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Evaluation of Two Commercial Broiler Male Lines Differing in Efficiency of Feed Utilization
Evaluation of Two Commercial Broiler Male Lines Differing in Efficiency of Feed Utilization A. CAHANER,1 E. A. DUNNINGTON, D. E. JONES, J. A. CHERRY,2 and P. B. SIEGEL3 Poultry Science Department, Virginia Polytechnic Institute & State University, Blacksburg, Virginia 24061 (Received for publication November 21, 1986)
1987 Poultry Science 66:1101-1110 INTRODUCTION
Genetic-environmental relationships involving the efficiency of feed utilization in broilers are biologically complex and economically important. Improvements in feed efficiency have resulted mainly from selection for growth which reduced age at marketing and thus energy-formaintenance requirements (McCarthy and Siegel, 1983; Cahaner and Siegel, 1986). Evidence of genetic variation for feed efficiency (e.g., Washburn etal., 1975; Pym, 1985;Sorensen, 1985; Leenstra et al., 1986; Sorensen and Leenstra, 1986) suggests that broilers with similar growth rates may differ in feed conversion ratios (FCR; grams feed consumed per gram body weight change). Weight gain depends on an excess of energy intake over maintenance requirements. While it may be assumed that these requirements are a function of body weight, factors such as motor activity, feather cover, and thermoregulatory ability can influence the amount of excess energy available to individuals of similar size and feed consumption. Further, conversion of excess energy to weight gain depends on the leamfat partition coefficient because, on a per gram basis, the energy cost of fatty tissue deposition is four times that of lean (Soller and Eitan, 1984). This reasoning is consistent with empirical evidence (Washburn etal., 1975; Pym
Present address: Hebrew University, Rehovot, Israel. Present address: University of Georgia, Athens, Geor-
2
gia.3
To whom correspondence should be addressed.
and Solvyns, 1979) demonstrating that meattype chickens selected for feed efficiency were leaner than their controls. Although genetic and nutritional factors influence fat deposition, genotype by nutrition interactions have been generally unimportant (see reviews: Siegel, 1984; Nir, 1984). Since dietary energy content can influence weight gain, use of the calorie conversion ratio (CCR; calories consumed per gram body weight change) as well as FCR as measurement criteria may be appropriate. Genetic variation in ability to respond to higher levels of dietary energy while maintaining the same CCR could contribute to differences in FCR. The purpose of this research was to examine possible reasons for differences in feed efficiency between two commercial broiler grandparent lines, and to evaluate effects of dietary energy on such differences.
MATERIALS AND METHODS
Stocks and Husbandry. Eight hundred early feathering males, 400 from each of two lines (A and B), were obtained as day-old chicks from a primary breeder. Chicks were wingbanded and housed as flocks of 50 in 16 litter-floor pens with continuous lighting. Diets H and L (which were fed ad libitum) were designed to influence feed conversion; they contained different amounts of metabolizable energy with a constant calorie-protein ratio (Table 1). Chicks in four pens from each line were fed Diet H and chicks in the other four pens Diet L. Starter diets were fed to 28 days of age and the grower diets thereafter.
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ABSTRACT Factors potentially influencing feed conversion were studied in two lines of broilers known to differ in efficiency of feed utilization. When fed diets differing in energy, line by diet interactions were seldom significant and line effects were greater than those between diets. Chicks from the more efficient line had less plumage cover, less fat, and spent more time sitting than those from the less efficient line. These data imply that improved feed conversion is a function of several traits. (Key words: feed efficiency, fat, chickens, behavior, body composition)
CAHANER ET AL
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TABLE 1. Composition of
Starter' Components
H
L
H
L
(\ nl/o}\ 61.45 31.50 2.00 2.00 1.40 .90 .25 .25 .05 .20
63.96 35.00 6.00 2.00 1.40 .90 .25 .25 .05 .19
66.30 26.82 2.00 2.00 1.50 .63 .25 .25 .05 .20
58.76 30.40 6.00 2.00 1.40 .70 .25 .25 .05 .19
22.00 3,050 139
23.00 3,200 139
20.19 3,125 154
21.21 3,275 154
1 Diets were designed to contain lower (L) or higher (H) amounts of metabolizable energy, with a constant calorie:protein ratio. 2 Vitamin and trace mineral premix provided per kilogram of diet: vitamin A, 8,000 IU; vitamin D 3 , 3,000 1U; vitamin E, 10 IU; vitamin K, 2 mg; riboflavin, 4 mg; d-pantothenic acid, 10 mg; thiamine, 1 mg; niacin, 35 mg; choline chloride, 350 mg; vitamin B , 2 , 8 Mg; folic acid, .5 mg; ethoxyquin, 250 mg; pyridoxine, 5 mg; biotin, .4 mg; manganese, 80 mg; zinc, 112 mg; iron, 62 mg; copper, 16 mg; iodine, .1 mg; selenium, .1 mg.
Traits Measured. Group body weights of all havior of each chick recorded as eating, drinkchicks in each pen were recorded at 1 day of ing, preening, standing, or sitting. These beage and at 7-day intervals thereafter to Day 49. havioral categories were mutually exclusive. After Day one, chicks had access to feed when At 39 days of age, eight chicks were selected body weights were obtained with the exception at random from each pen (with the restriction of 49-day body weights, which were obtained that color-marked chicks and those with severe after an 18-h feed withdrawal period. Feed con- leg problems were not included), fasted for 18 sumption was measured for each flock during h and killed by cervical dislocation. This procethe intervals between weighings. dure was repeated at 49 days of age using six At 1 day of age, random samples of 8 chicks of the color-marked chicks as samples. After per flock were individually weighed and their death, chicks were weighed, scalded, defeathbacks and wing plumage color-marked for iden- ered, and reweighed. Feather weight was calcutification. Commencing at 5 days of age, body lated as the difference between the body weight weights and cloacal and foot temperatures were just before and 30 min after defeathering. Two recorded for each of these chicks at 7-day inter- chicks from each pen were defeathered by hand vals to Day 40. Starting on Day 19 and at 7-day while the remainder were defeathered mechanintervals to Day 40, the length and proportion ically. Feathers from the hand-picked chicks of feather cover on the back pterylae were mea- were oven-dried for 48 h at 100 C, weighed to sured (Siegel et al., 1957b). Condition of legs determine dry matter, and returned to their rewas scored in ascending order from 1 (normal) spective carcasses for subsequent analysis. to 6 (lame) at 7-day intervals from Days 26 to 40. Defeathered carcasses were stored overnight Behaviors of the color-marked chicks were at 4 C before skin (with adhering and subcutanerecorded at 7-day intervals from 7 to 42 days ous fat), abdominal fat (leaf plus gizzard), liver, of age. Commencing at 0730 h, data were ob- digestive tract, breast, thighs and drumsticks, tained simultaneously by four observers located and shanks were removed and weighed. The in the hallway of four adjacent pens. After a remaining carcass was then weighed and length 5-min adjustment period, each flock was scan- of the right shank was measured. Subcutaneous ned three times at 5-min intervals with the be- fat was defined as major adipose tissues located
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Ingredient Ground yellow corn Dehulled soybean meal Hydrolyzed fat Menhaden fish meal Defluorinated phosphate Ground limestone NaCl Vitamin mix 2 Trace mineral mix 2 Methionine DL Calculated analysis Protein, % Energy, kcal metabolizable energy/kg Calorie:protein ratio
EFFICIENCY OF FEED UTILIZATION
Line by diet interactions for body weight, feed intake, FCR, and CCR were not significant (Table 2). Feed consumption values were similar for all line-diet subgroups, whereas body weights were heavier and FCR were lower for Line A than B and Diet H than L chicks (Table 2). The CCR were similar for both diets, but lower for Line A than B. The FCR and its two components, body weight gain and feed consumption, were calculated separately for each of three periods. Although there were no significant differences between lines for body weight, feed intake, or FCR to 21 days of age (Table 3), a pattern was developing in the data. Line A chicks, which were from a younger breeder flock, were 4 g (8%) lighter than Line B chicks at one day of age, and gained 8 g more than those from Line B by 21 days of age. From 22 to 35 days of age, they gained 34 g more than Line B chicks on the same amount of feed, which resulted in a superior FCR for Line A than B. This difference increased further between 36 and 49 days of age, when Line A chicks gained 80 g (9.4%) more weight than those from Line B on the same amount of feed.
