broiler population segregating for the Dw gene and, thus, consisting of normal-sized .... BT, for which differences of less than 0.5 C could be crucial, despite their ...
Genotype-by-Environment Interaction with Broiler Genotypes Differing in Growth Rate: 2. The Effects of High Ambient Temperature on Dwarf Versus Normal Broilers N. Deeb and A. Cahaner1 The Hebrew University of Jerusalem, Faculty of Agricultural, Food and Environmental Quality Sciences, Rehovot 76100, Israel ABSTRACT High ambient temperatures (AT) reduce feed consumption (FC) and BW in broilers, thereby leading to lower efficiency and profitability of poultry meat production in hot climates. These negative effects have been found to be more pronounced in chicken lines with high BW. The effects of high AT were investigated in a broiler population segregating for the Dw gene and, thus, consisting of normal-sized and dwarf broilers, which differed markedly in BW but had the same genetic background. All chicks were reared under normal AT (constant 22 C) up to Day 44, when AT was gradually raised, over 24 h, to 32 C and then held constant to Day 49. The dwarf chicks had 23% lower BW and BW gain (WG) at all ages until Day 44. During the first 24 h at 32 C, FC of the dwarf broilers was reduced by 35% and their average WG was 7.6 g/d, whereas FC of their normal-sized coun-
terparts was reduced by 46% and they lost BW (average WG: −42.1 g/d). Thereafter, however, the dwarf and normal broilers adapted similarly to the chronic heat stress, with FC and WG of both phenotypes averaging 72 and 35%, respectively, of the corresponding means at normal AT. The two phenotypes had similar body temperatures at normal AT, but following its increase, body temperature in the normal-sized broilers rose by 1.14 C, whereas in the dwarf ones it rose by only 0.47 C. This finding suggested better thermoregulation during acute heat stress, apparently due to the latter’s smaller body size. Under chronic heat stress, however, FC and WG were similarly reduced in the dwarf and normal broilers. We concluded that the dwarf gene has no value with regards to broiler tolerance to chronic heat stress, either for production or as a model.
(Key words: broiler, dwarf gene, heat stress, body weight gain, body temperature) 2001 Poultry Science 80:541–548
their overall genetic background and breeding history—the traits they were selected for and the intensity, duration, and conditions of selection. Yalcin et al. (1997) found that commercial broilers from three breeding companies differed significantly in their performances under hot, summer climate, despite their similar GR in a temperate spring climate, and suggested an effect of breeding under different climatic conditions. Deeb and Cahaner (2001) tested three groups of broilers at normal and high AT. The heat-induced reduction in GR and meat yield was highest in the progeny of hens from a commercial paternal line characterized by high GR and breast meat yield. The two other groups were progeny of hens from a commercial maternal line with somewhat lower GR and meat yield and from a GRrelaxed line derived from that maternal line, hence, with
INTRODUCTION High ambient temperatures (AT) reduce feed consumption (FC) and growth rates (GR) of broilers, thereby increasing the time needed to reach marketing weight and leading to lower efficiency and profitability of poultry meat production in hot climates (Geraert et al., 1996). These negative effects of high AT have been found to be more pronounced in chickens with higher BW and more rapid GR than in those with lower BW and GR (Emmans and Kyriazakis, 2000). Evidence for this association consists of comparisons between commercial broiler lines selected for rapid GR vs. relaxed broiler lines (Cahaner and Leenstra, 1992; Yunis and Cahaner, 1999) and broilers’ paternal vs. maternal lines (Deeb and Cahaner, 2001). However, the lines compared in each of these studies differed not only in their GR but also in
Abbreviation Key: AT = ambient temperature; BT = body temperature; %Change = percentage of change in trait’s mean due to increase in AT; Diff = difference between means of dwarf vs. normal broilers; %Diff = Difference as percentage of normal broilers’ mean; FC = feed consumption; FE = feed efficiency; GR = growth rate; MBW = metabolic BW; WG = weight gain.
2001 Poultry Science Association, Inc. Received for publication April 20, 2000. Accepted for publication December 18, 2000. 1 To whom correspondence should be addressed: cahaner@agri. huji.ac.il.
