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Genetic Variability in Growth Response of Chicks to Cold Brooding Temperature1
Genetic Variability in Growth Response of Chicks to Cold Brooding Temperature 1 G. M. RENWICK, K. W. WASHBURN,2 and G. M. LANZA Department of Poultry Science, University of Georgia, Athens, Georgia 30602 (Received for publication November 2, 1981)
1985 Poultry Science 64:785-788 INTRODUCTION
Current approaches to the conservation of energy in the brooding of broiler chicks have primarily involved management techniques such as partial house brooding. Genetic and strain variation in response to cool temperature brooding has received relatively little emphasis, although genetic variability in adaptation to cool temperature brooding sets the limits of the temperature at which chickens may be brooded without deleterious effects. Genetic involvement in thermoregulation is apparent from the varied adaptive advantages that favor survival of different organisms in their particular thermal environment. Differences in ability to survive extreme temperatures may be related to resistance to changes in body temperature or activation of thermoregulatory or homeostatic mechanisms to prevent body temperature changes. Survival of chickens in hot environments has been shown to be breed and family related (Hutt, 1938; Wilson et al, 1966). Sagher (1975) observed that acute cold stress caused by exposing day-old commercial broiler chicks to 10 C for 4 hr caused a marked reduction in growth rate during a subsequent 2-week period. Davison and Lickiss (1979), however, found that exposure of day-old Rhode Island Red X
1 Supported by State and Hatch funds allocated to the Georgia Agricultural Experiment Stations of the University of Georgia. 2 To whom correspondence should be addressed.
Light Sussex chicks to 10 C for 4 hr had no effect on subsequent growth of males and retarded the growth of females only through 11 days of age, indicating that breed differences may be responsible for conflicting results. The present study was conducted to determine the genetic variation and heritability of weight-gain response in chicks brooded under a control (32.2 C) or reduced temperature (26.7 C). MATERIALS AND METHODS
The chickens used were progeny of 28 sire families of the Athens-Canadian randombred population (Hess, 1952). In Trial 1, 846 progeny were identified by sire family only with an average number of 30 progeny per sire and a range of 12 to 37 progeny per sire. In Trial 2, 856 progeny were identified by sire and dam family with an average number of 31 progeny per sire and 5.3 per dam. The number of progeny for the dam families ranged from 2 to 12. After hatching, the chicks were sexed, weighed, and wing banded. Chicks within families were randomly assigned to either a control brooding temperature (32.2 C) or a reduced brooding temperature (26.7 C). These temperatures were maintained under the hover portion of electrically-heated, wire-floored, constantly-illuminated batteries maintained singly in four controlled environmental chambers in which temperatures diurnally cycled from 15.5 to 21.1 C. The progeny of each sire family appeared in four locations within each brooding temperature to randomize pen and micro-
785
Downloaded from http://ps.oxfordjournals.org/ at UPR Mayaguez Gen Lib on April 24, 2015
ABSTRACT Genetic variability in adaptation to brooding at a reduced temperature was examined by comparing 1 to 14-day body weight gains of progeny from 28 sire families of the Athens-Canadian randombred population brooded at 32.2 C and at 26.7 C. The 1 to 7-day gain of the chicks brooded at 26.7 C was significantly depressed, but 7 to 14-day gain was not significantly depressed by brooding at the reduced temperature. Heritability estimates averaged over the two trials were .31, .23, and .35 for the 1 to 7-, 7 to 14-, and 1 to 14-day weight gains of groups brooded at 32.2 C and .55, .59, and .67 for the gain of groups brooded at 26.7 C. The heritability estimates for the difference in weight gain of families brooded at the two temperatures was .78. (Key words: cold temperature, genetics, chickens)
786
RENWICK ET AL.
©
§•
The data were analyzed using either the Statistical Analysis System (Barr et al., 1979) or the Mixed Model Least-Squares and Maximum Likelihood computer programs (LSML76) (Harvey, 1977). Heritability estimates were calculated based on sib analysis using the sire component of variance.