3000 H
o ALL BIRDS •
SAMPLE
2000' BODY WT. (g)
1000-
RESULTS
Body Weight and Feed Conversion. Body weights approximated 2,000 g at 39 days of age and 2,600 g at 49 days (Table.2), with the latter weights obtained after an 18-h fast. The former were within the range of industry whole bird market weights, while the latter were consistent with those of broilers used for deboning and further processing. Body weights of the samples of chicks on which individual data were obtained were representative of the entire population (Figure 1).
0
5 7
12 14 19 21 26 28 33 35 4 0 42 AGE
49
(days)
FIG. 1. Growth curves of all chicks averaged by age (o) and for a random sample of chicks from each line-class subgroup (•). All body weights (WT.) were obtained in ad-libitum-fed chicks except at 49 days of age, when there was an 18-h feed withdrawal period.
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under the skin, including sartorial and clavicular as well as other large depots on the back and the abdominal wall. Individual composition analyses were conducted separately for each of the two chicks from each pen that were picked by hand. Skin plus subcutaneous fat was frozen in one bag, a sample of breast muscle in another, and the remainder of the chicken in a third bag. For analysis, skin plus subcutaneous fat were spread over a metal screen placed as a strainer over a 3-L can and oven dried for 48 h at 100 C to determine water content. During this process 20 to 50% of the fat liquidized by heating drained into the can, and its weight was recorded. The remaining tissue was crisp, easily homogenized, and its lipid-content determined in duplicates following extraction by chloroform:methanol (2:1) (Folch et al., 1957). These data were combined with the fat in the can and considered as lipid-content of skin plus subcutaneous fat for that chick. Samples of breast muscle were homogenized and lipid-content determined in duplicate (Folch et al., 1957). The remainder of each carcass was autoclaved, ground, homogenized, and lipid was determined as described for breast. Total lipid was calculated from the sum of values from breast, skin plus subcutaneous fat, and the remaining carcass. Analyses. Behavioral observations were analyzed as cumulative results over time by chi square; all other measurements were analyzed by two-way (2 lines x 2 diets) analysis of variance. There were four replicates of each line by diet combination for the pen data (body weight, feed consumption, and feed conversion), 32 replicates (4 pens X 8 chicks/pen) for measurements obtained weekly from the individual chicks, 24 or 32 replicates (4 pens x 6 or 8 chicks/pen) for the processing data, and 8 replicates (4 pens x 2 chicks/pen) for carcass composition.
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39 49
39 49
FCR, g/g
CCR, cal/g
2
5.24 5.87
5.27 5.94
3,362 4,941
3,280 4,848
Line B
5.38 6.15
1.66 1.89
3,297 4,866
2,030 2,617
H
Chicks were weighed after ad libitum feeding at 39 days and after 18 h food removal at 49 days of age.
5.51 6.22
1.79 2.01
1,929 2,510
L
2,065 2,727
H
1.62 1.81
Line A
1.71 1.92
3,331 4,974
1,993 2,638
L
L = Low energy diet; H = high energy diet.
39 49
Feed intake, g
1
39 49
(Day)
Age
Body weight,2 g
Variable
Diet 1
TABLE 2. Mean feed intake, body weight, and feed (FCR) and calorie (CCR) conversion ratio at 39 and 49 days of age and significance values from analyses of varianc
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EFFICIENCY OF FEED UTILIZATION
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TABLE 3. Mean body weight, feed intake, and feed conversion ratios (FCR) by lines, averaged over diets for various age periods Line1 Trait
Age
A
Difference2
B
(%)
(d)
Body weight,3 g Initial Final
AA* * *
0 49
Weight gains
2,682**
0to21 22 to 35 36 to 49
-8.0 4.6
706 958 854
1.1 3.5 9.4
0 to 49
4,911
4,904
.1
0 to 21 22 to 35 36 to 49
995 1,712 2,205
1,000 1,747 2,157
-.5 -2.0 2.0
FCR, feed:gain
Oto49
1.86**
1.95
-4.8
0to21 22 to 35 36 to 49
1.39 1.73** 2.37**
1.42 1.83 2.54
-2.2 -5.8 -7.2
1
Significances are given for comparisons of means within line pairs
2
( A - B ) X 100/B.
3
Chicks were weighed after 18 h food removal at 49 days of age and after ad libitum feed at all other ages.
*P«.05. **P<.01. ***P«.001.
resulting in line by diet interactions from 5 to 33 days of age. Line by diet interactions were also significant from 5 to 33 days of age for differences between cloacal and surface temperatures. As with surface temperatures, differ-
Body Temperatures. Cloacal temperatures were generally uniform. Surface (foot) temperatures were similar for birds fed both diets within Line A, while in Line B, means were generally lower for chicks fed Diet H than L (Table 4),
TABLE 4. Surface (foot) temperatures and differences between cloacal and surface temperatures of line-diet subgroups by age1 Cloacal minus surface
Surface H
L
5 12 19 26 33 40
H
L
(Day)
32.6** 34.5 32.9 32.0 32.8 33.8
2
33.8 35.0 32.8 32.1 32.6 33.2
33.0** 36.0 33.5* 32.9*** 33.4** 33.6*
Line B
Line A
Line B
Line A Age
L
H
L
8.1*
7.1 6.1 8.2 9.5 9.0 8.8
y p* * 5.0** 7.4* 8.6*** y o** * 8.9*
H
(°C) 32.3 35.3 32.6 31.0 31.8 32.2
6.5 8.2 9.5 8.7
7.8*
8.7 5.7 8.4
10.6 9.7 9.5
1 L = Low energy diet; H = high energy diet. Line by diet interactions were significant at all ages except 40 days. Significances are given for comparisons of means within diet pairs.
