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GENETICS |
The Hebrew University, Faculty of Agricultural, Food and Environmental Quality Sciences, PO Box 12, Rehovot 76100, Israel
2 Corresponding author: cahaner{at}agri.huji.ac.il
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: ascites • contemporary broiler • growth rate • selected line • slow-growing broiler
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In a pedigree population of commercial broiler lines with large genetic variation for GR, the incidence of AS (%AS) per sire family among progeny exposed to ascites-inducing conditions (AIC) was positively correlated with GR of their sibs under normal conditions (Deeb et al., 2002). Moghadam et al. (2001) also found a positive genetic correlation between the tendency of broilers to develop AS and their BW under normal conditions. Several earlier publications stating that AS develops in individuals with higher early GR were reviewed by Julian (2000). These findings led to the suggestion that further enhancement of broilers’ GR, by advanced management and also by continuous selective breeding, should be avoided because of a potential increase in the incidence of AS in contemporary broiler flocks (e.g., Julian, 1998; Moghadam et al., 2005).
The genetic association between GR and %AS was also investigated by comparing broiler lines differing in GR. In a study with males from 6 fast-growing broiler crosses and 2 lines of label-type slow-growing broilers, all reared together under a moderate cold challenge, mortality caused by AS was found only among the fast-growing broiler crosses. This correlation led the authors to suggest that AS is induced by high GR and high BW (Gonzales et al., 1998). However, there was no correlation between mean BW and %AS among the 6 fast-growing broiler crosses, although these crosses varied significantly in both traits. In addition, in other studies with broiler lines under moderate AIC, no clear association between the mean BW of the lines and %AS was found (Huchzermeyer et al., 1988; Silversides et al., 1997; Malan et al., 2003). These studies indicated that AS develops only in fast-growing broiler lines, but %AS varies significantly among lines. Wideman (1998) suggested that AS develops in broilers in which GR exceeds the rate at which their pulmonary vascular capacity increases, but these broilers do not necessarily have to be the fastest growing broilers in a flock. This claim was supported by Decuypere and Buyse (2005), who stated that AS is caused by an impaired oxygen supply that cannot sustain the rapid growth, rather than by an increased oxygen requirement (caused by rapid growth) per se.
Moreover, Pakdel et al. (2005) and Greef et al. (2001a, b) suggested that the genetic correlation between AS-related traits and GR (or the resulting BW) depends on the conditions under which broilers are reared, because only the actual GR under normal conditions reflects the genetic potential of the broilers, whereas the GR exhibited under cold conditions also depends on genetic susceptibility or resistance to AS. Therefore, in a study aimed at evaluating potential indicators for selection against susceptibility to AS in young broilers, all chicks were reared under standard conditions to d 19, and thereafter exposed to a novel protocol of high-challenge AIC that effectively induced AS in all susceptible individuals (Druyan et al., 2007a). This unique experimental procedure, based on exposing individually caged broilers to cool wind, revealed that the GR up to d 19 of broilers that later developed AS was similar to the GR of their counterparts that remained healthy, suggesting that within the population of that study (a commercial broiler line in the year 1999), high early GR was not associated with AS susceptibility. In another trial by Druyan et al. (2007b), approximately 250 broilers that later developed AS and approximately 650 that remained healthy to d 49, all reared together under moderate-challenge AIC, exhibited similar GR to the age of d 17, that is, before AS reduces the GR of the susceptible individuals.
On the basis of these results, it is hypothesized that the well-proven genetic variation in AS susceptibility within lines of fast-growing broilers (Lubritz et al., 1995; Wideman and French, 2000; de Greef et al., 2001b; Deeb et al., 2002; Pakdel et al., 2005; Druyan and Cahaner, 2007; Druyan et al., 2007a,b; Pavlidis et al., 2007) is not associated with genetic variation in GR and BW. According to this hypothesis, the continuous genetic enhancement of GR has not been increasing the proportion of AS-susceptible individuals in contemporary broiler stocks. Instead, the higher GR (whether caused by genetic selection, nutrition, etc.) and consequent higher oxygen demand triggers AS development in a larger proportion of the AS-susceptible individuals in these stocks. This hypothesis will be confirmed if AS-susceptible individuals are also found in slow-growing broiler lines, and if mean early GR is similar in the AS-susceptible vs. the AS-resistant individuals in contemporary fast-growing lines, and also in slow-growing lines.
