholesterol (HDLC) were inconsistent and were influenced by sex between 14 and 42 d of age. However, serum triglycerides and very low density lipoprotein cholesterol concentrations showed progressive increases over the 14 to 42 d period in birds that received dietary lard at either level in the S1 diet. These same serum constituents also increased to the greatest extents over the same period when birds were provided 3% added lard in the S2 diet. It was concluded that the response of broiler chickens between 14 and 42 d to different levels of dietary lard were influenced by age of feeding during the starter period. Furthermore, the specific effects of the diets on serum cholesterol, LDLC, and HDLC concentrations between 14 and 42 d varied with the sex and age of the bird.

(Key words: broiler, chick, dietary fat, lard, serum lipids) 1997 Poultry Science 76:1648–1654

INTRODUCTION Fat has been shown to be a practical and economical means by which to increase energy levels in poultry diets and stimulate growth (Hill and Dansky, 1954; Donaldson et al., 1957; Griminger, 1986; Latour et al., 1994). In a companion publication (Peebles et al., 1997b), feed conversion between 35 and 42 d was improved in broilers provided 3 or 7% added lard from 0 to 10 d of age. In addition, hematocrit and plasma protein concentrations responded differently to the 3 and 7% levels fed during that same time. Serum lipoprotein concentrations

Received for publication October 21, 1996. Accepted for publication July 9, 1997. 1This is Journal Article Number J-8997 from the Mississippi Agricultural and Forestry Experiment Station. 2Use of trade names in this publication does not imply endorsement by Mississippi Agricultural and Forestry Experiment Station of these products, nor similar ones not mentioned. 3To whom correspondence should be addressed: Poultry Science Department, Box 9665, Mississippi State, MS 39762. 4Current address: Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0306. 5Current address: Department of Animal Sciences, Purdue University, West Lafayette, IN 47907-1026.

are subject to change during the first few days of life when yolk absorption is high (Latour et al., 1995), and by adding fat to broiler diets (Hermier and Dillon, 1992). Furthermore, Peebles et al. (1997a) found that lard added to the starter diets of randombred broiler chickens elicited responses in serum low density lipoprotein cholesterol (LDLC) concentrations that varied with the sex and age of the bird. Although Yu et al. (1976) found much higher concentrations of cholesterol and LDL in sexually mature female chickens than in male chickens, there has been no previous research concerning LDL or LDLC concentrations in broilers between 14 and 42 d of age. In this study, the relationships between dietary fat, sex, age, and serum lipid concentration in commercial broilers were determined during the starter and grower periods. In addition, the effect of altered fat levels were examined more specifically by changing dietary fat regimens at 11 d of age. Because the effects of added dietary lard on serum lipids in randombred broiler chickens have been examined in an earlier study (Peebles et al., 1997a), these data may provide further insight as to the involvement of serum lipids, particularly LDLC, in the changes in response of broiler chickens to added dietary fat with selection.

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ABSTRACT The effects of lard added to starter diets on various serum lipids were determined in broiler chickens between 14 and 42 d of age. Nonisocaloric starter diets were formulated to contain either 0, 3, or 7% added lard, where the megacaloric percentages of all major nutrients were held constant. Birds received either 0, 3, or 7% added lard in starter diets through 10 d of age (S1), followed by either 3 or 7% added dietary lard through 21 d of age (S2). All possible combinations of the three S1 diets and two S2 diets yielded six total dietary treatments. A common grower diet was provided after 21 d. Concentrations of various serum lipids were determined weekly from 14 to 42 d of age. The effects of both the S1 and S2 diets on total cholesterol, low density lipoprotein cholesterol (LDLC),

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DIETARY FAT IN BROILERS

MATERIALS AND METHODS

TABLE 1. Ingredient percentage and calculated analysis of starter diets Added fat level Ingredients Ground yellow corn Soybean meal (48% CP) Fishmeal, menhaden (60% CP) Defluorinated phosphate Limestone Sodium chloride Micronutrient premix1 DL-methionine2 L-lysine·HCl3 Fat, lard Calculated analysis ME, kcal/kg CP Lysine TSAA Calcium Available phosphorus Sodium Total fat