*P<.05. **P<.01. ***P«.001.
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Feed intake, g/chick
714 992* 934*
48 2,563
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TABLE 5. Mean feather coverage of the back and wet and dry feather weights by lines averaged over diets at several ages Line1 Feather measurement
Difference2
Age (Day)
3
(%)
19 26 33 40
58.8 71.9 79.4* 81.2
Wet weight, % of body weight
39 49
5.91 5.66
Dry matter, % of wet weight
39 49
Dry weight, % of body weight
39 49
61.6 73.6 86.2 86.4 6.00 6.07
Significances are given for comparisons of means within line pairs. ( B - A ) X 100/A.
3
Proportion of the back length covered with feathers.
3.7 7.0
3.01 3.03
2.86 2.60***
1
1.5 7.3
50.9 50.2
49.1 46.9
2
4.8 2.4 8.6 6.4
5.2
16.5
*P<.05. ***P<.001.
ences between diets were similar for Line A chicks. In Line B, differences between cloacal and surface temperatures were consistently larger for chicks fed Diet H than Diet L.
Feathers. Diet did not influence feathering. Back feather cover was greater for Line B than A chicks at 33 days of age (Table 5). When expressed as a percent of body weight, dry
TABLE 6. Percentage of chicks sitting and standing by line-diet subgroups at various ages1 DietH
Diet L Variable
Line A
Line B
Line A
7 14 21 28 35 42
28 46 53 73 78 84
26 41 55 82 81 77
50 55 74 73 81 79
Mean
60
60
7 14 21 28 35 42
40 23 22 14 11 10
38 24 26 6 7 10
Mean
20
19
Age
Line A
LineB
40 49 64 66 66 74
39 51 64 73 80 82
33 45 59 74 73 76
69**
60
65*
60
27 18 10 21 6 7
40 28 14 28 20 14
33 20 16 17 9 9
39 26 20 17 14 12
15**
24
17*
21
(%)
(Day) Chicks sitting
Chicks standing
Diets L + H LineB
'Average of three observations taken 5 minutes apart on that day. L = Low energy diet; H = high energy diet. Significances are given for comparisons of means within line pairs. *P«.05. **P<.01.
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Back cover, %
EFFICIENCY OF FEED UTILIZATION
were more active than Line A chicks, but only when fed the high energy diet. Processing Traits. As expected, body weights of the chicks selected for processing were different from those of the entire population (Tables 2 and 7). The discrepancy at 39 days of age resulted from processed chicks having been fasted for 18 h before weighing, whereas the rest of the chicks were not fasted. Although all the chicks were fasted for 18 h at 49 days of age, those with leg problems were excluded from processing. These procedural items did not, however, change the weight relationships between lines. Because Line A chicks were heavier than Line B chicks, weights of parts and components were expressed as percent of body weight. Line A chicks, which had superior FCR, had less abdominal fat and subcutaneous plus skin
TABLE 7. Mean weights of body components for chicks processed at 39 and 49 days of age (line means averaged over diets) Line1 Variable
Age
Difference2
A
B
1,940* 2,770*
1,885 2,690
(%)
(Day) Body weight, g
39 49
2.9 3.0
Body components (% of body weight) Abdominal fat tissue
39 49
1.25** 1.16***
1.80 1.89
-30.6 -38.6
Subcutaneous + skin lipids
39 49
2.94* 2.71***
3.55 4.11
-17.2 -34.1
Total carcass lipids
39 49
8.02 o 57* * *
8.64 12.25
-7.2 -21.9
Lipids in breast meat 3
39 49
2.70 2.35
2.75 2.46
-1.8 -4.5
Breast
39 49
18.40* 19.11*
17.95 18.58
2.5 2.9
Thighs + drumsticks
39 49
20.87** 21.29***
20.43 20.49
2.2 3.9
Breast + thighs + drumsticks
39 49
39.27*** 40.39***
38.38 39.07
2.3 3.4
1
Significances are given for comparison between pairs of line means.
2
(A - B) X 100/B.
3
Wet weight.
*P«.05. **P«.01. ***P<.001.
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feather weight was also greater for Line B than A chicks at 49 days of age. Behaviors. The frequencies of chicks eating, drinking, and preening were minor in comparison to the incidence of sitting and standing behaviors. Accordingly, subsequent comments will focus on variations in the incidence of sitting and standing behavior. The proportion of chicks sitting increased from about 35% at 7 days of age to 80% at 42 days (Table 6). There were no consistent differences between lines for chicks fed Diet L. For Diet H, Line A chicks sat more than those from Line B. Data for standing mirrored those for sitting, with the proportion of chicks standing decreasing with age. As with sitting, differences between lines for chicks fed Diet L were inconsistent while for Diet H Line B chicks stood more than those from Line A (24 vs. 15%). It appears that Line B chicks
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chicks by diets. Differences in body weight were significant at 39 but not at 49 days of age. No differences were found between diets for the other traits. There were no differences between lines or between diets in the relative weights (percent of body weight) of the digestive tract, liver, shanks, and remaining carcass (back, wings, neck), or in shank length and leg scores. DISCUSSION
The range of body weights at which feed efficiency and carcass traits were studied was consistent with current broiler industry practices. Since growth rates generally exceeded those of current commercial broilers, results and conclusions may be relevant to broilers produced for the rest of this decade. Variation in the differences between cloacal and surface temperatures was not associated with FCR or carcass composition. Ambient temperatures, however, were generally optimal and
TABLE 8. Mean weights of body components for chicks processed at 39 and 49 days of age (diet means averaged over lines) Diets1 Age
Difference2
L
H
39 49
1,868** 2,698
1,958 2,762
Abdominal fat tissue
39 49
1.48 1.44
1.57 1.61
6.1 11.8
Subcutaneous + skin lipids
39 49
3.14 3.35
3.35 3.31
6.7 -.3
Total carcass lipids
39 49
8.04 10.81
8.62 11.01
7.2 1.9
Lipids in breast meat 3
39 49
2.76 2.54
2.69 2.28
-2.5 -10.2
Breast
39 49
18.08 19.05
18.27 18.65
1.0 -2.1
Thighs + drumsticks
39 49
20.69 20.92
20.60 20.87
-.4 -.2
Breast + thighs + drumsticks
39 49
38.77 39.97
38.88 39.52
.3 -1.1
Variable
(%)
(Day) Body weight, g
4.8 2.4
Body components (% of body weight)
'L means.
Low energy diet; H = high energy diet. Significances are given for comparisons between pairs of diet
2
(H - L) X 100/L.
3
Wet weight.
