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Group Selection for Adaptation to Multiple-Hen Cages: Production Traits During Heat and Cold Exposures1,2
Group Selection for Adaptation to Multiple-Hen Cages: Production Traits During Heat and Cold Exposures1,2 PATRICIA Y. HESTER, W. M. MUIR, J. V. CRAIG, and J. L. ALBRIGHT Department of Animal Sciences, Purdue University, West Lafayette, Indiana 47907 The selected line of chickens in multiple-hen cages showed an increased resistance to heat exposure, as indicated by lower mortality, when compared to the control and commercial lines housed in multiple-hen cages. Egg production 8 d prior to, during, and 8 d following either cold or heat exposures indicated that the selected line of chickens withstood social, handling, and environmental stressors better than the control line and, in some cases, the commercial line of chickens. It was concluded that the selected line of Leghorns showed evidence of stress resistance through lowered mortality and improved production.
{Key words: selection, adaptation, multiple-hen cages, Leghorns, production) 1996 Poultry Science 75:1308-1314
INTRODUCTION Not all strains of White Leghorns respond equally to a multiple-bird environment. For example, Cunningham and Ostrander (1982) compared two commercial strains of White Leghorns for responses to multiple bird cages. Fearfulness did not differ between the two strains, and both strains showed decreased feed usage per bird in higher densities; however, the colony size for maximum net profit differed between the strains. One strain produced higher net income with five birds per cage, whereas the other strain yielded higher income with four birds per cage. A comparison of five White Leghorn strains showed that the commercial strain with the least severe and the lowest frequency of agonistic behavior also demonstrated the highest survival percentage and the best egg production (Choudary et ah, 1972). A later study, however, showed no relationship between overt agonistic behavior and productivity of White Leghorns in multiple-hen cages (Al-Rawi et al., 1976). Other researchers have demonstrated that lines differ in their
Received for publication November 6, 1995. Accepted for publication July 1, 1996. ijournal Article Number 14,861 of the Purdue University Agricultural Research Programs, West Lafayette, IN 47907. financial support for this study was provided by USDA Award Number 58-3602-3118.
adaptability to multiple-hen cages (Wilson et ah, 1967; Choudary et al, 1972; Al-Rawi et al, 1976; Hansen, 1976; Ouart and Adams, 1982a,b; Craig et al, 1983; Craig and Lee, 1989, 1990). Although it is common practice to house commercial egg layers in multiple-hen cages, some primary breeders often select breeding stock in single-bird cages because measuring production in single-bird cages can maximize individual responses to selection, as an optimum index based on within- and between-family differences can be utilized (Garwood and Lowe, 1981). However, selection for improved individual performance may lead to a decline in group performance, especially if the selected traits also increase susceptibility to unfavorable environments (Griffing, 1967) and increase aggressiveness among cage mates (Lowry and Abplanalp, 1972; Craig et al, 1975; Bhagwat and Craig, 1977, 1978; Lee and Craig, 1981). White Leghorn strains that are high producers under less competitive situations may not be the best lines to house in multiple-hen cages. Group selection, defined as selection of birds based on group rather than individual performance, may be a more viable alternative for selecting caged breeder stock (Muir, 1996). A selected line of White Leghorns was developed by Muir (1996) that has shown improved survivability and reduced feather loss in multiple-hen cages (Craig and Muir, 1996). It was hypothesized that hens genetically selected for adaptation to a multiple-bird environment
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ABSTRACT A selected line of White Leghorns that has shown improved survivability and productivity and reduced feather loss in multiple-hen cages was evaluated for production traits under both stressed and unstressed conditions. It was hypothesized that hens selected for adaptation to multiple-bird cages would react less intensely to stressors and therefore lay more eggs and have lower mortality under stressed conditions. Three lines of chickens (selected, control, and commercial) were housed in either single-hen (1 hen) or multiple-hen cages (12 hens, social competition) at 16.7 or 17.1 wk of age. They were subsequently subjected to cold exposure at 33 wk of age and heat exposure at 44 wk of age.
PRODUCTION OF HENS ADAPTED TO MULTIPLE-HEN CAGES should react less intensely to social competition, and perhaps to all stressors in general, than the unselected controls. To test this hypothesis, the physiological response to social, thermal, and handling stress of selected, unselected control, and a commercial Leghorn line was evaluated (Hester et al., 1996). In many, but not all, instances, the selected line reacted less intensely to the stressors. Criteria for a less intensive response to stress included hemodilution (decreased packed cell volume) at the time of transfer to multiple hen cages and the lack of a leucocytic response due to social competition at 33 wk of age. The objective of the present study was to compare the production traits of a selected line of chickens developed by Muir (1996) with an unselected control line and a commercial line of Leghorns under both stressed and unstressed conditions.
Three genetic stocks of White Leghorns were compared. A line selected over seven generations for survival and hen-day egg production in multiple-bird cages was derived from and compared to an unselected control line, the North Central Randombred Control. The third stock was a commercial line of layers. The three strains of chicks were hatched on June 16, 1993 at the Purdue University Hatchery and reared in groups of 16, with others of their own strain and sex, in 61 x 91 cm wire cages. During the entire experimental period, feed and water were provided for ad libitum consumption. Beak trimming was not performed. More details on the origin of the genetic stocks, management of the birds, and assignment of the three genetic lines of pullets with respect to room, cage row within the room, and cage size (single- vs multiple-hen) are described by Craig and Muir (1996). At 16.7 to 17.1 wk of age, chickens of the three genetic lines were placed in either single-hen (1 pullet per cage, providing 1,085 cm 2 per bird) or multiple-hen (12 pullets per cage, which provided 362 cm 2 per bird) cages in one of four independently heated, ventilated, and lighted rooms of the Purdue University Layer Research Unit. Any observed effects of single- vs multiple-bird cage environments would be due to both the absence (1 bird) or presence (12 birds) of social competition as well as differences due to bird density (1,085 vs 362 cm 2 per bird), respectively. Each room contained eight rows of cages in a fourdeck, modified stair-step arrangement (North and Bell, 1990). Experimental units, consisting of either four consecutive single-bird cages or one multiple-hen cage, were repeated twice for each of the three genetic stocks within a cage row using a restricted randomization scheme. Water was provided through drip nipples with a single-caged hen having access to two drip nipples and the 12 hens of a multiple-bird cage having access to five drip nipples (Craig and Muir, 1996).