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even lower GR but similar meat yield. The broilers in the latter two groups were similarly affected by high AT, despite their different GR, possibly due to their similar overall genetic background. The authors concluded that the magnitude of the heat effect on broiler performance may depend not only on GR but also on differences between stocks in overall genetic background, as a result of different emphases during their breeding history. Therefore, the apparent conclusion that chickens become more sensitive to heat simply as their GR and BW increase is questionable because in all the reported studies, chicken lines compared for their responses to high AT had different genetic backgrounds. Rather, they appear to differ in many polygenes, typically accumulated during multi-generational selection for high GR in some lines, as compared to relaxed selection for GR in the other lines (Cahaner and Leenstra, 1992; Dunnington and Siegel, 1996; Cahaner et al., 1999; Yunis and Cahaner, 1999). However, GR in chickens is also strongly affected by the sex-linked recessive dwarf allele (Dw), first described by Hutt (1959). The GR (and consequently BW at any given age) of dw/dw males and dw/− females is about 30% lower than their Dw/Dw, Dw/dw and Dw/− counterparts, of equal genetic backgrounds (Guillaume, 1976). Because higher GR and BW of chickens make them more sensitive to high AT, it has been suggested that dwarf chickens should have inherent heat tolerance (Horst and Mathur, 1990). In some studies, egg production of dwarf hens is less depressed by high AT than that of normal hens, but other studies have not confirmed this conclusion (for comprehensive review see Merat, 1984, 1990; Decuypere et al., 1991). As for meat-type chickens, there are no published reports regarding the effect of high AT on dwarf vs. normal broilers at ages relevant to broiler production. In the present study, a broiler population segregating for the Dw gene, and therefore consisting of normal-size and dwarf broilers with the same genetic background, was utilized to evaluate the net effects of low vs. high GR on broiler response to high AT.
MATERIALS AND METHODS
the chicks (about 70 and 50 normal and dwarf broilers, respectively) were reared in a single deep-litter pen from hatch to 3 wk of age under a standard brooding regime, with AT gradually decreasing from 35 to 24 C. At 3 wk of age, the chicks were randomly assigned to individual cages in a temperature-controlled chamber, where AT was set to a constant 22 ± 1 C. On Day 44, AT was gradually raised, over 24 h, to 32 ± 1 C and then held constant to Day 49, when the experiment was terminated. Thus, the postbrooding growing period was actually divided into three phases according to AT: normal AT (constant 22 C; Days 21 to 44), increasing AT (changing from 22 to 32 C; Days 44 to 45), and high AT (constant 32 C; Days 44 to 49). A standard feeding program with commercial diets was used (21 and 18% CP, 3,200 and 3,300 cal/g ME from Days 1 to 21 and Days 21 to 49, respectively). Continuous light and feed and water were provided ad libitum throughout the experiment.
Measurements The BW of each chick and the weight of its individual feed trough were recorded at 21, 28, 35, 42, 44, 45, 46, 47, and 49 d of age. Average daily BW gain (WG), average daily FC and feed efficiency (FE = WG/FC) were calculated for each age interval. Rectal temperature, as an expression of body temperature (BT), was measured three times during the normalAT phase (Days 39, 42, and 43) and twice during the high-AT phase (Days 46 and 47). The BT was measured using a digital thermometer (Sika TT-7070; ± 0.1 C)3 with a 7-cm insertion probe and was recorded when the reading was stable for 15 s (Deeb and Cahaner, 1999).
Statistical Analysis Data within each AT phase were subjected to twoway ANOVA using JMP威 (SAS Institute, 1995), with Dw phenotype (dwarf vs normal body size) and sex (male vs female) as main effects, and their interaction, according to the model Y = µ + Dw + sex + Dw × sex + e
Stock, Temperature, and Husbandry Hemizygous dwarf females (dw/−) and heterozygous males (Dw/dw) were mated and produced chicks segregating for four dw genotypes and two body-size phenotypes: Dw/− (normal-sized females), dw/− (dwarf females), Dw/dw (normal-sized males) and dw/dw (dwarf males). Based on visual appraisal, especially by shank length, the chicks were genotyped at several ages, and only those having a consistent definitive normal or dwarf phenotype were included in the study. All of
3 Siebert and Kuhn GmbH and Co., PO Box 1113-34254, Kaufungen, Germany.
where Y is the dependent variable (BW, WG, FC, FE, or BT), µ is the grand mean, and e is the random residual error term.