**P«.01.
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lipids than those from Line B at both ages (Table 7). At 49 days of age,'total carcass lipids were less for Line A than Line B chicks. The difference between lines was mainly due to variation in adipose depots since lipid content in breast muscle was similar. The proportion of meat parts (breast, thighs plus drumsticks) was higher in Line A than B with differences in breast similar at both ages whereas those for thighs plus drumsticks increased from 39 to 49 days of age. When these carcass components were combined, percentage differences between lines ranged from 2.3 to 3.4 (.9 to 1.3% of body weight) at 39 and 49 days of age, respectively. Proportions of deboned breast meat were similar in all subgroups and ages, with an average of 84%. Since Line A chicks were larger and also had a higher proportion of meat parts than those from Line B, differences between them were even larger on an absolute basis. Table 8 summarizes data from processed
EFFICIENCY OF FEED UTILIZATION
(LeClercq, 1983; Whitehead and Griffin, 1984, Cahaner et al., 1986). Changes in fat stores with age were not consistent across depots or lines. Yet, overall total carcass lipid (as percent of body weight) of Line B chicks increased by 42% from 39 to 49 days of age, whereas the increase in Line A chicks was 19%. As a result, differences between lines in fat deposition increased. Similarly, the difference between lines in feed conversion also increased with age, emphasizing an association between these two traits. Superior growth rate and FCR of the chicks fed the higher energy Diet H resulted primarily from increased energy intake, as evidenced by similar CCR between diets within lines. The difference in dietary energy intake had little effect on carcass composition, perhaps due to the level of fat deposition of both lines, which was lower than that typically observed at these body weights in commercial broilers. Since the magnitude of differences in growth rate and FCR between the lines was similar to that of differences between the diets, it may appear that similar improvements in growth rate and FCR can be achieved by either genetic or nutritional means. Such inferences are not warranted. The genetically related superior FCR was associated, in part, with reduced fat deposition and increased meat yield, while the improvement of FCR by diet tended to have the opposite effect on meat yield. ACKNOWLEDGMENT
This research was supported, in part, by the John Lee Pratt Animal Nutrition Program and Binational Agricultural Research and Development Fund Project No. US-675-83C. The authors wish to thank N. B. Anthony, M. Katanbaf and A. Martin for their assistance in data collection. REFERENCES Cahaner, A., andP. B. Siegel, 1986. Evaluation of industry breeding programs for meat-type chickens and turkeys. Proc. 3rd World Congr. Genet. Appl. Livest. Prod. 10:337-346. Cahaner, A., Z. Nitsan, and I. Nir, 1986. Weight and fat content of adipose and nonadipose tissues in broilers selected for or against abdominal adipose tissue. Poultry Sci. 54:215-222. Chambers, J. R., A. Fortin, and A. A. Grander, 1983. Relationships between carcass fatness and feed efficiency and its component traits in broiler chickens. Poultry Sci. 62:2201-2207. Folch, J., M. Lees, and G. H. Sloane-Stanley, 1957. A simple method for isolation and purification of total lipids from animal tissues. J. Biol. Chem. 226:497-
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hence do not rule out that genetic variation in thermoregulation would be expressed under less optimal conditions and may be associated with FCR. Relevance of this reasoning is evident from the significant line by diet interactions for cloacal-surface differences through 33 days of age. There appear to be several factors contributing to the superior FCR of Line A to Line B chicks. Behavioral differences were observed only between chicks fed the H diet, while FCR differences between the lines were similar for both diets. Therefore, although the association between behavior and FCR could be expected, further study is needed. Differences in feathering (Line A had less feathering than Line B) were consistent with reports of considerable genetic variation in feathering of early feathering chickens (Siegel et al., 1957a). Under husbandry conditions of our experiment where hot-air brooding was used, heavy feather coverage was not needed for insulation and energy costs of producing plumage could have contributed to the inferior FCR of Line B. At 49 days of age, mean dry matter weights of feathers were 71 g for Line A and 81 g for Line B chicks. The wet weight equivalent of this 10 g difference in feather dry matter, which is mostly protein, is approximately 20 g of feathers or 50 g of muscle. Since mean live body weights of Line A and B chicks were 2,770 and 2,690 g, respectively (Table 7), it may be speculated that 30 of the 80 g difference in body weight between lines resulted from differences in feathering. The 30 g translated to about 2 points in FCR. Although feather volume may have positive effects in certain situations, their negative effect on feed conversion to live body weight, and more so on feed conversion to meat, deserves further study, particularly under different climatic conditions. Although neither of the lines studied would be considered excessively fat by today's standards (Nixey, 1986), the most pronounced difference between them was in the amount and distribution of fat. Although lipid-content of breast meat was similar for both lines, large differences existed for abdominal fat tissue and subcutaneous plus skin lipids, resulting in more carcass fat for Line B than Line A chicks. These results are consistent with those obtained from commercial broiler populations (Littlefield, 1972; Chambers et al., 1983) as well as from experimental lines selected on feed efficiency (Washburn et al., 1975; Pym, 1985) and on indirect and direct measures of fat deposition
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Siegel, P. B., 1984. Factors influencing excessive fat deposition in meat poultry 1. Genetics. Pages 51-52 in: Proc. XVII World's Poult. Congr., Helsinki, Finland. Siegel, P. B., J. V. Craig, and C. D. Mueller, 1957a. Heritabilities, sex differences and phenotypic correlations for six feathering characteristics. Poultry Sci. 36:621-628. Siegel, P. B., C. D. Mueller, and J. V. Craig, 1957b. Some phenotypic differences among homozygous, heterozygous, and hemizygous late feathering chicks. Poultry Sci. 36:232-239. Soller, M., and Y. Eitan, 1984. Why does selection for liveweight gain increase fat deposition? A model. World's Poult. Sci. J. 40:5-9. Sorensen, P., 1985. Influence of diet on response to selection for growth and efficiency. Pages 85-95 in: Poultry Genetics and Breeding. W. G. Hill, J. M. Manson, and D. Hewitt, ed. Br. Poult. Sci. Ltd., Longman Group, Harlow, UK. Sorensen, P., and r . Leenstra, 1986. Recent research on breeding of meat production stocks. Proc. 7th Eur. Poult. Conf. 1:60-69. Paris, France. Washburn, K. W., R. A. Guill, and H. M. Edwards, 1975. Influence of genetic differences in feed efficiency on carcass composition of young chickens. J. Nutr. 105:1311-1317. Whitehead, C. C , and H. D. Griffin, 1984. Development of divergent lines of lean and fat broilers using plasma very low density lipoprotein concentration as selection criterion: the first three generations.Br. Poult. Sci. 25:573-582.