Experiment 1 When the hens were 33 wk of age, an experiment dealing with a cold environmental temperature was initiated on January 31. No attempts were made to regulate humidity. The temperature of two of the four rooms was decreased to a mean temperature of 0 C for 72 h with a range of -5 to 7 C. It took 1.5 h to lower the temperature from control levels. Relative humidity for the two colder rooms varied from 50 to 74%. The other two rooms remained at their control temperature (x = 21 C; range of 18 to 24 C) with relative humidity varying from 35 to 44%. Recording thermographs and humidigraphs monitored the environment of each of the four rooms continuously throughout the experiment. Water provided to the hens via drip nipples remained unfrozen by allowing it to flow continuously through the polyvinyl chloride pipes. Handling stress involved the collection of blood samples from paired cold and control rooms on two occasions with different hens being bled on each occasion following the initiation of the colder temperatures. Hens were bled 4 to 6 h after a temperature of 0 C was achieved on Day 1 of the experimental period. Additional hens were bled 4 to 6 h following the end of the 72 h treatment of 0 C on Day 3. Data on blood parameters are reported in the companion paper of Hester et al. (1996). The comb of each hen exposed to cold was evaluated subjectively for the presence of frostbite on Day 4.
Experiment 2 The same hens that were exposed to a 0 C temperature at 33 wk of age were subjected to a mean environmental temperature of 38 C (range of 32.5 to 41.5 C) for 3 h at 44 wk of age. Relative humidity averaged 36% during the 3 h heating episode. The same two control rooms used in Experiment 1 were also used as controls in Experiment 2. Ambient temperature of the two control rooms was maintained at 30 C with a relative humidity of 39%. Hens of the two heated rooms were bled 1 to 3 h after a mean temperature of 38 C was achieved on Day 1 of the experimental period. One-half of the control birds was bled before the birds of the heated environment were sampled, with the remaining half of the controls bled immediately following the heating episode (Hester et al., 1996). Mortality was monitored continuously throughout the 3-h heating episode. Mortality was recorded for hens housed in the bottom three cage rows. Hens that succumbed to the 38 C temperature were removed from their cages. Twelve hours following the termination of the 38 C temperature, dead birds of the bottom three cage rows were replaced with hens from the top cage rows of their respective rooms so as to maintain constant bird density within the cages throughout the study. Hens previously subjected to 38 C for 3 h were exposed to a second heating episode 24 h later on Day 2 of the experimental period. Using the same schedule as described for the first heating episode, hens of two rooms
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MATERIALS AND METHODS
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were subjected to a mean temperature of 38 C (range of 35.5 to 40 C) for 3 h. Relative humidity of the two rooms averaged 32%. The two control rooms were the same as used in the first heating episode. Mean temperature of the two control rooms was 28 C with a relative humidity of 38%. Blood samples were obtained from hens not previously bled as described for the first heating episode.
Cold Environment
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Egg Production Egg records were maintained daily. Only eggs laid 8 d prior to, during, and 8 d following the heat and cold exposures were considered. For information on annual performance of the hens, see Muir and Liggett (1995).
Statistical Analysis
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FIGURE 1. Hen-day egg production of three genetic lines of laying hens (selected, control, and commercial) 8 d prior to, during (3 d), and 8 d following cold and handling exposures.
RESULTS Experiment 1. Cold Environmental Temperatures A cold environment of O C for 72 h did not result in any hen deaths, although 15.5% of the hens experienced mildly frostbitten combs. Single-caged hens (x of 42.4 + 2.9%) experienced a higher incidence of frostbite than hens of multiple cages (x of 6.5 ± 0.8%, P < 0.0001). The percentage incidences of frostbitten combs for the selected (4.5 ± 1.2%) and control (3.8 ± 1.1%) lines housed in multiple-bird cages were significantly lower than that of the commercial line (11.1 ± 1.9%) housed in multiple-hen cages (P < 0.0001). The percentage incidence of frostbite among genetic lines in single-hen cages was similar (43.8 ± 5.1%, 42.7 ± 5.1%, and 40.6 ± 5.0% for selected, control, and commercial lines, respectively). For egg production, a significant environmental temperature by time interaction (P < 0.00001) occurred when the selected vs control lines were compared before and during exposure to cold (Figure 1). Three days of cold exposure caused a decline in egg production for both genetic lines; however, the decrease was greater in the control line (10.5%) than in the selected line (7.6%).
Although handling and bleeding the birds of the control environment also caused a decline in egg production, the decrease was much less than that which occurred when hens were exposed to both handling and cold temperature. Handling and bleeding the selected birds in the control environment caused only a 1.2% decrease in egg production, whereas the control line experienced a 3.9% decrease in egg production. A comparison of the selected and commercial lines prior to and during cold exposure also resulted in a significant environmental temperature by time interaction for egg production (P < 0.0001, Figure 1). Both the selected (7.6%) and the commercial (8.8%) lines showed the same relative decrease in egg production as a result of cold temperature. However, within the control environment, the commercial line showed a greater decrease in eggs laid (3.6%) than did the selected line (1.3%) as a result of handling and blood sampling. The environmental temperature by time interaction was nonsignificant when the genetic lines were compared before and following cold exposure (Figure 1). A comparison of the selected with the control line 8 d prior to and during cold and handling exposures showed
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The incidence of frostbitten combs during cold exposure (Experiment 1) and mortality due to heat exposure (Experiment 2) were analyzed by chi-square (Steel and Torrie, 1980). The number of eggs produced 8 d prior to, during, and 8 d following cold (Experiment 1) and heat (Experiment 2) exposure was statistically evaluated using a categorical data analysis suitable for discrete data. Specifically, the CATMOD procedure of SAS® (SAS Institute, 1990) as outlined by Grizzle et al. (1969) was used. Planned comparisons on egg production were made with the selected vs the control lines as well as the selected vs the commercial lines. Each comparison of lines was made during pre-exposure and at the time of exposure as well as before and after exposure to environmental extremes.
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PRODUCTION OF HENS ADAPTED TO MULTIPLE-HEN CAGES Single-Hen Cages
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Although the commercial line always laid more eggs than the selected line of hens in both single- and multiple-hen cages and prior to and following exposure to cold, differences between the lines were more profound in single-hen cages before cold exposure. A comparison of the selected line with the control line prior to and following exposure to cold and handling did not show a significant interaction for cage size and genetic stock (P < 0.07).
Experiment 2. High Environmental Temperatures Time of Cold Stress
Multiple-Hen Cages
FIGURE 2. Hen-day egg production of three genetic lines of laying hens (selected, control, and commercial) housed in single- or multiplehen cages 8 d prior to, during (3 d), and 8 d following cold and handling exposures.
a cage size by genetic stock interaction for egg production (P < 0.05, Figure 2). The selected line always laid more eggs than the control line both in single- and multiple-hen cages and prior to and during exposure to cold or handling. However, within multiple-hen cages, the difference in egg production between the selected and control lines was much greater during 3 d of cold and handling stress. A comparison of the selected line with the commercial line 8 d prior to and during cold exposure showed a significant cage size by genetic stock interaction (P < 0.01, Figure 2). Eight days prior to the period of cold and handling, which was immediately after peak production, the commercial hens were laying more eggs than the selected line in both single- and multiple-hen cages. During exposure, egg production decreased; the commercial hens continued to outproduce the selected line in single-hen cages, but not in multiple-hen cages, where egg production between the two lines was similar. A comparison of the selected line with the commercial line prior to and following cold and handling exposures also showed a significant cage size by genetic stock interaction for egg production (P < 0.001, Figure 2).