In the present study, consisting of 2 similar trials in years 2002 and 2006, the experimental procedure of Druyan et al. (2007a, b) was used to identify the AS-susceptible individuals and to compare their early GR with that of their AS-resistant counterparts. These trials were conducted in broiler lines differing in potential GR (an unselected line derived from commercial broilers in 1986, and commercial broilers in 2002 and 2006) and in a pair of AS-susceptible and AS-resistant lines that were divergently selected from a commercial broiler line in the year 2000 (Druyan et al., 2007b).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Year 2002 Trial. The first trial was conducted in the year 2002, with the following genetic lines: the fourth generation of the AS-susceptible (AS-S) and AS-resistant (AS-R) lines, divergently selected from a commercial broiler dam-line in the year 2000 (Druyan et al., 2007b); the year 2002 broilers from a globally marketed line; and a research line that had been derived from a broiler line used commercially in the year 1986, and maintained since then without selection on GR or BW.
Year 2006 Trial. The second trial was conducted in the year 2006, with the same slow-growing research line, and the year 2006 broilers from the same globally marketed line, both as in the first trial.
Experimental Procedures
Year 2002 Trial. The trial included a total of 257 male chicks: 82 from the AS-S line, 42 from the AS-R line, 42 from the year 2002 broiler line, and 91 from the line representing 1986 commercial broilers. All the chicks were brooded together on wood shavings under standard brooding conditions (SBC). From 19 d of age, the chicks were exposed to the high-challenge AIC protocol that was used to select the lines, as described by Druyan et al. (2007a,b). Briefly, chicks were placed in individual cages with lightly cooled air blown over them by fans at approximately 3 m/s. The trial was conducted during the hot season; hence, the temperature of the cooled air ranged between 20 and 22°C. The GR of the chicks was enhanced with an accelerated feeding program consisting of crumbled prestarter feed from d 0 to 4 (instead of d 0 to 10), crumbled starter feed from d 5 to 14 (instead of d 11 to 21), crumbled grower feed from d 15 to 24 (instead of d 22 to 33) and a high-energy pelleted finisher feed from d 25 to 44. These 4 diets contained the following levels of CP (%) and energy (cal/kg of ME): 22 and 3,100, 20.5 and 3,125, 19.5 and 3,150, and 18.3 and 3,225, respectively. Feed and water were provided ad libitum. The GR was further enhanced by exposure to 23 h/d of light from hatch to the end of the trials. Broilers were kept to 42 d of age except those from the slow-growing 1986 line, which were kept to 54 d of age. The BW of each broiler was measured once a week, and the average daily weight gain (DWG) between each pair of consecutive BW measurements was calculated for each chick in every weekly interval.
Year 2006 Trial. This trial included a total of 175 male chicks: 97 from the 2006 broiler line, and 78 from the slow-growing 1986 line. All the chicks were brooded together on wood shavings under SBC. From 19 d of age, the chicks were exposed to the same high-challenge AIC protocol as in the year 2002. However, this trial was conducted during the cool season; hence, ambient temperatures were maintained at 18°C, lower than those in the year 2002 trial. The broilers were kept under those conditions up to the end of the trial, at 42 d of age for the 2006 broiler line, and at 54 d of age for the 1986 slow-growing line. The BW of each broiler was measured once a week, and the DWG between each pair of consecutive BW measurements was calculated for each chick in every weekly interval.
Diagnosis of AS
All the chicks that died from d 19 onward, throughout the phase under AIC, were necropsied and examined to determine the cause of death. Chicks with ascitic fluid or hydropericardium were diagnosed as having died because of AS and were recorded as being AS-susceptible (Sus). The few broilers that died from other causes were excluded from the data analyses. At the end of each trial, all surviving broilers were killed by cervical dislocation, necropsied, and visually examined. Broilers with ascitic fluid or hydro-pericardium were diagnosed as exhibiting AS and were also recorded as Sus; all other broilers were deemed healthy and recorded as being AS-resistant (Res).