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509. LeClercq, B., 1983. The influence of dietary protein content on the performance of genetically lean or fat growing chickens. Br. Poult. Sci. 24:581-587. Leenstra, F. R., P.F.G. Vereijken, and R. Pit, 1986. Fat deposition in a broiler sire strain. 1. Phenotypic and genetic variation in and correlations between abdominal fat, body weight and feed conversion. Poultry Sci. 65:1225-1235. Littlefield, L. H., 1972. Strain differences in growth of abdominal fat in broilers. Poultry Sci. 51:1829. (Abstr.) McCarthy, J. C , and P. B. Siegel, 1983. A review of genetical and physiological effects of selection in meattype poultry. Anim. Breed. Abstr. 51:87-94. Nir, I., 1984. Factors influencing excessive fat deposition in meat poultry 1. Nutrition. Pages 53-55 in: Proc. XVII World's Poult. Congr. Helsinki, Finland. Nixey.C, 1986. A comparison of growth and fat deposition of commercial avian species. Proc. 7th Eur. Poult. Conf. 2:671-680, Paris, France. Pym, R.A.E., 1985. Direct and correlated responses to selection for improved feed efficiency. Pages 97-112 in: Poultry Genetics and Breeding. W. G. Hill, J. M. Manson, and D. Hewitt, ed. British Poult. Sci. Ltd., Longman Group, Harlow, UK. Pym, R.A.E., and A. J. Solvyns, 1979. Selection for food conversion in broilers: Body composition of birds selected for increased body-weight gain, food consumption and food conversion ratio. Br. Poult. Sci. 20:87-97.
1987 Poultry Science 66:1101-1110 INTRODUCTION
Genetic-environmental relationships involving the efficiency of feed utilization in broilers are biologically complex and economically important. Improvements in feed efficiency have resulted mainly from selection for growth which reduced age at marketing and thus energy-formaintenance requirements (McCarthy and Siegel, 1983; Cahaner and Siegel, 1986). Evidence of genetic variation for feed efficiency (e.g., Washburn etal., 1975; Pym, 1985;Sorensen, 1985; Leenstra et al., 1986; Sorensen and Leenstra, 1986) suggests that broilers with similar growth rates may differ in feed conversion ratios (FCR; grams feed consumed per gram body weight change). Weight gain depends on an excess of energy intake over maintenance requirements. While it may be assumed that these requirements are a function of body weight, factors such as motor activity, feather cover, and thermoregulatory ability can influence the amount of excess energy available to individuals of similar size and feed consumption. Further, conversion of excess energy to weight gain depends on the leamfat partition coefficient because, on a per gram basis, the energy cost of fatty tissue deposition is four times that of lean (Soller and Eitan, 1984). This reasoning is consistent with empirical evidence (Washburn etal., 1975; Pym
Present address: Hebrew University, Rehovot, Israel. Present address: University of Georgia, Athens, Geor-
2
gia.3
To whom correspondence should be addressed.
and Solvyns, 1979) demonstrating that meattype chickens selected for feed efficiency were leaner than their controls. Although genetic and nutritional factors influence fat deposition, genotype by nutrition interactions have been generally unimportant (see reviews: Siegel, 1984; Nir, 1984). Since dietary energy content can influence weight gain, use of the calorie conversion ratio (CCR; calories consumed per gram body weight change) as well as FCR as measurement criteria may be appropriate. Genetic variation in ability to respond to higher levels of dietary energy while maintaining the same CCR could contribute to differences in FCR. The purpose of this research was to examine possible reasons for differences in feed efficiency between two commercial broiler grandparent lines, and to evaluate effects of dietary energy on such differences.
MATERIALS AND METHODS
Stocks and Husbandry. Eight hundred early feathering males, 400 from each of two lines (A and B), were obtained as day-old chicks from a primary breeder. Chicks were wingbanded and housed as flocks of 50 in 16 litter-floor pens with continuous lighting. Diets H and L (which were fed ad libitum) were designed to influence feed conversion; they contained different amounts of metabolizable energy with a constant calorie-protein ratio (Table 1). Chicks in four pens from each line were fed Diet H and chicks in the other four pens Diet L. Starter diets were fed to 28 days of age and the grower diets thereafter.
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ABSTRACT Factors potentially influencing feed conversion were studied in two lines of broilers known to differ in efficiency of feed utilization. When fed diets differing in energy, line by diet interactions were seldom significant and line effects were greater than those between diets. Chicks from the more efficient line had less plumage cover, less fat, and spent more time sitting than those from the less efficient line. These data imply that improved feed conversion is a function of several traits. (Key words: feed efficiency, fat, chickens, behavior, body composition)
CAHANER ET AL
1102
TABLE 1. Composition of
Starter' Components
H
L
H
L
(\ nl/o}\ 61.45 31.50 2.00 2.00 1.40 .90 .25 .25 .05 .20
63.96 35.00 6.00 2.00 1.40 .90 .25 .25 .05 .19
66.30 26.82 2.00 2.00 1.50 .63 .25 .25 .05 .20
58.76 30.40 6.00 2.00 1.40 .70 .25 .25 .05 .19
22.00 3,050 139
23.00 3,200 139
20.19 3,125 154
21.21 3,275 154
1 Diets were designed to contain lower (L) or higher (H) amounts of metabolizable energy, with a constant calorie:protein ratio. 2 Vitamin and trace mineral premix provided per kilogram of diet: vitamin A, 8,000 IU; vitamin D 3 , 3,000 1U; vitamin E, 10 IU; vitamin K, 2 mg; riboflavin, 4 mg; d-pantothenic acid, 10 mg; thiamine, 1 mg; niacin, 35 mg; choline chloride, 350 mg; vitamin B , 2 , 8 Mg; folic acid, .5 mg; ethoxyquin, 250 mg; pyridoxine, 5 mg; biotin, .4 mg; manganese, 80 mg; zinc, 112 mg; iron, 62 mg; copper, 16 mg; iodine, .1 mg; selenium, .1 mg.