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During Time of Cold Stress
Single-caged hens experienced higher mortality (x of 19.8 + 2.4%) than hens of multiple-bird cages (x of 14.1 ± 1.2%, P < 0.02). During the first heating episode, the selected line of chickens in multiple-hen cages showed an increased resistance to heat exposure, as indicated by lower mortality (9.7 ± 1.8%), when compared to the control (17.4 ±2.2%) and commercial (15.3 ±2.1%) lines housed in multiple-hen cages (P < 0.03). Single-caged hens showed similar mortality among the three genetic lines (17.7 ± 3.9%, 24.0 ±4.4%, and 17.7 ±3.9% for selected, control, and commercial lines, respectively). The second heating episode did not result in any hen mortality. Hen-day egg production 8 d prior to, during (2 d), and 8 d following heat exposure of three genetic lines of layers housed in either single- or multiple-hen cages is shown in Figure 3. A comparison of the selected and commercial lines prior to and during heat exposure showed a significant genetic stock by cage size by environmental temperature interaction (P < 0.00001). Two heating episodes of 3 h each suppressed egg production in both the selected and commercial lines of chickens; however, the decrease was greater in single-hen cages (10.0 and 12.4%, respectively) than in multiple-hen cages (7.6 and 5.1%, respectively). Although colony-caged hens of the selected and commercial lines always laid fewer eggs than the single-caged hens, the commercial line in colony cages of the control environment showed a marked reduction in egg production. Whereas the commercial line outperformed the selected line in single-hen cages in both the heated and control environments prior to and during heat exposure, the opposite occurred with hens of multiplebird cages, in that the selected line performed equally well or better than the commercial line. A comparison of the selected vs the commercial lines prior to and following high temperature also showed a significant genetic stock by cage size by environmental temperature interaction for egg production (P < 0.00001, Figure 3). The selected line laid more eggs than the commercial line in multiple-hen cages, with the opposite trend occurring in single-hen cages. A comparison of the 8-d period following heat and handling with the 8-d period prior to heat and handling showed that both genetic lines in single- and multiple-hen cages subjected to a high environmental temperature had a similar reduction in rate of lay, which averaged 22%. Part of the three-way interaction was because the hens of the control environ-
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HESTER ET AL. Heated Environment Multiple-Hen Cages
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FIGURE 3. Hen-day egg production of three genetic lines of laying hens (selected, control, and commercial) housed in single- or multiple-hen cages d prior to, during (3 d), and 8 d following heat and handling exposures.
ment, which were subjected to handling but not to a high environmental temperature during that stress period, also experienced a decrease in egg production, although less dramatic, with the colony-caged hens showing a greater decrease (11.5%) than the single-caged hens (5.0%). A comparison of the selected vs the control lines prior to and following heat exposure showed a significant genetic stock by cage size by environmental temperature by time interaction for egg production (P < 0.02, Figure 3). Eight days prior to heat exposure, the control line housed in multiple-hen cages laid an average of 4.1% fewer eggs than the selected line of multiple-hen cages. Following heat exposure, the control line housed in multiple-hen cages showed a much greater decrease in egg production (11.2%) than the selected line housed in multiple-hen cages. In single-hen cages during the 8 d prior to heat exposure, the selected line laid an average of 4.2% more eggs than the control line. Following heat exposure, in which egg production was reduced by 23.6 to 25.8% in both lines, the same trend of a greater rate of lay with the selected line than by the control line still prevailed. However, the differences between the lines following heat
exposure was greater, with the selected lines laying 6.4% more eggs than the control line.
DISCUSSION Our hypothesis was that hens selected for adaptation to a multiple-bird environment should react less intensely to stressors than do the unselected controls. Turkeys selected for high and low adrenal responses during cold stress support this hypothesis in that the low corticosterone lines were less excitable and had improved production traits (Brown and Nestor, 1973, 1974). Likewise, chickens selected for a high response to adrenocorticotropin had a shorter survival time of 12 min than the low response line when subjected to acute heat stress of 45 C (Edens and Siegel, 1975; Siegel, 1981). The selected line of Leghorns of the current study showed evidence of improved adaptation to multiplehen cages when compared to the other genetic stocks. The best evidence of the selected line's resistance to stress and improved adaptability to multiple-hen cages was the survival rate under high temperature condi-
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PRODUCTION OF HENS ADAPTED TO MULTIPLE-HEN CAGES
Although social stressors, induced by a colony cage of 12 hens, were detrimental to egg production immediately after peak production relative to production of hens in single-bird cages, the rate of lay of the selected line in multiple-hen cages was not as adversely affected as it was for the other two lines (Figure 2). A comparison of the three genetic lines at 33 wk of age (Experiment 1) showed that the commercial line's rate of lay was superior to those of the selected and control lines in single-hen cages before, during, and following cold exposure; however, in multiple-hen cages during cold exposure, the selected line performed equally as well as the commercial line, whereas the rate of lay of the control line was severely depressed. The selected line's smaller rate of decrease in egg production during social interaction, handling, and cold stress showed that it reacted less intensely. Egg production at 44 wk of age provided additional evidence that the selected line of chickens reacted less adversely to multiple stressors (Experiment 2). The selected line withstood social competition better than the control line, as indicated by a higher rate of lay in multiple-hen cages at 44 wk of age (Figure 3; Muir and Liggett, 1995). The same statement can be said when making statistical comparisons of the selected line with the commercial line. However, because the commercial line of multiple-bird cages in the control environment experienced an unexplained reduction in egg production both before (61.5%) and during (59.7%) the time of handling stress as compared to their counterparts of the heated environment (74.5 vs 69.4%, respectively, Figure 3), caution should be used in making this comparison. Within single-hen cages, the commercial line continued to lay more eggs than the other two genetic lines. For the chickens of colony cages following heat and handling, the selected line did not experience as great a decrease in egg