RESULTS AND DISCUSSION Cold temperature (26.7 C) brooding (CTB) increased 1 to 14-day mortality in each trial (Table 1). Because the mortality responses of families were similar, they were not ranked from resistant to susceptible on the basis of mortality response to CTB. Cold temperature brooding significantly depressed the 1 to 7-day weight gain in both trials; however, there was no significant effect of brooding temperature on weight gain from 7 to 14 or 1 to 14-days of age in either trial (Table 1). Weight gains, expressed as deviation
£ < H
Downloaded from http://ps.oxfordjournals.org/ at UPR Mayaguez Gen Lib on April 24, 2015
environmental effects. Feed and water were provided ad libitum, and accessory feed trays and water founts were placed directly under the hover. Previous studies (Renwick and Washburn, 1982) have shown this necessary to obtain a measure of response to temperature that is not confounded with feeding behavior. Individual body weight and mortality were recorded at 7 and 14 days of age and gains for the 1 to 7-, 7 to 14-, and 1 to 14-day periods were calculated. These data were utilized to classify the sire families according to the degree of resistance (high line) or susceptibility (low line) to brooding at 26.7 C. Estimates of variance components were obtained within trials separately for control groups, challenge groups brooded at a reduced temperature, and for deviations in weight gain. Deviations in gain were obtained by subtracting the gain of individuals reared at 26.7 C from the average gain of their full sibs (Trial 2) or half-sibs (Trial 1) reared in the control environment. These differences between sib progeny grown in the different environments were considered as the adaptation to cold stress and used to determine sire component heritability estimates. This type of estimate was obtained because a nonspecific parameter, body weight, was used to assess the adaptation to cold stress. Thus, heritability of body weight of the chicks grown in the cooler environment would include both the normal genetic variation in weight and the variation in ability to adapt to the cooler temperature.
GENETICS OF GROWTH RESPONSE TO COLD TEMPERATURE TABLE 2. First week weight gain (g) of sire families brooded at 26.7 C expressed as deviation from control (32.2 C) Deviations from control (g)
Rank 1
<,.
Trial 1
12 2 38 26 45 48 X
28 26 24 20 19 18
3 47 15 36 50 1 X
1 6 8 7 10 9
Trial 2
Trial 1
Trial 2
Low weight gain (susceptible) 25 26 15 24 23 20
-11.0 -10.0 -9.5 -6.3 -6.3 -6.2 -8.2
-9.0 -9.6 -4.5 -7.2 -7.1 -5.4 -7.1
High weight gain (resistant)
1
5 2 9 12 6 8
2.5 .3 -.8 0
.7 1.8
-1.9 -2.7
-2.1 -1.6
-1.1
-.9
-.4
-.6
Ranked 1 = Most resistant, 28 = least resistant.
from control, are presented in Table 2 for the six families most resistant and the six families most susceptible to CTB. The resistant families from the 26.7 C group gained at nearly the same rate as controls brooded at 32.2 C for the first week. However, the susceptible families from the group brooded at 26.7 C gained 8.2 and 7.1 g less for the first week in Trials 1 and 2, respectively (Table 2), than did the group brooded at 32.2 C. Heritability estimates for weight gain under control conditions (Table 1) were lower than most values previously reported for gain (Mer-
ritt, 1966). However, previous estimates for gain have usually been obtained over a longer time and at a later age; thus, they would not be exactly comparable to these estimates obtained at weekly intervals at an early age. Estimates obtained over the 2-week period were similar to those expected for body weight. The heritability estimates for weight gain at 26.7 C, which ranged from .40 to .80, were higher than for those obtained for groups brooded at 32.2 C and in each case the among sires variance components (Table 3) were substantially greater at 26.7 C than at 32.2 C. Cold temperature brooding increased the variation in weight gain response among sires, while having little consistent effect on within-sire variation. Theoretically, the best measure of response to CTB should be direct response (weight gain under CTB). However, variation in body weight would be influenced by the total genetic and environmental affect, not just by that specific portion influenced by adaptation to the cold temperature. Heritability estimates based on the difference between family means at 26.7 and 32.2 C should be a more reliable estimate of response than estimates of weight gain within the 26.7 C group, because the latter contains the total genetic variation in weight gain in addition to the genetic variation in response to cold. Heritability estimates (.34 and .49) for the weight gain difference in response to CTB for the first week were similar to estimates for birds at 26.7 C. Heritability estimates of weight gain difference for 7 to 14 and 1 to 14 days of age were considerably higher than that of birds brooded at 26.7 C and ranged from .74 to .90. Cold temperature brooding significantly depressed the mean 1 to 7-day weight gain yet did not affect the 7 to 14-day of age weight
TABLE 3. Variance components for weight gain response to brooding temperature Brooding temp
Gain 1-7 Days
Gain 1-14 Days
Gain 7-14 Days
"1
»w
Trial 1
32.2 26.7
4.0 13.6
123.6 105.0
7.1 14.6
192.5 132.2
34.3 46.9
287.2 301.1
Trial 2
32.2 26.7
9.1 19.4
65.9 99.5
15.6 35.0
254.0 143.6
43.7 91.6
427.7 365.3
1
2
CT
s = Among sires.