0%

3%

7%

68.27 26.45 2.50 1.50 0.50 0.27 0.25 0.23 0.03 0.00

(%) 62.81 28.78 2.50 1.58 0.50 0.29 0.25 0.25 0.04 3.00

57.06 30.30 2.50 1.73 0.50 0.31 0.25 0.30 0.05 7.00

3,047 20.14 1.10 0.88 0.85 0.44 0.21 3.16

3,259 20.95 1.15 0.91 0.88 0.46 0.22 5.96

3,334 21.08 1.21 0.97 0.93 0.48 0.23 9.75

1Supplied the following to each kilogram of finished feed: vitamin A (all-trans-retinol), 11,000 IU; cholecalciferol, 2,750 IU; vitamin E (dl-b, tocopherol), 22 IU; vitamin B12, 13.2 mg; riboflavin, 7.7 mg; niacin, 38.5 mg; d-pantothenic acid, 13.2 mg; choline chloride, 441 mg; selenium, 0.2 mg; d-biotin, 0.11 mg; manganese, 55 mg; zinc, 50 mg; iron, 30 mg; copper, 5 mg; iodine, 0.5 mg; menadione sodium bisulfite, 4.96 mg; folic acid, 1.1 mg; pyridoxine, 4.7 mg; thiamine, 1.8 mg; ethoxyquin, 55 mg. Supplied by Hoffmann-LaRoche, Inc., Nutley, NJ 07110. 2Supplied by Degussa Corp., Allendale, NJ 07401. 3Supplied by Heartland Lysine, Inc., Chicago, IL 60631.

Statistical Analysis A completely randomized experimental design, and a three-way split-plot analysis with a 3 (S1 diets) × 2 (S2 diets) × 2 (sex) factorial arrangement of treatments and sex split on age were used. Fisher’s Protected Least Significant Difference test was used to partition the means when a significant main effect was observed or to compare subclass means when age, S1 diet, S2 diet, or sex interactions were significant (Steel and Torrie, 1980). All SEM were based on pooled estimates of variance. All data were analyzed using the Mixed Procedure of SAS (SAS Institute, 1993). Statements of significance are based on P < 0.05 unless otherwise indicated.

RESULTS There was a significant age by S1 diet interaction for total serum cholesterol (P < 0.002). These data are shown

TABLE 2. Total serum cholesterol at 14, 21, 28, 35, and 42 d of age in broilers across sex that received 0, 3, or 7% added lard from 0 to 10 d of age (S1 diet) S1 diet Day

0%

3%

7%

14 21 28 35 42

118.11 118.8 121.9 120.4a 115.0

(mg/dL) 120.8yz 113.3z 119.7yz 120.3a,yz 125.5y

125.2yz 119.1yz 127.9y 99.3b,z 125.5yz

a,bMeans within day of age for each S1 diet with no common superscript differ significantly (P < 0.05). y,zMeans within S1 diet for each day of age with no common superscript differ significantly (P < 0.05). 1Mean; SEM = 4.26, based on pooled estimate of variance and n = 24 birds on each level of S1 diet on each day.

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A total of 3,600 broiler chicks were wing-banded and randomly assigned to six dietary treatment groups of 600 birds each. Males and females were grown separately with each treatment being allotted six replicates per sex, for a total of 72 pens, with 50 birds per pen. Each bird initially occupied 0.095 m2 of floor space. The pens were randomized with respect to dietary treatment. Fresh pine shavings were used as litter in floor pens. Birds were provided with continuous light. Feed and water were provided for ad libitum consumption. The birds were fed nonisocaloric starter diets that were formulated to contain various levels of added lard where the megacaloric percentages of all major nutrients were held constant. The diets were formulated in this manner so as to prevent a fiber dilution effect. Lard was heated to a liquid state and then added to the feed and mixed. Each pen of birds was fed a Starter 1 (S1) diet from 0 to 10 d of age that contained 0, 3, or 7% added lard, and a Starter 2 (S2) diet from 11 to 21 d of age that contained 3 or 7% added lard. Upon completion of one of three S1 period dietary regimens, birds were switched to one of two S2 period dietary regimens. All possible combinations of the three S1 diets and two S2 diets yielded six total dietary treatments. Treatment combinations were