production as the control line; the rate of decrease for the commercial line was similar to that of the selected line. In conclusion, production traits support the hypothesis that hens selected for adaptation to a multiple-bird environment are more stress resistant. The selected line in multiple-hen cages, as compared to the unselected control and commercial lines of chickens, reacted less intensely to stressors, as indicated by lower mortality during heat exposure, and a smaller rate of decline in egg production in multiple-hen cages as a result of handling and exposure to temperature extremes. Of the criteria used to evaluate stress in the current study and the one dealing with physiological appraisal (Hester et al., 1996), egg production and mortality provided the best evidence that the selected line of chickens was more stress resistant than the control and commercial lines. The physiological appraisal offered some evidence of stress resistance in that the selected line did not show an increase in the heterophil to lymphocyte ratio as a result of social stress, as did the other lines of chickens at 33 wk of age. However, trends were not always consistent; the selected line did not
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tions. During the first heating episode, the selected line in multiple-hen cages was more resistant to heat than were the unselected control and commercial lines in multiple-bird cages, as indicated by a lower mortality rate. Behavioral changes of the selected line as compared to the control and commercial lines, including reduced mortality from cannibalistic pecking and improved feathering, also supports the hypothesis of improved adaptation to multiple-hen cages (Craig and Muir, 1996). A total of 57 hens died in single cages compared to 122 hens of multiple-bird cages. When expressed on a percentage basis, significantly (P < 0.02) higher mortality occurred with birds of single cages (19.8%) than with those of multiple-hen cages (14.1%). These results are perplexing because the single-caged hens should have been able to dissipate heat more easily due to more space allocation (1,085 cm 2 per single-caged bird vs 362 cm 2 per bird of multiple-hen cages). In addition, each single-caged hen had access to more drip nipple waterers on a per bird basis than the colony-caged hens. The acuteness of the first heating episode may have eliminated the advantages of less crowding and more waterers for the single-caged hens. Birds of multiple-bird cages experienced a lower incidence of frostbitten combs than hens of single cages, most likely because of their ability to transfer body heat to one another and maintain a higher core body temperature during cold exposure. The reason for the higher incidence of frostbite in the commercial stock than in the control and selected lines in multiple-bird cages is unknown, especially as a similar effect was not apparent in the single cages. Comb size was not measured; there were no apparent visual differences among the three lines in comb size at the time that cold exposure was initiated. Using egg production as a criterion, the selected line of chickens withstood social, handling, and environmental stressors better than the control line and, in some cases, the commercial line of chickens. At 33 wk of age (Experiment 1), the commercial line laid at a higher rate of lay than the selected line; however, at 44 wk of age, the selected line in multiple-hen cages was more persistent in that it laid more eggs than the commercial line in multiple-hen cages (Figure 3, Muir and Liggett, 1995). Even at 33 wk of age, when the commercial line was laying more eggs than the other two lines, the selected line seemed more able to cope with handling than the commercial and control lines, as evidenced by the sharper decline in rate of lay in these latter two groups during the handling period as compared to prior to handling (Figure 1, control environment). A combination of handling and cold was more detrimental to egg production than handling alone, suggesting that the stressors were additive (McFarlane and Curtis, 1989). The selected line reacted less intensely to handling and cold exposure, as indicated by the less severe reduction in egg production when compared to the control line of chickens (Figure 1, cold environment).
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s h o w similar leucocytic responses to cage size at 18 or 44 w k of age. It is b e c a u s e of the lack of consistency in the h e t e r o p h i l t o l y m p h o c y t e ratios a n d the fact t h a t a d r e n a l function w a s similar a m o n g the three genetic stocks u n d e r a v a r i e t y of stressful conditions (Hester et al., 1996) t h a t p r o d u c t i o n s e r v e d as a better indicator of stress resistance t h a n the physiological appraisal.
ACKNOWLEDGMENTS Technical assistance from M a r i s u e Freed, Julie L a d d , Jean Craig, Brent L a d d , D e e n a Liggett, Debbie Miles, Mollie M c C o m b , Jamie Carrigan, a n d K i m Berry w a s greatly a p p r e c i a t e d . G r a t i t u d e is also expressed to Ken W o l b e r for the m a n a g e r i a l care of the b i r d s . H a t c h i n g e g g s of the commercial strain w e r e k i n d l y d o n a t e d b y DeKalb® P o u l t r y Research, Inc., DeKalb, IL 60115.
Al-Rawi, B., J. V. Craig, and A. W. Adams, 1976. Agonistic behavior and egg production of caged layers: genetic strain and group-size effects. Poultry Sci. 55:796-807. Bhagwat, A. L., and J. V. Craig, 1977. Selecting for age at first egg: Effects on social dominance. Poultry Sci. 56:361-363. Bhagwat, A. L., and J. V. Craig, 1978. Selection for egg mass in different social environments 3. Changes in agonistic activity and social dominance. Poultry Sci. 57:883-891. Brown, K. I., and K. E. Nestor, 1973. Some physiological responses of turkeys selected for high and low adrenal response to cold stress. Poultry Sci. 52:1948-1954. Brown, K. I., and K. E. Nestor, 1974. Interrelationships of cellular physiology and endocrinology with genetics 2. Implications of selection for high and low adrenal response to stress. Poultry Sci. 53:1297-1306. Choudary, M. R., A. W. Adams, and J. V. Craig, 1972. Effects of strain, age at flock assembly, and cage arrangement on behavior and productivity in White Leghorn type chickens. Poultry Sci. 51:1943-1950. Craig, J. V., T. P. Craig, and A. D. Dayton, 1983. Fearful behavior by caged hens of two genetic stocks. Appl. Anim. Ethol. 10:263-273. Craig, J. V., and H.-Y. Lee, 1989. Research note: Genetic stocks of White Leghorn type differ in relative productivity when beaks are intact versus trimmed. Poultry Sci. 68:1720-1723. Craig, J. V., and H.-Y. Lee, 1990. Beak trimming and genetic stock effects on behavior and mortality from cannibalism in White Leghorn-type pullets. Appl. Anim. Behav. Sci. 25: 107-123. Craig, J. V., and W. M. Muir, 1996. Group selection for adaptation to multiple-hen cages: Beak-related mortality, feathering, and body weight responses. Poultry Sci. 75: 294-302. Craig, J. V., M.-L. Jan, C. R. Polley, A. L. Bhagwat, and A. D. Dayton, 1975. Changes in relative aggressiveness and social dominance associated with selection for early egg production in chickens. Poultry Sci. 54:1647-1658.