CT
w = Within sires.
Downloaded from http://ps.oxfordjournals.org/ at UPR Mayaguez Gen Lib on April 24, 2015
family
787
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RENWICK ET AL.
TABLE 4. Phenotypic andgenotypic correlations of gain during the 1 to 7 and 7 to 14-day periods Actual data Phenotypic
Genetic
32.2 C 26.7 C
-.02 .31
2.17 .65
Trial 2
32.2 C 26.7 C
.45 .54
.79 .71
Trial 1 Trial 2
32.2 C 26.7 C
.33 .57
Difference — .75 .88
gain. Despite t h e lack of an effect of CTB o n t h e 7 to 14-day weight gain, heritability estimates increased c o m p a r e d t o estimates for t h e 1 t o 7-day period. This m a y indicate t h a t variation in response to a d a p t a t i o n to CTB is substantial after 2 weeks of age even w h e n actual weight gains are n o t depressed. It is also possible t h a t weight gain over the 7 to 14-day period is measuring t h e capacity for comp e n s a t o r y g r o w t h and n o t response to b r o o d i n g t e m p e r a t u r e . C o m p e n s a t o r y g r o w t h was evident in Trial 1 of this s t u d y and has b e e n s h o w n t o occur in o t h e r studies in which CTB was used (Renwick and Washburn, 1 9 8 2 ; D e a t o n et al, 1 9 7 3 ) . T h u s , over t h e 2-week period of this study, there are t w o i m p o r t a n t aspects of gain: 1) h o w m u c h it was depressed during t h e first week and 2) h o w m u c h t h e depression was c o m p e n s a t e d for during t h e second week. T h e same genetic system would n o t necessarily be affecting t h e response to b o t h aspects. T h u s , o n e m i g h t o b t a i n different results if selection were based on t h e response during the first week, t h e second week, o r t h e overall 2-week response. S o m e insight into this m a y be provided b y the correlations of weight and gain for p e r t i n e n t periods (Table 4 ) . T h e correlations between 7- and 14-day weights were high (.76 t o .87), as would be e x p e c t e d from previous studies. However, t h e 7-day weight would be a contributing variable t o t h e 14-day weight and t h u s w o u l d bias t h e estimate. T h e correlation b e t w e e n 1 t o 7 and 7 t o 14-day gain provides a
REFERENCES Barr, A. J., J. H. Goodnight, J. P. Sail, W. N. Blair, and D. M. Chilko, 1979. SAS User's Guide 1979 ed. SAS Insti., Inc., Raleigh, NC. Davison, T. F., and P. A. Lickiss, 1979. The effect of cold stress on the fasted, water-deprived, neonate chicken (Gallus domesticus). J. Therm. Biol. 4:113-120. Deaton, J. W„ F. N. Reece, L. F. Kubena, B. D. Lott, and J. D. May, 1973. The ability of the broiler chicken to compensate for early growth depression. Poultry Sci. 52:262-265. Harvey, W. R., 1977. User's guide for LSML76 mixed model least-squares and maximum likelihood computer program. Ohio State Univ., Columbus, OH. Hess, C. W., 1952. Randombred populations of the Southern Regional Poultry Breeding Project. World's Poult. Sci. J. 18:147-152. Hutt, F. B., 1938. Genetics of the fowl. VII. Breed differences