RESULTS As expected, sex significantly affected most traits but did not interact with the Dw phenotypes, hence means over sexes are presented in all tables and figures. No mortality was observed during the experimental period. The change in each trait due to the increase in AT from normal to high was calculated from the difference in the phenotypic mean for each high-AT period and the normal AT period (Days 42 to 44), expressed as percent-
EFFECTS OF HEAT STRESS ON DWARF VS. NORMAL BROILERS
FIGURE 1. Effect of ambient temperature (AT) on growth and BW of normal and dwarf broilers, reared under constant 22 C up to Day 44, when AT was gradually raised, over 24 h (Days 44 to 45), to 32 C, and held constant thereafter (Days 45 to 49).
age of the latter (%Change). The difference between means of the two phenotypes (Diff) was calculated for each period and also was expressed as percentage of the mean of normal sized broilers (%Diff). The expressions %Diff and %Change were calculated for all traits except BT, for which differences of less than 0.5 C could be crucial, despite their negligible magnitude on a percentage scale.
Normal AT Phase (Days 21 to 44) The dwarf and normal chicks differed significantly in BW at all ages (Figure 1), with the difference increasing from 107 g at 21 d of age to 457 g (P < 0.001) on Day 44 (Table 1). However, the %Diff in BW between the Dw phenotypes remained approximately 23% during the entire normal AT phase (Table 1). The SD of BW also increased with age in both phenotypes, maintaining a similar magnitude relative to the mean (11 to 12% in the normal birds and 13% in the dwarf ones). The differences in BW between the normal chicks and their dwarf sibs resulted from their different GR, expressed as weekly averages of daily WG at each age interval during the normal AT phase (Figure 2a). During the last 2 d of this phase (Days 42 to 44), WG was 54.2 and 39.8 g/d in the normal and dwarf chicks, respectively, a highly significant difference of 26.6% (Table 1). The FC increased with age and was higher in the normal chicks than in their dwarf sibs under the normal AT (Figure 2b). At the end of this phase (Days 42 to 44), FC of the normal broilers was 32 g/d higher than that of their dwarf sibs (Table 1), a highly significant difference of about 20%. As expected, FE decreased with age in both phenotypes (Figure 2c), from about 0.5 during Days 21 to 28, to about 0.3 at the end of this phase (Days 42 to 44), with only a small, nonsignificant difference between the two phenotypes (Table 1).
Increasing AT Phase (Day 44 to 45) During this 24-h phase, AT was gradually raised, starting from 22 C immediately after BW and feed trough
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weights were recorded on Day 44, and reaching 32 C exactly 24 h later, when BW and feed trough weights were recorded for Day 45. Mean BW of both groups continued to increase from Days 44 to 45 (Figure 1), as WG during this phase was only slightly lower than on Days 42 to 44 (Table 1 and Figure 2a). However, the variation in WG among chicks within each phenotype increased substantially, with SD values of WG for Days 44 to 45 being two to three times larger than those of WG for Days 42 to 44 (Table 1). Due to this higher variation, the 24.8% difference in WG for Days 44 to 45 between the normal and dwarf broilers was not significant (Table 1). Means, and also SD of FC during this phase were very similar to those of the previous period, whereas SD of FE, like that of WG, was more than twofold larger than in the normal AT phase (Table 1). As on Days 42 to 44, the two phenotypes did not differ significantly in mean FE for Days 44 to 45.