0% S1-3% S2, 0% S1-7% S2, 3% S1-3% S2, 3% S1-7% S2, 7% S1-3% S2, and 7% S1-7% S2. Ingredient percentage and calculated analysis of the 0, 3, and 7% starter diets are provided in Table 1. At 21 d of age, all birds were given a common grower diet which contained 2.75% added lard. All diets met or exceeded National Research Council (1994) recommendations. At 14, 21, 28, 35, and 42 d of age, blood samples were randomly collected at the same time each day from two birds per pen. Blood was collected from each bird into nonheparinized tubes for analysis of serum total cholesterol, high density lipoprotein cholesterol (HDLC), and triglycerides. Individual serum samples within each pen or replicate were pooled prior to analysis. Concentrations of very low density lipoprotein cholesterol (VLDLC) were estimated as one-fifth of the concentration of triglycerides, and LDLC as the difference between total cholesterol and the combined concentrations of HDLC and VLDLC (Friedewald et al., 1972).

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PEEBLES ET AL. TABLE 3. Serum cholesterol (CHOL) and low density lipoprotein cholesterol (LDLC) at 14, 21, 28, 35, and 42 d of age in male and female broilers that received 3 or 7% added lard from 11 to 21 d of age (S2 diet) CHOL

LDLC

Male

Female

Male

Day

3% S2

7% S2

3% S2

7% S2

3% S2

14 21 28 35 42

133.21,a,x 121.4a,xy 127.2xy 117.5a,y 131.2a,x

125.1ab,xy 126.1a,x 127.2x 120.7a,xy 110.9b,y

114.1b 114.5ab 116.3 112.2ab 113.8b

(mg/dL) 113.1b,yz 31.012,y 106.2b,z 36.12y 121.9xy 39.77y 102.8b,z 37.80ab,y 132.1a,x 57.58a,x

Female

7% S2

3% S2

7% S2

28.68z 36.02xyz 40.85xy 46.90a,x 29.46c,yz

26.76y 31.33xy 37.13xy 41.03a,x 36.03bc,xy

28.68z 29.72yz 40.35xy 26.79b,z 43.02b,x

a,bMeans

within parameter and day of age for each sex-S2 diet combination with no common superscript differ significantly (P < 0.05). within parameter, sex, and S2 diet for each day of age with no common superscript differ significantly (P < 0.05). 1Mean; SEM = 4.80, based on pooled estimate of variance and n = 18 birds of each sex on each level of S2 diet on each day. 2Mean; SEM = 4.64, based on pooled estimate of variance and n = 18 birds of each sex on each level of S2 diet on each day. x–zMeans

females fed 3% S2 diets had higher LDLC concentrations than 7% S2 females. Nevertheless, there were inconsistent fluctuations in cholesterol and LDLC with age between 14 and 42 d in males and females that received 3 or 7% S2 diets. Significant interactions between age and S1 diet were found for serum triglycerides (P < 0.0009) and VLDLC (P < 0.0006) concentrations (Table 4). Overall, triglycerides and VLDLC concentrations increased over the 14 to 42 d period in all birds, regardless of diet. Significant increases occurred between certain ages; however, increases were not progressive over time in the birds that received no lard in the starter diet. Furthermore, there were significant differences in triglyceride and VLDLC concentrations among the three dietary groups at Days 35 and 42. At 35 d, birds fed 3% S1 diets had higher serum triglyceride and VLDLC concentrations than no added fat controls, with the 7% birds being intermediate. At 42 d, the controls exhibited higher concentrations of both serum lipids than 7% birds, with