in susceptibility to extreme heat. Poultry Sci. 17:454-462. Merritt, E. S., 1966. Estimates by sex of genetic parameters for body weight and skeletal dimensions in a randombred strain of meat type fowl. Poultry Sci. 4 5 : 1 1 8 - 1 2 5 . Renwick, G. M., and K. W. Washburn, 1982. Adaptation of chickens to cool brooding temperature. Poultry Sci. 61:1279-1289. Sagher, B. M., 1975. The effect of cold stress on muscle growth in young chicks. Growth 39: 281-288. Wilson, H. R., A. E. Abramas, I. J. Ross, R. W. Dorminey, and C. J. Wilcox, 1966. Familial differences of Single Comb Leghorn chickens in tolerance to high ambient temperatures. Poultry Sci. 45:784-788.
Downloaded from http://ps.oxfordjournals.org/ at UPR Mayaguez Gen Lib on April 24, 2015
Trial 1
better measure of response over these two phases. These correlations were m o d e r a t e (.31 t o .55), indicating t h a t a p p r o x i m a t e l y 10 to 30% of t h e variation ( R 2 ) in 7 to 14-day gain could b e explained b y variation in 1 t o 7-day gain. This suggests that selection for a d a p t a t i o n t o CTB should be a p p r o a c h e d by use of an index involving minimal response during t h e first week and m a x i m a l c o m p e n s a t o r y g r o w t h during t h e second w e e k . T h e increased mortality, decreased gain, and efficiency of feed utilization are i m p o r t a n t considerations in CTB. However, some depression in early g r o w t h and efficiency could be tolerated if c o m p e n s a t o r y g r o w t h and improved efficiency occurred a t a later stage.
1985 Poultry Science 64:785-788 INTRODUCTION
Current approaches to the conservation of energy in the brooding of broiler chicks have primarily involved management techniques such as partial house brooding. Genetic and strain variation in response to cool temperature brooding has received relatively little emphasis, although genetic variability in adaptation to cool temperature brooding sets the limits of the temperature at which chickens may be brooded without deleterious effects. Genetic involvement in thermoregulation is apparent from the varied adaptive advantages that favor survival of different organisms in their particular thermal environment. Differences in ability to survive extreme temperatures may be related to resistance to changes in body temperature or activation of thermoregulatory or homeostatic mechanisms to prevent body temperature changes. Survival of chickens in hot environments has been shown to be breed and family related (Hutt, 1938; Wilson et al, 1966). Sagher (1975) observed that acute cold stress caused by exposing day-old commercial broiler chicks to 10 C for 4 hr caused a marked reduction in growth rate during a subsequent 2-week period. Davison and Lickiss (1979), however, found that exposure of day-old Rhode Island Red X
1 Supported by State and Hatch funds allocated to the Georgia Agricultural Experiment Stations of the University of Georgia. 2 To whom correspondence should be addressed.