High AT Phase (Days 45 to 49) In Figure 2 and Table 1, results of this phase are divided into three 1-d or 2-d periods (Days 45 to 46, Days 46 to 47, and Days 47 to 49). Means of all traits changed drastically, mainly during the first 24 h after AT reached 32 C (Days 45 to 46), representing typical acute heat stress. The means gradually increased over the following 3 d (Days 46 to 47 and Days 47 to 49), apparently because the birds gradually adapted to the high AT, as happens under chronic heat stress. Acute Heat Stress (Days 45 to 46). Relative to the normal AT period (Days 42 to 44), FC was drastically reduced in both phenotypes, but the reduction was significantly larger in the normal birds (by 46.3%) than in their dwarf counterparts (by 34.6%), hence the two phenotypes had very similar FC for Days 45 to 46 (Figure 2), despite the 23.3% difference in their BW (Table 1). The reduction in FC was accompanied by a more drastic reduction in WG, and therefore FE was also much lower. In the dwarf broilers, WG for Days 45 to 46 averaged 7.6 g/d compared to 39.8 g/d during the normal AT period, 2 d earlier. The normal broilers were more severely affected by the acute heat. Many of them lost BW during this 24-h period, hence their WG for Days 45 to 46 averaged −42.1 g/d, compared to +54.2 g/d in the previous normal AT period (Table 1). Despite the reduction in BW of the normal birds, they were still heavier than the dwarf birds on Day 46 (Figure 1) but by only 21.3%, as compared to 23.3% on Days 44 and 45 (Table 1). Chronic Heat Stress (Days 46 to 49). These 3 d were characterized by the apparent gradual adaptation of the broilers to the chronic heat stress, which was reflected by increasing FC, WG, and FE (Figure 2, Table 1). The FC means on Days 47 to 49 of the normal and dwarf broilers were 118 and 94 g/d, respectively, which was 72% of the corresponding means at the end of the normal AT phase (Days 42 to 44) in both phenotypes (Table 1). For WG, the effect of chronic high AT was similar in both phenotypes as well, with WG for Days 47 to 49
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being 65% of the WG for Days 42 to 44. Mean FE for Days 47 to 49 was slightly lower than FE for Days 42 to 44 in each phenotype, but Figure 2c suggests that this reduction in FE reflects the expected decline in FE due to age, rather than due to the high AT, and that this effect was similar in both phenotypes. For all three traits, the difference between the two phenotypes (Diff and %Diff) under constant high AT (Days 47 to 49) was equal to that at the end of the normal AT phase (Days 42 to 44), indicating that the two body-size phenotypes were similarly affected by the chronic heat stress (Table 1).
DISCUSSION Dwarf vs. Normal Broilers Under Normal AT Previously reported effects of the Dw gene were studied with egg-type (hence slow-growing) chickens or broiler stocks with very low GR, such as Athens-Canadian Randombred (Hess, 1962). Therefore, the results in those reports may not be comparable to those from the fast-growing broilers used in the present study. Under normal AT, mean BW of the dwarf broilers in the present study was about 23% lower than the mean BW of their normal-sized sibs, whereas larger differences (25 to 29%) were found in the Athens-Canadian Randombred stock at similar ages (Marks, 1980). However, BW of the normal (nondwarf) chickens at about 6 wk of age in the aforementioned study was three times lower than that of their counterparts in the present study. Indeed, a series of studies showed that the effect of the dwarfing gene on BW is lower in chickens with more rapid GR (Mohammadian and Jaap, 1972; Khan et al., 1975; Reddy and Siegel, 1977; Cherry and Siegel, 1978, 1979, 1981; Marks, 1987; Decuypere et al., 1991). Mean FE of the dwarf chicks during the normal AT phase was similar to that of the normal-sized broilers, in agreement with previous reports that the Dw gene does not affect FE (Marks, 1980; Zulkifli et al., 1993). The two Dw pheno-
Body Temperature Body temperature was measured three times during the normal AT phase; similar values were found on Days 39, 42, and 43, averaging 41.4 C for both phenotypes (Table 2). The SD values were also similar in the dwarf and normal broilers. During the high-AT phase (Days 46 and 47), BT averaged 41.87 C in the dwarf broilers and 42.54 C in the normal broilers. The highly significant difference of 0.67 C between the two Dw phenotypes resulted from a much larger effect of high AT on the BT of normal vs. dwarf broilers. The BT of the normal-sized broilers increased by 1.14 C, whereas that of the dwarf birds increased by only 0.47 C. The SD of BT under high AT was also higher in the normal broilers than in the dwarf ones (Table 2).