TABLE 4. Serum triglycerides (TG) and very low density lipoprotein cholesterol (VLDLC) at 14, 21, 28, 35, and 42 d of age in broilers across sex that received 0, 3, or 7% added lard from 0 to 10 d of age (S1 diet) TG

VLDLC

Day

0% S1

3% S1

7% S1

0% S1

14 21 28 35 42

104.91,yz 128.9x 121.9xy 103.1b,z 163.8a,w

88.5z 110.6y 116.9xy 132.0a,wx 144.9ab,w

(mg/dL) 102.4x 20.982,yz 117.3x 25.78x 117.5x 24.36xy 121.8ab,wx 20.62bz 135.6b,w 32.77aw

3% S1

7% S1

17.33z 22.09y 23.38xy 26.39a,wx 28.99ab,w

20.50x 23.47x 23.52x 24.36ab,wx 27.12b,w

a,bMeans within parameter and day of age for each S1 diet with no common superscript differ significantly (P < 0.05). w–zMeans within parameter and S1 diet for each day of age with no common superscript differ significantly (P < 0.05). 1Mean; SEM = 6.79, based on pooled estimate of variance and n = 24 birds on each level of S1 diet on each day. 2Mean; SEM = 1.38, based on pooled estimate of variance and n = 24 birds on each level of S1 diet on each day.

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in Table 2. Serum cholesterol changed with age only in the 3 and 7% S1 dietary groups. Cholesterol increased between 21 and 42 d in birds fed the 3% S1 diet, and decreased between 28 and 35 d in birds fed the 7% S1 diet. In addition, at 35 d, serum cholesterol was lower in 7% birds than in 0 and 3% S1 birds. When sex was examined as a main effect, serum cholesterol (P < 0.003) and LDLC (P < 0.04) were higher in males than in females. Also, significant age by sex by S2 diet interactions were apparent for serum cholesterol (P < 0.002) and LDLC (P < 0.002) concentrations (Table 3). At 14 d, males that received the 3% S2 diet had higher serum cholesterol concentrations than females that received either 3 or 7% S2 diets; whereas at 21 and 35 d, males fed either S2 diet had higher serum cholesterol concentrations than females fed 7% S2 diets. At 42 d, 3% S2 males and 7% S2 females had cholesterol concentrations that exceeded those of 7% S2 males and 3% S2 females. Serum LDLC also tended to follow this same pattern at 42 d; however, at 35 d, males fed 7% S2 and

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DIETARY FAT IN BROILERS TABLE 5. Serum triglycerides (TG) and very low density lipoprotein cholesterol (VLDLC) at 14, 21, 28, 35, and 42 d of age in broilers across sex that received 3 or 7% added lard from 11 to 21 d of age (S2 diet) TG

VLDLC

Day

3% S2

7% S2

3% S2

7% S2

14 21 28 35 42

91.81,z 128.5a,y 125.4y 125.5y 148.8x

(mg/dL) 105.5y 109.4b,y 112.0y 112.4y 147.4x

18.362,z 25.71a,y 25.11y 25.11y 29.76x

20.84y 21.86b,y 22.39y 22.47y 29.49x

those fed 3% S1 diets being intermediate. There were also significant age by S2 diet interactions for serum triglyceride (P < 0.04) and VLDLC (P < 0.05) concentrations (Table 5). Overall, triglyceride and VLDLC concentrations increased over the 14 to 42 d period in all birds, but these increases were greater in birds that received the 3% rather than the 7% S2 diet. This result occurred because, although both 3 and 7% groups had significant increases between Days 35 and 42, increases between Days 14 and 21 were primarily in the 3% S2 birds. Triglyceride and VLDLC concentrations were also higher in birds fed the 3% S2 diets compared to the 7% at 21 d of age. Significant main effects of sex (P < 0.0004) and S1 diet (P < 0.01) on serum HDLC were found. Males exhibited higher HDLC concentrations than females, and dietary lard in the S1 diet caused serum HDLC in the birds to be higher than in birds given no lard in the S1 diet. However, significant age by sex by S1 diet (P < 0.05) and