Light Sussex chicks to 10 C for 4 hr had no effect on subsequent growth of males and retarded the growth of females only through 11 days of age, indicating that breed differences may be responsible for conflicting results. The present study was conducted to determine the genetic variation and heritability of weight-gain response in chicks brooded under a control (32.2 C) or reduced temperature (26.7 C). MATERIALS AND METHODS
The chickens used were progeny of 28 sire families of the Athens-Canadian randombred population (Hess, 1952). In Trial 1, 846 progeny were identified by sire family only with an average number of 30 progeny per sire and a range of 12 to 37 progeny per sire. In Trial 2, 856 progeny were identified by sire and dam family with an average number of 31 progeny per sire and 5.3 per dam. The number of progeny for the dam families ranged from 2 to 12. After hatching, the chicks were sexed, weighed, and wing banded. Chicks within families were randomly assigned to either a control brooding temperature (32.2 C) or a reduced brooding temperature (26.7 C). These temperatures were maintained under the hover portion of electrically-heated, wire-floored, constantly-illuminated batteries maintained singly in four controlled environmental chambers in which temperatures diurnally cycled from 15.5 to 21.1 C. The progeny of each sire family appeared in four locations within each brooding temperature to randomize pen and micro-
785
Downloaded from http://ps.oxfordjournals.org/ at UPR Mayaguez Gen Lib on April 24, 2015
ABSTRACT Genetic variability in adaptation to brooding at a reduced temperature was examined by comparing 1 to 14-day body weight gains of progeny from 28 sire families of the Athens-Canadian randombred population brooded at 32.2 C and at 26.7 C. The 1 to 7-day gain of the chicks brooded at 26.7 C was significantly depressed, but 7 to 14-day gain was not significantly depressed by brooding at the reduced temperature. Heritability estimates averaged over the two trials were .31, .23, and .35 for the 1 to 7-, 7 to 14-, and 1 to 14-day weight gains of groups brooded at 32.2 C and .55, .59, and .67 for the gain of groups brooded at 26.7 C. The heritability estimates for the difference in weight gain of families brooded at the two temperatures was .78. (Key words: cold temperature, genetics, chickens)
786
RENWICK ET AL.
©
§•
The data were analyzed using either the Statistical Analysis System (Barr et al., 1979) or the Mixed Model Least-Squares and Maximum Likelihood computer programs (LSML76) (Harvey, 1977). Heritability estimates were calculated based on sib analysis using the sire component of variance.
RESULTS AND DISCUSSION Cold temperature (26.7 C) brooding (CTB) increased 1 to 14-day mortality in each trial (Table 1). Because the mortality responses of families were similar, they were not ranked from resistant to susceptible on the basis of mortality response to CTB. Cold temperature brooding significantly depressed the 1 to 7-day weight gain in both trials; however, there was no significant effect of brooding temperature on weight gain from 7 to 14 or 1 to 14-days of age in either trial (Table 1). Weight gains, expressed as deviation
£ < H
Downloaded from http://ps.oxfordjournals.org/ at UPR Mayaguez Gen Lib on April 24, 2015
environmental effects. Feed and water were provided ad libitum, and accessory feed trays and water founts were placed directly under the hover. Previous studies (Renwick and Washburn, 1982) have shown this necessary to obtain a measure of response to temperature that is not confounded with feeding behavior. Individual body weight and mortality were recorded at 7 and 14 days of age and gains for the 1 to 7-, 7 to 14-, and 1 to 14-day periods were calculated. These data were utilized to classify the sire families according to the degree of resistance (high line) or susceptibility (low line) to brooding at 26.7 C. Estimates of variance components were obtained within trials separately for control groups, challenge groups brooded at a reduced temperature, and for deviations in weight gain. Deviations in gain were obtained by subtracting the gain of individuals reared at 26.7 C from the average gain of their full sibs (Trial 2) or half-sibs (Trial 1) reared in the control environment. These differences between sib progeny grown in the different environments were considered as the adaptation to cold stress and used to determine sire component heritability estimates. This type of estimate was obtained because a nonspecific parameter, body weight, was used to assess the adaptation to cold stress. Thus, heritability of body weight of the chicks grown in the cooler environment would include both the normal genetic variation in weight and the variation in ability to adapt to the cooler temperature.
GENETICS OF GROWTH RESPONSE TO COLD TEMPERATURE TABLE 2. First week weight gain (g) of sire families brooded at 26.7 C expressed as deviation from control (32.2 C) Deviations from control (g)
Rank 1
<,.