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Cunningham, D. L., and C. E. Ostrander, 1982. The effects of strain and cage shape and density on performance and tearfulness of White Leghorn layers. Poultry Sci. 61: 239-243. Edens, F. W., and H. S. Siegel, 1975. Adrenal responses in high and low ACTH response lines of chickens during acute heat stress. Gen. Comp. Endocrinol. 25:64-73. Garwood, V. A., and P. C. Lowe, 1981. A comparison of combination and family selection in chickens. Poultry Sci. 60:285-288. Griffing, B., 1967. Selection in reference to biological groups. I. Individual and group selection applied to populations of unordered group. Aust. J. Biol. Sci. 20:129-139. Grizzle, J. E., C. F. Starmer, and C. G. Koch, 1969. Analysis of categorical data by linear models. Biometrics 25:489-504. Hansen, R. S., 1976. Nervousness and hysteria of mature female chickens. Poultry Sci. 55:531-543. Hester, P. Y., W. M. Muir, J. V. Craig, and J. L. Albright, 1996. Group selection for adaptation to multiple-hen cages: Hematology and adrenal function. Poultry Sci. 75: 1295-1307. Lee, Y.-P., and J. V. Craig, 1981. Agonistic and nonagonistic behaviors of pullets of dissimilar strains of White Leghorns when kept separately and intermingled. Poultry Sci. 60:1759-1768. Lowry, D. C , and H. Abplanalp, 1972. Social dominance difference, given limited access to common food, between hens selected and unselected for increased egg production. Br. Poult. Sci. 13:365-376. McFarlane, J. M., and S. E. Curtis, 1989. Multiple concurrent stressors in chicks. 3. Effects on plasma corticosterone and the heterophihlymphocyte ratio. Poultry Sci. 68:522-527. Muir, W. M., 1996. Group selection for adaptation to multiplehen cages: Selection program and direct responses. Poultry Sci. 75:447-458. Muir, W. M., and D. Liggett, 1995. Group selection for adaptation to multiple-hen cages: Comparisons with control and commercial lines. Poultry Sci. 74:101. (Abstr.) North, M. O., and D. D. Bell, 1990. Cage management. Pages 297-370 in: Commercial Chicken Production Manual. 4th ed. Van Nostrand Reinhold, New York, NY. Ouart, M. D., and A. W. Adams, 1982a. Effects of cage design and bird density on layers. 1. Productivity, feathering, and nervousness. Poultry Sci. 61:1606-1613. Ouart, M. D., and A. W. Adams, 1982b. Effects of cage design and bird density on layers. 2. Bird movement and feeding behavior. Poultry Sci. 61:1614-1620. SAS Institute, 1990. SAS/STAT® User's Guide. Version 6, 4th ed. SAS Institute Inc., Cary, NC. Siegel, H. S., 1981. Adaptation of poultry to modern production practices. Pages 57-66 in: World Poultry Production: Where and How? C. W. Scheele and C. W. VeerKamp, ed. Spelderholt Institute for Poultry Research, Beekbergen, The Netherlands. Steel, R.G.D., and J. H. Torrie, 1980. Principles and Procedures of Statistics. A Biometrical Approach. 2nd ed. McGrawHill Book Co., Inc., New York, NY. Wilson, H. R., J. E. Jones, and R. W. Dorminey, 1967. Performance of layers under various cage regimes. Poultry Sci. 46:422^25.
{Key words: selection, adaptation, multiple-hen cages, Leghorns, production) 1996 Poultry Science 75:1308-1314
INTRODUCTION Not all strains of White Leghorns respond equally to a multiple-bird environment. For example, Cunningham and Ostrander (1982) compared two commercial strains of White Leghorns for responses to multiple bird cages. Fearfulness did not differ between the two strains, and both strains showed decreased feed usage per bird in higher densities; however, the colony size for maximum net profit differed between the strains. One strain produced higher net income with five birds per cage, whereas the other strain yielded higher income with four birds per cage. A comparison of five White Leghorn strains showed that the commercial strain with the least severe and the lowest frequency of agonistic behavior also demonstrated the highest survival percentage and the best egg production (Choudary et ah, 1972). A later study, however, showed no relationship between overt agonistic behavior and productivity of White Leghorns in multiple-hen cages (Al-Rawi et al., 1976). Other researchers have demonstrated that lines differ in their
Received for publication November 6, 1995. Accepted for publication July 1, 1996. ijournal Article Number 14,861 of the Purdue University Agricultural Research Programs, West Lafayette, IN 47907. financial support for this study was provided by USDA Award Number 58-3602-3118.
adaptability to multiple-hen cages (Wilson et ah, 1967; Choudary et al, 1972; Al-Rawi et al, 1976; Hansen, 1976; Ouart and Adams, 1982a,b; Craig et al, 1983; Craig and Lee, 1989, 1990). Although it is common practice to house commercial egg layers in multiple-hen cages, some primary breeders often select breeding stock in single-bird cages because measuring production in single-bird cages can maximize individual responses to selection, as an optimum index based on within- and between-family differences can be utilized (Garwood and Lowe, 1981). However, selection for improved individual performance may lead to a decline in group performance, especially if the selected traits also increase susceptibility to unfavorable environments (Griffing, 1967) and increase aggressiveness among cage mates (Lowry and Abplanalp, 1972; Craig et al, 1975; Bhagwat and Craig, 1977, 1978; Lee and Craig, 1981). White Leghorn strains that are high producers under less competitive situations may not be the best lines to house in multiple-hen cages. Group selection, defined as selection of birds based on group rather than individual performance, may be a more viable alternative for selecting caged breeder stock (Muir, 1996). A selected line of White Leghorns was developed by Muir (1996) that has shown improved survivability and reduced feather loss in multiple-hen cages (Craig and Muir, 1996). It was hypothesized that hens genetically selected for adaptation to a multiple-bird environment
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ABSTRACT A selected line of White Leghorns that has shown improved survivability and productivity and reduced feather loss in multiple-hen cages was evaluated for production traits under both stressed and unstressed conditions. It was hypothesized that hens selected for adaptation to multiple-bird cages would react less intensely to stressors and therefore lay more eggs and have lower mortality under stressed conditions. Three lines of chickens (selected, control, and commercial) were housed in either single-hen (1 hen) or multiple-hen cages (12 hens, social competition) at 16.7 or 17.1 wk of age. They were subsequently subjected to cold exposure at 33 wk of age and heat exposure at 44 wk of age.
PRODUCTION OF HENS ADAPTED TO MULTIPLE-HEN CAGES should react less intensely to social competition, and perhaps to all stressors in general, than the unselected controls. To test this hypothesis, the physiological response to social, thermal, and handling stress of selected, unselected control, and a commercial Leghorn line was evaluated (Hester et al., 1996). In many, but not all, instances, the selected line reacted less intensely to the stressors. Criteria for a less intensive response to stress included hemodilution (decreased packed cell volume) at the time of transfer to multiple hen cages and the lack of a leucocytic response due to social competition at 33 wk of age. The objective of the present study was to compare the production traits of a selected line of chickens developed by Muir (1996) with an unselected control line and a commercial line of Leghorns under both stressed and unstressed conditions.
Three genetic stocks of White Leghorns were compared. A line selected over seven generations for survival and hen-day egg production in multiple-bird cages was derived from and compared to an unselected control line, the North Central Randombred Control. The third stock was a commercial line of layers. The three strains of chicks were hatched on June 16, 1993 at the Purdue University Hatchery and reared in groups of 16, with others of their own strain and sex, in 61 x 91 cm wire cages. During the entire experimental period, feed and water were provided for ad libitum consumption. Beak trimming was not performed. More details on the origin of the genetic stocks, management of the birds, and assignment of the three genetic lines of pullets with respect to room, cage row within the room, and cage size (single- vs multiple-hen) are described by Craig and Muir (1996). At 16.7 to 17.1 wk of age, chickens of the three genetic lines were placed in either single-hen (1 pullet per cage, providing 1,085 cm 2 per bird) or multiple-hen (12 pullets per cage, which provided 362 cm 2 per bird) cages in one of four independently heated, ventilated, and lighted rooms of the Purdue University Layer Research Unit. Any observed effects of single- vs multiple-bird cage environments would be due to both the absence (1 bird) or presence (12 birds) of social competition as well as differences due to bird density (1,085 vs 362 cm 2 per bird), respectively. Each room contained eight rows of cages in a fourdeck, modified stair-step arrangement (North and Bell, 1990). Experimental units, consisting of either four consecutive single-bird cages or one multiple-hen cage, were repeated twice for each of the three genetic stocks within a cage row using a restricted randomization scheme. Water was provided through drip nipples with a single-caged hen having access to two drip nipples and the 12 hens of a multiple-bird cage having access to five drip nipples (Craig and Muir, 1996).