DISCUSSION As indicated in Table 3 of this study, higher levels of serum cholesterol and LDLC were found in males than in females. This result is similar to results of studies in mammals, in which males had LDL concentrations that were higher than those in females (Brown and Goldstein, 1986, 1989). Nevertheless, numerous studies have indicated that total blood lipid levels, including LDL, may be strongly enhanced by estrogen (Landauer et al., 1941; McDonald and Riddle, 1945; Balnave, 1971; Nirmalan and George, 1972; Bruce and Anastassiadis, 1977). The increase in blood lipids during reproduction appears to be necessary for the deposition of the relatively large amounts of fat needed for yolk formation (Griminger, 1986).

TABLE 6. Serum high density lipoprotein cholesterol at 14, 21, 28, 35, and 42 d of age in male and female broilers that received 0, 3, or 7% added lard from 0 to 10 d of age (S1 diet) Male

Female

Day

0% S1

3% S1

7% S1

14 21 28 35 42

96.381,by 85.92ab,z 87.88ab,yz 85.00ab,z 78.17bc,z

105.62a,y 89.42a,z 88.04ab,z 87.50ab,z 81.33abc,z

96.00by 89.58ay 91.83ay 93.33ay 78.67bcz

0% S1 (mg/dL) 82.50c,yz 82.75ab,y 83.63ab,y 82.33b,yz 73.00c,z

3% S1

7% S1

81.25c 80.75b 82.29ab 86.83ab 89.17a

93.88b,y 85.08ab,z 82.00b,z 87.00ab,yz 87.50ab,yz

a–cMeans within day of age for each sex-S1 diet combination with no common superscript differ significantly (P < 0.05). y,zMeans within sex and S1 diet for each day of age with no common superscript differ significantly (P < 0.05). 1Mean; SEM = 3.28, based on pooled estimate of variance and n = 12 birds of each sex on each level of S1 diet on each day.

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a,bMeans within parameter and day of age for each S2 diet with no common superscript differ significantly (P < 0.05). x–zMeans within parameter and S2 diet for each day of age with no common superscript differ significantly (P < 0.05). 1Mean; SEM = 5.62, based on pooled estimate of variance and n = 36 birds on each level of S2 diet on each day. 2Mean; SEM = 1.11, based on pooled estimate of variance and n = 36 birds on each level of S2 diet on each day.

age by S1 diet by S2 diet (P < 0.005) interactions also occurred for serum HDLC concentration. Sex-influenced effects of S1 dietary fat on HDLC between 14 and 42 d are apparent in Table 6. Males fed 3% S1 diets at 14 and 21 d and 7% S1 diets at 28 d had higher HDLC concentrations than females at the same times and under the same dietary regimens. However, at 42 d, this trend was reversed, in that females fed added fat in the S1 diet exhibited the highest HDLC concentrations. Age-related decreases in HDLC occurred between 14 and 21 d in males that received 0 or 3% S1 diets and in females given 7% S1 diets. Concentrations in males decreased again between 35 and 42 d after having received 7% S1 diets. Nonetheless, significant decreases in HDLC occurred between 14 and 42 d only in males, regardless of diet. Table 7 shows HDLC concentrations within S1-S2 diet combinations across sex at each time period. Peak HDLC concentrations were observed at 14 and 42 d in birds that were provided 3% S1 and S2 diets, and at 21 and 35 d in birds that were provided 7% S1 and S2 diets. In general, HDLC concentrations decreased between 14 and 42 d, with initial declines primarily occurring between 14 and 21 d in most S1-S2 diet combinations.