TABLE 1. Means and SD of BW, BW gain (WG), feed consumption (FC), and feed efficiency (FE) of dwarf and normal-sized broilers at three ambient temperatures (AT): normal AT (22 C, Days 42 to 44), increasing AT (22 to 32 C, Days 44 to 45), and high AT (32 C, Days 45 to 49) Mean
%Change1
SD
Dwarf-Normal
AT (C)
Age (d)
Dwarf
Normal
Dwarf
Normal
Dwarf
Normal
Diff2
%Diff3
BW (g)
22 22 22 to 32 32 32 32
42 44 45 46 47 49
1,426 1,506 1,543 1,551 1,563 1,614
1,853 1,963 2,013 1,971 1,988 2,060
179 195 220 230 242 212
211 231 227 208 208 207
. . . . . .
. . . . . .
−427*** −457*** −470*** −420*** −425*** −446***
−23.0 −23.3 −23.3 −21.3 −21.4 −21.7
WG (g/d)
22 22 to 32 32 32 32 22
42–44 44–45 45–46 46–47 47–49 42–44
39.8 37.9 7.6 11.7 25.8 130
54.2 50.4 −42.1 17.5 35.6 162
13 33 26 24 13 26
22 48 60 30 20 22
... −4.8 −80.9 −70.6 −35.2 ...
... −7.0 −177.7 −67.7 −34.3 ...
−14.4*** −12.5 49.7*** −5.8 −9.8** −32***
−26.6 −24.8 118.1 −33.1 −27.5 −19.8
22 to 32 32 32 32 22 22 to 32
44–45 45–46 46–47 47–49 42–44 44–45
128 85 79 94 0.31 0.30
163 87 92 118 0.33 0.31
24 16 17 15 0.06 0.17
25 21 25 19 0.09 0.18
−1.5 −34.6 −39.2 −27.7 ... −3.3
0.6 −46.3 −43.2 −27.2 ... −7.8
−35*** −2 −13** −24*** −0.03 −0.01
−21.5 −2.3 −14.1 −20.3 −8.7 −4.2
32 32 32
45–46 46–47 47–49
0.09 0.15 0.27
−0.48 0.19 0.30
0.32 0.27 0.13
0.44 0.26 0.15
−70.9 −51.6 −11.4
−244.5 −43.3 −9.9
Variable
FC (g/d)
FE (g/g)
1
Deviation in percentage from the normal AT phase (Days 42 to 44). Dwarf − normal. 3 100 × (dwarf − normal)/normal. **0.001 < P(F) < 0.01. ***P(F) < 0.001. 2
. . . . . .
. . . . . .
. . . . . .
. . . . . .
0.57*** −0.04 −0.03
118.4 −22.1 −10.3
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EFFECTS OF HEAT STRESS ON DWARF VS. NORMAL BROILERS
types also did not differ significantly in BT during the normal AT phase, despite the 23% differences in WG and BW, presumably due to natural selection for optimal BT, independent of body mass (Dunnington and Siegel, 1984).
Dwarf vs. Normal Broilers Under Acute High AT
FIGURE 2. Effects of ambient temperature (AT) on BW gain (a), feed consumption (b), and feed efficiency (c) of normal and dwarf broilers, reared under constant 22 C up to Day 44, when AT was gradually raised, over 24 h (Days 44 to 45), to 32 C, and held constant thereafter (Days 45 to 49).