Statistical Analyses
The data on AS mortality and morbidity were analyzed by a 
2 test to compare the incidence of Sus vs. Res broilers within the 1986, 2002, and 2006 lines, and (in the first trial) between the selected lines (AS-S vs. AS-R). In the AS-S line, data from the few Res individuals (5/82) were excluded from the following analyses and Sus individuals in the AS-R line (5/42) were also excluded.
In the broiler lines from the years 1986, 2002, and 2006, differences between Sus and Res individuals in BW and DWG data were tested by 1-way ANOVA according to the following model:
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with the fixed effect of AS (Sus or Res).
The BW and DWG data of the 2 divergently selected lines, AS-S and AS-R, were subjected to 1-way ANOVA according to the following model:
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with the fixed effect of Line (AS-S vs. AS-R). All the statistical analyses were conducted by using JMP software (SAS Institute, 2005).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Mortality and Morbidity.
By d 28, 9 d after the beginning of the AIC protocol, 8 broilers from the AS-S selected line and 1 broiler from the year 2002 line died from AS (Table 1
). During the following week, substantial AS mortality was observed only in the AS-S line, reaching 30.5% by d 35, whereas in the 1986 and 2002 lines, AS mortality was approximately 5%, and was only 2.4% (1 broiler) in the AS-R line. By d 42, the highest mortality from AS was recorded in the AS-S line (62.2%) and the lowest in the AS-R line (7.1%); the 1986 broiler line also had low AS mortality (11.0%), whereas the 2002 broiler line had intermediate mortality (21.4%). With no selection on GR for 16 yr, the healthy (Res) broilers in the 1986 line exhibited approximately 30% lower mean BW compared with the healthy (Res) 2002 broilers (1,708.2 vs. 2,447.9 g; Table 2); therefore, the broilers from the 1986 line were kept longer under the AIC and reached 24.2% AS mortality by d 54 (Table 1).
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), despite the large difference in their GR.
BW and DWG.
Table 2 presents the mean BW at several ages of AS-susceptible (Sus) and AS-resistant (Res) individuals within each of the 2 broiler lines (from year 1986 and year 2002), and the mean BW of the AS-S and AS-R lines. The starting number of individuals in each group is also presented (n); these numbers decreased from d 19 to later ages because of AS mortality under AIC in the Sus and AS-S groups, and remained unchanged in the Res and AS-R groups. Mean DWG in each age interval was calculated for the individuals that survived during that interval. Because BW and DWG values increased with age, the Sus vs. Res contrasts were also expressed as a percentage.
The mean BW of all chicks upon hatch (d 0) was higher in the 1986 line than in the 2002 line (43.1 vs. 39.7 g, respectively) because the latter were progeny of young hens. However, the Sus and Res individuals exhibited similar BW on d 0 in the 1986 broiler line (43.6 vs. 42.6 g) and in the 2002 broiler line (39.5 vs. 39.9 g; Table 2). During the following period under SBC, similar GR was exhibited by the Sus and Res broilers in the 1986 line (DWG equaled 11.1 vs. 10.9, 23.7 vs. 23.3, and 34.9 vs. 35.0 g/d in the first, second, and third week, respectively), and on d 19 the 2 groups reached a similar mean BW (Table 2). In the year 2002 broiler line, GR (mean DWG from d 0 to 7, 7 to 14, and 14 to 19) was approximately 8% higher in the Sus broilers than in their Res counterparts. Consequently, mean BW on d 19 was 8.1% higher in the Sus broilers than in the Res broilers (Table 2). The mean DWG from d 14 to 19 for the Sus and Res groups was approximately 50 g/d in the 2002 broiler line, compared with 35 g/d in the 1986 broiler line. Thus, mean GR under SBC of the broilers from the 1986 line was approximately 30% lower than that of the contemporary 2002 broilers. In the selected lines, hatching chicks from the AS-R line were heavier than the chicks from the AS-S line (52.2 vs. 49.6 g). However, mean DWG up to d 19 and mean BW on d 19 were higher in the AS-S line than in the AS-R line, but only by approximately 5% (Table 2).