Trial 1
12 2 38 26 45 48 X
28 26 24 20 19 18
3 47 15 36 50 1 X
1 6 8 7 10 9
Trial 2
Trial 1
Trial 2
Low weight gain (susceptible) 25 26 15 24 23 20
-11.0 -10.0 -9.5 -6.3 -6.3 -6.2 -8.2
-9.0 -9.6 -4.5 -7.2 -7.1 -5.4 -7.1
High weight gain (resistant)
1
5 2 9 12 6 8
2.5 .3 -.8 0
.7 1.8
-1.9 -2.7
-2.1 -1.6
-1.1
-.9
-.4
-.6
Ranked 1 = Most resistant, 28 = least resistant.
from control, are presented in Table 2 for the six families most resistant and the six families most susceptible to CTB. The resistant families from the 26.7 C group gained at nearly the same rate as controls brooded at 32.2 C for the first week. However, the susceptible families from the group brooded at 26.7 C gained 8.2 and 7.1 g less for the first week in Trials 1 and 2, respectively (Table 2), than did the group brooded at 32.2 C. Heritability estimates for weight gain under control conditions (Table 1) were lower than most values previously reported for gain (Mer-
ritt, 1966). However, previous estimates for gain have usually been obtained over a longer time and at a later age; thus, they would not be exactly comparable to these estimates obtained at weekly intervals at an early age. Estimates obtained over the 2-week period were similar to those expected for body weight. The heritability estimates for weight gain at 26.7 C, which ranged from .40 to .80, were higher than for those obtained for groups brooded at 32.2 C and in each case the among sires variance components (Table 3) were substantially greater at 26.7 C than at 32.2 C. Cold temperature brooding increased the variation in weight gain response among sires, while having little consistent effect on within-sire variation. Theoretically, the best measure of response to CTB should be direct response (weight gain under CTB). However, variation in body weight would be influenced by the total genetic and environmental affect, not just by that specific portion influenced by adaptation to the cold temperature. Heritability estimates based on the difference between family means at 26.7 and 32.2 C should be a more reliable estimate of response than estimates of weight gain within the 26.7 C group, because the latter contains the total genetic variation in weight gain in addition to the genetic variation in response to cold. Heritability estimates (.34 and .49) for the weight gain difference in response to CTB for the first week were similar to estimates for birds at 26.7 C. Heritability estimates of weight gain difference for 7 to 14 and 1 to 14 days of age were considerably higher than that of birds brooded at 26.7 C and ranged from .74 to .90. Cold temperature brooding significantly depressed the mean 1 to 7-day weight gain yet did not affect the 7 to 14-day of age weight
TABLE 3. Variance components for weight gain response to brooding temperature Brooding temp
Gain 1-7 Days
Gain 1-14 Days
Gain 7-14 Days
"1
»w
Trial 1
32.2 26.7
4.0 13.6
123.6 105.0
7.1 14.6
192.5 132.2
34.3 46.9
287.2 301.1
Trial 2
32.2 26.7
9.1 19.4
65.9 99.5
15.6 35.0
254.0 143.6
43.7 91.6
427.7 365.3
1
2
CT
s = Among sires.
CT
w = Within sires.
Downloaded from http://ps.oxfordjournals.org/ at UPR Mayaguez Gen Lib on April 24, 2015
family
787
788
RENWICK ET AL.