After the elevation in AT (from 24 to 32 C within 24 h, Day 44 to 45), broiler WG, FC, and FE during the acute heat stress (Day 45 to 46) was much reduced in both Dw phenotypes (Figure 2). However, the reduction was more drastic in the normal-sized broilers than in their dwarf siblings. Horst and Mathur (1990) suggested that the dwarf gene provides the inherent ability to more efficiently utilize nutrients under high AT by reducing the energy requirements for maintenance, thus leading to superior production under such conditions. This suggestion was confirmed by the results obtained during the acute heat stress period in the present study. The two Dw phenotypes consumed similar amounts of feed on Days 45 to 46, but only the dwarf broilers were able to maintain their BW—and even increase it—whereas the same amount of feed was not sufficient for the normal-sized broilers to regulate their BT, thus leading to a reduction in their mean BW. These results illustrate the high energetic efforts invested by normal broilers in facilitating the increase in their BT in order to survive the heat stress. The higher thermoregulatory efficiency of the dwarf broilers, apparently due to their lower BW, was more evident with the BT data. With the change from normal to high AT, mean BT of the dwarf broilers increased by only 0.47 C, as compared to the 1.14 C increase in mean BT of the normal-sized broilers. The difference in BT between the two Dw phenotypes under high AT was similar to the differences found by Touchburn et al. (1980) in a warm environment with a highprotein diet.
TABLE 2. Means and SD of body temperature (BT) of dwarf and normal broilers, measured during normal ambient temperature (AT) (22 C, Days 39, 42, and 43) and high AT (32 C, Days 46 and 47)
Variable BT (C)
BT change (C)3
AT (C)
Age (d)
22 22 22 22 32 32 32 22 to 32
39 42 43 Mean2 46 47 Mean2
Mean Dwarf
Normal
Dwarf
Normal
Diff1
41.37 41.41 41.40 41.40 41.81 41.93 41.87 0.48
41.40 41.40 41.39 41.40 42.47 42.62 42.54 1.15
0.18 0.21 0.24 0.17 0.45 0.44 0.43 0.45
0.20 0.19 0.29 0.18 0.57 0.64 0.58 0.59
−0.03 0.01 0.01 0.00 −0.66*** −0.69*** −0.66*** −0.67***
Dwarf − normal. Average BT. 3 [Average BT (Days 46, 47)] − [average BT (Days 39, 42, 43)]. ***P(F) < 0.001. 1 2
SD
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Dwarf vs. Normal Broilers Under Chronic High AT After losing BW during acute heat stress (Day 45 to 46), the normal-sized broilers apparently managed to partially adapt to the high AT. On Days 47 to 49, WG of the normal broilers averaged 35.6 g/d, about 35% below their WG under normal AT. Similar differences were found in the WG of normal commercial broilers in temperate (spring or fall) vs. hot (summer) seasons in Turkey (Yalcin et al., 1997; Settar et al., 1999). However, despite the lower BT of the dwarf vs. normal-sized broilers during this phase, the relative differences in WG, FC, and FE between the two Dw phenotypes in the last growing period (Days 47 to 49) were similar to those exhibited at the end of the normal AT phase (Days 42 to 44). The superior thermoregulation of the dwarf broilers, as evidenced by their lower BT under high AT (0.66 C below their normal-sized counterparts, Table 2), was not reflected in their WG, which was similarly depressed by the heat stress in the two Dw phenotypes. Based on evidence of a negative association between FC and BT under heat stress (Deeb and Cahaner, 1999), a lower reduction in FC was expected in the dwarf broilers than in their normal-sized sibs due to the lower BT of the dwarfs. However, FC of the dwarf and normal broilers was equally reduced (by 27%) from Days 42 to 44 to Days 47 to 49 (Table 1). A similar reduction in FC of dwarf and normal layers due to high AT (5.8 and 5.4%, respectively) was found in 7-mo-old hens reared at 22 or 30 C for 18 wk (Ahmad et al., 1974). A higher reduction in FC of dwarf and normal layers at high AT (34.1 and 36.5%, respectively) was reported by Horst and Petersen (1977). Likewise, Simon (1972) found that dwarf birds, in contrast to their normal sibs, were unable to increase their FC when fed ad libitum following 2 d of feed withdrawal, during a period of six consecutive cycles of 2 d of feed withdrawal followed by 4 d of feeding ad libitum. Barbato et al. (1980) suggested that dwarfism results in altered feeding behavior, as evidenced by a change in the amount of time spent on feeding activity. It seems that the reduction in FC of dwarf broilers at high AT should be attributed to their lower appetite, which was not increased by their lower BT relative to that of the normal-sized broilers. When FC was expressed relative to metabolic BW (MBW = BW0.75), the FC:MBW ratio during the normal AT phase (Day 42 to 44) was similar in the dwarf and normal broilers (0.56 and 0.57, respectively). During the acute heat-stress period (Days 45 to 46), this ratio was reduced to 0.35 and 0.29 for the dwarf and normal broilers, respectively, leveling off at around 0.40 in both phenotypes under the chronic heat stress (Days 47 to 49). It seems that under stable conditions, when the broilers became accustomed to the AT (normal or high), FC was similarly determined by MBW in both phenotypes and did not reflect the difference in BT at high AT.