Under the AIC protocol, GR after d 28, and especially after d 35, was substantially lower in the Sus and AS-S broilers, compared with their Res or AS-R counterparts (Table 2). The healthy broilers (Res groups and the AS-R selected line) exhibited the genetic potential for GR and BW after d 19 of the commercial broilers in the years 1986 and 2002, and in the year 2000 (when the AS-R selected line was initiated). The Res broilers from the 2002 broiler line and the AS-R broilers exhibited their highest GR during the fifth week, with mean DWG of 92.3 and 76.2 g/d, respectively (Table 2). The Res broilers from the 1986 line reached their highest GR (mean DWG = 65.4 g/d) during the sixth week.
On d 42, the mean BW of the year 2002 Res broilers was approximately 250 g higher than that of the year 2000-derived AS-R line. The mean BW on d 42 of the 1986 Res broilers was only 1,708.2 g, lower than that of the 2002 Res broilers by approximately 740 g. Because of the low BW on d 42 of the 1986 broilers, they were kept under AIC until d 54, when the Res broilers averaged 2,421.8 g, similar to the mean BW of the 2002 Res broilers on d 42 (Table 2).
Year 2006 Trial
Mortality and Morbidity.
By d 28, 2 broilers from the 2006 line had died from AS, whereas none from the 1986 line had died (Table 3). During the following week, AS mortality began to occur in the 1986 line (3 broilers, 3.8%) and 8 more broilers in the 2006 line died from AS, for a 10.3% cumulative mortality. By d 42, the cumulative AS mortality was 26.8% among the 2006 broilers and only 15.4% in the 1986 broiler line. In this trial, as in the year 2002 trial, the broilers from the 1986 line were kept longer under the AIC, to 54 d of age, when they reached a mean final BW closer to that of the 2006 broilers at 42 d of age. By then (d 54), a total of 15 broilers (19.2%) in the 1986 line had died because of AS, and 11 more were diagnosed with AS. Thus, the incidence of AS mortality and morbidity in the 1986 line accumulated to 33.3%. In the 2006 broiler line, 20 broilers were diagnosed with AS on d 42; thus, the AS mortality and morbidity in this line accumulated to 47.4% (Table 3).
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presents the means of BW and DWG in the year 2006 trial, but it has the same structure and content as Table 2. The mean BW of all chicks upon hatch (d 0) was higher in the 2006 contemporary broiler line than in the 1986 line (43.35 vs. 37.1 g, respectively) because the latter were progeny of very young hens. However, the Sus and Res individuals exhibited similar BW on d 0 in the 1986 broiler line (36.6 and 37.8 g) and in the 2006 broiler line (43.4 and 43.3 g; Table 4). During the following 19 d under SBC, similar GR was exhibited by the Sus and Res broilers in the year 1986 line, and on d 19 the 2 groups exhibited similar mean BW (362.6 and 378.7 g). Also in the 2006 broiler line, similar mean GR was exhibited by the Sus and Res broilers under SBC, and on d 19 the 2 groups exhibited similar mean BW (675.9 and 676.1 g; Table 4). As expected, 2006 contemporary broilers exhibited much higher GR under SBC than the broilers from the 1986 broiler line, resulting in mean BW on d 19 of 676 g vs. approximately 370 g, respectively.
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). The groups of healthy (Res) broilers supposedly exhibited the genetic potential for GR and BW after d 19 of commercial broilers in the years 1986 and 2006. The 2006 Res broilers exhibited their highest GR during the fifth week, with mean DWG of 80 g/d, whereas the Res broilers from the 1986 line reached their highest mean DWG (57 g/d) during the sixth week (Table 4).
On d 42, mean BW of the 1986 Res broilers was only 1,481.5 g (Table 4), approximately 860 g lower than the mean BW (2350.4 g) of the year 2006 Res broilers. Because of this 63% difference, reflecting almost 2 decades without selection on BW in the 1986 line, the broilers from this line were kept under AIC until d 54 (as in the year 2002 trial) to allow them to gain more BW. Mean BW of the Res broilers from the 1986 line was 2,188.6 g on d 54, only approximately 160 g less than the mean BW of the Res broilers from the 2006 line on d 42 (Table 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The second hypothesis, suggested by Druyan at al. (2007b) based on data from that study and on several previous reports (Wideman, 1998; Wideman and French, 2000; Decuypere and Buyse, 2005), asserts that the tendency to develop AS is associated with high actual GR only because the latter increases oxygen demand in the genetically susceptible individuals above a threshold reflecting their inherent lower capacity for oxygen supply. This hypothesis is in agreement with several studies (recently reviewed by Druyan and Cahaner, 2007) suggesting that the tendency to develop AS is controlled by a few major genes, and that these genes are independent of the genes that control GR; hence, there is no "true" (i.e., symmetrical) genetic correlation between AS and GR. According to this hypothesis, there should be high-GR broilers that do not develop AS despite their high oxygen demand, because they are genetically resistant. Similarly, there should be broilers with genetically low-GR yet that are susceptible to AS, although they require special environmental conditions to express this susceptibility.