Traits Measured. Group body weights of all havior of each chick recorded as eating, drinkchicks in each pen were recorded at 1 day of ing, preening, standing, or sitting. These beage and at 7-day intervals thereafter to Day 49. havioral categories were mutually exclusive. After Day one, chicks had access to feed when At 39 days of age, eight chicks were selected body weights were obtained with the exception at random from each pen (with the restriction of 49-day body weights, which were obtained that color-marked chicks and those with severe after an 18-h feed withdrawal period. Feed con- leg problems were not included), fasted for 18 sumption was measured for each flock during h and killed by cervical dislocation. This procethe intervals between weighings. dure was repeated at 49 days of age using six At 1 day of age, random samples of 8 chicks of the color-marked chicks as samples. After per flock were individually weighed and their death, chicks were weighed, scalded, defeathbacks and wing plumage color-marked for iden- ered, and reweighed. Feather weight was calcutification. Commencing at 5 days of age, body lated as the difference between the body weight weights and cloacal and foot temperatures were just before and 30 min after defeathering. Two recorded for each of these chicks at 7-day inter- chicks from each pen were defeathered by hand vals to Day 40. Starting on Day 19 and at 7-day while the remainder were defeathered mechanintervals to Day 40, the length and proportion ically. Feathers from the hand-picked chicks of feather cover on the back pterylae were mea- were oven-dried for 48 h at 100 C, weighed to sured (Siegel et al., 1957b). Condition of legs determine dry matter, and returned to their rewas scored in ascending order from 1 (normal) spective carcasses for subsequent analysis. to 6 (lame) at 7-day intervals from Days 26 to 40. Defeathered carcasses were stored overnight Behaviors of the color-marked chicks were at 4 C before skin (with adhering and subcutanerecorded at 7-day intervals from 7 to 42 days ous fat), abdominal fat (leaf plus gizzard), liver, of age. Commencing at 0730 h, data were ob- digestive tract, breast, thighs and drumsticks, tained simultaneously by four observers located and shanks were removed and weighed. The in the hallway of four adjacent pens. After a remaining carcass was then weighed and length 5-min adjustment period, each flock was scan- of the right shank was measured. Subcutaneous ned three times at 5-min intervals with the be- fat was defined as major adipose tissues located
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Ingredient Ground yellow corn Dehulled soybean meal Hydrolyzed fat Menhaden fish meal Defluorinated phosphate Ground limestone NaCl Vitamin mix 2 Trace mineral mix 2 Methionine DL Calculated analysis Protein, % Energy, kcal metabolizable energy/kg Calorie:protein ratio
EFFICIENCY OF FEED UTILIZATION
Line by diet interactions for body weight, feed intake, FCR, and CCR were not significant (Table 2). Feed consumption values were similar for all line-diet subgroups, whereas body weights were heavier and FCR were lower for Line A than B and Diet H than L chicks (Table 2). The CCR were similar for both diets, but lower for Line A than B. The FCR and its two components, body weight gain and feed consumption, were calculated separately for each of three periods. Although there were no significant differences between lines for body weight, feed intake, or FCR to 21 days of age (Table 3), a pattern was developing in the data. Line A chicks, which were from a younger breeder flock, were 4 g (8%) lighter than Line B chicks at one day of age, and gained 8 g more than those from Line B by 21 days of age. From 22 to 35 days of age, they gained 34 g more than Line B chicks on the same amount of feed, which resulted in a superior FCR for Line A than B. This difference increased further between 36 and 49 days of age, when Line A chicks gained 80 g (9.4%) more weight than those from Line B on the same amount of feed.
3000 H
o ALL BIRDS •
SAMPLE
2000' BODY WT. (g)
1000-
RESULTS
Body Weight and Feed Conversion. Body weights approximated 2,000 g at 39 days of age and 2,600 g at 49 days (Table.2), with the latter weights obtained after an 18-h fast. The former were within the range of industry whole bird market weights, while the latter were consistent with those of broilers used for deboning and further processing. Body weights of the samples of chicks on which individual data were obtained were representative of the entire population (Figure 1).
0
5 7
12 14 19 21 26 28 33 35 4 0 42 AGE
49
(days)
FIG. 1. Growth curves of all chicks averaged by age (o) and for a random sample of chicks from each line-class subgroup (•). All body weights (WT.) were obtained in ad-libitum-fed chicks except at 49 days of age, when there was an 18-h feed withdrawal period.
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under the skin, including sartorial and clavicular as well as other large depots on the back and the abdominal wall. Individual composition analyses were conducted separately for each of the two chicks from each pen that were picked by hand. Skin plus subcutaneous fat was frozen in one bag, a sample of breast muscle in another, and the remainder of the chicken in a third bag. For analysis, skin plus subcutaneous fat were spread over a metal screen placed as a strainer over a 3-L can and oven dried for 48 h at 100 C to determine water content. During this process 20 to 50% of the fat liquidized by heating drained into the can, and its weight was recorded. The remaining tissue was crisp, easily homogenized, and its lipid-content determined in duplicates following extraction by chloroform:methanol (2:1) (Folch et al., 1957). These data were combined with the fat in the can and considered as lipid-content of skin plus subcutaneous fat for that chick. Samples of breast muscle were homogenized and lipid-content determined in duplicate (Folch et al., 1957). The remainder of each carcass was autoclaved, ground, homogenized, and lipid was determined as described for breast. Total lipid was calculated from the sum of values from breast, skin plus subcutaneous fat, and the remaining carcass. Analyses. Behavioral observations were analyzed as cumulative results over time by chi square; all other measurements were analyzed by two-way (2 lines x 2 diets) analysis of variance. There were four replicates of each line by diet combination for the pen data (body weight, feed consumption, and feed conversion), 32 replicates (4 pens X 8 chicks/pen) for measurements obtained weekly from the individual chicks, 24 or 32 replicates (4 pens x 6 or 8 chicks/pen) for the processing data, and 8 replicates (4 pens x 2 chicks/pen) for carcass composition.
1103
39 49
39 49
FCR, g/g
CCR, cal/g
2
5.24 5.87
5.27 5.94
3,362 4,941
3,280 4,848
Line B
5.38 6.15
1.66 1.89
3,297 4,866
2,030 2,617
H
Chicks were weighed after ad libitum feeding at 39 days and after 18 h food removal at 49 days of age.
5.51 6.22
1.79 2.01
1,929 2,510
L
2,065 2,727
H
1.62 1.81
Line A
1.71 1.92
3,331 4,974
1,993 2,638
L
L = Low energy diet; H = high energy diet.
39 49
Feed intake, g
1
39 49
(Day)
Age
Body weight,2 g
Variable
Diet 1
TABLE 2. Mean feed intake, body weight, and feed (FCR) and calorie (CCR) conversion ratio at 39 and 49 days of age and significance values from analyses of varianc
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EFFICIENCY OF FEED UTILIZATION
1105
TABLE 3. Mean body weight, feed intake, and feed conversion ratios (FCR) by lines, averaged over diets for various age periods Line1 Trait
Age
A
Difference2
B
(%)
(d)
Body weight,3 g Initial Final
AA* * *
0 49
Weight gains
2,682**
0to21 22 to 35 36 to 49
-8.0 4.6
706 958 854
1.1 3.5 9.4
0 to 49
4,911
4,904
.1
0 to 21 22 to 35 36 to 49
995 1,712 2,205
1,000 1,747 2,157
-.5 -2.0 2.0
FCR, feed:gain
Oto49
1.86**
1.95
-4.8
0to21 22 to 35 36 to 49
1.39 1.73** 2.37**
1.42 1.83 2.54
-2.2 -5.8 -7.2
1
Significances are given for comparisons of means within line pairs
2
( A - B ) X 100/B.
3
Chicks were weighed after 18 h food removal at 49 days of age and after ad libitum feed at all other ages.