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PEEBLES ET AL. TABLE 7. Serum high density lipoprotein cholesterol at 14, 21, 28, 35, and 42 d of age in broilers across sex that received 0, 3, or 7% added lard from 0 to 10 d of age (S1 diet) and 3 or 7% added lard from 11 to 21 d of age (S2 diet) 0% S1 Day 14 21 28 35 42

3% S2 86.631,bc,x 85.75abc,x 85.63xy 85.67ab,xy 76.00b,y

3% S1

7% S1

7% S2

3% S2

92.25bc,x 82.92bc,yz 85.88xy 81.67b,yz 75.17b,z

(mg/dL) 103.00a,x 83.87c 80.92c,y 89.25ab 81.83y 88.50 84.00ab,y 90.33ab 87.00a,y 83.50ab

7% S2

3% S2

7% S1

94.75ab,x 83.75abc,y 86.83xy 86.83ab,xy 80.33ab,y

95.13ab 90.92a 87.00 93.50a 85.83ab

Early exposure to added fat in starter diets between 0 and 10 d of age affected total cholesterol, but did not affect LDLC, in the present study. However, there was no interaction between S1 diet and sex with regard to cholesterol. High levels of cholesterol intake concomitant with yolk absorption during this time (Noble and Tullett, 1988) may have masked any sex differences in the serum cholesterol concentrations of chicks fed added fat. The 7% S1 diet also had a depressing effect on cholesterol in comparison to the 3% S1 diet. Resistance to diet-induced hypercholesterolemia can also be increased by stimulation of cholesterol catabolism (Subbiah et al., 1983). Stimulation of cholesterol catabolism has been found to occur naturally in the early life of birds after exposure to high cholesterol concentrations during yolk sac absorption (Latour et al., 1994; Peebles et al., 1996). In older birds, resistance may occur through dietary exposure to cholesterol (Keeley, 1979) or indirectly through inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase (Qureshi et al., 1983a,b). However, dietary cholesterol has been shown to increase cholesterol concentrations in various species of birds (Morrisey and Donaldson, 1977; Keeley, 1979; Allen and Wong, 1993). Males maintained higher cholesterol concentrations than females regardless of S2 fat level through 35 d, but a shifting balance between diet-induced hypercholesterolemia and diet-induced cholesterol catabolism may explain the lack of consistent S2 effects within both males and females. This shifting balance may also help explain the anomalous cholesterol peak at 42 d in females that had received the 7% S2 diet. Several studies have reported gradual increases in cholesterol over time in chickens fed high cholesterol diets (Keeley, 1979; Allen and Wong, 1993). However, the S1 and S2 diets in our study did not contain added cholesterol per se; thus, the variations in cholesterol seen over time may be due to the effects of the dietary fat on cholesterol biosynthetic pathways, though the precise mechanisms involved are not known. Differences in the responses of

serum cholesterol and LDLC concentrations to the S2 lard, particularly at 14, 21, and 35 d, are largely related to the fact that HDL particles, rather than LDL, are the main cholesterol carriers in birds. Furthermore, these differences may be related to results showing that the induction of hypercholesterolemia by dietary saturated fats does not impede in vivo clearance of LDL particles (Hermier and Dillon, 1992). It is also interesting to note that LDLC in randombred chickens respond differently to those same diets, in that males were not affected by the added dietary lard and that in females, level of S1 diet interacted with that of the S2 diet at 14 d of age (Peebles et al., 1997a). Added dietary fat given at different times in the growing period may elicit divergent responses in various blood composition indices. Broiler chicks that receive no additional fat have been found to have higher triglyceride and VLDLC concentrations between 0 and 10 d of age than birds fed added fat (Latour et al., 1992). Lower serum concentrations of triglycerides and VLDLC at 42 d in birds fed added lard in the S1 diet, as indicated in Table 4, may be due to an overcompensation in these birds for the extra added dietary lard. This over-compensation may be related to the down-regulation of lipoprotein lipase. This contrast is accentuated by the rapid increases in triglyceride and VLDLC concentrations in the 0% S1 birds between Days 35 and 42. These increases also occur in conjunction with slow, steady increases in triglyceride and VLDLC concentrations in the 3 and 7% S1 birds between Days 14 and 42. When the 3% and 7% S2 birds are examined separately in Table 5, higher serum triglyceride and VLDLC concentrations are seen in the 3% S2 birds between 21 and 35 d of age. These differences between the two S2 diets are significant at Day 21. Very low density lipoprotein concentrations are relatively good indicators of fat deposition in the bird (Griffin and Whitehead, 1982; Grunder et al., 1987; Grunder and Chambers, 1988). Hargis and Creger (1980) found that prolonged feeding (10 to 14 d) of a low-fat diet during