To test these 2 alternative hypotheses, the present study consisted of 2 similar trials in the years 2002 and 2006, in which a high-challenge protocol of AIC was used to identify the AS-susceptible individuals and to compare their GR to that of their AS-resistant counterparts. These studies were conducted in broiler lines that differed in their genetic potential for GR. The contemporary fast-growing commercial broilers in the years 2002 and 2006 were tested in the respective years, along with a line that was derived from a commercial broiler line in the year 1986 and kept since then without selection on GR or BW. The trial in the year 2002 also included the AS-S and AS-R lines, which were divergently selected from a year 2000 commercial broiler line (Druyan et al., 2007b).
Mean GR under SBC and mean BW on d 19 were approximately 5% higher in the chicks from the AS-S line than in those from the AS-R line (Table 2). A difference in GR and BW (under SBC) was also found between AS-susceptible (SUS) and AS-resistant (RES) lines that were divergently selected on AS mortality under hypobaric conditions simulating an altitude of 2,900 m above sea level (Pavlidis et al., 2007). The higher GR in AS-susceptible lines than in AS-resistant lines found in these 2 similar yet independent studies may appear to support the hypothesis that AS susceptibility and resistance are genetically associated with higher and lower GR, respectively. However, it was suggested by Druyan et al. (2007b) that the difference in mean BW between the AS-S and AS-R lines could be related to the relatively low-challenge AIC under which the initial cycle of their divergent selection was conducted. The overall AS mortality in the base population (generation S0) was only 17.2%, substantially less than the 44% AS in a previous study with the same commercial line under high-challenge AIC (Druyan et al., 2007a). The high heritability of AS in the latter study suggested that the 44% of individuals that developed AS were genetically susceptible. Thus, the lower incidence of AS in the S0 population suggests that the environmental conditions during the first cycle of the divergent selection did not induce AS in all AS-susceptible broilers. Instead, AS developed only in the AS-susceptible broilers with genetically higher GR (hence higher oxygen demand), whereas the AS-susceptible broilers with genetically lower GR managed to supply their inherent lower oxygen demand, and hence did not develop AS.
With this GR-dependent differential genotype-to-phenotype expression, the observed incidence of AS per family (%ASF) was apparently biased downward in slow-growing families, and upward in fast-growing families. Consequently, the divergent selection on %ASF under these conditions also resulted in a correlated divergence of 6.5% in GR (Druyan et al., 2007b), but not because of a true genetic association between high GR and AS. This conclusion was supported by the results of the following selection cycles in the AS-S and AS-R lines. The divergence in %AS increased from 37% in S1 (Druyan et al., 2007b) up to 84.4% (93.9% vs. 9.5%) in S4 (Table 1), whereas the difference in GR and BW on d 19 between the 2 selected lines in S4 was only 5% (Table 2), similar to the 6.5% difference in BW on d 17 and 28 between the high-%ASF and low-%ASF families that were selected from the S0 population to establish the AS-S and AS-R lines, respectively (Druyan et al., 2007b). Thus, the increasing divergence in %AS between the selected lines, from the S1 to S4 generation (the present study), had no correlated effect on their divergence in GR, probably because all the selection cycles from S1 to S4 were conducted under a highly efficient AIC that also induced AS in the AS-susceptible individuals with inherent low GR.
Moreover, from the data presented by Pavlidis et al. (2007), it appears that their divergent lines exhibited similar GR and BW under normal conditions up to generation S7, whereas the divergence in %AS between the SUS and RES lines increased to approximately 60% (90 vs. 30%, respectively). These results indicate that the response to selection on AS under the high-challenge hypobaric conditions was not associated with correlated divergence in GR. Only the selection cycle between S7 and S8 resulted in a significant divergence in GR between the SUS and RES lines. In addition, in the last 2 selection cycles in that study (S8 to S9 and S9 to S10), the difference in BW between the SUS and RES lines did not increase, thus further supporting the hypothesis that there is no true genetic correlation between the propensity of broilers to develop AS and their potential GR and BW.