Dw as a Model to Investigate the Relationship Between BW and Heat Tolerance In a number of studies, stocks or lines with lower BW were always more tolerant to heat, i.e., their FC and WG were less depressed than in the high BW stocks (Adams and Rogler, 1968; Washburn et al., 1992; Eberhart and Washburn, 1993). However, due to differences in breeding history, these stocks also differed in other traits and some of these differences, rather than BW, might have been responsible for differences in heat tolerance (Washburn, 1985). In the present study, hemizygous dwarf females and heterozygous males were mated, producing full-sib families of chicks segregating for the two Dw phenotypes but sharing equal genetic backgrounds. Assuming that the Dw gene only modifies GR and BW, we hypothesized that this model would be appropriate to investigate the relationship between GR and BW of broilers and their heat tolerance. This hypothesis was confirmed only during the 24-h phase of acute heat stress. Later, under chronic heat stress, the small-sized dwarf broilers exhibited superior thermoregulation over their normal-sized counterparts, but without the expected advantage in FC and WG. Hormonal aspects of dwarf chickens are well documented (Decuypere et al., 1991; Tixier-Boichard et al., 1992), but have not been tested under high AT, hence the role of hormones in the suppression of FC and WG in dwarf broilers is not known. It can be concluded, however, that with regard to heat tolerance, low BW due to the dwarf gene does not resemble low BW due to polygenic variation in GR.
New Variation is Exposed Under High AT Immediately following the elevation in AT (Days 44 to 46), a drastic increase in variation (higher SD values, Table 1) was observed for WG and FE, but not for FC, in both phenotypes. With the rise in AT, there was also an increase in variation (among individuals within the two phenotypic groups) in BT, with more than twofold the SD values (Table 2). After the birds had adapted to the heat (Days 47 to 49), SD values of WG returned to their level under normal AT (13 and 20 in the dwarf and normal birds, respectively; Table 1). Apparently, great variation in adaptation to acute heat stress was exposed by the acute, but not chronic, heat stress. A similar increase in SD values of BT were observed when nakedneck and normally feathered broilers were reared under AT that fluctuated daily between 24 and 32 C (Deeb and Cahaner, 1999); BT of the normally feathered birds increased by about 1 C, and SD values were almost doubled during the high AT period. A higher variation in egg production (as reflected by higher CV values) was found in normal vs. dwarf hens when pullets were reared under summer heat stress (Khan et al., 1987). Unfortunately, BT was not measured on Days 48 and 49, hence it is not known whether the BT of the heatadapted broilers would have maintained this high level
EFFECTS OF HEAT STRESS ON DWARF VS. NORMAL BROILERS
of variation or would have returned to the levels observed during the normal AT phase. However, if the greater variation in WG and FE on Days 44 to 46 has a significant genetic component, it may suggest the feasibility of selection for better adaptation of broilers to acute heat stress. In conclusion, the results of Days 47 to 49 suggest that the Dw gene has no practical value in terms of improving the tolerance of fast-growing broilers to chronic heat stress. Moreover, dwarf broilers are not useful as a model to investigate the association between BW and heat tolerance. It is suggested that heat adaptation in broilers can be improved by applying selection in a hot environment. However, based on previous reports, such a selection may lead to reduced potential GR (GR at normal AT). In order to reliably evaluate the association between GR and heat tolerance, differences in GR must reflect the polygenic variation that is exploited by the standard selection procedures applied by commercial broiler breeders.
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