*P«.05. **P<.01. ***P«.001.
resulting in line by diet interactions from 5 to 33 days of age. Line by diet interactions were also significant from 5 to 33 days of age for differences between cloacal and surface temperatures. As with surface temperatures, differ-
Body Temperatures. Cloacal temperatures were generally uniform. Surface (foot) temperatures were similar for birds fed both diets within Line A, while in Line B, means were generally lower for chicks fed Diet H than L (Table 4),
TABLE 4. Surface (foot) temperatures and differences between cloacal and surface temperatures of line-diet subgroups by age1 Cloacal minus surface
Surface H
L
5 12 19 26 33 40
H
L
(Day)
32.6** 34.5 32.9 32.0 32.8 33.8
2
33.8 35.0 32.8 32.1 32.6 33.2
33.0** 36.0 33.5* 32.9*** 33.4** 33.6*
Line B
Line A
Line B
Line A Age
L
H
L
8.1*
7.1 6.1 8.2 9.5 9.0 8.8
y p* * 5.0** 7.4* 8.6*** y o** * 8.9*
H
(°C) 32.3 35.3 32.6 31.0 31.8 32.2
6.5 8.2 9.5 8.7
7.8*
8.7 5.7 8.4
10.6 9.7 9.5
1 L = Low energy diet; H = high energy diet. Line by diet interactions were significant at all ages except 40 days. Significances are given for comparisons of means within diet pairs.
*P<.05. **P<.01. ***P«.001.
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Feed intake, g/chick
714 992* 934*
48 2,563
CAHANER ET AL.
1106
TABLE 5. Mean feather coverage of the back and wet and dry feather weights by lines averaged over diets at several ages Line1 Feather measurement
Difference2
Age (Day)
3
(%)
19 26 33 40
58.8 71.9 79.4* 81.2
Wet weight, % of body weight
39 49
5.91 5.66
Dry matter, % of wet weight
39 49
Dry weight, % of body weight
39 49
61.6 73.6 86.2 86.4 6.00 6.07
Significances are given for comparisons of means within line pairs. ( B - A ) X 100/A.
3
Proportion of the back length covered with feathers.
3.7 7.0
3.01 3.03
2.86 2.60***
1
1.5 7.3
50.9 50.2
49.1 46.9
2
4.8 2.4 8.6 6.4
5.2
16.5
*P<.05. ***P<.001.
ences between diets were similar for Line A chicks. In Line B, differences between cloacal and surface temperatures were consistently larger for chicks fed Diet H than Diet L.
Feathers. Diet did not influence feathering. Back feather cover was greater for Line B than A chicks at 33 days of age (Table 5). When expressed as a percent of body weight, dry
TABLE 6. Percentage of chicks sitting and standing by line-diet subgroups at various ages1 DietH
Diet L Variable
Line A
Line B
Line A
7 14 21 28 35 42
28 46 53 73 78 84
26 41 55 82 81 77
50 55 74 73 81 79
Mean
60
60
7 14 21 28 35 42
40 23 22 14 11 10
38 24 26 6 7 10
Mean
20
19
Age
Line A
LineB
40 49 64 66 66 74
39 51 64 73 80 82
33 45 59 74 73 76
69**
60
65*
60
27 18 10 21 6 7
40 28 14 28 20 14
33 20 16 17 9 9
39 26 20 17 14 12
15**
24
17*
21
(%)
(Day) Chicks sitting
Chicks standing
Diets L + H LineB
'Average of three observations taken 5 minutes apart on that day. L = Low energy diet; H = high energy diet. Significances are given for comparisons of means within line pairs. *P«.05. **P<.01.
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Back cover, %
EFFICIENCY OF FEED UTILIZATION
were more active than Line A chicks, but only when fed the high energy diet. Processing Traits. As expected, body weights of the chicks selected for processing were different from those of the entire population (Tables 2 and 7). The discrepancy at 39 days of age resulted from processed chicks having been fasted for 18 h before weighing, whereas the rest of the chicks were not fasted. Although all the chicks were fasted for 18 h at 49 days of age, those with leg problems were excluded from processing. These procedural items did not, however, change the weight relationships between lines. Because Line A chicks were heavier than Line B chicks, weights of parts and components were expressed as percent of body weight. Line A chicks, which had superior FCR, had less abdominal fat and subcutaneous plus skin
TABLE 7. Mean weights of body components for chicks processed at 39 and 49 days of age (line means averaged over diets) Line1 Variable
Age
Difference2
A
B
1,940* 2,770*
1,885 2,690
(%)
(Day) Body weight, g
39 49
2.9 3.0
Body components (% of body weight) Abdominal fat tissue
39 49
1.25** 1.16***
1.80 1.89
-30.6 -38.6
Subcutaneous + skin lipids
39 49
2.94* 2.71***
3.55 4.11
-17.2 -34.1
Total carcass lipids
39 49
8.02 o 57* * *
8.64 12.25
-7.2 -21.9
Lipids in breast meat 3
39 49
2.70 2.35
2.75 2.46
-1.8 -4.5
Breast
39 49
18.40* 19.11*
17.95 18.58
2.5 2.9
Thighs + drumsticks
39 49
20.87** 21.29***
20.43 20.49
2.2 3.9
Breast + thighs + drumsticks
39 49
39.27*** 40.39***
38.38 39.07
2.3 3.4
1
Significances are given for comparison between pairs of line means.
2
(A - B) X 100/B.
3
Wet weight.
*P«.05. **P«.01. ***P<.001.
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feather weight was also greater for Line B than A chicks at 49 days of age. Behaviors. The frequencies of chicks eating, drinking, and preening were minor in comparison to the incidence of sitting and standing behaviors. Accordingly, subsequent comments will focus on variations in the incidence of sitting and standing behavior. The proportion of chicks sitting increased from about 35% at 7 days of age to 80% at 42 days (Table 6). There were no consistent differences between lines for chicks fed Diet L. For Diet H, Line A chicks sat more than those from Line B. Data for standing mirrored those for sitting, with the proportion of chicks standing decreasing with age. As with sitting, differences between lines for chicks fed Diet L were inconsistent while for Diet H Line B chicks stood more than those from Line A (24 vs. 15%). It appears that Line B chicks
1107
CAHANER ET AL.
1108
chicks by diets. Differences in body weight were significant at 39 but not at 49 days of age. No differences were found between diets for the other traits. There were no differences between lines or between diets in the relative weights (percent of body weight) of the digestive tract, liver, shanks, and remaining carcass (back, wings, neck), or in shank length and leg scores. DISCUSSION
The range of body weights at which feed efficiency and carcass traits were studied was consistent with current broiler industry practices. Since growth rates generally exceeded those of current commercial broilers, results and conclusions may be relevant to broilers produced for the rest of this decade. Variation in the differences between cloacal and surface temperatures was not associated with FCR or carcass composition. Ambient temperatures, however, were generally optimal and
TABLE 8. Mean weights of body components for chicks processed at 39 and 49 days of age (diet means averaged over lines) Diets1 Age
Difference2
L
H
39 49
1,868** 2,698
1,958 2,762
Abdominal fat tissue
39 49
1.48 1.44
1.57 1.61
6.1 11.8
Subcutaneous + skin lipids
39 49
3.14 3.35
3.35 3.31
6.7 -.3
Total carcass lipids
39 49
8.04 10.81
8.62 11.01
7.2 1.9
Lipids in breast meat 3
39 49
2.76 2.54
2.69 2.28
-2.5 -10.2
Breast
39 49
18.08 19.05
18.27 18.65
1.0 -2.1
Thighs + drumsticks
39 49
20.69 20.92
20.60 20.87
-.4 -.2
Breast + thighs + drumsticks
39 49
38.77 39.97
38.88 39.52
.3 -1.1
Variable
(%)
(Day) Body weight, g
4.8 2.4
Body components (% of body weight)
'L means.