The testing of the hypotheses regarding an inherent association between AS and the genetic potential for high GR was broadened in the present study by including contemporary commercial broilers in the years 2002 and 2006, and an experimental low-GR slow-growing line. All the lines were tested under the same experimental protocol that allowed measurement of GR under SBC up to d 19, and then efficiently distinguished between the AS-susceptible individuals and the AS-resistant ones—those that remained healthy under the same high-challenge AIC. The 47% AS among the year 2006 broilers (Table 3) was similar to %AS among fast-growing commercial broilers in another trial with the same experimental protocol (Druyan et al., 2007a) and in several other studies (e.g., (Lubritz et al., 1995; Wideman and French, 2000; Hadad et al., 2006), suggesting that the majority (if not all) of the genetically AS-susceptible contemporary broilers developed AS in the year 2006 trial. The Sus and Res individuals had similar mean BW and GR (DWG) from hatch to d 19 under standard conditions (Table 4), in agreement with the hypothesis that genetic susceptibility to AS is not associated with higher GR.
Yet in the trial with year 2002 broilers, mean BW and GR (DWG) from hatch to d 19 were approximately 8% higher in the AS-susceptible (Sus) than in the AS-resistant (Res) individuals (Table 2). However, %AS among the contemporary year 2002 commercial broilers was only 31% (Table 1) compared with 47% in the 2006 trial (Table 3). In the 2002 trial, because of high outside temperatures, the inside ambient temperatures ranged between 20 to 22°C, higher than the target of 18°C that was successfully maintained in the 2006 trial. It appears that because of less challenging AIC, the AS-susceptible individuals with the lowest GR did not develop AS, hence the lower %AS. The inclusion of these low-GR individuals in the Res group reduced its mean BW and GR, leading to the observed difference of approximately 8% (Table 2). Deeb et al. (2002) studied the association between AS susceptibility and GR; they induced AS by moving 37-d-old pedigreed commercial broilers to individual cages under cold conditions, yet only 18% of them developed AS, probably because the cold challenge started too late. It appears that also in this study, the heavier susceptible individuals developed AS despite the late challenge, but the small-body (i.e., low-GR) susceptible individuals escaped the challenge and remained healthy, leading the authors to the erroneous conclusion that AS susceptibility was genetically correlated with high GR.
In the present study, in both the 2002 and 2006 trials, AS was developed in approximately 32% of the individuals in the 1986 experimental line, despite the low GR of the line, which equaled the genetic potential of commercial broilers in the mid-1980s. Already in the 1970s, AS was found in contemporary broiler populations, but only when reared at high altitude (reviewed by Huchzermeyer et al., 1988). In the present study, AS was induced in slow-growing broilers under low altitude by enhancing energy lost (housing in individual cages under cool wind) and enhancing metabolism (high-energy pelleted feed). The current results, as well as those from the 1970s and 1980s reports on high-altitude studies, clearly indicate that AS-susceptible individuals existed in the broiler populations of 20 to 30 yr ago. This indication further supports the hypothesis that genetic variation in GR is not correlated with genetic variation in susceptibility or resistance to AS.
In summary, the results of the present study, supported by several previous ones, suggest that there is no true genetic correlation between the potential GR of broilers and their propensity to develop AS. In view of this conclusion, it is suggested that breeding against AS susceptibility should not be aimed at selecting for reduced GR, but rather at identifying and eliminating all the AS-susceptible individuals in the selected population and selecting for high GR among the AS-resistant ones. To accurately identify all AS-susceptible individuals, including those with lower GR that usually remain healthy and hence are wrongly selected as AS-resistant, the selection must be conducted under an effective high-challenge system, preferably in individual cages that ensure uniform exposure to the AIC. In the future, once major genes that control the AS-related genetic variation (Wideman and French, 2000; Druyan and Cahaner, 2007) are found and sequenced, a molecular identification of all AS-susceptible individuals will be possible, independent of their potential or actual GR.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received for publication January 2, 2008. Accepted for publication January 30, 2008.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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