TABLE 4. Phenotypic andgenotypic correlations of gain during the 1 to 7 and 7 to 14-day periods Actual data Phenotypic
Genetic
32.2 C 26.7 C
-.02 .31
2.17 .65
Trial 2
32.2 C 26.7 C
.45 .54
.79 .71
Trial 1 Trial 2
32.2 C 26.7 C
.33 .57
Difference — .75 .88
gain. Despite t h e lack of an effect of CTB o n t h e 7 to 14-day weight gain, heritability estimates increased c o m p a r e d t o estimates for t h e 1 t o 7-day period. This m a y indicate t h a t variation in response to a d a p t a t i o n to CTB is substantial after 2 weeks of age even w h e n actual weight gains are n o t depressed. It is also possible t h a t weight gain over the 7 to 14-day period is measuring t h e capacity for comp e n s a t o r y g r o w t h and n o t response to b r o o d i n g t e m p e r a t u r e . C o m p e n s a t o r y g r o w t h was evident in Trial 1 of this s t u d y and has b e e n s h o w n t o occur in o t h e r studies in which CTB was used (Renwick and Washburn, 1 9 8 2 ; D e a t o n et al, 1 9 7 3 ) . T h u s , over t h e 2-week period of this study, there are t w o i m p o r t a n t aspects of gain: 1) h o w m u c h it was depressed during t h e first week and 2) h o w m u c h t h e depression was c o m p e n s a t e d for during t h e second week. T h e same genetic system would n o t necessarily be affecting t h e response to b o t h aspects. T h u s , o n e m i g h t o b t a i n different results if selection were based on t h e response during the first week, t h e second week, o r t h e overall 2-week response. S o m e insight into this m a y be provided b y the correlations of weight and gain for p e r t i n e n t periods (Table 4 ) . T h e correlations between 7- and 14-day weights were high (.76 t o .87), as would be e x p e c t e d from previous studies. However, t h e 7-day weight would be a contributing variable t o t h e 14-day weight and t h u s w o u l d bias t h e estimate. T h e correlation b e t w e e n 1 t o 7 and 7 t o 14-day gain provides a
REFERENCES Barr, A. J., J. H. Goodnight, J. P. Sail, W. N. Blair, and D. M. Chilko, 1979. SAS User's Guide 1979 ed. SAS Insti., Inc., Raleigh, NC. Davison, T. F., and P. A. Lickiss, 1979. The effect of cold stress on the fasted, water-deprived, neonate chicken (Gallus domesticus). J. Therm. Biol. 4:113-120. Deaton, J. W„ F. N. Reece, L. F. Kubena, B. D. Lott, and J. D. May, 1973. The ability of the broiler chicken to compensate for early growth depression. Poultry Sci. 52:262-265. Harvey, W. R., 1977. User's guide for LSML76 mixed model least-squares and maximum likelihood computer program. Ohio State Univ., Columbus, OH. Hess, C. W., 1952. Randombred populations of the Southern Regional Poultry Breeding Project. World's Poult. Sci. J. 18:147-152. Hutt, F. B., 1938. Genetics of the fowl. VII. Breed differences in susceptibility to extreme heat. Poultry Sci. 17:454-462. Merritt, E. S., 1966. Estimates by sex of genetic parameters for body weight and skeletal dimensions in a randombred strain of meat type fowl. Poultry Sci. 4 5 : 1 1 8 - 1 2 5 . Renwick, G. M., and K. W. Washburn, 1982. Adaptation of chickens to cool brooding temperature. Poultry Sci. 61:1279-1289. Sagher, B. M., 1975. The effect of cold stress on muscle growth in young chicks. Growth 39: 281-288. Wilson, H. R., A. E. Abramas, I. J. Ross, R. W. Dorminey, and C. J. Wilcox, 1966. Familial differences of Single Comb Leghorn chickens in tolerance to high ambient temperatures. Poultry Sci. 45:784-788.
Downloaded from http://ps.oxfordjournals.org/ at UPR Mayaguez Gen Lib on April 24, 2015
Trial 1
better measure of response over these two phases. These correlations were m o d e r a t e (.31 t o .55), indicating t h a t a p p r o x i m a t e l y 10 to 30% of t h e variation ( R 2 ) in 7 to 14-day gain could b e explained b y variation in 1 t o 7-day gain. This suggests that selection for a d a p t a t i o n t o CTB should be a p p r o a c h e d by use of an index involving minimal response during t h e first week and m a x i m a l c o m p e n s a t o r y g r o w t h during t h e second w e e k . T h e increased mortality, decreased gain, and efficiency of feed utilization are i m p o r t a n t considerations in CTB. However, some depression in early g r o w t h and efficiency could be tolerated if c o m p e n s a t o r y g r o w t h and improved efficiency occurred a t a later stage.