Low energy diet; H = high energy diet. Significances are given for comparisons between pairs of diet
2
(H - L) X 100/L.
3
Wet weight.
**P«.01.
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lipids than those from Line B at both ages (Table 7). At 49 days of age,'total carcass lipids were less for Line A than Line B chicks. The difference between lines was mainly due to variation in adipose depots since lipid content in breast muscle was similar. The proportion of meat parts (breast, thighs plus drumsticks) was higher in Line A than B with differences in breast similar at both ages whereas those for thighs plus drumsticks increased from 39 to 49 days of age. When these carcass components were combined, percentage differences between lines ranged from 2.3 to 3.4 (.9 to 1.3% of body weight) at 39 and 49 days of age, respectively. Proportions of deboned breast meat were similar in all subgroups and ages, with an average of 84%. Since Line A chicks were larger and also had a higher proportion of meat parts than those from Line B, differences between them were even larger on an absolute basis. Table 8 summarizes data from processed
EFFICIENCY OF FEED UTILIZATION
(LeClercq, 1983; Whitehead and Griffin, 1984, Cahaner et al., 1986). Changes in fat stores with age were not consistent across depots or lines. Yet, overall total carcass lipid (as percent of body weight) of Line B chicks increased by 42% from 39 to 49 days of age, whereas the increase in Line A chicks was 19%. As a result, differences between lines in fat deposition increased. Similarly, the difference between lines in feed conversion also increased with age, emphasizing an association between these two traits. Superior growth rate and FCR of the chicks fed the higher energy Diet H resulted primarily from increased energy intake, as evidenced by similar CCR between diets within lines. The difference in dietary energy intake had little effect on carcass composition, perhaps due to the level of fat deposition of both lines, which was lower than that typically observed at these body weights in commercial broilers. Since the magnitude of differences in growth rate and FCR between the lines was similar to that of differences between the diets, it may appear that similar improvements in growth rate and FCR can be achieved by either genetic or nutritional means. Such inferences are not warranted. The genetically related superior FCR was associated, in part, with reduced fat deposition and increased meat yield, while the improvement of FCR by diet tended to have the opposite effect on meat yield. ACKNOWLEDGMENT
This research was supported, in part, by the John Lee Pratt Animal Nutrition Program and Binational Agricultural Research and Development Fund Project No. US-675-83C. The authors wish to thank N. B. Anthony, M. Katanbaf and A. Martin for their assistance in data collection. REFERENCES Cahaner, A., andP. B. Siegel, 1986. Evaluation of industry breeding programs for meat-type chickens and turkeys. Proc. 3rd World Congr. Genet. Appl. Livest. Prod. 10:337-346. Cahaner, A., Z. Nitsan, and I. Nir, 1986. Weight and fat content of adipose and nonadipose tissues in broilers selected for or against abdominal adipose tissue. Poultry Sci. 54:215-222. Chambers, J. R., A. Fortin, and A. A. Grander, 1983. Relationships between carcass fatness and feed efficiency and its component traits in broiler chickens. Poultry Sci. 62:2201-2207. Folch, J., M. Lees, and G. H. Sloane-Stanley, 1957. A simple method for isolation and purification of total lipids from animal tissues. J. Biol. Chem. 226:497-
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hence do not rule out that genetic variation in thermoregulation would be expressed under less optimal conditions and may be associated with FCR. Relevance of this reasoning is evident from the significant line by diet interactions for cloacal-surface differences through 33 days of age. There appear to be several factors contributing to the superior FCR of Line A to Line B chicks. Behavioral differences were observed only between chicks fed the H diet, while FCR differences between the lines were similar for both diets. Therefore, although the association between behavior and FCR could be expected, further study is needed. Differences in feathering (Line A had less feathering than Line B) were consistent with reports of considerable genetic variation in feathering of early feathering chickens (Siegel et al., 1957a). Under husbandry conditions of our experiment where hot-air brooding was used, heavy feather coverage was not needed for insulation and energy costs of producing plumage could have contributed to the inferior FCR of Line B. At 49 days of age, mean dry matter weights of feathers were 71 g for Line A and 81 g for Line B chicks. The wet weight equivalent of this 10 g difference in feather dry matter, which is mostly protein, is approximately 20 g of feathers or 50 g of muscle. Since mean live body weights of Line A and B chicks were 2,770 and 2,690 g, respectively (Table 7), it may be speculated that 30 of the 80 g difference in body weight between lines resulted from differences in feathering. The 30 g translated to about 2 points in FCR. Although feather volume may have positive effects in certain situations, their negative effect on feed conversion to live body weight, and more so on feed conversion to meat, deserves further study, particularly under different climatic conditions. Although neither of the lines studied would be considered excessively fat by today's standards (Nixey, 1986), the most pronounced difference between them was in the amount and distribution of fat. Although lipid-content of breast meat was similar for both lines, large differences existed for abdominal fat tissue and subcutaneous plus skin lipids, resulting in more carcass fat for Line B than Line A chicks. These results are consistent with those obtained from commercial broiler populations (Littlefield, 1972; Chambers et al., 1983) as well as from experimental lines selected on feed efficiency (Washburn et al., 1975; Pym, 1985) and on indirect and direct measures of fat deposition
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509. LeClercq, B., 1983. The influence of dietary protein content on the performance of genetically lean or fat growing chickens. Br. Poult. Sci. 24:581-587. Leenstra, F. R., P.F.G. Vereijken, and R. Pit, 1986. Fat deposition in a broiler sire strain. 1. Phenotypic and genetic variation in and correlations between abdominal fat, body weight and feed conversion. Poultry Sci. 65:1225-1235. Littlefield, L. H., 1972. Strain differences in growth of abdominal fat in broilers. Poultry Sci. 51:1829. (Abstr.) McCarthy, J. C , and P. B. Siegel, 1983. A review of genetical and physiological effects of selection in meattype poultry. Anim. Breed. Abstr. 51:87-94. Nir, I., 1984. Factors influencing excessive fat deposition in meat poultry 1. Nutrition. Pages 53-55 in: Proc. XVII World's Poult. Congr. Helsinki, Finland. Nixey.C, 1986. A comparison of growth and fat deposition of commercial avian species. Proc. 7th Eur. Poult. Conf. 2:671-680, Paris, France. Pym, R.A.E., 1985. Direct and correlated responses to selection for improved feed efficiency. Pages 97-112 in: Poultry Genetics and Breeding. W. G. Hill, J. M. Manson, and D. Hewitt, ed. British Poult. Sci. Ltd., Longman Group, Harlow, UK. Pym, R.A.E., and A. J. Solvyns, 1979. Selection for food conversion in broilers: Body composition of birds selected for increased body-weight gain, food consumption and food conversion ratio. Br. Poult. Sci. 20:87-97.