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a–cMeans within day of age for each S1–S2 diet combination with no common superscript differ significantly (P < 0.05). x–zMeans within S1 and S2 diet for each day of age with no common superscript differ significantly (P < 0.05). 1Mean, SEM = 3.28, based on pooled estimate of variance and n = 12 birds on each level of S1 and S2 diet combination on each day.

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DIETARY FAT IN BROILERS

ACKNOWLEDGMENTS The authors wish to extend appreciation to Janice Orr and Becky Rudolph for expert secretarial assistance in the preparation of this manuscript.

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the starter period produced more abdominal fat (and presumably higher VLDLC concentrations) at 49 d of age than did feeding a low-fat diet for a short period of time (7 d). Thus, higher triglyceride and VLDLC concentrations do not necessarily occur in birds fed a high-fat diet. They may be more efficient in clearing the added lard from their systems. Similar relationships have been observed in young chicks under 10 d of age (Latour et al., 1994). However, the amount of dietary fat during the first 10 d of age appears to have a direct effect, not an inverse effect, on the percentage of abdominal fat pad at 44 d of age (Brake et al., 1993). High density lipoprotein cholesterol concentrations would be expected to be affected more than LDLC concentrations as the bird enters sexual maturity, because HDL is the main cholesterol carrier in birds (Hermier and Dillon, 1992). However, Yu et al. (1976) found no differences in HDLC concentrations in mature birds. Sex differences were detected in HDLC concentrations between 14 and 42 d in Table 6. This effect was even reversed after Day 35. Concentrations of HDLC tended to be higher in females rather than in males at Day 42. Nonetheless, dietary lard had no influence on relative ovary or testicular weights in these birds through 42 d (Peebles et al., 1997b). However, this study concluded at 42 d of age, long before sexual maturity. Decreases in HDLC concentrations from 14 to 42 d of age in all birds that had received the 3% S2 diet and in birds that had received the 0% S1-7% S2 combination are in agreement with previous studies (Castillo et al., 1992). As shown in Table 7, the highest HDLC concentrations were more often seen in birds that received constant amounts of added fat throughout the starter period (i.e., the 3% S1-3% S2 and the 7% S1-7% S2 combinations). Higher concentrations were evident in the 3% S1 and 3% S2 birds at Days 14 and 42, and in the 7% S1 and 7% S2 birds at Days 21 and 35. This improved utilization would indicate that extra-hepatic cholesterol anabolic pathways may be more efficient in these birds, along with improved utilization of cholesterol by the liver, perhaps in the form of bile. This would also facilitate HDL-dependent reverse cholesterol transport (Steinberg, 1978; Eisenberg, 1984). However, HDL-dependent reverse cholesterol transport in mammals is dependent upon apolipoprotein E (apoE) (Julien et al., 1988), and birds are incapable of producing apoE (Hermier and Dillon, 1992). Reverse cholesterol transport is not known to take place in birds. Understanding of the various possible roles of avian HDL in cholesterol transport requires further investigation. In conclusion, broilers responded to lard that was added to the starter diet between 0 and 21 d through changes in serum triglycerides and VLDLC between 14 and 42 d of age. However, responses in these parameters varied with the age of feeding. The sex, as well as the age, of the bird exerted further influences on the responses of other serum lipids, including cholesterol, LDLC and HDLC, to added lard in the starter diet.

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