Experiment 1 When the hens were 33 wk of age, an experiment dealing with a cold environmental temperature was initiated on January 31. No attempts were made to regulate humidity. The temperature of two of the four rooms was decreased to a mean temperature of 0 C for 72 h with a range of -5 to 7 C. It took 1.5 h to lower the temperature from control levels. Relative humidity for the two colder rooms varied from 50 to 74%. The other two rooms remained at their control temperature (x = 21 C; range of 18 to 24 C) with relative humidity varying from 35 to 44%. Recording thermographs and humidigraphs monitored the environment of each of the four rooms continuously throughout the experiment. Water provided to the hens via drip nipples remained unfrozen by allowing it to flow continuously through the polyvinyl chloride pipes. Handling stress involved the collection of blood samples from paired cold and control rooms on two occasions with different hens being bled on each occasion following the initiation of the colder temperatures. Hens were bled 4 to 6 h after a temperature of 0 C was achieved on Day 1 of the experimental period. Additional hens were bled 4 to 6 h following the end of the 72 h treatment of 0 C on Day 3. Data on blood parameters are reported in the companion paper of Hester et al. (1996). The comb of each hen exposed to cold was evaluated subjectively for the presence of frostbite on Day 4.
Experiment 2 The same hens that were exposed to a 0 C temperature at 33 wk of age were subjected to a mean environmental temperature of 38 C (range of 32.5 to 41.5 C) for 3 h at 44 wk of age. Relative humidity averaged 36% during the 3 h heating episode. The same two control rooms used in Experiment 1 were also used as controls in Experiment 2. Ambient temperature of the two control rooms was maintained at 30 C with a relative humidity of 39%. Hens of the two heated rooms were bled 1 to 3 h after a mean temperature of 38 C was achieved on Day 1 of the experimental period. One-half of the control birds was bled before the birds of the heated environment were sampled, with the remaining half of the controls bled immediately following the heating episode (Hester et al., 1996). Mortality was monitored continuously throughout the 3-h heating episode. Mortality was recorded for hens housed in the bottom three cage rows. Hens that succumbed to the 38 C temperature were removed from their cages. Twelve hours following the termination of the 38 C temperature, dead birds of the bottom three cage rows were replaced with hens from the top cage rows of their respective rooms so as to maintain constant bird density within the cages throughout the study. Hens previously subjected to 38 C for 3 h were exposed to a second heating episode 24 h later on Day 2 of the experimental period. Using the same schedule as described for the first heating episode, hens of two rooms
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MATERIALS AND METHODS
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were subjected to a mean temperature of 38 C (range of 35.5 to 40 C) for 3 h. Relative humidity of the two rooms averaged 32%. The two control rooms were the same as used in the first heating episode. Mean temperature of the two control rooms was 28 C with a relative humidity of 38%. Blood samples were obtained from hens not previously bled as described for the first heating episode.
Cold Environment
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Egg Production Egg records were maintained daily. Only eggs laid 8 d prior to, during, and 8 d following the heat and cold exposures were considered. For information on annual performance of the hens, see Muir and Liggett (1995).
Statistical Analysis
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FIGURE 1. Hen-day egg production of three genetic lines of laying hens (selected, control, and commercial) 8 d prior to, during (3 d), and 8 d following cold and handling exposures.
RESULTS Experiment 1. Cold Environmental Temperatures A cold environment of O C for 72 h did not result in any hen deaths, although 15.5% of the hens experienced mildly frostbitten combs. Single-caged hens (x of 42.4 + 2.9%) experienced a higher incidence of frostbite than hens of multiple cages (x of 6.5 ± 0.8%, P < 0.0001). The percentage incidences of frostbitten combs for the selected (4.5 ± 1.2%) and control (3.8 ± 1.1%) lines housed in multiple-bird cages were significantly lower than that of the commercial line (11.1 ± 1.9%) housed in multiple-hen cages (P < 0.0001). The percentage incidence of frostbite among genetic lines in single-hen cages was similar (43.8 ± 5.1%, 42.7 ± 5.1%, and 40.6 ± 5.0% for selected, control, and commercial lines, respectively). For egg production, a significant environmental temperature by time interaction (P < 0.00001) occurred when the selected vs control lines were compared before and during exposure to cold (Figure 1). Three days of cold exposure caused a decline in egg production for both genetic lines; however, the decrease was greater in the control line (10.5%) than in the selected line (7.6%).
Although handling and bleeding the birds of the control environment also caused a decline in egg production, the decrease was much less than that which occurred when hens were exposed to both handling and cold temperature. Handling and bleeding the selected birds in the control environment caused only a 1.2% decrease in egg production, whereas the control line experienced a 3.9% decrease in egg production. A comparison of the selected and commercial lines prior to and during cold exposure also resulted in a significant environmental temperature by time interaction for egg production (P < 0.0001, Figure 1). Both the selected (7.6%) and the commercial (8.8%) lines showed the same relative decrease in egg production as a result of cold temperature. However, within the control environment, the commercial line showed a greater decrease in eggs laid (3.6%) than did the selected line (1.3%) as a result of handling and blood sampling. The environmental temperature by time interaction was nonsignificant when the genetic lines were compared before and following cold exposure (Figure 1). A comparison of the selected with the control line 8 d prior to and during cold and handling exposures showed
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The incidence of frostbitten combs during cold exposure (Experiment 1) and mortality due to heat exposure (Experiment 2) were analyzed by chi-square (Steel and Torrie, 1980). The number of eggs produced 8 d prior to, during, and 8 d following cold (Experiment 1) and heat (Experiment 2) exposure was statistically evaluated using a categorical data analysis suitable for discrete data. Specifically, the CATMOD procedure of SAS® (SAS Institute, 1990) as outlined by Grizzle et al. (1969) was used. Planned comparisons on egg production were made with the selected vs the control lines as well as the selected vs the commercial lines. Each comparison of lines was made during pre-exposure and at the time of exposure as well as before and after exposure to environmental extremes.
/
PRODUCTION OF HENS ADAPTED TO MULTIPLE-HEN CAGES Single-Hen Cages
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Although the commercial line always laid more eggs than the selected line of hens in both single- and multiple-hen cages and prior to and following exposure to cold, differences between the lines were more profound in single-hen cages before cold exposure. A comparison of the selected line with the control line prior to and following exposure to cold and handling did not show a significant interaction for cage size and genetic stock (P < 0.07).
Experiment 2. High Environmental Temperatures Time of Cold Stress
Multiple-Hen Cages
FIGURE 2. Hen-day egg production of three genetic lines of laying hens (selected, control, and commercial) housed in single- or multiplehen cages 8 d prior to, during (3 d), and 8 d following cold and handling exposures.
a cage size by genetic stock interaction for egg production (P < 0.05, Figure 2). The selected line always laid more eggs than the control line both in single- and multiple-hen cages and prior to and during exposure to cold or handling. However, within multiple-hen cages, the difference in egg production between the selected and control lines was much greater during 3 d of cold and handling stress. A comparison of the selected line with the commercial line 8 d prior to and during cold exposure showed a significant cage size by genetic stock interaction (P < 0.01, Figure 2). Eight days prior to the period of cold and handling, which was immediately after peak production, the commercial hens were laying more eggs than the selected line in both single- and multiple-hen cages. During exposure, egg production decreased; the commercial hens continued to outproduce the selected line in single-hen cages, but not in multiple-hen cages, where egg production between the two lines was similar. A comparison of the selected line with the commercial line prior to and following cold and handling exposures also showed a significant cage size by genetic stock interaction for egg production (P < 0.001, Figure 2).
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During Time of Cold Stress
Single-caged hens experienced higher mortality (x of 19.8 + 2.4%) than hens of multiple-bird cages (x of 14.1 ± 1.2%, P < 0.02). During the first heating episode, the selected line of chickens in multiple-hen cages showed an increased resistance to heat exposure, as indicated by lower mortality (9.7 ± 1.8%), when compared to the control (17.4 ±2.2%) and commercial (15.3 ±2.1%) lines housed in multiple-hen cages (P < 0.03). Single-caged hens showed similar mortality among the three genetic lines (17.7 ± 3.9%, 24.0 ±4.4%, and 17.7 ±3.9% for selected, control, and commercial lines, respectively). The second heating episode did not result in any hen mortality. Hen-day egg production 8 d prior to, during (2 d), and 8 d following heat exposure of three genetic lines of layers housed in either single- or multiple-hen cages is shown in Figure 3. A comparison of the selected and commercial lines prior to and during heat exposure showed a significant genetic stock by cage size by environmental temperature interaction (P < 0.00001). Two heating episodes of 3 h each suppressed egg production in both the selected and commercial lines of chickens; however, the decrease was greater in single-hen cages (10.0 and 12.4%, respectively) than in multiple-hen cages (7.6 and 5.1%, respectively). Although colony-caged hens of the selected and commercial lines always laid fewer eggs than the single-caged hens, the commercial line in colony cages of the control environment showed a marked reduction in egg production. Whereas the commercial line outperformed the selected line in single-hen cages in both the heated and control environments prior to and during heat exposure, the opposite occurred with hens of multiplebird cages, in that the selected line performed equally well or better than the commercial line. A comparison of the selected vs the commercial lines prior to and following high temperature also showed a significant genetic stock by cage size by environmental temperature interaction for egg production (P < 0.00001, Figure 3). The selected line laid more eggs than the commercial line in multiple-hen cages, with the opposite trend occurring in single-hen cages. A comparison of the 8-d period following heat and handling with the 8-d period prior to heat and handling showed that both genetic lines in single- and multiple-hen cages subjected to a high environmental temperature had a similar reduction in rate of lay, which averaged 22%. Part of the three-way interaction was because the hens of the control environ-
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HESTER ET AL. Heated Environment Multiple-Hen Cages
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FIGURE 3. Hen-day egg production of three genetic lines of laying hens (selected, control, and commercial) housed in single- or multiple-hen cages d prior to, during (3 d), and 8 d following heat and handling exposures.
ment, which were subjected to handling but not to a high environmental temperature during that stress period, also experienced a decrease in egg production, although less dramatic, with the colony-caged hens showing a greater decrease (11.5%) than the single-caged hens (5.0%). A comparison of the selected vs the control lines prior to and following heat exposure showed a significant genetic stock by cage size by environmental temperature by time interaction for egg production (P < 0.02, Figure 3). Eight days prior to heat exposure, the control line housed in multiple-hen cages laid an average of 4.1% fewer eggs than the selected line of multiple-hen cages. Following heat exposure, the control line housed in multiple-hen cages showed a much greater decrease in egg production (11.2%) than the selected line housed in multiple-hen cages. In single-hen cages during the 8 d prior to heat exposure, the selected line laid an average of 4.2% more eggs than the control line. Following heat exposure, in which egg production was reduced by 23.6 to 25.8% in both lines, the same trend of a greater rate of lay with the selected line than by the control line still prevailed. However, the differences between the lines following heat
exposure was greater, with the selected lines laying 6.4% more eggs than the control line.
DISCUSSION Our hypothesis was that hens selected for adaptation to a multiple-bird environment should react less intensely to stressors than do the unselected controls. Turkeys selected for high and low adrenal responses during cold stress support this hypothesis in that the low corticosterone lines were less excitable and had improved production traits (Brown and Nestor, 1973, 1974). Likewise, chickens selected for a high response to adrenocorticotropin had a shorter survival time of 12 min than the low response line when subjected to acute heat stress of 45 C (Edens and Siegel, 1975; Siegel, 1981). The selected line of Leghorns of the current study showed evidence of improved adaptation to multiplehen cages when compared to the other genetic stocks. The best evidence of the selected line's resistance to stress and improved adaptability to multiple-hen cages was the survival rate under high temperature condi-
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PRODUCTION OF HENS ADAPTED TO MULTIPLE-HEN CAGES
Although social stressors, induced by a colony cage of 12 hens, were detrimental to egg production immediately after peak production relative to production of hens in single-bird cages, the rate of lay of the selected line in multiple-hen cages was not as adversely affected as it was for the other two lines (Figure 2). A comparison of the three genetic lines at 33 wk of age (Experiment 1) showed that the commercial line's rate of lay was superior to those of the selected and control lines in single-hen cages before, during, and following cold exposure; however, in multiple-hen cages during cold exposure, the selected line performed equally as well as the commercial line, whereas the rate of lay of the control line was severely depressed. The selected line's smaller rate of decrease in egg production during social interaction, handling, and cold stress showed that it reacted less intensely. Egg production at 44 wk of age provided additional evidence that the selected line of chickens reacted less adversely to multiple stressors (Experiment 2). The selected line withstood social competition better than the control line, as indicated by a higher rate of lay in multiple-hen cages at 44 wk of age (Figure 3; Muir and Liggett, 1995). The same statement can be said when making statistical comparisons of the selected line with the commercial line. However, because the commercial line of multiple-bird cages in the control environment experienced an unexplained reduction in egg production both before (61.5%) and during (59.7%) the time of handling stress as compared to their counterparts of the heated environment (74.5 vs 69.4%, respectively, Figure 3), caution should be used in making this comparison. Within single-hen cages, the commercial line continued to lay more eggs than the other two genetic lines. For the chickens of colony cages following heat and handling, the selected line did not experience as great a decrease in egg production as the control line; the rate of decrease for the commercial line was similar to that of the selected line. In conclusion, production traits support the hypothesis that hens selected for adaptation to a multiple-bird environment are more stress resistant. The selected line in multiple-hen cages, as compared to the unselected control and commercial lines of chickens, reacted less intensely to stressors, as indicated by lower mortality during heat exposure, and a smaller rate of decline in egg production in multiple-hen cages as a result of handling and exposure to temperature extremes. Of the criteria used to evaluate stress in the current study and the one dealing with physiological appraisal (Hester et al., 1996), egg production and mortality provided the best evidence that the selected line of chickens was more stress resistant than the control and commercial lines. The physiological appraisal offered some evidence of stress resistance in that the selected line did not show an increase in the heterophil to lymphocyte ratio as a result of social stress, as did the other lines of chickens at 33 wk of age. However, trends were not always consistent; the selected line did not
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tions. During the first heating episode, the selected line in multiple-hen cages was more resistant to heat than were the unselected control and commercial lines in multiple-bird cages, as indicated by a lower mortality rate. Behavioral changes of the selected line as compared to the control and commercial lines, including reduced mortality from cannibalistic pecking and improved feathering, also supports the hypothesis of improved adaptation to multiple-hen cages (Craig and Muir, 1996). A total of 57 hens died in single cages compared to 122 hens of multiple-bird cages. When expressed on a percentage basis, significantly (P < 0.02) higher mortality occurred with birds of single cages (19.8%) than with those of multiple-hen cages (14.1%). These results are perplexing because the single-caged hens should have been able to dissipate heat more easily due to more space allocation (1,085 cm 2 per single-caged bird vs 362 cm 2 per bird of multiple-hen cages). In addition, each single-caged hen had access to more drip nipple waterers on a per bird basis than the colony-caged hens. The acuteness of the first heating episode may have eliminated the advantages of less crowding and more waterers for the single-caged hens. Birds of multiple-bird cages experienced a lower incidence of frostbitten combs than hens of single cages, most likely because of their ability to transfer body heat to one another and maintain a higher core body temperature during cold exposure. The reason for the higher incidence of frostbite in the commercial stock than in the control and selected lines in multiple-bird cages is unknown, especially as a similar effect was not apparent in the single cages. Comb size was not measured; there were no apparent visual differences among the three lines in comb size at the time that cold exposure was initiated. Using egg production as a criterion, the selected line of chickens withstood social, handling, and environmental stressors better than the control line and, in some cases, the commercial line of chickens. At 33 wk of age (Experiment 1), the commercial line laid at a higher rate of lay than the selected line; however, at 44 wk of age, the selected line in multiple-hen cages was more persistent in that it laid more eggs than the commercial line in multiple-hen cages (Figure 3, Muir and Liggett, 1995). Even at 33 wk of age, when the commercial line was laying more eggs than the other two lines, the selected line seemed more able to cope with handling than the commercial and control lines, as evidenced by the sharper decline in rate of lay in these latter two groups during the handling period as compared to prior to handling (Figure 1, control environment). A combination of handling and cold was more detrimental to egg production than handling alone, suggesting that the stressors were additive (McFarlane and Curtis, 1989). The selected line reacted less intensely to handling and cold exposure, as indicated by the less severe reduction in egg production when compared to the control line of chickens (Figure 1, cold environment).
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HESTER ET Ah.
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s h o w similar leucocytic responses to cage size at 18 or 44 w k of age. It is b e c a u s e of the lack of consistency in the h e t e r o p h i l t o l y m p h o c y t e ratios a n d the fact t h a t a d r e n a l function w a s similar a m o n g the three genetic stocks u n d e r a v a r i e t y of stressful conditions (Hester et al., 1996) t h a t p r o d u c t i o n s e r v e d as a better indicator of stress resistance t h a n the physiological appraisal.
ACKNOWLEDGMENTS Technical assistance from M a r i s u e Freed, Julie L a d d , Jean Craig, Brent L a d d , D e e n a Liggett, Debbie Miles, Mollie M c C o m b , Jamie Carrigan, a n d K i m Berry w a s greatly a p p r e c i a t e d . G r a t i t u d e is also expressed to Ken W o l b e r for the m a n a g e r i a l care of the b i r d s . H a t c h i n g e g g s of the commercial strain w e r e k i n d l y d o n a t e d b y DeKalb® P o u l t r y Research, Inc., DeKalb, IL 60115.
Al-Rawi, B., J. V. Craig, and A. W. Adams, 1976. Agonistic behavior and egg production of caged layers: genetic strain and group-size effects. Poultry Sci. 55:796-807. Bhagwat, A. L., and J. V. Craig, 1977. Selecting for age at first egg: Effects on social dominance. Poultry Sci. 56:361-363. Bhagwat, A. L., and J. V. Craig, 1978. Selection for egg mass in different social environments 3. Changes in agonistic activity and social dominance. Poultry Sci. 57:883-891. Brown, K. I., and K. E. Nestor, 1973. Some physiological responses of turkeys selected for high and low adrenal response to cold stress. Poultry Sci. 52:1948-1954. Brown, K. I., and K. E. Nestor, 1974. Interrelationships of cellular physiology and endocrinology with genetics 2. Implications of selection for high and low adrenal response to stress. Poultry Sci. 53:1297-1306. Choudary, M. R., A. W. Adams, and J. V. Craig, 1972. Effects of strain, age at flock assembly, and cage arrangement on behavior and productivity in White Leghorn type chickens. Poultry Sci. 51:1943-1950. Craig, J. V., T. P. Craig, and A. D. Dayton, 1983. Fearful behavior by caged hens of two genetic stocks. Appl. Anim. Ethol. 10:263-273. Craig, J. V., and H.-Y. Lee, 1989. Research note: Genetic stocks of White Leghorn type differ in relative productivity when beaks are intact versus trimmed. Poultry Sci. 68:1720-1723. Craig, J. V., and H.-Y. Lee, 1990. Beak trimming and genetic stock effects on behavior and mortality from cannibalism in White Leghorn-type pullets. Appl. Anim. Behav. Sci. 25: 107-123. Craig, J. V., and W. M. Muir, 1996. Group selection for adaptation to multiple-hen cages: Beak-related mortality, feathering, and body weight responses. Poultry Sci. 75: 294-302. Craig, J. V., M.-L. Jan, C. R. Polley, A. L. Bhagwat, and A. D. Dayton, 1975. Changes in relative aggressiveness and social dominance associated with selection for early egg production in chickens. Poultry Sci. 54:1647-1658.
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